Motor Protection

[jraef]

Here in Canada Our Code book states in section 28-306 Rating or trip selection of overload devices for motors is 125% of the full load current rating of a motor having a marked service factor of 1.15 or greater. This is the maximum you are allowed to set your overload device. I understand it may be different for you folks in the USA who follow the NEC. Fuses here in Canada are not sized from the above. We have a separate chart for fuses and yet another chart for circuit breakers.

A lot of people misinterpret this on both sides of the 49th parallel. That is the point at which the OL protection must pick up and begin the I2t trip curve, not what you SET the OL relay for. Many, if not most, OL relays have ALREADY factored the 125% into the relay design. So adding 125% to the FLC will result in a pick up point that starts at 172%, putting the motor at risk. The real rule is to read and follow the instructions of the OL relay, which vary from mfr to mfr.

 

[nehpets]

A brief, simple explanation.
Most O/Load relays are thermal devices the current pullerd by the motor heats up bi-metal strips that open the circuit when the heat generated by the excess current being drawn bends the bi-metal strip to the trip mechanism. this heating effect introduces an inherent time delay.

 

[gusterminator]

I couldn't agree more jraef.....I have replaced heaters in one of the old style overloads that took W53 heaters and in the overload installation manual it stated when choosing the heater on there chart the 125% was already accounted for and to use the FLA of the motor..........I have seen a 200HP motor burn up because somebody applied the 125% to the FLA and then chose the heater on that value.

 

[dogleg43]

Here are a couple of mechanical torque limiting clutches:
http://www.autogard.com/amer/
http://www.bostongear.com/products/clutch/centric.html

Another source for current switches is NK: http://www.nktechnologies.com/current-sensing-switches/

Just my 2 cents worth. I like Tom's idea the best.

The other suggestions seem to be trying to fix a mechanical problem with an electrical solution.