Slow-speed synchronous timing motors

Representative are low-torque synchronous motors with a multi-pole hollow cylindrical magnet (internal poles) surrounding the stator structure. An aluminum cup supports the magnet. The stator has one coil, coaxial with the shaft. At ????? so they are parallel with the shaft. They are the stator poles. One of the pair of dirtyryrty a common shading coil. The poles are rather narrow, and between the poles leading from one end of the coil are an identical set leading from the ???? end. In all, this creates a repeating sequence of four poles, unshaded alternating with shaded, that creates a circumferential traveling field to which the rotor's magnetic poles rapidly synchronize. Some stepping motors have a similar structure.

Watthour-meter motors

These are essentially two-phase induction motors with permanent magnets that retard rotor speed, so their speed is quite accurately proportional to wattage of the power passing through the meter. The rotor is an aluminum-alloy disc, and currents induced into it react with the field from the stator. One phase of the stator is a coil with many turns and a high inductance, which causes its magnetic field to lag almost 90 degrees with respect to the applied (line/mains) voltage. The other phase of the stator is a pair of coils with very few turns of heavy-gauge wire, hence quite-low inductance. These coils are in series with the load.

The core structure, seen face-on, is akin to a cartoon mouth with one tooth above and two below. Surrounding the poles ("teeth") is the common flux return path. The upper pole (high-inductance winding) is centered, and the lower ones equidistant. Because the lower coils are wound in opposition, the three poles cooperate to create a "sidewise" traveling flux. The disc is between the upper and lower poles, but with its shaft definitely in front of the field, so the tangential flux movement makes it rotate.

Electronically commutated motors

Such motors have an external rotor with a cup-shaped housing and a radially magnetized permanent magnet connected in the cup-shaped housing. An interior stator is positioned in the cup-shaped housing. The interior stator has a laminated core having grooves. Windings are provided within the grooves. The windings have first end turns proximal to a bottom of the cup-shaped housing and second end turns positioned distal to the bottom. The first and second end turns electrically connect the windings to one another. The permanent magnet has an end face rom the bottom of the cup-shaped housing. At least one galvano-magnetic rotor position sensor is arranged opposite the end face of the permanent magnet so as to be located within a magnetic leakage of the permanent magnet and within a magnetic leakage of the interior stator. The at least one rotor position sensor is designed to control current within at least a portion of the windings. A magnetic leakage flux concentrator is arranged at the interior stator at the second end turns at a side of the second end turns facing away from the laminated core and positioned at least within an angular area of the interior stator in which the at least one rotor position sensor is located.

ECM motors are increasingly being found in forced-air furnaces and HVAC systems to save on electricity costs as modern HVAC systems are running their fans for longer periods of time (duty cycle).^[6] The cost effectiveness of using ECM motors in HVAC systems is questionable, given that the repair (replacement) costs are likely to equal or exceed the savings realized by using such a motor.^{[citation needed]}

Hysteresis synchronous motors

These motors are relatively costly, and are used where exact speed (assuming an exact-frequency AC source) as well as rotation with a very small amount of fast variations in speed (called 'flutter" in audio recordings) is essential. Applications included tape recorder capstan drives (the motor shaft could be the capstan). Their distinguishing feature is their rotor, which is a smooth cylinder of a magnetic alloy that stays magnetized, but can be demagnetized fairly easily as well as re-magnetized with poles in a new location. Hysteresis refers to how the magnetic flux in the metal lags behind the external magnetizing force; for instance, to demagnetize such a material, one could apply a magnetizing field of opposite polarity to that which originally magnetized the material.

These motors have a stator like those of capacitor-run squirrel-cage induction motors. On startup, when slip decreases sufficiently, the rotor becomes magnetized by the stator's field, and the poles stay in place. The motor then runs at synchronous speed as if the rotor were a permanent magnet. When stopped and re-started, the poles are likely to form at different locations.

For a given design, torque at synchronous speed is only relatively modest, and the motor can run at below synchronous speed.

Single-phase AC synchronous motors

Small single-phase AC motors can also be designed with magnetized rotors (or several variations on that idea; see "Hysteresis synchronous motors" below). The rotors in these motors do not require any induced current so they do not slip backward against the mains frequency. Instead, they rotate synchronously with the mains frequency. Because of their highly accurate speed, such motors are usually used to power mechanical clocks, audio turntables, and tape drives; formerly they were also much used in accurate timing instruments such as strip-chart recorders or telescope drive mechanisms. The shaded-pole synchronous motor is one version.

