This is a 150hp screw compressor motor that was running fine. We were checking the power quality when the amp load went to a locked rotor level and the fuses blew. Any ideas why? After checking everything and replacing the fuses, the unit started and ran without issue. This is a wye start-delta run. Any ideas why the amps went so high? I hope the attachments went well.

That's a weird one all right.

If current goes to locked rotor amps but there's no machine damage, I would guess the load torque had exceed motor breakdown torque:

• Perhaps the load torque suddenly increased dramatically for some mechanical reason or malfunction in the air system.
  
• If no abnormal mechanical overload, perhaps reduced motor torque capability due to motor being in wye for some reason (never auto-shifted from wye to delta during start... or else shifted back to wye due to something you disturbed while you were in there).

Those might be longshots, but it's all I can think of.

Lake Man

Check the timer in your starting circuit. If you remained in wye, then the motor was operating undervoltage.

Another possibility is that the starter dropped back into wye for some reason.

Howard

MotorDoc- The compressor had been running for a couple of days at the time it happened. The compressor next to it fried the drive that it runs on and we were checking the power quality to look for problems. This compressor does not use a drive just cross the line. We thought about it going back into wye, but it has a mech. interlock.????