Got a shock at work and a little puzzled on how. 2

[eric34]

I got a call to a VFD drive not working.I found that the drive had lost it's smoke(burnt up) and a load reactor was also burnt.I tested the motor to ground and everything was good.I wired up a new drive and everything was good.About an hour later I got a call saying that the drive was working properly slowing down or stopping,I when to the drive and found that it was humming and hunting back and forth,touched it with my hand and got a shock.347/600v 4 wire A little puzzled on what happened.

 

[Skogsgurra]

It depends.

If it was the normal "shaking" shock feeling, you probably have a bad PE connection.

If it was a "hot and burning" feeling, it may be that your PE is bad or very long.

To the high-frequency components of the inverter PWM waveform, a long PE conductor is a high impedance, it is not unusual to have between 50 to 100 V HF on the motor frame if the cable is long (or PE not properly connected).

I once had a case where there was some confusion about how to ground the cable shield (which was also the PE conductor). The old audiophile rule "Connect one end, only" won over the "Allow no breaks in the PE" rule. Result: A potentially lethal installation.



Gunnar Englund

http://www.gke.org

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

 

[DickDV]

Getting a shock from touching the drive has to mean that the drive is not grounded properly. I'm assuming, of course, that you weren't touching across two input or output leads but rather to the enclosure or a grounded surface like a concrete floor.

Using your AC voltmeter, check for voltage from a known-good ground to the drive frame. Anything over a couple of voltages is too much.

Skogs' comment about the motor being grounded to the drive is a good point but will not usually cause a shock hazard. Both ends of the motor/ground conductor or shield must be grounded for proper drive operation.

After checking the drive frame for voltage, you probably should check you incoming power for balance to ground. You mentioned 347V balanced to ground and you just might find that one leg is grounded so you have 600/600/0 or something close to it. In that case you have a floating power supply with one leg grounded and you will want to check several recent threads on this board about floating supplies. If you've got one, your headaches are just beginning.

Good luck and please keep us posted on what you find.

 

[JBinCA]

I agree with Dick DV. Regardless of the source of the voltage, if you got shocked by touching the equipment frame or door it's because the area you touched wasn't properly grounded. Preventing that is a main reason why we do ground equipment.

If it was the door, there should be a cable that connects the door to the cabinet frame - you shouldn't have to rely on door hinges as part of your grounding path.

I would also say floating power supplies are a good place to start looking.

I think Skogs concern about HF voltages becomes more of an issue as cable length increases. Isn' that the case Skogs? And while you're answering questions, what does PE stand for? It sounds like your describing what I'd call a ground lead.

 

[Skogsgurra]

Yes. A long cable has a high PE (Protective Earth lead) impedance and that makes it difficult for the HF current (coupled capacitively from stator windings to frame) to return to the source. So, it is not unusual to get a "burning" feeling when you touch the motor lightly. A firm grip makes that feeling less noticable.

Gunnar Englund

http://www.gke.org

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

 

[Marke]

There are effectively two sources of power here, (two generators) One is the incoming feed, and the other is the inverter itself. The inverter generates a high frequency component and this is passed out to the motor. If the motor is not earthed back to the inverter except via the main supply, there will be a high voltage AC in the inverter frame due to the high impedance in the earthing at the high frequency. The problem is that there is considerable high frequency current from the motor windings to the motor frame due to the capacitance within the motor.
It is imperative that the motor is earthed back to it's supply, which is the inverter. The cable should be a screened cable with the screen correctly terminated at both ends, and preferably, the screened cable will also contain a distributed earth conductor earthed at both ends.
I agree with Gunnar, if the voltage is high frequency, it is a problem with the earthing between the motor and the drive.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[DickDV]

The original post plainly states that touching the INVERTER resulted in a shock.

Regardless how the motor is grounded back to the drive or even if it isn't grounded at all, the problem is the INVERTER not being properly grounded. Whether the voltage is high or low frequency, a properly grounded INVERTER frame will not cause a shock when touched.

I do agree that the motor needs to be grounded back to the inverter frame but this is primarily an issue regarding nuisance faulting of the drive or possibly, if the motor has no other grounding, a potential shock hazard at the motor.

Or, an I still missing something?

 

[Marke]

Hello DickDV

The problem is that there is a reasonably high impedance to earth for the High Frequency component of the drive output. If the bond between the motor and the drive is very poor, there will be a high HF current caused by the switching frequency and the capacitance between the motor windings and the motor frame which will flos around the circuit from the inverter to the motor, to the earth and back to the inverter. This current is flowing through the earth connections to the motor and to the inverter and a voltage will be developed across these "impedances". The switching currents are quite high resulting in high voltages across a moderate impedance. If there is a good bond between the motor and the inverter, there will be almost no switching currents flowing in the earth circuits and therefore little or no earth voltage to the inverter frame.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[Marke]

The circuit did not come out.
Here it is:

http://www.lmphotonics.com/images/Inverter motor earthing.gif

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[Skogsgurra]

Yes, Mark. That picture says what it is about. (Or could be, we do not know for sure).

Dick, I do not think that it is the inverter: "it was humming and hunting back and forth,touched it with my hand and got a shock". Inverters do not hum much - and do definitely not hunt back and forth" :=)

Gunnar Englund

http://www.gke.org

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

 

[ozmosis]

If the drive is not grounded correctly then this would possibly create a 'hunting' of the motor if there is an analogue input to the drive providing a feedback to an internal PID loop or a direct setpoint. The sensor could be mounted elsewhere and with a different ground to the drive and thus set-up the hunting. The 'humming' noise could simply be the correlation of the hunting as load is applied to the VFD. If the ground is not secure from the drive then this is when you will get a shock. If the motor screen or armour is not connected all the way through, then you would certainly get a jolt from the motor, even if you have a seperate PE cable running through.
It could also be from you (eric) to the drive due to static build up.

 

[DickDV]

Skpgs, if the shock came from the motor, then I am in complete agreement with you and Marke.

Kind of wonder what became of the original poster! Maybe he could comment on this.

 

[waross]

Grounding and codes:
In North America, the ground conductor, Protective Earth conductor, or equipment grounding conductor must be capable of withstanding fault currents. The minimum ampacity of the equipment grounding conductor is specified by the NEC in the U.S. and by the CEC in Canada. If the screen or shield is not rated as to ampacity, and/or the ampacity is insufficient to meet the codes, then both a code rated equipment grounding conductor (PE) and a shield or screen may be needed to comply with the codes and make the installation safe.
respectfully

 

[itsmoked]

Kind of wonder what became of the original poster! Maybe he could comment on this.

He probably touched it again, while standing on a wet floor this time....

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com