VFD's and Motor In Rush Currents

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faresos

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Hello everyone,

I have been asked if a maintenance person shuts off the safety disconnect at the motor (20HP) but does not shut off the VFD disconnect, turning on the safety disconnect while the VFD is on will result in inrush current of 8 to 10 times the amp draw going through the VFD. The drives are not rated for this inrish current. The result will be damage or destruction of the drive and possibly catastrophic failure of the MCC. The VFD is located in the motor control center (MCC).The contractor suggested to install auxiliary contacts in the safety disconnects that would disable the VFD's.

Anythoughts on this ??

Thanks,
 
faresos said:
Hello everyone,

I have been asked if a maintenance person shuts off the safety disconnect at the motor (20HP) but does not shut off the VFD disconnect, turning on the safety disconnect while the VFD is on will result in inrush current of 8 to 10 times the amp draw going through the VFD. The drives are not rated for this inrish current. The result will be damage or destruction of the drive and possibly catastrophic failure of the MCC. The VFD is located in the motor control center (MCC).The contractor suggested to install auxiliary contacts in the safety disconnects that would disable the VFD's.

Anythoughts on this ??

Thanks,
Click to expand...
We always use early break auxiliary contacts. And the VSD is NEVER enabled with the o/p open circuit to avoid the possibility of the fault situation that concerns you.
 
faresos said:
Hello everyone,

I have been asked if a maintenance person shuts off the safety disconnect at the motor (20HP) but does not shut off the VFD disconnect, turning on the safety disconnect while the VFD is on will result in inrush current of 8 to 10 times the amp draw going through the VFD. The drives are not rated for this inrish current. The result will be damage or destruction of the drive and possibly catastrophic failure of the MCC. The VFD is located in the motor control center (MCC).The contractor suggested to install auxiliary contacts in the safety disconnects that would disable the VFD's.

Anythoughts on this ??

Thanks,
Click to expand...
Yes, you need the aux contact in the safety switch, wired back to the VFD Run Command or E-Stop input do that the transistors are disabled as soon as the handle of the disconnect is moved. On all UL98 style safety switches (knife switch styles), the aux contacts that attach to the handle operating mechanism are always like this; they make only after the main contacts close, they break before the main contacts open. All UL98 style safety switches I am aware of are like this, but you have to be more careful of the cheaper little UL508 listed rotary disconnects. Some are, some are not.

The concept is correct, and in fact is actually even worse than described. When an AC induction motor is initially connected to any supply, for a brief fraction of a second each winding is a short circuit, only the wire resistance itself will reduce the current flow. In that instant, current can jump to as much as 2200% of the motor nameplate amps (in theory) before magnetic flux is created so that induction and thereby impedance is established. This is the true definition of inrush current, it is the MAGNETIC inrush current going into the motor coils. This rapid rise of high current, however brief, is referred to as a high "dI/dt" ( d= delta or change, I = current, t = time) and can kill transistors very quickly.
 
Thanks you all for the reply. It seems like I will specify the disconnect switch with Aux. Contacts.

Thanks,

Sam
 
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