Switching motor from vfd to contactor while running. 4

[mcdou]

I have an application in which it is desireable to switch the feed of a motor between a vfd and a line contactor while the motor is running. This is desired so that one vfd can be used to ramp more than one motor up to speed. Does anyone have experience with this? I am concerned about switching the power source of the motor at full load. Is there a danger to the vfd or the motor?
Any insight would be appreciated.

 

[CBII]

I will assume that you have two or more pumps, equally sized, on common suction and discharge manifolds. Be careful to set your VFD minimum speed for the worst case operating condition, all pumps running and the VFD at minimum speed due to demand. The VFD driven pump will make steam without some flow.

If you have energy consumption as a consideration, my experiance is that usually running all pumps on VFD's will be the lowest operating cost. You can run a cost analysis to justify use of VFD's on each pump.

 

[jbartos]

Suggestion: Since various drives have efficiency less than 1, power factor less than one, and lower reliability than electromagnetic contactors, some consideration can be given and was given in literature to have the drive bypass by a contactor at a proper time. Then, the drive is free for another motor control, e.g. start-up. The clue appears to be in achieving an optimal motor state at which the transition to its contactor power feed is smooth with the minimum harm to the motor.

 

We switch from vfd to bypass on our drives quite frequently, but we make sure the drive gets shut down at this transition so if the switch is thrown back to vfd the drive will have to go through the start sequence to come on. Our drives can start on the fly or into a spinning motor. our contactor scheme is set up to completly isolate the drive at bypass(input and output)

 

[jbartos]

Suggestion: If the drive has an intermediate DC section, i.e. AC-DC-AC converter, then the output AC frequency synchronization may be somewhat off from the input AC that is actually a side the contactor is using. Any mismatch in frequency during the transition is somewhat affecting the motor in terms of speed adjustment, current adjustment and mechanical stress adjustments. It would be preferable to have the output AC synchronized with the input AC of the converter (vfd).

 

[jraef]

Actually, this application is done all the time with VFDs, in fact many now have canned "application macros" that do the sequencing control in the VFD processor. But you WILL have a transition spike when connecting the motor across-the-line. You MUST isolate the VFD output from the line power, so there is no (economical) way to avoid an open transition. Robicon VFDs offer a "synchronous transfer" option, but it is very expensive and usually applied to large motors.
Going the other way, from X-Line back to VFD control is also problematic unless your VFD has frequency matching, a.k.a. "speed search" or "flying restart" which detects and matches the VFD output to the rotor frequency. Look for this feature for this application.
Another solutionoften used is to put the lead pump on the VFD permanently. When it gets near full speed, switch on a lag pump full speed and slow down the VFD. Pressure surges can be a problem, but using soft starters on the lag pumps can help mitigate that as well. That which does not kill me, makes me stronger... and pissed off!