Synchronous Motor Question

[dereckbc]

Ok another dumb question from a mod I know, just that motors are not my field of expertise.

I was under the impression for variable speed control of AC motors an asynchronous should be used as they are best suited for the job in constant speed and torque applications. But as I get into learning more, EV applications, synchronous motors especially 400 Hz types are also good choices for their EV application.

So I guess what my question is or what I am over looking is how do you control the speed and torque of a synchronous motor without loosing a lot of efficiency?

So far I have done a bit of work with DC series and shunt wound motors and understand the controls (PWM), but I lack in AC motors. Any good link would be appreciated.

 

[gar]

090814-1603 EST

The speed of a synchronous motor is defined by the frequency of the applied power.

See if you can get access to "Alternating-Current Machinery", by Bailey and Gault, 1951, McGraw-Hill. Chapter 8, p124, Synchronous Motors.

Google books can give you information on the closest library that contains a copy. In my area there are about 4 libraries within 40 miles with a copy. There should a total of 146 libraries with the book. My number 1 library is listed as 8 miles but it is really nearer to 4 miles.

Torque loading causes the angle between the rotating magnetic field vector from the stator and the magnetic vector of the rotor to increase until sync is lost.

A stepping motor is a synchronous motor. A brushless DC motor is a synchronous motor. By appropriately adjusting DC current to the stator windings you can adjust the rotor position to any desired angle. This is how you get microstepping with a stepping motor. Same with fine positioning with a brushless DC motor.

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[mikeames]

Speed is controled by the change in frequency. 120 X frequency divided by the number of poles is the RPM. Torque will decreas as well by not by the same ratio. There will be a limit on how low you can go before the motor drops sync with the field. The solution would be to then get a motor with more poles. I dont remember the specifics because its been a while since I reviewed it .

 

[LarryFine]

A stepping motor is a synchronous motor. A brushless DC motor is a synchronous motor. By appropriately adjusting DC current to the stator windings you can adjust the rotor position to any desired angle. This is how you get microstepping with a stepping motor. Same with fine positioning with a brushless DC motor.

Sounds like you're describing a servo.

 

[gar]

090814-2044 EST

Larry:

The internal parts of a brushless DC motor are a permanent magnet rotor (could be an electromagnet, but performance would not be as good), a multi-pole stator, and also commutation sensors and likely an encoder.

This becomes a servo when feedback to control position and velocity is included in the overall package.

But fundamentally the motor part is a synchronous motor.

It is possible to take a standard synchronous motor and add appropriate circuitry and make it a servo.

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[dereckbc]

The speed of a synchronous motor is defined by the frequency of the applied power.

Gar thanks I understand that part perhaps I did not make myself clear.

My real interest is in a 400 Hz synchronous vs say a induction asynchronous. As a hobby I modify golf carts, which are mostly series wound DC motors with PWM controllers. I got that down to an art.

Now I want to use an AC motor and a VFD controller to get the higher efficiency and constant torque of an AC motor. I think the best candidate is an induction asynchronous. However I drop in a very active EV forum and the rage now seems to be using 400 Hz synchronous motors and it puzzles me. I can understand why they would look at a 400 Hz motor as the core material, windings, and overall size is reduced. But what is bothering me is the efficiency affects of using a VFD and the lower frequencies that would be required for variable speed,. The permeability of the core being designed for 400 Hz in my minds eye would make the lower speeds very inefficient. So I am missing something, and do not know what it is?

 

[gar]

090815-2229 EST

dereckbc:

If you use what are generally described as brushless DC motors, then construction is with a high energy, high flux density, permanent magnet on the rotor. This in of itself makes this a synchronous motor. Virtually no heat is generated in the rotor and therefore does not have to be transferred out thru other structures. Thus, most of the heat is generated in the stator and this has better heat transfer to the outside of the motor. For comparable output horsepower the motor can be smaller than other motors.

No matter what speed you run the motor the current in the stator windings determines torque. Since the real problem is thermal this means that the limitation at any speed as a first order approximation is some maximum continuous current. However, at low speeds the internal fan is insufficient for cooling and you need an external blower.

Note: HP = Constant*RPM*Torque.

You can operate the motor to full torque down to zero speed with an external fan. You must provide appropriate current control.

Note: this sounds just like a DC motor. The conventional DC motor generates most of its heat in the rotor. Thus, needs to be bigger for the same output HP.

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[Besoeker]

090815-2229 EST

If you use what are generally described as brushless DC motors, then construction is with a high energy, high flux density, permanent magnet on the rotor. This in of itself makes this a synchronous motor. Virtually no heat is generated in the rotor and therefore does not have to be transferred out thru other structures. Thus, most of the heat is generated in the stator and this has better heat transfer to the outside of the motor. For comparable output horsepower the motor can be smaller than other motors.

[/QUOTE]

We make brushless dc motors (and controllers) up to about 150kW. As you say, for comparable output they are smaller than other motor technologies and, with high operating speeds, very much smaller. The ones we make are normally either 10,000 rpm or 20,000 rpm. We use a carbon fibre sleeve to keep the magnets attached to the rotor - that was something of a learning curve...

Some the motors have gone into the EV field but most of our customers have been machine tool manufacturers. Here, dynamic performance is a must have - stopping for a tool change is non-productive time.
With the small size and inertia, we get from 10,000 rpm to zero in half a second within half a degree position accuracy.

 

[gar]

090816-1651 EST

To experiment and see how a rotating magnetic field is generated you could try the following experiment:

Two coils, two adjustable current power supplies, and one Boy Scout compass.

The coils are identical and wound on a 5/8" square cardboard tube 3/4" wide. The outside of the coil is about 1.25". Each coil has 700 turns of #30 enamel wire. Mounting the coil axes perpendicular to each other allows the generation of a magnetic vector that can be rotated in space by adjusting the relative currents in the coils.

The outside of the compass is octagonal with 2" across the flats. If the compass is orientated so that one flat is north-south and the coil is placed against the east flat, then about 15 MA of current produces a magnetic vector of about the intensity of the earth's magnetic field. This causes the needle to move 45 deg to the east or west when the current is flowing.

One reference has a criteria of 115 MA for maximum current. I have not done temperature rise measurements, but 100 MA is probably safe.

Setting two coils at 90 deg relative to each other and adjacent to the compass and using a maximum of 100 ma should allow you to position the compass needle to any desired angle fro 0 thru 360.

A 100 MA current produces a flux density about 7 times greater than the earth and thus the earth's field is not a large factor in the experiment.

You figure out the how to adjust the currents for a given angle and maintain an approximately constant magnitude magnetic vector.

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