If a conventional squirrel-cage rotor has flats ground on it to create salient poles and increase reluctance, it will start conventionally, but will run synchronously, although it can provide only a modest torque at synchronous speed.

Because inertia makes it difficult to instantly accelerate the rotor from stopped to synchronous speed, these motors normally require some sort of special feature to get started. Some include a squirrel-cage structure to bring the rotor close to synchronous speed. Various other designs use a small induction motor (which may share the same field coils and rotor as the synchronous motor) or a very light rotor with a one-way mechanism (to ensure that the rotor starts in the "forward" direction). In the latter instance, applying AC power creates chaotic (or seemingly chaotic) jumping movement back and forth; such a motor will always start, but lacking the anti-reversal mechanism, the direction it runs is unpredictable. The Hammond organ tone generator used a non-self-starting synchronous motor (until comparatively recently), and had an auxiliary conventional shaded-pole starting motor. A spring-loaded auxiliary manual starting switch connected power to this second motor for a few seconds.

Generator and Motor

A hand-cranked generator can be used to generate voltage to turn a motor. This is an example of energy conversion from mechanical to electrical energy and then back to mechanical energy.

As the motor is turning, it also acts as a generator and generates a "back emf". By Lenz's law, the emf generated by the motor coil will oppose the change that created it. If the motor is not driving a load, then the generated back emf will almost balance the input voltage and very little current will flow in the coil of the motor. But if the motor is driving a heavy load, the back emf will be less and more current will flow in the motor coil and that electric power being used is converted to the mechanical power to drive the load.

AC Generator

The turning of a coil in a magnetic field produces motional emfs in both sides of the coil which add. Since the component of the velocity perpendicular to the magnetic field changes sinusoidally with the rotation, the generated voltage is sinusoidal or AC. This process can be described in terms of Faraday's law when you see that the rotation of the coil continually changes the magnetic flux through the coil and therefore generates a voltage.

How Alternators Work

An alternator consists of a rotor, stator, rectifier, housing, and shaft. The shaft is usually connected to the car engine by a belt and pulleys. The rotor is fixed to the shaft, and consists of several wire bundles wound around an iron core. Current is either induced, or passed through sliprings and brushes on the shaft, to create a magnetic field in the rotor. This assembly spins inside the stator winding which is fixed to the housing. A current is induced in the stator, which is then converted to direct current by the rectifier. The stator usually produces three-phase alternating current to allow for a smoother direct current.

An alternator is different to a regular generator in two ways. First, it creates it's own magnetic field using the rotor winding instead of using magnets. This makes alternators smaller and lighter than most generators. Second, it's main winding is fixed to the housing, rather than on the shaft. This allows for more windings in the stator which results in more current.

Terminals

Alternators usually have four terminals marked with letters. The "B" terminal is the main output which connects to the battery. The "S" terminal also connects to the battery and measures the voltage. The "IG" terminal is connected to the ignition switch, and the "L" terminal is connected the charging light.

Regulator

A regulator controls the output of the alternator by changing the current in the rotor. This changes the strength of it's magnetic field, and the amount of current induced in the stator. It monitors the battery voltage via the "S" terminal and compares it to the output voltage. Modern alternators have the regulator inside the case, but older models had an external regulator.

Troubleshooting

If the voltage at the battery terminals is lower than expected, the problem is usually a break in one of the stator coils. If there is no voltage, the rotor winding may have a break. Another cause of low voltage is seized bearings.

Alternator Uses

When the engine is running, the alternator powers the spark plugs, charges the battery, and powers the accessories. An alternator is essential because the battery would quickly go flat doing all this, leaving no charge to start the engine next time.

Alternators - what are they, how do they work and what breaks??

If you have been around cars for a while you might have heard the term generator. Well, those were the old days and the good old generator is history. What a generator did for the old cars, an alternator now does. You see, a car has and electrical system that carries power to such essential things as headlights, the ignition coils, engine cooling fans and other non-essential things as the radio (my son would argue that one), air conditioning fans (my wife would argue that one), and all of those other things upon which we have become accustomed to depend. All of that power has to come from somewhere!! A lot of people might think that power comes from the battery, and that is true to some extent. But the real answer is that the power to run all of those electrical things comes from Saudi Arabia! Huh? Saudi Arabia? Well, maybe Dallas, or Oklahoma. But the point is that the source for all of that energy is the gas tank. Yep. And the link from the gas tank to the battery is that mysterious thing called the alternator. It takes mechanical power from the crankshaft, transmits it via a "fan" belt, (it used to run the cooling fan as well) or serpentine belt as it is called in most of the newer vintage cars, and turns the alternator. So, the main function of the alternator is to convert power from the gasoline engine that drives you along the road, to electrical energy to keep the battery in tip-top condition.

