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Joined: Dec 2004
Posts: 78
C
Cinner Offline OP
Member
Does a motor disconnect need to be before(line side) the Variable Frequency Drive or can it be on the secondary of a VFD?

Joined: Jun 2006
Posts: 7
E
Junior Member
It depends on if you want to safely isolate the whole system or just the motor.

Joined: Sep 2001
Posts: 806
N
Member
If there is a switch installed on the motor side, it must NEVER be opened while the motor is running.

The voltage spikes developed by the inductance of the motor windings can destroy the semiconductors in the VFDs power bridge.

Joined: Feb 2003
Posts: 939
F
Member
also i will like to add this to this disscussion that if you use the disconnect switch [ non fused type ] i suggest that you add a "polit" switch or microswitch on the camshaft of the disconnection switch so it will send the signal to the VFD contoller to shut off the power to prevent spikeing voltage and sorta like lock out to prevent the VFD firing up with very high voltage with low HZ it can wreack hovac and also during running if disconnected it will spike as well


Merci , Marc


Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

Joined: Mar 2005
Posts: 48
R
Member
A disconnect is required on the line side of the VFD. Quite often, a fused disconnect is furnished as part of the VFD equipment. For small VFD's, the disconnect needs to provide the short circuit and fault current protection. The VFD feeder breaker will provide this over current protection.

A disconnect (safety) switch on the load side, for disconnecting the motor is not required as long as the VFD line side OCP device can be locked in the open position.

It is standard practice to provide a LOCKOUT STOP pushbutton at the motor, whether or not a motor disconnect switch is provided.

I also like to install a JOG or START/STOP control station at the motor in order to locally check rotation, etc.

Joined: Jan 2003
Posts: 4,391
I
Moderator
Quote
The voltage spikes developed by the inductance of the motor windings can destroy the semiconductors in the VFDs power bridge.

I am having a hard time with that.

How is a spike from the motor that was caused by the switch opening getting back to the motor through an open switch?

I do agree that opening a switch on the load side of a VFD is not a great idea but I do not think that this spike is the reason.

That said it is common to have disconnect switches on the load side of VFDs when the VFDs are located at a central MCC and the motors are not in sight.

As 'Frenchelectrician' mentioned installing an auxiliary contact in the remote disconnect to disable the VFD is common.


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Joined: Sep 2001
Posts: 806
N
Member
Quote
How is a spike from the motor that was caused by the switch opening getting back to the motor through an open switch?

By arcing back across the contacts as they open? The voltage developed by rapidly dumping current through a motor winding can easily get up into the several kV range.

I've repaired several VFDs that got wiped out by this kind of abuse. In one incident, it was a simple safety disconnect switch at the motor that killed it. In the others (machine tool retrofit applications), it was the installation (by a well-meaning machinist) of the VFD upstream of a reversing contactor or drum switch, to try to keep as much of the original setup in use as possible. Those problems were solved by wiring the VFD right to the motor, and handling the reversing at a low level, with a logic input to the VFD.

Joined: Jan 2005
Posts: 5,316
Cat Servant
Member
Exactly where is it required to have the disconnect on the load side?

Are we saying that we are required to do something that will harm the VFD? Maybe creating a hazard?

Joined: Sep 2004
Posts: 167
S
Member
It sounds as if a disconnecting means on the load side of a VFD could meet the conditions in 430.102(B)exception (a). Since opening it would fry the VFD.

"...introduces additional or increased hazards to persons or property.


Larry LeVoir
Inspector
City of Irvine, CA
Joined: Jan 2003
Posts: 4,391
I
Moderator
In this area it is common to find disconnects located at the motors with the VFDs in MCCs at some remote location.

These disconnects will have auxiliary contacts to disable the VFD.

IMO these disconnects are not intended to be used to control the motor only provide a way to have a switch for service work at the motor.

If they are used in an emergency situation and it fries the VFD so be it.

Sandsnow, I am not sure the inspectors here would buy into the increased hazard to just the VFD as enough of a problem to warrant less protection of personal at the motor.

JMO, Bob


[This message has been edited by iwire (edited 02-09-2007).]


Bob Badger
Construction & Maintenance Electrician
Massachusetts
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