2 Pole Motors

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yesterlectric

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A 5 hp 230v 1 phase blower motor is marked 3450 rpm. It was found to have only a 30 amp breaker at the panel supplying it, and it has been tripping at startup. 30 amp breaker seems small.

My question is: table 430.248 says ampacity and short circuit ground fault protection is supposed to be based on a 28 amp full load current. But the table says it is for “normal” speed motors. Most induction motors I’m familiar with are 4 pole 1745 or so RPM. I’m not aware of the nec defining what “normal” speeds are. Is a 2 pole motor that spins at 3450 rpms considered a “normal” speed motor, where I can use the table 430.248, or not? Sure would be nice if they would define normal speed.
 
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2 pole and 4 pole motors are common motors and the currents in Table 430.248 apply to both. There is a bit of direction in 430.6 where it says:
... Motors built for low speeds (less than 1200 RPM) or high torques may have higher full-load currents, and multispeed motors will have full-load current varying with speed, in which case the nameplate current ratings shall be used. ...
Click to expand...
 
My 5 HP air compressor (nameplate 23.8A) has never tripped its 30A breaker. Going on 27 years now.

As stated, it’s lower speed motors that have higher currents. Has this been operating OK and just now started having problems, or a new installation?
 
I’m told it started failing after they replaced a motor with one of the same horsepower but a different brand. Haven’t seen it yet. This makes some sense though. The NEC would allow this motor to be on a 70 amp breaker and putting it on the 30 amp might work for one brand of motor but not another of the same horsepower. Additionally, having it on the 30 amp may work for one brand of motor but only for a while because as the motor ages it may draw more current and also if the circuit breaker is undersized and is seeing a lot of inrush it could become weak over time too.

This is an application with I believe quite heavy use which may be more than some uses of air compressors.
 
Several possibilities to consider:
1) It may be that the original motor failed because of a mechanical issue which has still not been fixed, putting additional load on the new motor.
2) It may be that a 5 Hp 4 pole motor was replaced with a 5 Hp 2 pole motor, which will cause all sorts of grief.
3) The new motor might have higher efficiency when running, but also higher starting current initially.

-Jon
 
Josh111 said:
I’m told it started failing after they replaced a motor with one of the same horsepower but a different brand. Haven’t seen it yet.
Click to expand...
Do they still have the old motor or at least a picture of its nameplate? That could help determine if the replacement motor is suitable.
 
With so many small air compressors, motor HP is a joke; I'd guess the motor was more like a 3HP or even less with that "fake" 5HP label. Your real 5HP is higher starting current. The @synchro suggestion of confirming the nameplate is essential.
 
How many amps is the new motor rated at and is the actual voltage 230? I can find Baldor motors matching your specs at 19.5 amps and at 23 amps. Maybe the old motor was cap start and run but new one is cap start only?

19.5 amp single phase motor on a 30 amp breaker might cut it. 23 amp motor probably not. Lots of possible reasons, I'm just throwing out another.

Funny as on Monday I had to convert over an air compressor with a "special" non-HP rated, non-standard frame, 3450 rpm motor. The old motor kept breaking at the welds on the rigid base. Darn thing ripped loose, locked up, smoked the belts, set off the fire alarm, and took out several AHU's with pneumatic controls in a hospital over the weekend. The motor isn't available anymore. Had to pull a rabbit out of my.....hat:) Doubled the motor sheave/pulley and went with a standard 1750 rpm, 5 HP, 3 phase, 184T frame motor. Worked out well considering there is no adjustment possible for belt tension. A little pie times D math and some dumb luck let me drill the new motor mounting holes in just the perfect spot. Don't worry, God re-paid me by kicking my butt later that day when.......well, that is another story!
 
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