I replaced a psc condenser fan motor. The bad original was a half horse motor, 1050 rpm, psc, 208-230 motor, totally enclosed, rated amp draw was 2.9. The incoming voltage is 208 and the compressor is a 3 phase. The new motor has identical specs except that rla is 3.0A. I took an amp draw with 2 differnt meters and found that it was about 3.6, about 20 percent more than rated amps. I thought that maybe I was getting some interference so I connected a long wire and took the amp draw outside the cabinet and found the same 3.6. I will probably replace the motor but in the few warm days that it has run it has not gone out on internal overload. I believe the SF is 1. I presume the motor is bad but anyone have any suggestions why the motor has not gone out on overload or why I might be getting a false reading? By the way I took an amp draw of the old motor and it was 3 amps when it would run so I imagine that the readings are not being influenced by being too close to the compressor--or else that motor's reading was about 20 percent over as well. Thanks
I would verify the SF. Also, check the rated temperature rise of the motor. If the temp rise is not over 40ºC, then the required overload setting is FLA * 125%, otherwise it's FLA * 115%. [NEC 430-32(c)]
So if your FLA is 30 amps, in all likelyhood the OL setting is 37.5 or thereabouts.
Just my opinion, but even if the OL isn't tripping, I think having a 30 amp motor with a 1.0 SF running at 36 amps is an incorrect application. I'd plan a replacement. I'd also tach down the fan speed, if possible. Something's causing the overload condition, and I wouldn't jump on a motor defect as the first suspect (although anything's possible).
There are 10 types of people. Those who know binary, and those who don't.
#130521 - 07/14/0605:07 PMRe: high amp draw by motor
I believe the rated temp rise is 60 degrees and the capacitor is correct (10mf). I also checked the capacitance with my meter and it was good. Yes, the motor is similar to the webpage cited by Bjarney. I would like your opinions--do you think the amp draw reading is accurate? If it is then changing out the motor is the thing to do. One other question. Will the amp draw be more at 230 volts or at 208 volts?
#130523 - 07/15/0605:11 AMRe: high amp draw by motor
Be careful on the True RMS issue. Some meters say True RMS on the outside, but they mean only for voltage, not current. The Fluke 87 is like that. So when you add a current probe to it, it is not reading true RMS current. Some condensers are coming with electronic motor controllers now, small VFDs designed to work with 1 phase PSC motors. They will give you trouble when trying to read current.
#130526 - 07/18/0609:02 AMRe: high amp draw by motor