I know it is possible to serve multiple motors from one VFD however I have an application where I have two motors of identical horsepower however one will be a standby in case motor 1 fails, they are also deigned to alternate hours to provide each motor with equal run time. Typically when two motors are connected to a single drive both will operate at the same speed (correct me if I am wrong) would it be possible to serve both motors from a single VFD sized for one-motor and thru a BMS signal and a contactor supplied with the motor circuit protector (one for each motor) alternate the motors and arrange it so if one were to fail the other would start? Would there be any disadvantages to this arrangement? thanks.
1 VFD 2 Motors??
- Thread starter Mike01
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hillbilly1
Senior Member
- Location
- North Georgia mountains
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- Owner/electrical contractor
Yes, multiple motors can be run off of one vfd, if I remember correctly though, seems like a choke is needed. But what you are doing is different because only one motor at a time will be running. The problem is, if one motor goes bad, and the VFD trips out, it will have to be reset to run the other motor. It could be done, but it's going to be a somewhat complicated control scheme.
hillbilly1
Senior Member
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- North Georgia mountains
- Occupation
- Owner/electrical contractor
I remember now, I believe it's called a reactor.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
This is really the only problem, other than also making sure the VFD is turned off (Stop command, done ramping down or braking if enabled) before switching motors. Lets think this through a bit.
The VFD is the primary means of protecting the motor from overloads and faults. Let's say the load jams somehow (you didn't say what the load was). When the VFD senses an OL in the motor and shuts down, it switches to the backup motor but will require overriding the VFDs thermal memory so that it knows the new motor is cool. If you do that, and the 2nd motor then immediately faults on overload too because the load is still jammed, the VFD switches back to motor 1, clears the thermal memory again and fries motor one. Then it detects that new fault, switches back to motor 2 again, but clears the thermal memory again and then fries motor 2.
There are a few VFDs that allow you to program multiple motor parameters into them, but they still may have only have one motor thermal register (the memory that keeps track of the motor heat for OL purposes). So the way to do it if you don't have that ability in the VFD or it doesn't have two thermal registers would be to disable the motor protection features in the VFD and use external motor overload relays for each motor. You will also need separate mechanically interlocked contactors for the motors, that way you can't accidentally turn on a second one while one is running and fry the VFD transistors. By the time you are done with all that, you may have been better off just buying a second VFD, but it depends on what size you are looking at. 10HP and under, I would just buy the 2nd VFD and be done.
Thanks for all the opinions I think the best thing to do for the reasons stated above (overloads, fault sensing, etc.) and simplicity is to just provide the additional VFD, just to rework the controls alone and the user having to adopt to a new setup than they are typically used to everywhere else at their facility, definitely do not want to overcomplicate things I will just go with the KISS method. thanks again.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
we have a few lift stations that use a simple Alternating Relay, the relay changes to the other motor each time the motor is turned off, so that the VFD will be starting the new motor fresh when its called for the next time.
Macromatic Alternating Relays
we use these because of the great tech support, and design on the fly if you need a special relay.
in these lift stations If the PLC detects a motor fault, it will shut down the Alternating Relay and leave it on the good motor, but this can be done through the overload contacts also without a PLC.
Macromatic Alternating Relays
we use these because of the great tech support, and design on the fly if you need a special relay.
in these lift stations If the PLC detects a motor fault, it will shut down the Alternating Relay and leave it on the good motor, but this can be done through the overload contacts also without a PLC.
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