# Can a motor (off a VFD) turn after power is removed but capacitors havent drained?



## ashley.perry.20 (Sep 1, 2017)

Hey I was wondering if someone could answer this question. Where I work when a lockout is performed and power is removed from a VFD we wait until the screen on the VFD shuts off before we get the operator to re-bump the motor. For example the other day we removed power from a 150hp drive by opening the disconnect switch. At the time the 600V was removed the DC bus voltage on the drive was above 800V. It took approx. 1 full minute for the DC bus voltage to drop to the point the screen shut off (capacitors had discharged). If the operator had attempted to restart this motor while there was still 600V+ on the DC bus, could the motor turn? I'm asking this as myself and a co-worker had varying opinions. Just looking for another opinion. Thanks


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## JRaef (Mar 23, 2009)

No. The bus charge stays showing a charge because there is nothing draining it off besides the tiny burden of the PC boards of the drive (in some cases). The instant you attempted to "run" the drive, the first transistor that fired would drain all of the energy out of that bus instantly, but would be there on one phase of the motor for maybe one pulse, so at 4kHz carrier frequency, 1/4000th of a second, not enough time to create any torque. But in reality, the VFD firing board will not even allow that to take place. If you hit the Run command when the line power is not there, the drive will not even send power to the base emitters of the transistors.

That doesn't mean the DC bus is not dangerous, it is, if you make contact with it. I have seen someone killed by that on a 600HP drive. I'm just responding to your question about that remaining DC bus energy being capable of generating torque.

Re-applying line power too soon is dangerous to the DRIVE, because there will be some sort of "pre-charge circuit" involved in the rectifier- to-DC Bus connection that limits the capacitor charging current so that the caps don't damage themselves or the rectifier diodes. That is usually in the form of a current limiting resistor that is in the circuit for a second when you first energize, then is bypassed by a relay contact a second later. That relay contact remains closed until the DC Bus voltage drops below some threshold value. If you re-energize before that contact opens, the resistor is not in the circuit and the capacitor charging current spike can be big enough to cause damage. So waiting until the LEDs go out is the best practice. technically you could re-energize sooner than that, but you will not know when that is.


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## Jeff B (Sep 30, 2017)

Input phase loss would/should disable output and register fault.


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## bill39 (Sep 4, 2009)

I'm going to say yes because we had a project with ABB drives and if the large drive e-stopped or lost power damage to the system would occur if the the small drive stopped at the same time. The DC bus from the large drive powered the small drive just long enough for the motor to continue driving its conveyor just a small distance. That kept things from being damaged. 

A very simplified explanation but it's all I can remember about it.


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## Jeff B (Sep 30, 2017)

bill39 said:


> I'm going to say yes because we had a project with ABB drives and if the large drive e-stopped or lost power damage to the system would occur if the the small drive stopped at the same time. The DC bus from the large drive powered the small drive just long enough for the motor to continue driving its conveyor just a small distance. That kept things from being damaged.
> 
> A very simplified explanation but it's all I can remember about it.


That sounds like the common DC buss package that ABB uses. I serviced many of the ACS600 and ACS800 VFD's that share a separate "Common" DC buss. The individual drives do not contain their own DC capacitor buss. All DC links to the IGBT's were tied to the common DC buss which made for a simple solution for regenerative current back to the main lines of the power utility. These did not use dynamic braking with the resistor. Is this what you had, Bill?

Stand alone VFD's typically do not operate without all input phases present and at threshold. Line sags and brownouts should fault the drive. Phase loss would certainly do it unless the VFD phase loss protection has been deactivated, if deactivation is available.


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## bill39 (Sep 4, 2009)

Jeff B said:


> That sounds like the common DC buss package that ABB uses. I serviced many of the ACS600 and ACS800 VFD's that share a separate "Common" DC buss. The individual drives do not contain their own DC capacitor buss. All DC links to the IGBT's were tied to the common DC buss which made for a simple solution for regenerative current back to the main lines of the power utility. These did not use dynamic braking with the resistor. Is this what you had, Bill?
> 
> Stand alone VFD's typically do not operate without all input phases present and at threshold. Line sags and brownouts should fault the drive. Phase loss would certainly do it unless the VFD phase loss protection has been deactivated, if deactivation is available.


Yes, I believe the ACS800 package you entioned may be it. I don't think it had the brake though.


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## paulengr (Oct 8, 2017)

Some VFDs have a power loss ride through option...they actually go into regen and hold up the line using the motor as a lower source (an asynchronous generator) or using the DC bus power (almost nothing). Its enough if you have enough to ride through a "blip" but that's it. The DC bus caps store maybe a couple cycles (milliseconds) of power but that's it whether or not the drive will fault or attempt to run.

Bus capacitors on a big enough drive can be an electrical hazard but not a mechanical one.

Sent from my Galaxy Tab 2 using Tapatalk


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