So, what happens when an alternator goes bad? Well, at first, nothing. That is because the battery has some reserve power in it, enough to keep the engine running for quite some time, many many miles in fact. So a bad alternator doesn't necessarily mean a tow truck should be called right away. As long as energy is conserved elsewhere, like turning off the blower motor, the rear window de-fogger, the stereo and the headlights (if possible), you could make it for some distance on just the battery reserve alone.

One major problem which will finally occur as the battery loses its charge is that there will not be sufficient voltage to keep the engine running well. Many years ago I was in California and saw a car coming down the street with its catalytic converter glowing white hot and flames coming from beneath the car. What had happened is the alternator quit, the battery ran down, the engine was not firing on all eight cylinders and the unburned fuel was being burned in the catalytic converter! It had been long overdue for the driver to call a tow truck!

Before we get started diagnosing alternator problems there is one thing I must mention. Alternators use an "exciter" voltage to get the alternator working when you start your car. Now get this! About 90% of the cars made today run that 12 volts through the "battery" or "alternator" bulb (AKA the idiot light). So you need to check to see if this bulb is not burned out. It should light when the key is turned on! If it doesn't then there is a very good chance that the alternator will not put out!! Replace the bulb before beginning the rest of the diagnosis.

So, how do you know when your alternator is going bad? Most of the time the alternator fails in stages. A little techie talk here. The alternator gets its name from the fact that it generates alternating current (AC). The old generators I mentioned before generated direct current (DC). Well the battery can't use alternating current so the alternator output is fed into what are called diodes, which convert the AC into DC. The alternator has a unique feature in that it is able to generate a relatively high voltage while the engine is at idle. The old generators needed to be running at a fast pace before they got up to 13 or 14 volts. The alternator can do this since it is really three alternators in one body. Each of the three sections of the alternator generates its voltage out of phase with the other two sections. Since the complete cycle (one revolution) of the alternator is 360 degrees, each phase is shifted by 120 degrees from the next phase. So in one revolution of the alternator it puts out three separate voltages.

OK, back to the failure mode. Each of the three phases has its own windings in the alternator and each of the windings has its own pair of diodes. Each of these windings and/or diodes can fail, one set at a time. If this happens the alternator can still charge the battery, but only with a limited current, approximately 2/3 of its original capacity if one system fails. If two systems fail, then it puts out only 1/3 of its rated capacity. What that means to you is that you can go a long time on a limping alternator. Chances are if you don't need headlights or air conditioning or other high current using accessories, you would never know that the alternator was in the process of failing! The time you will find out is when it is 10 below zero and you wear down the battery by cranking the starter, then put the fan on high for heat, and then drive in the dark.

So, how can you tell if the alternator is failing without taking it apart and doing some measuring inside the alternator? It's really pretty simple. You will need a simple voltmeter. You can get one at Radio Shack for under ten dollars. Here's what you do - start the car, make sure all the accessories are off and rev up the motor to a fast idle. Set the Voltmeter to the DC scale (not AC or Ohms). Measure the voltage across the battery terminals - red lead of the voltmeter on the positive terminal, black on the negative (ground in most cars). The voltage should, and probably will, read around 14 volts. If it reads less than 12 volts you may indeed have a failed alternator and you can skip the next step. Next, turn on the heater, the rear window de-fogger, the radio, the headlights and anything else that draws power. Now rev up the motor and watch the voltmeter. It should still be reading around 14 volts. If it reads lower than 13 volts the chances are that your alternator is not up to snuff.

One last failure mode is of course noise. The rotor inside the alternator rotates on bearings, normally very high precision needle bearings, and these can fail. When they do you will hear a loud grinding noise associated with the alternator. To isolate the noise take a length of tubing, heater hose will do fine, put one end to your ear and move the other around in the vicinity of the alternator. The noise will be much louder when you point it at the alternator if that is the culprit. Other possibilities are the water pump and the power steering pump which are also driven by the engine belt. To further isolate the noise disconnect the drive belt and spin the alternator by hand. If you hear a rumble or grinding noise then the bearings are shot. If you don't hear a noise the problem may still be in the alternator since the bearing might be quiet without the loading of the drive belt tension. Check for side play in the pulley. If you are pretty certain the noise came from the alternator it is a relatively simple task to take it apart and visually inspect the bearings, else swap it in for a rebuilt. Your auto supply store will normally bench test the alternator free of charge and can tell you at that time if the bearings are noisy.

