Rule of thumb, amps on motor with no load?

[hockeyoligist2]

Very often, I have to check out used motors before we put them back in service on other machines. I meg them, check bearings and end play, power them up and check the amps with no load.

Is there a "rule of thumb" that the motor amps should be on a no-load motor compared to the FLA?

Example: Today I had a 5 HP 460v motor with a FLA of 6.9 amps it was drawing 3.5 amps with no load, only a pulley on it. Megged good, bearings seemed OK. Installed it on a compressor and it blew a fuse instantly on start-up, compressor spins freely. Megged again at the starter to eliminate a bad wire, checked out OK, replaced the fuse, blew the same leg, 10 amp fuse, double element, slow blow fuse. Unhooked the wires pulled in the starter and the Voltage was 470 phase to phase, 271 to ground on all three legs.

 

[sgunsel]

Starting current under load is probably 40+ amps. You can use 20 amp fuses and should be OK if the motor is good. It sounds like your no-load current is in the right ball park, but you should check the manufacturers data for actual no-load current. You also need thermal overloads if not internal to the motor.

 

[Jraef]

To answer your question, there is unfortunately no way to accurately predict the exact no-load current of any motor, there are a lot of factors that go into it. Could be anywhere from 20-60% of FLA.

9 times out of 10 on a compressor application like that is that there is a problem with an unloader. You can spin it freely by hand because it is un-powered. When you go to start it electrically, the unloader is getting power when it is not supposed to, i.e. a relay contact somewhere is welded etc. etc. Unloader valves are usually held open by springs and energized to close a few seconds after start once the motor is at speed. If that unloader closes immediately, the motor locks up under the compressor pressure.

 

[a.bisnath]

compressor starting loads

compressor starting loads

In all systems, starting current and load is the highest for compressors,choice of fuse type and fuse value is important ,If there are any record or values in the schematic drawings it should tell you the type and value I have seen for normal fuses it is rated at 300% the full load curent and for delay blow there are several design types,for a slow blow I would use the closest to 185% of the full load say 13-15 amps-remember line fuses are for lock rotor or jam and earth fault conditions,the overload unit is for protection is for actual sensed loads with a less sharp current time graph e.g bearing wear,rotor stator rubbing actual abnormal load conditions vibration e.t.c.