air compressor motor

[mtnelectrical]

I have a 3 phase motor 208/230/460 working on 208. Nameplate says 6.4/12.8 amps. When the starts pulls 14.5 and start increasing till it reaches 19.8 amps at 175 psi . When starts to compensate for air loss it pulls near the 19 amps. Is this normal for a motor in this application? And what value would you use for the overload proteccion and fuses?

 

[raider1]

Starting current for a motor can be 6 times the running load amps.

For sizing the overloads check out 430.32.

Chris

 

[mtnelectrical]

Starting current is 37 amps of so. But what puzzles me is that motor nameplate says 12.8 and motor runs at 14.5 after starts.

 

[jim dungar]

Starting current is 37 amps of so. But what puzzles me is that motor nameplate says 12.8 and motor runs at 14.5 after starts.

Your nameplate lists 3 different voltages, but only 2 different currents.

My guess is the 12.8A is for the 230V connection, and you should expect the running current to be higher if the source is only 208V.

 

[texie]

Your nameplate lists 3 different voltages, but only 2 different currents.

My guess is the 12.8A is for the 230V connection, and you should expect the running current to be higher if the source is only 208V.

I would agree except that the OP indicates it is pulling 19 amps at design pressure. That would be about 30% above name plate.
We don't know if this is factory packaged system with a "special" motor or home brewed system with maybe the wrong pulleys.

 

[mtnelectrical]

Nope. It is not a special motor. It has been replaced recently. And the pulleys they look like they were there from day one. Now I'm thinking may by they install the wrong motor. My question. How much would affect runinig this motor at this high amperage?

 

[kwired]

I have a 3 phase motor 208/230/460 working on 208. Nameplate says 6.4/12.8 amps. When the starts pulls 14.5 and start increasing till it reaches 19.8 amps at 175 psi . When starts to compensate for air loss it pulls near the 19 amps. Is this normal for a motor in this application? And what value would you use for the overload proteccion and fuses?

As mentioned we don't know if 208 volt rating is allowable to be higher than 12.8, often they will be about 10% more than 240 volt rating. But the fact the current increases as pressure increases sounds normal to me. If anything they either needed a lower ratio for speed of compressor shaft or they put on about the smallest motor they thought would drive the thing to cut cost even though it probably should have been a little larger. Sounds like you probably have a 5 hp motor when a 7.5 may have been better. They may be intentionally running it into it's service factor.

 

[electric_cal]

Nope. It is not a special motor. It has been replaced recently. And the pulleys they look like they were there from day one. Now I'm thinking may by they install the wrong motor. My question. How much would affect runinig this motor at this high amperage?

I would agree with the previous posts. The nameplate information indicates a 5 HP motor, but the operating full load amps you are reading would indicate that you have a 7 1/2 HP load. You are operating this motor at almost 50% above the nameplate FLA. Some special motors will allow you to run at 1.15% to 1.25% above the nameplate FLA for a short time, but I would expect this motor to fail if you continue to operate at this current level.

 

[kwired]

Nope. It is not a special motor. It has been replaced recently. And the pulleys they look like they were there from day one. Now I'm thinking may by they install the wrong motor. My question. How much would affect runinig this motor at this high amperage?

What is the setting of overload protection? If still set for original motor and is near 7.5 hp values, that may mean they did replace with a smaller motor.

 

[mtnelectrical]

Nope. it was a 5 HP motor, I saw it on the floor but it was a really old one. An old frame not available anymore and physically it was bigger than this new one.

 

[kwired]

Nope. it was a 5 HP motor, I saw it on the floor but it was a really old one. An old frame not available anymore and physically it was bigger than this new one.

Couple things come to mind here.

1. 5 hp is still 5 hp, but that older motor may or may not take more abuse than some newer ones, or could even have a special rating, or so could a new one for that matter, but that is more common in single phase motors for this kind of application than for three phase motors.

2. If compressor is same age and has had no major work done to it, it may take more to drive it than it once did, because of wear. Could even be part of reason for old motor failure.

 

[electric_cal]

Nope. it was a 5 HP motor, I saw it on the floor but it was a really old one. An old frame not available anymore and physically it was bigger than this new one.

Could you possibly get the original motor nameplate information? This would help you in learning what the original operating full load current was. I wonder if your original motor could have been rewound to a 7 1/2 HP in the past, and the nameplate was not changed? It is also possible that the compressor is faulty and that is why the new motor is overloaded. Regarding your original question, if you set the overload at the new motors nameplate FLA, but you are reading a 19 amp load, then the overload will trip. That is its purpose. It is there to save the motor from failing due to an overload condition, which is what you presently have. I would not set the overload above your motors nameplate FLA, it would probably lead to your motor failing. I think you need to look into this deeper.

 

[kwired]

I wonder if your original motor could have been rewound to a 7 1/2 HP in the past, and the nameplate was not changed?

Need to have some input from some of the motor experts, as I am not all that certain this is something that can be done in most cases. 7.5 would likely require more copper, more steel, and likely different designed rotor as well than a 5 would require, then you have to fit all of it into the same space the 5 was originally in.

 

[mtnelectrical]

Thank you guys for all responses. Love this forum, always full of knowledge that give you ideas to troubleshoot your problems. I am going to look into mechanical part a little more. I kwox sometimes there are some valves inside that create an extra drag in the normal operation of the compressor.

 

[hbiss]

My guess is that the pulley on the motor was changed (larger) to run the compressor at a higher RPM to get more air out of it. Probably done many years ago (which is why it looks original) and that old motor, being better built than the crap today, took the abuse until now. You won't be so lucky now. What I would do is either replace the motor with a 7 1/2 hp or install a smaller pulley, slow it down by 25% and see what the current draw is.

PS- I always get a little nervous with old compressors. When was the last time the receiver (tank) was hydro tested? Most times the safe thing is to replace the whole compressor.

-Hal

 

[kwired]

My guess is that the pulley on the motor was changed (larger) to run the compressor at a higher RPM to get more air out of it. Probably done many years ago (which is why it looks original) and that old motor, being better built than the crap today, took the abuse until now. You won't be so lucky now. What I would do is either replace the motor with a 7 1/2 hp or install a smaller pulley, slow it down by 25% and see what the current draw is.

PS- I always get a little nervous with old compressors. When was the last time the receiver (tank) was hydro tested? Most times the safe thing is to replace the whole compressor.

-Hal

I still think before increasing motor size one would want to look into the rating of the compressor. Like I mentioned earlier, maybe the compressor has problems leading to higher current draw and purchasing the larger motor may be a waste of time and money because the compressor will fail soon anyway - especially if overdriven. There is a valid concern over the tank.