Before you go running down to the parts store for a new alternator make sure to check the connections at the battery terminals and also check to see that the voltage is the same at the alternator terminal (the big fat one with the heavy wire attached) {also, read the article, dead battery}. Check to make sure the belts are tight and not slipping. Replace them if they are cracked or shiny on the side that faces the alternator pulley.

One final thing to check - the field voltage. In order for the alternator to generate electricity it must be supplied with a field voltage. If you know which wire is the one that supplies the field (normally labeled 'F') then simply check with a voltmeter to see if there is 12 volts at the field. Another check is to use a hacksaw blade or a lightweight screwdriver , anything magnetic, and hold it near the side of the alternator with the ignition switch turned in the on position. If there is a field voltage present then the metal will be attracted magnetically to the side of the alternator, not very strongly, but you will feel it pull the metal to the side of the alternator.

So, what are you going to ask the mechanic when he tells you that you need a new alternator?

1. Did you perform a load test on the alternator? If you did, what were the voltage readings? Were they all below specification?? (mechanics will use a load testing machine instead of turning on all the accessories.)

2. Did you check to see if the belts were old and cracked or possibly slipping?

3. Did you measure the voltage at the alternator connector or at the battery? Were the readings the same at both places or is there a voltage drop somewhere in the system. You can tell him the "Dead Battery" story if you want to.

4. Finally, did you check the price on a rebuilt as well as a new alternator? (rebuilt alternators are just as good as new if they are done correctly and usually cost about 1/3 as much)

Now that you know all about alternators you can feel confident that you can discuss the failure modes with a mechanic and not get shafted. It is also fun to watch the faces of a mechanic when you ask questions like those above. He will soon figure out that you know more about the electrical system of your car than how to turn the lights on!

A Very Simple AC Motor

Here is a photo of a very simple eddy current AC motor I put together. I think this one wins the prize for the Simplest AC Motor you can make. It works great and is very easy to build. I found the original plans in a book titled: "Physics Demonstration Experiments" by Harry F. Meiners, Vol 2, Ronald Press Co., NY, 1970, LCCC #69-14674. With some experimentation, I found that the can spins faster when the nut is on the end of the bolt than when the nut is removed. What do you think will happen if the rotor is moved to the other side of the bolt? It consists of a coil mounted onto a 3/4" bolt. The coil is about 100' of 20AWG wire, on a form about 1.5" long, with a dc resistance of about 1.2 ohms, and an inductance of about 2.4mH as an air-core inductor. The voltage supplied to the coil is 19Vac from a plug-in transformer and supplies about 2.5Aac to the coil. The rotor is an aluminum film canister (today they use plastic, but you might still find a few of these around - ask your friends) with a dimple in the bottom of it, resting on a pencil. (I figured that the graphite in the pencil will lubricate the rotor.)

The eddy current motor on the left has two rotors, they spin in opposite directions. The set-up on the right shows a variac, multimeter, eddy current motor, and a calibrated strobe. With this, we could plot speed vs. voltage. We found that the rotor would spin about 1000 rpm with 120V applied to it. Can't keep it there for long, since the coil and bolt get real hot. On these two coils, a smaller diameter wire was used, so the dc resistance was about 11.2 ohms, and 24mH as an air-core inductor. With this, we could apply 120Vac to it and only 2 amps would be drawn.

This shows the basic construction. The bolt is a 4" long 3/4-13 bolt, the wood is 3/4" thick. I put a small dimple into the bottom of the aluminum film canister so it would sit onto the pencil point. The red strips of tape helped with the strobe and looks cool as it spins. I found that the nut on the end of the bolt makes it go faster.

A Shaded Pole AC Motor

Here is a photo of a typical shaded pole motor. See the close-up of the notch in the laminations and the extra heavy winding of two turns creating the phase difference between the two sections of the laminations, giving the magnetic field a directional motion. The rotor spins CW as seen from the end with the screw on the shaft. Motors like this are used in thousands of applications.

Another Shaded Pole AC Motor

Here is a photo of a ceiling fan motor, also shaded pole, but with six windings instead of only one as seen above. The rotor laminations are skewed to provide smoother torque. The pole pieces with the windings have a slot in them to create a delayed flux, creating a direction for rotation.

