Drive bypass

[ccjersey]

I am looking at installing a VSD on a single motor 15hp 1800rpm service is open y to 120/240V 3 phase open delta. motor will be driving a positive displacement lobe blower in vacuum service and controlled by analog 4-20ma signal. Leads are short less than 20 feet. I think this will be a constant torque application. I have 2 older drives one is a "new" Square D Altivar66 and the other is a used Danfoss I believe.

Any problems with the open delta 3 phase and the drive?
Can I install a 3 pole double throw switch ahead of the drive as a means of emergency bypass. Will the output of the drive have to have a bypass as well? or will the bypass conductors feed into the output of the drive directly without problems. I want to have a middle of the night-over the phone solution to any drive problem.

Bypass wouldn't be switched under power.

Thanks

Jim

 

[Marke]

Hello ccjersey

Firstly, I would be very careful about bypassing an inverter.
There are several things to be careful of.
1. Do not under any circumstances feed voltage into the output of the drive. You can do this with a soft starter but you will cause damage if you do it to a drive.
2. Switching on the output of a drive is gnerally considered to be a NO. Some drive manufacturers claim that they protect against this, but in most cases, if you switch while the drive is running, you can cause damage to the drive.
3. If you wish to be able to bypass the drive if it fails, then you must use appropriate interlocks to prevent the output from being connected to the supply, and to prevent ouptut switching while there is power on the input to the drive.

Operating the drive from an open delta supply will work, but depending on the supply, you may have a higher ripple current through the main DC bus capacitors. To prevent this from cuasing damage, I would suggest that you ensure the drive is at least one size larger than the motor.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[DickDV]

In addition to Mark's comments, inverters on floating (ungrounded) delta supplies will not be able to detect motor lead ground faults properly.

Worse, since there is no convenient ground return path for output high frequency pulse currents capacitively coupled to ground (common mode noise), you set yourself up for nasty noise problems elsewhere in your facility. These ground pulses must find a way back to the DC bus and, lacking a grounded wye source, will take whatever path is available---maybe that shield around your instrumentation leads or that power supply in your plant computer.

I'd try it and see what happens. Brace yourself for common mode problems tho and leave space beside the drive cabinet for a drive isolation transformer, just in case. That would be a transformer with a grounded wye secondary, of course.

 

[Skogsgurra]

There is one aspect that quite often is forgotten - that of thermal protect. You should see to it that the protect works also in a by-pass situation. Otherwise all that has been said above holds very true.

 

[jraef]

Skogs point is very important. If you bypass with a double pole switch, you would need 2 of them, one for line bypass and one for load isolation. There are companies selling "inverter bypass switches" which are multi-pole cam switches designed to facilitate line and load isolation and bypass in one movement. But both systems neglect the issue of motor protection. You must have something that will open the power circuit in the event of an overload. It is usually done with a contactor because is is simple to add a thermal overload relay and drop out the coil. You can also do it with a Motor Protective Switch that has a trip mechanism to open all 3 phases. These are IEC devices that are only recently becoming more accepted in the US, but you will need to ask a decent distributor about them if you are not already familiar.

"Venditori de oleum-vipera non vigere excordis populi"