- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
For those who are unfamiliar with this concept, the resistors are indicative of a different variant of Wye Delta starter called a "Closed Transition" starter. The resistors are tied in using a 4th contactor during that transition to absorb the potential HUGE spike in current that can take place in an open transition. That's the dirty little secret of Wye-Delta starting; it usually doesn't really reduce the MAGNITUDE of the starting current peak, in fact it can actually make it worse, upwards of 2000% of FLA. All it really does is to decrease the DURATION of the current spike because the motor is already spinning. So when the load has a possibility of significantly showing down during that open transition and the residual magnetism in the motor core makes it a generator that is out of phase with the Delta re-connection, that high current spike can then create a torque spike that is known to damage loads and even the motors themselves. I have had two cases where motor shafts have sheared off, one on a 500HP Toshiba motor on a refrigeration screw compressor. So the Closed Transition variant has the resistors absorb that spike of current rather than the motor. The funny part is, a Solid State Soft Stater is ALWAYS less expensive than a Closed Transition Wye-Delta starter. So the people who chose Open Transition Y-D over RVSS are basically gambling that the little bit they save by going Y-D might be seriously offset by having to switch to Closed Transition, and they will usually not realize it until AFTER something breaks!
