motor protection

We have some equipment that was rebuilt by an outside company. There is a three-phase 480-volt cabinet exhaust blow they installed rate at ½ HP. The only protection device that they installed is a three-phase circuit breaker rated at 1 amp. This exhaust motor is always tripping out and therefore they never run the exhaust blow. Correct me if I’m wrong, but I thought the circuit breaker was for short-circuit protection and the thermal overloads were for over-current protection. If I use my trusty Square D Motor Data Calculator I come up with a circuit breaker rated at 15 Amp and the thermal overloads rated at B1.45.
Is this correct?

Thanks

rmonroe

Overcurrent protection

It appears your circuit breaker is under sized.

However, more information is required (i.e. motor nameplate current), but circuit breakers can be used to protect motors. There are two type of overcurrents that motors must be protected against.

Note: the following statements do not address all of the possible protective options nor sizing criteria.

First there is the low level long time frame overcurrent caused by the normal operation of the motor. This type usually protected against by the use of overload relays, however for motors less than 1HP, the US NEC and UL allow the use of a circuit breaker sized at not more than 115% of the actual motor full load current.

Second there are the short circuit and ground fault overcurrents caused by the failure of the motor. These are usually protected against by the use of circuit breakers or fuses. The US NEC and UL allow the use of a thermal-magnetic circuit breaker sized at not more than 250% of the published table motor full load current (in this case 1.1A).

I would replace the circuit breaker with a manual starter, which is rated for thermal overload protection and short circuit protection. Most motor control manufacturers make them. They are common in this type of 'always on' application, as well as in front of contactors for dual duty. Here's one: http://web2.automationdirect.com/static/specs/fujimcselection.pdf

I dunno, if this is a 1/2 hp 480vac motor then the FLA may be around 1 amp so I would look at sizing it a slightly larger or fusing it at 1.5, maybe higher but that would depend on the FLA.

I dont see where you got 15amps, even at 120vac single phase the FLA shouldnt be much more than 8amps.

Recheck your FLA and size the fuses or circuit breakers accordingly. Wouldnt hurt to add a manual or electric starter to the circuit that has overloads. For an application with current this low I would use an IEC style starter with adjustable overload. Check the draw and adjust your overloads accordingly.

May want to verify motor has no shorts to ground just for GP.

rsdoran said:

I dont see where you got 15amps, even at 120vac single phase the FLA shouldnt be much more than 8amps.

Click to expand...

That's just for branch circuit protection. The Square D 'slide rule' assumes 15A is the smallest branch circuit breaker you'll use in a service panel.

The fan is probably drawing just slightly more than 1A, so the breaker trips after a period of time (as it should).

I would recommend going the motor starter route as well.

beerchug

-Eric

I did check the current draw on the motor and was drawing between 1.2 to 1.5 amp. I'm going to install a motor starter and eliminate the 1 amp breaker from this system.

Thanks to all for the advice.

rmonroe