Starting currents when shifting speeds

[RI951]

Hi,

My facility has several two-speed squirrel cage induction motor fans that have occasionally tripped on thermal overload during shifts from fast speed to slow speed, and once from slow speed to fast speed.

I am trying to determine the starting/in-rush current during the three modes: stop-to-fast, slow-to-fast, and fast-to-slow.

Stop-to-fast mode data can be obtained from the vendor tech manual. But my electrical engineer is somewhat stumped when trying to figure out what the relative magnitudes would be for the other two modes, without going out in the field and measuring amps directly.

Thanks for any input. I just discovered this site, and plan to access it often. And I did try the search feature before posting...

Pat

 

[dpc]

The problem of transient current on fast to slow transition is pretty well known. Quantifying the magnitude is difficult. We typically provide a time-delay relay for transition from fast to slow to allow motor time to slow down.

For slow to fast, the problem is less common. but again, a very short time delay would allow motor residual voltage to decay and avoid very high transients. With no motor terminal voltage, you would have an inrush that would not be greater than the normal starting current.

 

[RI951]

Thanks dpc. Will investigate the time delay angle. Currently, we are limiting operators to stopping the fan for two minutes, then restarting them in the other speed. Yet the fans, which are safety grade equipment, will auto-shift to slow on an accident signal, so they should be robust enough to withstand high transient currents...

 

[busbar]

Motor sizes are not given, but there could be problems with similar high currents occuring during bus transfer. One paper is Amit Raje, et al, Bus Transfer Systems: Requirements, Implementation, and Experiences, IEEE Transactions in Industry Applications, Vol. 39, No. 1, January/February 2003

Spin-down characteristics of a momentarily open-circuited bus my contribute to the undesired operation.