Portable Generator Stator

Greetings All:
I have been in the industrial electrical business for a long time. I
am very familiar with motors being rewound and machined. I have been
in business for myself now for about 8 months. We supply, service,
install, lease and rent Emergency Standby Generators. I am also a
licensed electrician in Alabama and Georgia, USA. We recieved a
portable generator the other day that would not hold a "one step" load
drop. It is a GB5000-2 Devilbiss portable. It has one 20A 240vac
duplex recpt and one 20A 120vac duplex recpt. We disconnected these
and ran straight to the stator wires for the load test. We one step it
and it DIED. after restarting it we got it to produce about 15% of
load regulated, anything else would drop the frequency to as low as
50Hz. We proceeded to meg the stator. When we did this smoke came
from the stator. The stator has 4 wires and 2 capacitor wires. We
measured with a VOM across L1 and L2=infinity, L1 to Neutral = .4 ohms,
L2 to Neutral=infinity, L1 to ground wire=.2ohms, L2 to ground
wire=infinity. VOM on capacitor read 49.9 and cap is rated for
50.microfarads. I am assuming that there was a slight breakdown in the
winding and the generator was ok at small load then as load increased
leakage to ground increased. Then we it was megged at 1000VDC it
shorted completely. My question is, Is this a good assumption and if
it is can we rewind this stator. iT is simply a 2pole 3600rpm stator.
Or do we have to take it to a motor shop. Any help is appreciated.
robert mcmahon
global energies
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Must be some megger you used to make smoke come from the windings!
The stator has 4 wires and 2 capacitor wires. We
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My guys said they megged it at 1000vdc....what truly happened is not really in question....I understand that a megger does really have the ability to burn a winding but this is the story I got. The question is can I rewind it and are my assumptions true based on the VOM readings that the stator is shorted.
Reply to
Global Energies
That kind of equipment, I would have only used a 500V meggar. But I've never known of a *meggar* that could pass enough current to open-circuit a main winding.
You mention 4 wires. That *could* be two 120V windings connected in series to give you 240/120. But that's only a guess. Without looking into the machine, it *could* be wired other ways as well. Then you mention L1, L2, and neutral. So what's the fourth wire if those are three of them?
Is the regulator powered from just the 120/240 line, or is it powered from another winding? If the regulator is electronic and connected to L1/L2, did you *really* connect 1000VDC to the poor thing and fry it too??
Yes, a machine like that could be re-wound. Question is, find a shop to do it, and is it worth it?
Generally, you need to have some experience to do a good job. Carefully removing the old winding can be hard if the unit has been dipped and baked in varnish. Enough magnet wire, slot paper, wedges, *patience*, and knowing how to connect/ place the coils is all that's needed.
But open up the regulator and see if it's any good anymore before going too far.
As far as your 'one step' load failure, you don't mention how much load you were putting on it. If the *engine* is what was slowing down/ stalling, you either have too much load for the engine, or the engine has mechanical problems. Not all small engine governors handle 0-100% load changes equally. But it should have been able to recover.
Just because a machine has a 20A / 240V and a 20A/120V outlet on it, doesn't really mean it is designed for 4800 +2400 = 7200 watts either. Although the '5000' in GB5000 suggests the machine is rated for 5000 watts.
Adding a load to the generator would heat up the winding but not necessarily increase 'leakage to ground' unless there are *two* faults in the machine, or the neutral is tied to ground on purpose. (or your load is grounded). But that kind of fault would have 'smoked' while the generator was running, not under a meggar test.
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L1...L2...Neutral..."Ground" On the receptacles they have both L1 & L2 going to the 240v recpt and they split the 120v recpt so that L1 is supply one recpt on top and L2 the recpt on bottom and the neutral connected to neutral. However on this recpt the neutral and the "Ground" are bonded.
There is no per say "regulator" the voltage and freq are regulated by the speed of the motor. 3600rpm (2 pole) 60 Hz. 240/120v. On the rotor the two sets of windings connect on one side to a metal disc on the shaft and the other side of the windings are soldered together to a Diode that is then connected to the same metal disc. And there is a capacitor on the stator with two wires connected in the stator but I dont know if they are series or paralleled with the windings.
The generator is 5000 watts. It can only carry 20A. We applied a load bank at 5000watts in one step. The unit slowed drastically (frequency 20hz) then it just stalled. It is a 10hp Briggs and Stratton engine with a Mechanical Governor.
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Global Energies
Well, the speed of the engine will control frequency, I agree. But using speed control to set voltage is dicey at best, and terrible at worst. Most likely those extra windings are some sort of resonant circuit on the stator to induce an AC on the rotor that is then rectified by the diode to provide field current. More load current affects the amount of current in the second stator winding and that in turn affects rotor power.
Sounds like a design to provide some self-regulation by automatically boosting the field power as load is applied. But such systems are susceptible to the power factor of the load, and may have a response-time problem.
Two windings on the rotor, I'm having a hard time visualizing. No slip-rings or brushes on the rotor? So what is this 'metal disc', just a termination point?
The capacitor on the stator is connected to a different winding that L1/L2/Neutral?
Stepping a 5000 watt load on is equivalent to a 6.7 HP load on the engine. If the frequency dropped that bad and then 'stalled', that's definitely an engine/governor issue, nothing in the generator itself. Considering the size overall, that Briggs&Stratton engine may not be capable of much better. Any adjustments in such a small governor?
Larger machines, the governor often has a 'gain' and 'response' adjustment that can be tweaked so that the governor will rapidly apply full throttle anytime the speed drops even 10% or more (your's apparently dropped 66%). But if this machine doesn't have such an adjustment, then your out of luck.
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