archibaldtuttle
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Need to replace a compressor motor that has always struggled with cold starts and melted down when the overload failed on a cold start.
I could have this motor rebuilt again. theoretically it was matched to the load in the day (1950s refrigeration compressor) Its a massive Westinghouse 5 HP single phase. In normal operation in refrigeration,it would, ironically never be cold and would have had short cycle timers to prevent starting against pressure, but i read in various threads elsewhere, e.g., that folks would add soft starter to refrigerator compressors that didn't come that way OEM to ease struggling on start and high start current.
this is Westinghouse 456113A Sser. # 4803 - 1710 RPM on a 254 frame (which these days by NEMA standards would be on a 15HP motor)and it draws 28 running load amps. Don't know if that indicates the efficiency of modern motors that draw 21 amps for 5HP rating or it indicates conservative rating in the old days -- or a little of both. I have some motor guys look at it and say I'd need a 10 HP motor to do equivalent work.
don't know if that is knowing homage to these antiques or just the size of the motor is so much bigger than equivalent HP today it begs comparison to much higher HP motors.
Using contemporary tables the assumed torque for horsepower is about linear with 5 HP at 2000 RPMs (guess they picked 2000 cause its even #. motors are really in the 1700 to 1800 range) 13.1 ft. pounds and 7.5 HP is 20 ft. pounds and 10 HP is 26 ft. pounds.
So theoretically if I want more torque i just replace with higher horsepower, but those figures are torque at speed not locked rotor or starting torque which seem more relevant to me. obviously i don't have a really good idea of the theoretical load of the compressor i'm running and can't find any specs on the original motor - maybe there is some ole book somewhere but the model doesn't come up on the internet anywhere.
In nibbling around the edges on the starting problem, i always thought the other hard starting solution was a motor manufactured with extra start windings that dropped out on a centrifugal switch. But i don't really seem to find any offerings that indicate this design. Maybe I'm not using the right terminology. Assuming i could use the soft starter linked above, that theoretically can handle 7 HP. its RLA load is up to 32 and none of the 7.5 HP motors i've looked at go above 31 so don't know if i could get away with it on 7.5 HP.
The obvious other answer is that, for the price of a motor or rebuild, i could buy an entire chinese compressor set up that wouldn't have the same capacity as this unit but would have enough capacity for most of my uses. But this old Compressor runs absolutely smooth as a cucumber at speed and it really has a lot of capacity and i've already got it paired with 150 gallon tank so I'm at least trying to understand approaches to hard starting loads before ditching it.
thanks for any thoughts if you happen to be sitting around on the 4th thinking abstractly about motors like me.
brian
I could have this motor rebuilt again. theoretically it was matched to the load in the day (1950s refrigeration compressor) Its a massive Westinghouse 5 HP single phase. In normal operation in refrigeration,it would, ironically never be cold and would have had short cycle timers to prevent starting against pressure, but i read in various threads elsewhere, e.g., that folks would add soft starter to refrigerator compressors that didn't come that way OEM to ease struggling on start and high start current.
this is Westinghouse 456113A Sser. # 4803 - 1710 RPM on a 254 frame (which these days by NEMA standards would be on a 15HP motor)and it draws 28 running load amps. Don't know if that indicates the efficiency of modern motors that draw 21 amps for 5HP rating or it indicates conservative rating in the old days -- or a little of both. I have some motor guys look at it and say I'd need a 10 HP motor to do equivalent work.
don't know if that is knowing homage to these antiques or just the size of the motor is so much bigger than equivalent HP today it begs comparison to much higher HP motors.
Using contemporary tables the assumed torque for horsepower is about linear with 5 HP at 2000 RPMs (guess they picked 2000 cause its even #. motors are really in the 1700 to 1800 range) 13.1 ft. pounds and 7.5 HP is 20 ft. pounds and 10 HP is 26 ft. pounds.
So theoretically if I want more torque i just replace with higher horsepower, but those figures are torque at speed not locked rotor or starting torque which seem more relevant to me. obviously i don't have a really good idea of the theoretical load of the compressor i'm running and can't find any specs on the original motor - maybe there is some ole book somewhere but the model doesn't come up on the internet anywhere.
In nibbling around the edges on the starting problem, i always thought the other hard starting solution was a motor manufactured with extra start windings that dropped out on a centrifugal switch. But i don't really seem to find any offerings that indicate this design. Maybe I'm not using the right terminology. Assuming i could use the soft starter linked above, that theoretically can handle 7 HP. its RLA load is up to 32 and none of the 7.5 HP motors i've looked at go above 31 so don't know if i could get away with it on 7.5 HP.
The obvious other answer is that, for the price of a motor or rebuild, i could buy an entire chinese compressor set up that wouldn't have the same capacity as this unit but would have enough capacity for most of my uses. But this old Compressor runs absolutely smooth as a cucumber at speed and it really has a lot of capacity and i've already got it paired with 150 gallon tank so I'm at least trying to understand approaches to hard starting loads before ditching it.
thanks for any thoughts if you happen to be sitting around on the 4th thinking abstractly about motors like me.
brian
