Magnetic Overloads and Service factor

[vanskat]

Forgive me if I am missing something fundamental here.

Doesn't a magnetic overload relate to inrush current only and has no bearing on the service factor of a motor?

I ask this because we have been trying to start a piece of equipment for over a week but the magnetic overload kept tripping the motor. This is an obvious overload condition which is rooted in mechanical issues. However, is my notion flawed in thinking that if I adjust the time delay and trip point seting on the dashpot to just below the service factor (15%) current rating I will allow for more inrush and hence a higher start up torque to turn the equipment?

 

[dpc]

Typically a motor has two forms of electrical protections:

Short-circuit and overload.

In a typical motor starter, the overload relay protects against sustained overload conditions and a circuit breaker or fuse protects against short-circuits. This is the "magnetic" trip if you have a breaker.

If you are experiencing trips that occur as soon as the motor is energized, this is the short-circuit protection and probably has nothing to do with the adjustment of the overload relay.

If you have a standard molded-case circuit breaker, you should be able to adjust the setting of the instantaneous "magnetic" trip element to eliminate this.

Adjustment of the overload relay setting won't help this problem.

Motor inrush includes an asymmetrical current that is determined by the phase angle of the voltage when the motor is energized. This is why these trips can appear to be random. (I'm assuming the motor has been meggered and is OK.)

Regards,

dpc

 

[vanskat]

Well, an MCE test has been done and the motor could never be better. The architecture is that of a pringle switch which feeds a size 6 starter etc....

We "seemed" to have sorted out the problem. It lay with the fluid coupling being over-lubricated which added additional torque during start-up or so we suspect. Physically, start-up seems much easier and seamless now.