Disconnect on VFD output

[thinker]

Hello colleagues,
One of our customers insistss that the VFD based dynamometer system we develop must have a disconnect switch
in line of sight of the motor (between inverter and motor)
with reference to NEC 430.102(B). Our opinion is that the above article is not applicable to VFD driven motors, but NEC does not have clearly stated exception. Any opinion or experience to support our position?

 

[gordonl]

Motors need to be disconnected for worker safety whether the supply is variable or fixed frequency.

 

[Dopler]

That is only for safety reasons, it is up to you to install it.

Some times we install a disconnect switch near the motor, because it was not near the VFD, to perform hands on the motor with safety.

 

[rbulsara]

Drive or no drive..motor disconnect is required by NEC
Also a disconncet for motor controller is requierd by NEC.

You can have both items disonnected by a single disconnect (ahead of or built-in the controller).

The VFD is a type of motor controller...a disconncet for VFD is required..whether is is integral to VFD or a separate unit..it is a different matter of choice..

And it is not "up to" anybody, it is required where NEC is adopted as a Code.

 

[DickDV]

Motor lead disconnects are OK if there is a pre-action switch on the handle to stop the drive.

Also, some manufacturers are approving of disconnects without pre-action switches. ABB is one of those.

For those brands that do accept disconnects, they usually state that the drive will likely not fault when the disconnect is opened but will surely fault on Overcurrent when the disconnect is reclosed.

If you expect to open the disconnect with the motor loaded, you better be ready for a huge flash. At least, the disconnect had better be rated for load-break duty.

 

[CCST3]

The disconnect is an NEC safety issue. It should be there.

However, disconnecting a VFD under load is not healthy for the drive. Use a disconnect switch with (or a field adder) "early break" contacts. Wire into drive logic to gain a head start for a VFD "controlled shutdown" before the disconnect opens on the VFD load.

Most vendor manuals will have this somewhere or contact the factory drive "expert".

 

[7carisfast]

To my knowledge, as long as you have a lockable switch for the motor, you do not need a local disconnect per the NEC. If the MCC bucket can be locked, then that is your lockable switch. A local switch is just a better safety measure since you can trace the motor leads to the switch to ensure lockout is acquired. But it is not required.

 

[gordonl]

One method I've used is puttinng a drive E-stop button next to the disconnect with a label on the disconnect to push switch first. The E-Stop then did a drive shutdown before opening the disconnect.

 

[rbulsara]

The disconnect of NEC is for maintenance purposes...unless the disconnct is fully HP rated, it is not to be used under load...

Emergnecy stops are different application...if some one wants to do a maintenance...he/ she would stop the motor in a proper way...E-stop is not for that purpose..

Btw, 7carisfast is also correct, as it is an permissible exception in the NEC..you have to read up Article 430...

 

[Skogsgurra]

We do it a little bit different here in Sweden. Or rather; we are allowed to do it differently. The problem is that a "Full Load Rated" disconnect has to handle other frequencies than 50 or 60 Hz. It is not unlikely that you have to break a full load current with a 10 Hz frequency if the drive is running slowly.

Normal switches are designed to break an AC current where zero crossings occur every tenth millisecond (or every 8.33rd in the US). When the drive is running with low frequency like ten Hz, there are no zero crossings every tenth ms. The current looks more like DC to the switch and the arcing continues for three - five times the normal time. The result is melted down switch poles and general catastrophe.

In order to avoid these problems, there is a disconnect with an ordinary STOP button that you press first. Only after the drive has shut down properly does an interlock allow you to open the main (motor) circuit. There is a supervisory circuit that checks the interlock.

This arrangement avoids designing the breaker for full load low frequency interruption. It also makes stopping from the safety switch a legal and natural action. For large drives, especially CSI drives, it is the only possible solution.