A Universal Motor

And here is a photo of a universal motor. It has brushes like a DC motor, but will operate on AC or DC.

A 3-Phase AC Motor Demonstrator

Here is a project my daughter is working on. It shows how a 3-phase AC motor works with a rotating magnetic field and a permanent magnet rotor, making it a synchronous AC motor. We have pushbuttons which allow the user to turn on any one of the pairs of opposite coils, in either a N-S or a S-N orientation. For example, the green button turns on the horizontal pair of coils in a S-N orientation. The yellow button turns on the horizontal pair of coils in a N-S orientation. On each coil is a bi-color LED to indicate the magnetic polarity of the coil when it is turned on. The power to the coils (each pair connected in parallel) is supplied by a 5v computer power supply. The coils draw about 4amps at 5Vdc each, so a supply with 23amps available is a great match. Each coil is mounted on a 3/4" bolt, attached to a hinge. This way, sets of coils can be folded down out of the way to show how a shaded pole motor works. The rotor is a bar of steel with a NIB magnet on each end. The rotor does oscillate a bit when going from coil to coil.

Here's more photos:

By pressing the colored buttons in the correct sequence, the rotor will follow the magnetic field in a clockwise fashion. The faster you go through the sequence, the faster the rotor will rotate. This shows that the speed of this motor is dependant on the frequency of the power applied to it. The higher the frequency, the faster it goes. At 60Hz, it would rotate at 1 revolution/cycle * 60 cycles/sec * 60 sec/min = 3600 revolutions per minute or rpm.

Three Phase AC Motor Stator

Industrial AC Motors

These are cut-aways of actual industrial three phase AC motors. They have different HP ratings, from 5hp, 2hp, 900hp. They are manufactured by Reliance Electric (used to be part of Rockwell Automation, now part of Baldor Electric).

Linear motors

A linear motor is like an ac motor, but it is unwrapped and laid out flat. The photos show parts of linear motors. Some have flat coils and magnet sections, others are "T" shaped. Check www.anorad.com for more info.
More information is available in these two excellent articles:

Repulsion motor

are wound-rotor single-phase AC motors that are similar to universal motors. In a repulsion motor, the armature brushes are shorted together rather than connected in series with the field. By transformer action ,the stator induces currents in the rotor, which create torque by repulsion instead of attraction as in other motors. Several types of repulsion motors have been manufactured, but the repulsion-start induction-run (RS-IR) motor has been used most frequently. The RS-IR motor has a centrifugal switch that shorts all segments of the commutator so that the motor operates as an induction motor once it has been accelerated to full speed. Some of these motors also lift the brushes out of contact with the commutator once the commutator is shorted. RS-IR motors have been used to provide high starting torque per ampere under conditions of cold operating temperatures and poor source voltage regulation. Few repulsion motors of any type are sold as of 2005.

Permanent-split capacitor motor

Another variation is the permanent-split capacitor (PSC) motor (also known as a capacitor start and run motor).^[5] This motor operates similarly to the capacitor-start motor described above, but there is no centrifugal starting switch,^[5] and what correspond to the the start windings (second windings) are permanently connected to the power source (through a capacitor), along with the run windings.^[5] PSC motors are frequently used in air handlers, blowers, and fans (including ceiling fans) and other cases where a variable speed is desired.

A capacitor ranging from 3 to 25 microfarads is connected in series with the "start" windings and remains in the circuit during the run cycle.^[5] The "start" windings and run windings are identical in this motor,^[5] and reverse motion can be achieved by reversing the wiring of the 2 windings,^[5] with the capacitor connected to the other windings as "start" windings. By changing taps on the running winding but keeping the load constant, the motor can be made to run at different speeds. Also, provided all 6 winding connections are available separately, a 3 phase motor can be converted to a capacitor start and run motor by commoning two of the windings and connecting the third via a capacitor to act as a start winding.

[edit]

Split-phase induction motor

Another common single-phase AC motor is the split-phase induction motor,^[4] commonly used in major appliances such as washing machines and clothes dryers. Compared to the shaded pole motor, these motors can generally provide much greater starting torque by using a special startup winding in conjunction with a centrifugal switch.

