Full wave bridge rectifier waveform

[freak]

I have a 120V floor buffer that uses a GBPC5006 full wave bridge rectifier to power it's motor.  Its rated draw is 15A.  The unit runs but under a normal load the current jumps up over 25A and blows the breaker.  I pulled the rectifier and it checks ok with my meter.  The waveform doesn't look like a textbook example but I'm not an expert.  The image is with the rectifier alone powered through a isolation transformer.  Is this waveform normal or does it indicate a problem?

[Stray Electron]

   FIRST, I wouldn't use an oscilloscope on that kind of circuit. The motor inductance will cause voltage spikes and they could kill your scope.

   Second, no those aren't normal looking waveforms. It looks like you may have at least one shorted diode in the bridge. Which probably also explains the excess current draw.

[DavidAlfa]

That waveform definitely looks strange.
Disconnect the motor and turn it on, does it consume any power?
Looks like one half wave is partially filtered, while the other it's not, does it have a filtering capacitor at the output?

[fmashockie]

That definitely looks strange.  If one of the diodes in the rectifier failed open, you'd have a half wave rectifier.  Which clearly isn't what you have on the scope.  Now if one of diodes is shorted, you are going to draw a massive amount of current.  But you said you got the above waveform by applying voltage across the BR with an isolation transformer.  Does your isolation transformer have a fuse (most do)?  If it does, it would have likely blown if one of the diodes was shorted in this rectifier.  If we use the photo below as an example, the positive going side of the AC sinewave being rectified by D1 and D4 would not be affected, but the negative side would.  You'd effectively be shorting the secondary winding of the transformer.  So either the transformer would eventually fail open or diode D3 would fail open due to the overcurrent. 

[PA0PBZ]

The image is with the rectifier alone powered through a isolation transformer.  Is this waveform normal or does it indicate a problem?

Did you put a small load on the rectifier? If not you will not see a normal waveform due to capacitance, leakage and such.

[freak]

   FIRST, I wouldn't use an oscilloscope on that kind of circuit. The motor inductance will cause voltage spikes and they could kill your scope.

   Second, no those aren't normal looking waveforms. It looks like you may have at least one shorted diode in the bridge. Which probably also explains the excess current draw.

This is the rectifier alone on the bench.  Nothing else connected.

[freak]

The image is with the rectifier alone powered through a isolation transformer.  Is this waveform normal or does it indicate a problem?

Did you put a small load on the rectifier? If not you will not see a normal waveform due to capacitance, leakage and such.

No there is no load on the rectifier in my example.

[freak]

That definitely looks strange.  If one of the diodes in the rectifier failed open, you'd have a half wave rectifier.  Which clearly isn't what you have on the scope.  Now if one of diodes is shorted, you are going to draw a massive amount of current.  But you said you got the above waveform by applying voltage across the BR with an isolation transformer.  Does your isolation transformer have a fuse (most do)?  If it does, it would have likely blown if one of the diodes was shorted in this rectifier.  If we use the photo below as an example, the positive going side of the AC sinewave being rectified by D1 and D4 would not be affected, but the negative side would.  You'd effectively be shorting the secondary winding of the transformer.  So either the transformer would eventually fail open or diode D3 would fail open due to the overcurrent.

Yes the isolation transformer is fused.

[freak]

That waveform definitely looks strange.
Disconnect the motor and turn it on, does it consume any power?
Looks like one half wave is partially filtered, while the other it's not, does it have a filtering capacitor at the output?

If i disconnect the motor there is nothing other than the rectifier.  There are no capacitors.

[freak]

The image is with the rectifier alone powered through a isolation transformer.  Is this waveform normal or does it indicate a problem?

Did you put a small load on the rectifier? If not you will not see a normal waveform due to capacitance, leakage and such.

I would like to re-test it with a load.  Seems like an incandescent light bulb would be suitable?

[bdunham7]

I would like to re-test it with a load.  Seems like a light bulb would be suitable?

An incandescent bulb, even a small one, would be good.  LED/CFL/etc won't be good!

[freak]

I would like to re-test it with a load.  Seems like a light bulb would be suitable?

An incandescent bulb, even a small one, would be good.  LED/CFL/etc won't be good!

Yeah I just clarified that.  I'll re-test it in a bit.

[DavidAlfa]

Yeah, or a heater, resistive load are the easiest to test.

[freak]

Well here is how it looks with a 100W load on it.  So that looks good I guess I'll have to think about the motor being the cause of the problem.  Unless of course that rectifier has issues with a larger load.

[bdunham7]

That type of rectifier is unlikely to have intermittent or variable problems when connected to mains--it will either work or it will pop entirely and permanently.

[fmashockie]

Nice work! Check for a short between windings on the motor.  What is the make/model of the floor buffer?

[freak]

It's a Tornado Glazer 1500.

[fmashockie]

I'd be checking that Triac first actually.  Easy to test with a DMM.  If it went short, it could be causing your overcurrent issues.

[freak]

That type of rectifier is unlikely to have intermittent or variable problems when connected to mains--it will either work or it will pop entirely and permanently.

I pulled 10 amps through it today on the bench and you were right.  The waveform looked normal and nothing strange happened.

[freak]

I'd be checking that Triac first actually.  Easy to test with a DMM.  If it went short, it could be causing your overcurrent issues.

If the triac shorted wouldn't it be just like having it on high speed?  Instead of chopping the waveform the full sine wave passes?

[Swake]

The breaker could have failed too. These things are not eternal unfortunately. Or there might be another unexpected load on the same circuit. Issues in the wiring or plugs can also be a cause. Check for loose or corroded connections.

[fmashockie]

Swake is right.  Fortunately, that is fairly easy to test - just plug the buffer machine into a different circuit.  You could take it a step further and hook it up to a power analyzer or try a clamp meter on the incoming mains hot wire to check the amperage. 

But they are right in checking for loose connections.  I usually go off the deep end and start checking for more 'complex' sources of fault, when really 9/10 its usually something stupid like a loose connection or cold joint.  You'd think I would learn by now 

[freak]

Swake is right.  Fortunately, that is fairly easy to test - just plug the buffer machine into a different circuit.  You could take it a step further and hook it up to a power analyzer or try a clamp meter on the incoming mains hot wire to check the amperage. 

But they are right in checking for loose connections.  I usually go off the deep end and start checking for more 'complex' sources of fault, when really 9/10 its usually something stupid like a loose connection or cold joint.  You'd think I would learn by now 

This unit exhibits the same problem in multiple buildings.  The first thing I did was check the amp draw.  When freewheeling it draws about 10 amps.  Place it on the floor and amps goes over 25 and trips the breaker.

[mikerj]

Sounds like either a mechanical issue or possibly shorted turns in the motor, 10 Amps with no load is really excessive.  Can you easily spin the motor by hand when it's not powered?  Are you seeing an unusual amount of sparking from the brushes when it's running?

[fmashockie]

I would guess something mechanical in nature as well.  Now that you provided the symptoms (I assumed it was immediately tripping the breaker when you powered it up).  A bad pulley, belt, or bearing.