Motor Control Circuit

[Boiler79]

Have motor control circuit and being told that it is burning up motors and I do not see it.
For an example controls for 20hp motor FLA 48amp. Disconnect to a distribution block, using #6 from distribution block to motor starter contactor. From starter contactor to motor using #4 in sealtite. Distribution block and contactor in control cabinet. I am being told that because we are going from a #6 to #4 wire the motor is burning up. If I am missing something want to knowwhat it is.

 

[charlie b]

A change in wire size will not cause this problem, especially since both wire sizes have sufficient ampacity to handle a 20 HP motor. Whoever is giving you that story is either sadly ignorant of all things electrical, or is trying to shift the blame away from themself.

But something is not adding up with your description. Table 430.250 (the information source we are supposed to use for sizing motor-related wires and breakers) does not correlate 20 HP to 48 amps at any voltage level. What is the voltage rating of the motor, and what is the voltage of the power source?

Also, what exactly do you mean by saying the motor is buring up? What are the symptoms? Are the windings overheated to the point of destruction? Did the motor run abnormally hot, but has not yet actually failed? Are the motor bearings hot, but not the windings? What is the nature of the mechanical load? Is the motor correctly sized to serve that load?

I can't really give you any assistance, without knowing more about the circumstances.

 

[mnbiker]

first thing i would check is all connections from disconect, distribution block, motor starter/contactor to motor. looked for discolerd wires and signs of heat. bad contacts or connections can be tough on a motor, especialy on start up. throw a clamp on it and see if the in rush current is within reason.

 

[Jraef]

... Table 430.250 (the information source we are supposed to use for sizing motor-related wires and breakers) does not correlate 20 HP to 48 amps at any voltage level. What is the voltage rating of the motor, and what is the voltage of the power source? ...

I'd think it's most likely a 230V 3 phase motor. 430.52 says that we must size the conductors for 54A at that HP and voltage, but that doesn't mean the motor FLA on the nameplate will be that high. It's usually a little lower.

Still, I agree with everything charlie b said. #6 wire should be fine and nothing changes when going to a larger size down stream, other than it costing a little more.

One thing though Boiler79,
I don't see you mention an overload relay. Is that just an oversight on your posting ("From starter contactor to motor using #4 in sealtite.") or do you not have one in the circuit? If not, THAT might be the reason why you are burning up the motor. If the motor overloads and there is no OL relay, the motor may be smoking before the fuses clear. I know that TECHNICALLY you can use the fuses as the OCPD in a motor circuit, but it RARELY works out that you can find the EXACT size fuse to properly protect the motor. That's why we have overload relays.

If you have one, is it set properly for the FLA that is lower than the standard 54A? That might be another issue. Even a small increase in the OL trip point can seriously affect the motor life if the load is straining the motor's capabilities.

 

[hurk27]

I take it this is a new install, I might also question the design of the use of the motor, like is the motor being cycled often? I have found motors that never reach full speed and are being bumped continuously can over heat, you should never use a squarle cage motor as a stepping motor. the next question is? is there any speed control and is the motor design for reduced speed?

 

[weressl]

I take it this is a new install, I might also question the design of the use of the motor, like is the motor being cycled often? I have found motors that never reach full speed and are being bumped continuously can over heat, you should never use a squarle cage motor as a stepping motor. the next question is? is there any speed control and is the motor design for reduced speed?

How long is the feeder? What are the overload settings in relationship to the FLA?

 

[a.bisnath]

line and load protection

line and load protection

all in all what I am seeing is improper line or load protection ,a motor is burning,has the line fuses or breaker been sized and adjusted correctly?Has the overload of the correct current rage and also is it adjusted properly?These are the elementary factors in motor protection design and selection,what about the actual load/type of load(starting on full torque) the motor is driving? this can also cause motors to overheat and burn if it is more in watts or horsepower than the motor can supply.

 

[BJ Conner]

Check the Location

Check the Location

Nothing in the controls sound like the problem.
Check the location, Is it excessively hot? Are the vents on the motor open? Is in in an enclosure of some kind?
Whats the load?