In the split-phase motor, the startup winding is designed with a higher resistance than the running winding. This creates an LR circuit which slightly shifts the phase of the current in the startup winding. When the motor is starting, the startup winding is connected to the power source via a set of spring-loaded contacts pressed upon by the stationary centrifugal switch. The starting winding is wound with fewer turns of smaller wire than the main winding, so it has a lower inductance (L) and higher resistance (R). The lower L/R ratio creates a small phase shift, not more than about 30 degrees, between the flux due to the main winding and the flux of the starting winding. The starting direction of rotation may be reversed simply by exchanging the connections of the startup winding relative to the running winding..

The phase of the magnetic field in this startup winding is shifted from the phase of the mains power, allowing the creation of a moving magnetic field which starts the motor. Once the motor reaches near design operating speed, the centrifugal switch activates, opening the contacts and disconnecting the startup winding from the power source. The motor then operates solely on the running winding. The starting winding must be disconnected since it would increase the losses in the motor.

[edit] Capacitor start motor

A capacitor start motor is a split-phase induction motor with a starting capacitor inserted in series with the startup winding, creating an LC circuit which is capable of a much greater phase shift (and so, a much greater starting torque). The capacitor naturally adds expense to such motors.

[edit]

Single-phase AC induction motors

Shaded-pole motor

A common single-phase motor is the shaded-pole motor, which is used in devices requiring low starting torque, such as electric fans or other small household appliances. In this motor, small single-turn copper "shading coils" create the moving magnetic field. Part of each pole is encircled by a copper coil or strap; the induced current in the strap opposes the change of flux through the coil (Lenz's Law). This causes a time lag in the flux passing through the shading coil, so that the maximum field intensity moves across the pole face on each cycle. This produces a low level rotating magnetic field which is large enough to turn both the rotor and its attached load. As the rotor accelerates the torque builds up to its full level as the principal (rotationally stationary) magnetic field is rotating relative to the rotating rotor. Such motors are difficult to reverse without significant internal alterations or disassembly and (if possible) reassembling the stator, but flipped over.

A reversible shaded-pole motor was made by Barber-Colman several decades ago. It had a single field coil, and two principal poles, each split halfway to create two pairs of poles. Each of these four "half-poles" carried a coil, and the coils of diagonally-opposite half-poles were connected to a pair of terminals. One terminal of each pair was common, so only three terminals were needed in all.

The motor would not start with the terminals open; connecting the common to one other made the motor run one way, and connecting common to the other (only) made it run the other way. These motors were used in industrial and scientific devices.

An unusual, adjustable-speed, low-torque shaded-pole motor could be found in traffic-light and advertising-lighting controllers. The motor drove a shaft through a reduction gear train; the shaft carried cams to operate switches for the lights. The rotor was simply an aluminum disc roughly 15 cm in diameter. The stator had one coil, and the general shape of a letter C, but with a only a small gap at the ends.

The pole faces were parallel and relatively close to each other, with the disc centered between them, something like the disc in a watthour meter. Each pole face was split, and had a shading coil on one part; the shading coils were on the parts that faced each other. Both shading coils were probably closer to the main coil; they could have both been farther away, without affecting the operating principle, just the direction of rotation.

Applying AC to the coil created a field that progressed in the gap between the poles. The plane of the stator core was approximately tangential to an imaginary circle on the disc, so the traveling magnetic field dragged the disc and made it rotate.

The stator was mounted on a pivot so it could be positioned for the desired speed and then clamped in position. Keeping in mind that the effective speed of the traveling magnetic field in the gap was constant, placing the poles nearer to the center of the disc made it run relatively faster, and toward the edge, slower.

It's possible that these motors are still in use in some older installations.

[edit]

Two-phase AC servo motors

A typical two-phase AC servo-motor has a squirrel cage rotor and a field consisting of two windings:

1. a constant-voltage (AC) main winding.
2. a control-voltage (AC) winding in quadrature with the main winding so as to produce a rotating magnetic field. Reversing phase makes the motor reverse.

An AC servo amplifier, a linear power amplifier, feeds the control winding. The electrical resistance of the rotor is made high intentionally so that the speed/torque curve is fairly linear. Two-phase servo motors are inherently high-speed, low-torque devices, heavily geared down to drive the load. In the World War II Ford Instrument Company naval analog fire-control computers, these motors had identical windings and an associated phase-shift capacitor. AC power was fed through tungsten contacts arranged in a very simple bridge-topology circuit to develop reversible torque.

