Induction generator motor starting issue

Thread Starter

l0vot

Joined Apr 29, 2013
107
I am attempting to build a stand-alone generator using a working 3 phase motor, it needs around 20MFD to operate in it's current configuration, but I am running it with 25MFD until I get some 20 MFD caps. The generator is self exciting, it produces around 5V open circuit with no caps because of residual magnetism in the core, which is enough to get things going. Everything appeared to be fine, until I tried adding a load: an induction motor 1/3 of the generator's size; the motor gave a little spin, and then the generator essentially stopped producing power. I figured the motor was eating too much reactive power, and basically starved the generator, so I doubled the cap bank to 50MFD, predictably the generator's voltage spiked well past it's normal operating voltage, but as soon as the motor was switched on, it quit, just like before. I was not deterred, so I rapidly switched the motor off and on, since the generator is self-exciting the motor would spin a little faster every time it was switched until it was spinning fast enough that the generator no longer died with the motor on.

It seems like the generator can't handle motor starting, is there any reasonable way to fix this? This is basically a low-cost test-bed, it's only 6HP 460V 120hz.

Note: solutions that might destroy the generator are acceptable, within reason.
 
Last edited:

ebp

Joined Feb 8, 2018
2,332
Oh lord, flashbacks to an earlier lifetime. I designed a controller for a 3 phase permanent magnet variable speed generator for charging lead acid batteries many years ago. I know my client had played with using off the shelf motors as generators but abandoned the idea. I don't know why since I had no involvement with that. Their generator was explosion proof.

A motor is a difficult load to start, because of course it is also a generator. Before it spins up, it is mostly just the winding resistance effectively in series with some inductance. As the speed rises, it generates increasing back EMF that "bucks" the input current. Do you have any numbers for the stall current? My guess is the reactive nature of the load probably isn't a big player, but I may be completely wrong.

It sounds like you will need some sort of soft-start circuit, probably a phase-angle control that slowly increases the conduction angle from zero to full over a period of a few seconds. The alternative (no pun intended) might be to manage the amount of capacitance with a control circuit. This would probably involve switching capacitors out as the load spins up. This actually may be more difficult than phase angle control. Getting the motor going if it has the "wrong" type of load (don't even be thinkin' about a compressor or a big flywheel) will add to the challenge.

Will the generator run at fixed or variable speed?

You might try some incandescent light bulbs as test loads for the gen. Typically they take around 10 times as much current when they are cold than when running at rated voltage, which sort of emulates a motor except they are non-reactive.
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
If this is a standard induction motor, how is the rotor providing excitation?
Max.
Capacitors, caps provide reactive power, the generator has residual magnetic fields, the tiny voltage and current the generator is able to produce without any help is enough to get a tiny amount of energy in the caps, the caps feed this energy back into the generator as reactive power, the small amount of reactive power increases the strength of the magnetic fields in the generator, which increases the voltage and amperage the generator is able to produce, which recharges the caps to a higher voltage and the whole thing starts anew, this happens many times a second until the generator is at full power.
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
Can you provide more details on the generator?
I'll get you the whole nameplate later, but it's just an ordinary 3HP 3 phase motor that is being driven at double the speed it's supposed to. Double the speed=double the frequency&double the voltage&double the power, so instead of 230V 60Hz, now it's 460V 120Hz.
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
Oh lord, flashbacks to an earlier lifetime. I designed a controller for a 3 phase permanent magnet variable speed generator for charging lead acid batteries many years ago. I know my client had played with using off the shelf motors as generators but abandoned the idea. I don't know why since I had no involvement with that. Their generator was explosion proof.

A motor is a difficult load to start, because of course it is also a generator. Before it spins up, it is mostly just the winding resistance effectively in series with some inductance. As the speed rises, it generates increasing back EMF that "bucks" the input current. Do you have any numbers for the stall current? My guess is the reactive nature of the load probably isn't a big player, but I may be completely wrong.

It sounds like you will need some sort of soft-start circuit, probably a phase-angle control that slowly increases the conduction angle from zero to full over a period of a few seconds. The alternative (no pun intended) might be to manage the amount of capacitance with a control circuit. This would probably involve switching capacitors out as the load spins up. This actually may be more difficult than phase angle control. Getting the motor going if it has the "wrong" type of load (don't even be thinkin' about a compressor or a big flywheel) will add to the challenge.

Will the generator run at fixed or variable speed?

You might try some incandescent light bulbs as test loads for the gen. Typically they take around 10 times as much current when they are cold than when running at rated voltage, which sort of emulates a motor except they are non-reactive.


Fixed speed, I will be testing other types of loads later, will post results.
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
I tried a bigger setup, it took less doing to get the load motor started, but it still couldn't handle the starting current it pulls, it looks like motor starting requires an induction generator many times larger than the induction motor, or requires some sort of energy storage device that CAN handle temporary overloads. i would try for a synchronous condenser, but those things are both too large, and too expensive to be practical, or useful in this situation (unless I could make a crude one by sticking permanent magnets inside a modified induction motor rotor or something). VFDs soft start their loads, so that would probably solve that particular issue, but a VFD substantially increases the cost of this entire project, since it would be more expensive than the engine I would need to get to power the final product.
 
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