Control.com - Nerds in Control

We have a fan with a 37 kW electric motor works. To prevent the start up hums of motor we used a VFD and bypass topology. When the motor intended to drive the nominal speed. Drive is removed and a bypass contactor closes. But when the bypass contactor closes a shock is observed in foundation of electric motor and causing trip in bypass circuit. Please help me to solve this problem.

Amin... is this a commissioning problem, or is it something new in an existing unit?

Regards, Phil Corso

If you are just bring the fan up to speed slowly and do not intend to vary the speed during operation try a Soft Start device. It's cheaper and less complicated then a VFD. It should stop the hum at start up and save on the electric bill if the fan is starting in a Peak Demand billing period. If you want to keep the VFD is there a reason why the fan can not just operate through the VFD?

Steve King
Plateau Controls and Service, LLC

I would have to agree with Steve King. My best guess is that (and I may be pointing out the obvious here)... that the incoming three phase is "phase-shifted" from the output of the VFD, when you switch over the phases have to come into alignment and bam it jerks.

I would just let the VFD control it and be done with it. however if you must switch to bypass mode... you are going to have to come up with a phase sense circuit to advance or retard the output phases to the input... even then it will be difficult to get it to work... unless there is already something out there to do this... in fact I think I saw a circuit board that may do it... let me check on that... and I'll get back with you.