Three-phase AC synchronous motors

If connections to the rotor coils of a three-phase motor are taken out on slip-rings and fed a separate field current to create a continuous magnetic field (or if the rotor consists of a permanent magnet), the result is called a synchronous motor because the rotor will rotate synchronously with the rotating magnetic field produced by the polyphase electrical supply.

The synchronous motor can also be used as an alternator.

Nowadays, synchronous motors are frequently driven by transistorized variable-frequency drives. This greatly eases the problem of starting the massive rotor of a large synchronous motor. They may also be started as induction motors using a squirrel-cage winding that shares the common rotor: once the motor reaches synchronous speed, no current is induced in the squirrel-cage winding so it has little effect on the synchronous operation of the motor, aside from stabilizing the motor speed on load changes.

Synchronous motors are occasionally used as traction motors; the TGV may be the best-known example of such use.

One use for this type of motor is its use in a power factor correction scheme. They are referred to as synchronous condensers. This exploits a feature of the machine where it consumes power at a leading power factor when its rotor is over excited. It thus appears to the supply to be a capacitor, and could thus be used to correct the lagging power factor that is usually presented to the electric supply by inductive loads. The excitation is adjusted until a near unity power factor is obtained (often automatically). Machines used for this purpose are easily identified as they have no shaft extensions. Synchronous motors are valued in any case because their power factor is much better than that of induction motors, making them preferred for very high power applications.

Some of the largest AC motors are pumped-storage hydroelectricity generators that are operated as synchronous motors to pump water to a reservoir at a higher elevation for later use to generate electricity using the same machinery. Six 350-megawatt generators are installed in the Bath County Pumped Storage Station in Virginia, USA. When pumping, each unit can produce 563,400 horsepower (420 megawatts).^[3]

Wound rotor

An alternate design, called the wound rotor, is used when variable speed is required. In this case, the rotor has the same number of poles as the stator and the windings are made of wire, connected to slip rings on the shaft. Carbon brushes connect the slip rings to an external controller such as a variable resistor that allows changing the motor's slip rate. In certain high-power variable speed wound-rotor drives, the slip-frequency energy is captured, rectified and returned to the power supply through an inverter.

Compared to squirrel cage rotors, wound rotor motors are expensive and require maintenance of the slip rings and brushes, but they were the standard form for variable speed control before the advent of compact power electronic devices. Transistorized inverters with variable-frequency drive can now be used for speed control, and wound rotor motors are becoming less common. (Transistorized inverter drives also allow the more-efficient three-phase motors to be used when only single-phase mains current is available, but this is never used in household appliances, because it can cause electrical interference and because of high power requirements.)

Several methods of starting a polyphase motor are used. Where the large inrush current and high starting torque can be permitted, the motor can be started across the line, by applying full line voltage to the terminals (Direct-on-line, DOL). Where it is necessary to limit the starting inrush current (where the motor is large compared with the short-circuit capacity of the supply), reduced voltage starting using either series inductors, an autotransformer, thyristors, or other devices are used. A technique sometimes used is (Star-Delta, YΔ) starting, where the motor coils are initially connected in star for acceleration of the load, then switched to delta when the load is up to speed. This technique is more common in Europe than in North America. Transistorized drives can directly vary the applied voltage as required by the starting characteristics of the motor and load.

This type of motor is becoming more common in traction applications such as locomotives, where it is known as the asynchronous traction motor.

The speed of the AC motor is determined primarily by the frequency of the AC supply and the number of poles in the stator winding, according to the relation:

N_s = 120F / p

where

N_s = Synchronous speed, in revolutions per minute

F = AC power frequency

p = Number of poles per phase winding

Actual RPM for an induction motor will be less than this calculated synchronous speed by an amount known as slip, that increases with the torque produced. With no load, the speed will be very close to synchronous. When loaded, standard motors have between 2-3% slip, special motors may have up to 7% slip, and a class of motors known as torque motors are rated to operate at 100% slip (0 RPM/full stall).

The slip of the AC motor is calculated by:

S = (N_s − N_r) / N_s

where

N_r = Rotational speed, in revolutions per minute.

S = Normalised Slip, 0 to 1.

As an example, a typical four-pole motor running on 60 Hz might have a nameplate rating of 1725 RPM at full load, while its calculated speed is 1800 RPM.

The speed in this type of motor has traditionally been altered by having additional sets of coils or poles in the motor that can be switched on and off to change the speed of magnetic field rotation. However, developments in power electronics mean that the frequency of the power supply can also now be varied to provide a smoother control of the motor speed.

