Motor Protection

[ctrane]

I am looking for a little help with motor protection. I recently read the article by Mr. Holt in EC&M titled, "Motor Calculations Part 1: Motors and Branch-Circuit Conductors." I am having trouble understanding the last example in Part 1. I understand the Short-Circuit/Ground Fault protection, and am comfortable with the Branch Circuit Conductor sizing. But, what about the Overload Protection? By this calculation, does it mean that the fuse to protect the Overload will be a 15A Fuse? If this is the case, won't it trip on start-up? I have been trying to locate Fuse Trip Curves, and the ones from GE show a 15A Fuse having a 10,000sec+ capability of 30A. Is this consistent with Fuse Trip Curves? Also, what is the difference between FLA and FLC? I am new to this game, and any and all help will be greatly appreciated. Thanks.

 

[A/A Fuel GTX]

Check out 430.32. It covers overload ratings. Then check out 430.52. It covers OCPD ratings. They are two different protections. Overloads do not trip on ground faults, they trip on current draw above the values in 430.32.

FLA and FLC are one in the same.

 

[bdarnell]

I'll take your questions one at a time. I'm sure to get one at least partly wrong and someone will chime in to help out.

1. Overload protection for a motor is typically provided by a melting or non-melting resettable thermal unit (the latter being more prevalant). They are sized based on the full-load current of the motor and are responsive to motor current. When they sense a motor overload, they open the circuit to protect the motor. Some smaller, internal ones are automatically reset, while others have to be manually reset.

2. Since I don't have the fuse melt curves for the fuse you mentioned, I can't say if that is correct or not, but it doesn't seem out of line. A 30 amp Class J has a 300 sec. opening time at 60 amps. That's as far out as my curve goes.

3. FLC and FLA are the same. Full Load Current - Full Load Amps

Hope most of that is correct and hope it helps.

 

[eric stromberg]

Overloads

Overloads

As has been indicated earlier, overloads are covered in 430.32.

It also depends on the type and size of motor that you have. Many smaller, single phase, motors have an overload in the windings. A sink disposal unit is an example of this. If something gets caught in the mechanism, the overload will trip. After waiting a bit, it can be reset.

Three phase motors are different. The overloads for three phase motors are typically found in the starters. The starter consists of a disconnect, short-circuit/ground-fault fuses(or circuit breakers), a contactor, a set of overloads (either bi-metallic strip, eutectic alloy, or electronic), and a small transformer that powers the control equipment necessary.

Eutectic alloy (solder pot) overloads are an approximate model of the heating of the motor during start-up. The inrush of starting current heats the solder in the overloads, but not enough to melt it and cause a trip. If, however, the motor is started several times in rapid succession, the overloads will heat up and trip.

As an example, for a three phase motor that is rated with a service factor of 1.15 and has a degC rise of 40 or less, the overloads are sized at 125% of nameplate motor current. The short-circuit/ground-fault protection (let's say we're using dual element fuses) is 175% of the 430.250 current for this motor.

Eric Stromberg, P.E.
Lake Jackson, Texas

 

[ctrane]

Thank you all for the kind repliesl. It has helped me a great deal.