- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
While I don't disagree, I still see no allowances for that fact in the NEC. As I see it, you still need to have the SCPD sized for the motor circuit.weressl said:How much short circuit can an ASD deliver? I bet no properly sized OCPD would trip before the ASD's own overcurrent limit would cut the power supply.
Nothing wrong with what you are saying here, except that this is true only with vector drives that do the motor circuit calculations and adjustments you are speaking of.I disagree that one can connect any number of motors on a single drive. Drive characterization portion of the software will try to calculate the motor parameters, including the cable parameters and optimizes the power supply parameters accordingly. Different cable lengths to the individual motors will result in overdriving certain motors and underdriving others in the same group while everything is running. When you start taking motors out the situation gets even more complex, especially if it is desirable to have the motors running in synch.
So this points out that I failed to mention that you MUST put the VFD in open loop V/Hz (a.k.a. Scalar) operating mode in order to run multiple motors from one VFD. You cannot run them in any kind of vector control mode, open or closed loop. All of the VFD manuals will tell you that as well. As Besoeker said, you will lose the accuracy and torque performance that vector drives are capable of, but it will work to drive all of the motors at the same commanded speed. In the conveyor system, as a single load moves down the conveyor and loads the individual motors differently, the load speed may not remain the same, but in a relative sense it will work. Roller conveyor manufacturers do this all the time.
Besoeker,
Its an experience thing, but I think the extra harmonic problems come from the fact that each individual motor circuit will be a different length of cable and no two motors are exactly alike, so different total circuit impedances, cable capacitances, ringing effects etc. Only 2 motors on one drive is not usually a problem, 9 on one drive is. I have done numerous tunnel freezers where we had upwards of 15 motors on one drive and the motors would fry at less than 1/2 load, which can only be attributed to harmonics. I have also done this on utility power transformer cooling fans where we had only 4 fans on each drive, still a problem. Both situations were solved with reactors. But I have done dozens of pump stations with only 2 motors on one drive with no reactor and had no problems. So the crossover point is somewhere between 2 and 4 motors on one drive, but I can't say for sure what all of the circumstances are, so it's just prudent to always put the reactor in.