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Slip

If the rotor of a squirrel-cage motor were to run at synchronous speed, the flux in the rotor at any given place on the rotor would not change, and no current would be created in the squirrel cage. For this reason, ordinary squirrel-cage motors run at some tens of rpm slower than synchronous speed, even at no load. Because the rotating field (or equivalent pulsating field) actually or effectively rotates faster than the rotor, it could be said to slip past the surface of the rotor. The difference between synchronous speed and actual speed is called slip, and loading the motor increases the amount of slip as the motor slows down slightly.

Squirrel-cage rotors

Most common AC motors use the squirrel cage rotor, which will be found in virtually all domestic and light industrial alternating current motors. The squirrel cage refers to the rotating exercise cage for pet animals. The motor takes its name from the shape of its rotor "windings"- a ring at either end of the rotor, with bars connecting the rings running the length of the rotor. It is typically cast aluminum or copper poured between the iron laminates of the rotor, and usually only the end rings will be visible. The vast majority of the rotor currents will flow through the bars rather than the higher-resistance and usually varnished laminates. Very low voltages at very high currents are typical in the bars and end rings; high efficiency motors will often use cast copper in order to reduce the resistance in the rotor.

In operation, the squirrel cage motor may be viewed as a transformer with a rotating secondary. When the rotor is not rotating in sync with the magnetic field, large rotor currents are induced; the large rotor currents magnetize the rotor and interact with the stator's magnetic fields to bring the rotor almost into synchronization with the stator's field. An unloaded squirrel cage motor at rated no-load speed will consume electrical power only to maintain rotor speed against friction and resistance losses; as the mechanical load increases, so will the electrical load - the electrical load is inherently related to the mechanical load. This is similar to a transformer, where the primary's electrical load is related to the secondary's electrical load.

This is why, for example, a squirrel cage blower motor may cause the lights in a home to dim as it starts, but doesn't dim the lights on startup when its fan belt (and therefore mechanical load) is removed. Furthermore, a stalled squirrel cage motor (overloaded or with a jammed shaft) will consume current limited only by circuit resistance as it attempts to start. Unless something else limits the current (or cuts it off completely) overheating and destruction of the winding insulation is the likely outcome.

In order to prevent the currents induced in the squirrel cage from superimposing itself back onto the supply, the squirrel cage is generally constructed with a prime number of bars, or at least a small multiple of a prime number (rarely more than 2). There is an optimum number of bars in any design, and increasing the number of bars beyond that point merely serves to increase the losses of the motor particularly when starting.

Virtually every washing machine, dishwasher, standalone fan, record player, etc. uses some variant of a squirrel cage motor.

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Three-phase AC induction motors

Where a polyphase electrical supply is available, the three-phase (or polyphase) AC induction motor is commonly used, especially for higher-powered motors. The phase differences between the three phases of the polyphase electrical supply create a rotating electromagnetic field in the motor.

Through electromagnetic induction, the time changing and reversing rotating magnetic field induces a time changing and reversing current in the conductors in the rotor; this sets up a time changing and opposing moving electromagnetic field that causes the rotor to turn with the field. The rotor always rotates slightly behind the phase peak of the primary magnetic field of the stator and, thus, is always moving slower than the rotating magnetic field produced by the polyphase electrical supply.

Induction motors are the workhorses of industry and motors up to about 500 kW (670 hp) in output are produced in highly standardized frame sizes, making them nearly completely interchangeable between manufacturers (although European and North American standard dimensions are different). Very large induction motors are capable of tens of megawatts of output, for pipeline compressors, wind-tunnel drives, and overland conveyor systems.

There are two types of rotors used in induction motors: squirrel cage rotors and wound rotors.

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AC motor

An induction motor (IM) is a type of asynchronous AC motor where power is supplied to the rotating device by means of electromagnetic induction. Another commonly used name is squirrel cage motor because the rotor bars with short circuit rings resemble a squirrel cage (hamster wheel).

An electric motor converts electrical power to mechanical power in its rotor (rotating part). There are several ways to supply power to the rotor. In a DC motor this power is supplied to the armature directly from a DC source, while in an induction motor this power is induced in the rotating device. An induction motor is sometimes called a rotating transformer because the stator (stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is the secondary side. Induction motors are widely used, especially polyphase induction motors, which are frequently used in industrial drives.

Induction motors are now the preferred choice for industrial motors due to their rugged construction, absence of brushes (which are required in most DC motors) and — thanks to modern power electronics — the ability to control the speed of the motor.