AC induction motor generating voltage ???????????

I have an AC induction motor with a stator winding (3 phase) operating with
a phase shift capacitor to provide comutation. The rotor is a copper bar
design with copper end rings brazed together on M19 laminations. Input is
via rectified DC through a full wave bridge followed by the caps in line.
Phase 1 & 2 have the same turns & wire size and phase 3 is more turns of
bigger wire.
Problem is I can get this motor to generate 130 volts P-P while being back
driven when connected to the bridge/cap circuit, no other electrical
connections exist. Without the circuit, simply backdriving the motor
produces 2.24 V P-P. Which could be resdual magnetism in the lams.
As speed is raised on the back driving motor (it's a PMDC on a separate
power supply) voltage on a scope will be nill until at the same speed
(approx. 12800 RPMs) something happens. The voltage jumps to 130 (in a sine
wave) and the speed of the back driving motor slows as if the AC motor is
working as a brake. The AC get hot to the touch so something is going on
electrically in the AC.
The current of the generated voltage is less than 500 milliamps.
This motor has a brake (not in the circuit for this test) that is used to
stop the rotation after power is cut. Motor powers an actuator (ball screw)
and there is a spec for stopping time. Some motors pass some do not. The
generated voltage sometimes keeps the brake from providing torque to stop in
time. The 2.24 P-P witout the circuit will not keep the brake open, it
should stop the shaft.
After all said how can an AC motor (induction, no magnetic field) produce
produce a voltage when connected to the bridge and cap circuit but not
without it ?
Whew, if anybody answers this I will be very grateful as I am stumped. Not
claiming to be an AC expert. I know enough to be dangerous. My specialty is
mechanical followed by DC motors.
Thanks in advance,
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Hi Kevin,
In years gone by I worked as an electrical engineer for a motor company so I think I can help you. My comments are in the text below:
I don't quite follow what you are saying here. Are you saying that the stator phases are wound with different turns and wire sizes - I've never seen an induction motor like that. Are you saying that the DC you are feeding into the stator is somehow different for the three phases? I've seen DC feed into induction motors for the purpose of dynamic breaking. It's common to do this with induction motors - but why the capacitors and why do you say they are used for phase shift / commutation?
If the motor gets hot to the touch I doubt that you are only generating 500 milliamps. Also, exactly what are you using this motor to do? Are you using it to turn a ball screw? Why the separate PMDC motor? A 3 phase induction motor can be reversed by switching two of the motor's leads, although not all of them are designed to run in reverse. I need a better picture of what you are doing.
Again - I need a better idea of the application.
Induction motors can act as generators under the right conditions. One way is when they are connected to the line and are driven above their synchronous speed. Above synchronous speed they will feed power back into the line so they are called induction generators. But there is also a way to make an induction motor generate even if it isn't connecteed to a power source - you add some capacitors across the motor leads. The capacitors provide some phase shift and allow the motor to self excite ( there has to be a small amount of residual magnetism in the rotor lams to get things started) and generate voltage. This is probably what you are seeing. By the way, the capacitors have to be rated for this application, if not they could be destroyed and even explode under the right conditions - so be careful. Induction generators (especially with a low resistance copper rotor) can generate a lot of current if there is an electrical load (like the secondary of your transformer winding). If you say the motor is getting hot - that's most likely what's happening.
If the motor leads aren't connected to anything the induction motor can only generate a couple of volts, as you said, from residual magnetism. Once the power is removed from an induction motor the voltage at the open leads will die off within several seconds - if there is nothing connected to the motor leads.
You can use DC to dynamically brake an induction motor. Usually, DC is supplied by a transformer / rectifier combo by a contactor AFTER the main contactor disconnects the motor leads from the AC line. A large power resistor is normally put in series with the DC supply to limit the DC current to the stator winding and control the rate of breaking. Capacitors are not usually included in the DC supply.
An induction motor can certainly turn a ball screw, but they have limits. For a 60 Hz line powered induction motor, the maximum speed you can get is just under 3600 RPM. They also have fairly poor low speed torque when compared to DC motors. Although you can get around both of these problems by using a modern vector drive.
Hope this helps, Big John
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Big John
| After all said how can an AC motor (induction, no magnetic field) produce | produce a voltage when connected to the bridge and cap circuit but not | without it ?
Without the connection, you've got 3 separate unconnected windings, right? But with the connection of the bridge and cap circuit, voltage being generated can be fed back into the windings and create a stronger field, which in turn induces more current in the rotor, which in turn ... on and on. That's my guess as to what is happening. You're exciting the generator from it's own voltage. All it needs is just a little magnetic field to get started and you admitted the small residual magnetism giving you 2.24 V in the unconnected case.
I don't know what is going on to create the heating, but maybe something is changing, or an arc is developing, at higher levels.
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Short story is that it is working as an AC Induction Generator, check out
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Big John .... yes 2 of the phases same turns and wire while the 3rd has a different, the 2 phases that determin direction are the same the 3rd phase the odd one is always powered while the other 2 are switched depending on direction. On the insertion of the stator is a WYE or sometimes called starr winding. not to be confuesed with a delta insertion. The DC goes into the bridge and AC comes out. In a true 2 phase or 3 phase motor there are the same number of independent supplies as phases. In my motor there is only 1 power supply. In the true versions the supply voltages are shifted by X number of electrical degrees each to provind the rotating field. In my motor the cap does that.
When I said hot I should have said warm sorry.
The motor turns a ball screw actuator that operates 4 external air inlet/outlet doors on the boeing 777. The PMDC is used to replicate the air pressure back driving the geartrain and speeding the motor above its sync speed & becoming a generator.
The supply is a 400 Hz at 115 Vrms.
The construction of this motor elec/mech is aerospace rated so don't worry on those lines. It has been in production/use for 12 years. If Boeing uses and quals it you know it is rugged.
Thanks to all for your response to my post, Engineering can be the best/worst field depending on whether things work and if you can figure them out when they don't. Thanks to you guys problem is identified now I move onto solution which will have to be on my "customers" drive circuit and they ultimatly own that fix. I just advise from here on out ........ shook the monkey of again.
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