All motors are generators, right? 3

[Renovator1]

Sorry about wandering over from the circuit engineering section, but something occurred to me that I could not find the answer to using google...

If all motors can be generators (and I can clearly understand how even an induction motor can be a generator - simply apply a lower frequency to it than it is currently spinning at), how, exactly, can a shaded pole motor be a generator? Is it simply a matter of over-speeding the rotor, as in a conventional induction machine, relying on residual magnetism to initiate a then self-reinforcing magnetic field build-up?

A bit obtuse of a question, I know, but it really has stumped me this evening!

 

[TurbineGen]

For a series wound generator, parhaps a compound motor? Perhaps you could use the shunt winding as a buck/boost system? The field generated from the shunt winding would induce a current in the armatuers of the machine. That would then transfer to the series field windings. I guess it's not a perfect series generator, but it eliminates the fast switching issue due to the L/R time constant. Would this have a practical purpose? Maybe if you could find a decent way to control it.

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[Renovator1]

Hey TurbineGen, thanks for joining in. There may not be much practical application for this thread but my aim was to have some fun and stretch out the few brain cells that are still firing in a non-random fashion.

That said, even though I started off wondering about how in the heck a shaded pole motor could even *theoretically* act as a generator, the series motor/generator issue really intrigues me because there actually are applications where it would be practical to recover energy such as in vehicles, cranes, an even elevators, I suppose.

waross' post really got me to thinking about this (probably let some of the "magic smoke" out of those few brain cells in the process) so I took a few minutes today to do an experiment on a 20hp series DC motor we have at the shop. I connected my scope across A1 and F1 (A2 and F2 bonded) then powered it up with 12V from a car battery and the motor spun up to some rather sedate RPM. When a 300R/10W resistor was connected across the A1/F1 terminals the voltage dropped to just under 2V immediately after disconnecting the battery then continued dropping to zero along with the shaft rpm. I suspect that if I cut the resistor value in half I will see at least twice the voltage present at A1/F1 immediately after removing the battery power... Unfortunately, I didn't have a slew of different power resistors on hand to try this, but at least it showed that, yes, a series motor will certainly perform as a generator...

Now, waross - there is one assumption you made in your poignant explanation that doesn't exactly hold true in the real world - as the series motor cum generator delivers more power the rotor will decelerate more quickly. Only if there is infinite inertia in the rotor can there be infinite power delivered from a series generator.

So it would seem that with a high-speed analog or digital smps controller it shouldn't be too difficult to extract power from of a series motor when it is called upon to decelerate a load (whether that load be a car, crane hoist or elevator cabin, etc.).

Of course, it does seem to hold true that to do so you will need to short the motor terminals for the time it takes to allow the field/armature current to build up enough to overcome the supply voltage when the short is released. Hey, it can't be any more difficult than it was to get the Saab Gripen stabilized, right?

 

[waross]

A generator usually has a prime mover that supplies sufficient if not infinite power for generation.
When motors (Cranes, elevators) are overhauled the load serves as the prime mover and increases the speed enough to regenerate back into the source. No free coasting motor can regenerate more power than is available in the inertia of the motor load combination. Some of that power will be dissipated as heat in the motor windings.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Reno! You got my drift.

I happen to know the guy who was responsible for the aerodynamics of that plane. He was on a conference in SF, CA when the Gripen hit ground and disintegrated the first time. He got an early phone call (time difference 9 hours) and that kind of ruined his day, no more conference but lots of phone calls. Anyhow, he and his collegue needed something to eat, so they went to the Chinese on the corner. When he opened his fortune cookie it said: "You have a big problem - but you will solve it". He then did just that. Perhaps he could also build a controller for a series DC generator.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

If you add a shunt winding to the field poles, the field strength may be increased. This greatly aids regeneration. It is a simple matter to design the field poles so as to allow room for the additional winding. Problem solved. Except that you now have a compound machine rather than a series machine.
What I am trying to say is that the best way, (but not the only way) to regenerate with a series machine is to use a compound machine.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[roydm]

I hate to nit pick but we should use the correct terminology, Generators are DC Alternators AC

 

[TurbineGen]

Rennovator, I must admit this is an intriguing thread. I love motor theory and design. These types of threads get us engineers thinking. As for the experiment, I would guess that the voltage on the resistor would be non linear? Perhaps you could hook your series machine up to another prime mover such as another motor and keep adding resistors in parallel and see what happens? I am curious now.

I agree with Waross that shunt winding is probably the best way to make a series generator functional. Admittedly, it technically isn't a purely series generator, but one you have your field established with the shunt winding, you can deenergize that coil and let the machine run as a series machine. So it still is a series generator, just the field is established within the machine rather than the controller. However with proper switching within a controller you might be able to get it to generate by using the residual magnetism to get the field to proper strength.

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[Renovator1]

waross/TurbineGen - Adding a shunt field coil to the series motor/generator will certainly make it behave better, but wouldn't that be cheating?? )

Anyway, TurbineGen, I am planning on doing just such a test. I have a box of 4 ohm 53W resistors which will make it convenient to increase the loading in a stepwise fashion. I just have to rig up a second, identical, DC motor as the prime mover and monitor its speed/amp draw.

Things I am wondering which this test should answer:

1. Will the series motor/generator "self-excite" when driven by a prime mover rather than energizing the motor and loading it as it coasts?

2. Can the motor/generator be used the inductor in a boost converter?

3. Does the output voltage increase as more 4 ohm resistors are paralleled, assuming the RPM of the prime mover does not change significantly?

One last thing - I can find no definitive information about using a series motor as a generator anywhere... is it just me or does someone have any dusty old tomes that might shed some light on such?

 

[Renovator1]

Edit Q2: meant to write, "as the inductor/voltage source in a boost converter".