Synchronous Motor Question - Mike Holt's Forum

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. - 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, , 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.

. 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 . - 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.

.

gar; said: 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? - 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.

.

- 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.

Fellow members, I am a Mechanical engineer, who has to deal with motors attached to mechanical devices, and who knows just enough to be dangerous about motors. I have even posted to threads in this forum.

However, one thing I cannot remember from my old college days is the difference between induction motors, (which I have a lot of familiarity with) and synchronous motors, which I encounter from time to time in small fractional HP as well as 1-10 HP ranges.

I have looked in all the text books and reference books I have in my possession, as well as googling the words. Still I don't know what I would like to know about synchronous motors. Differences, advantages, disadvantages, etc.

I'll bet you electrical types can straighten me out. Comment: The differences, advantages, disadvantages, etc. should be available in textbooks, e.g.
1. M.G. Say "Alternating Current Machines"
2. S. R. Slemon "Magnetoelectric Devices, Transducers, Transformers, and Machines"
3. A. E. Fitzgerald, Charles Kingsley, Jr., Stephen D. Umans "Electric Machinery"
4. Etc.
Differences: Asynchronous motor runs with slip, does not have field winding supplied by a power source called "Exciter", has a different speed - torque characteristics, it is less expensive, etc.
Advantages: Asynchronous motor is simpler and rugged, less expensive, cannot drop out of synchronism, is often simply started with Direct On Line (DOL) start, etc.
Disadvantages: Asynchronous motors usually do not power very high HP loads, experience slip, tend to burn rather than drop out of synchronism and stop, etc.

Also, visit technical library and read various Encyclopedia, Technical Dictionary, and popular magazines/journals similar to popular mechanics. Besides the speed difference the synchronous motor needs field excitation with DC. Then you have a three phase supply for the armature and a Dc supply for the field.

The synchronous motor has constant speed from no load to full load or until it is pulled out of synchronism, that speed is exactly 120*Line Frequency/Poles.

Synchronous motors can be applied to very large loads of constant type achieving high efficiency and adjustable power factor. One disadvantage of synchronous motors is the lack of accelerating
Torque, then auxiliary windings or pony motors are required to start.
For you that are contributing to this thread, how does this discussion pertain to the 1-10 HP synchronous motors that I mentioned in the thread starter. Remember that it is a ME asking the question, so you have to try to couch your answers in such a way that it is understandable to someone outside your disipline.

Do concepts like the reactive power issue, which I actually understand a little, have any real affect in that small HP range. What I know about the motors that I have in mind in asking the question above, is that they are slow speed, and draw very little current, and can drive into a stall and not burn up very rapidly, (if at all-and that is in question in my application.).

I have already noted above that the reference information readily available to me has failed me on the topic, and the nearest technical library at an engineering university, or such, is some distance away, so I was hoping to get answers and input from peers, rather than having to make the trip.

rmw Small synch motors such as you describe are typically used when the necessity of maintaining an accurate fixed speed is important to the application without the need for complex speed control electronics, i.e. clock and timer motors, projector drives, turntables, indexing conveyors etc. They are also slightly more efficient and smaller than their induction counterparts, and can turn at lower speeds, again without additional electronics. By simply controlling the field strength, either with permanent magnets or slip rings on the rotor, a specific speed can be accurately maintained with relatively high torque for the physical size. They are sometimes used in servo applications, especially when slow motor speeds are necessary (but of course then you add back in the complex electronics).

The downside is usually cost (significantly higher) and off-the-shelf availability (significantly lower) in one-off applications compared to the ubiquitous induction motor. You will however find synchronous motors more often in OEM applications because the cost and availability issues then become negotiating points.

If you are going to need adjustable speed, the latest generation of vector control drives for induction motors makes synch motors less attractive unless again, slow motor speeds without forced cooling are necessary. They are also less valuable in high speed applications and/or those where output shaft speed is lowered through a gearbox in order to multiply torque, because in most cases you may as well use an induction motor anyway.

Once you get into high HP and power factor issues, they become attractive again, but for different reasons.

Was this what you were looking for?

"Venditori de oleum-vipera non vigere excordis populi"


Hi jraef,

Not sure if I'm just mis-reading your post here -

[blue]By simply controlling the field strength, either with permanent magnets or slip rings on the rotor, a specific speed can be accurately maintained with relatively high torque for the physical size."[/blue]

How does control of the field strength determine motor speed? Stator frequency governs the speed of a synchronous machine; field strength can be used to vary the speed of a DC machine.

For mains-supplied application, synchronous machines are fixed-speed drives. They normally rotate slightly faster than the equivalent induction machine because the synchronous machine rotates at a speed determined by the supply frequency, while the induction machine rotates at a slightly lower speed - it 'slips' relative to the supply. If an induction motor was going faster than the equivalent synchronous motor, it would in fact be a generator!




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If we learn from our mistakes,
I'm getting a great education! ScottyUK,
Hmmm, I can't believe I wrote that...
This is one of those times when you learn how someone else read your words and you see the completely different and unintended viewpoint they can get from it. My bad.

The intent behind that statement was not to imply that changing field strength changes speed (although I DID apparently say that). It was meant more as a design issue in either selecting the PM or the field coil design. Sorry to be misleading, thanks for pointing it out.

"Venditori de oleum-vipera non vigere excordis populi"