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Electric Motors/Thermal overload relay


What would cause a 1.7-2.4 Amp relay for a motor starter to smoke and fry the internal components?

Neil, good question, it was totally burnt, you could peal the insulation and even the magnet wire back and it is all burnt means it was either a short term high current, or a long term semi high current    or the wrong relay, or the wrong voltage

Was it a new or existing relay?   If existing and aged it could simply have shorted,  if the motor is only protected by a circuit breaker,  breakers are way too slow to react  if the starter had overloads,  were they gone too?  Did the motor have an internal self resetting thermal? Or a push button reset thermal?

Neither are worth a damn most of the time, especially the manual reset type,   you made sure the voltage met the relay voltage tolerance?  AT THE MOTOR?

 a shorted winding could cause the starting circuit amperage to rise and the relay is in series series/parallel with the relay,

An overload to the motor shaft will send all kinds of crazy currents through the start circuit as it "hunts" to release or close

I've seen installs where the magnetic center of the motor is not taken into account or scribed, especially sleeve bearing or plain bearing motors,

 but even ball/conrad type bearings have to have room for thermal growth, so it can happen to ball bearing motors not that often, but say they have a spring type thrust washer in one end and it is setup to where the load crushes the spring and takes away any thermal expansion room,

 even with a pulley and belts   I have looked at installs where the belt pull was so eschewed the belt drag on the pulley had a binding effect that can cause false loads and high current

other possibilities:

Incorrect applied voltage,   voltage too high    too low,   

Incorrect voltage on a dual voltage motor, where motor is connected for high volts normally rated 220-240  and  low 110-120 for single phase,  and either way  high or low,  230 on 110 connected  or 110 on 220 connected will cause high current

some single phase motors are really difficult to read the connections, on even a new plate  they just do a crap job of designing the plate, and the run winding is someway left open so only the start circuit sees the applied voltage but no winding to swap to   

The wrong relay even in a brand new motor,  wrong tag on relay

Grounded windings from dust dirt contaminates, could even be intermittent   would allow for high current leakage to ground,

A failed capacitor or both if so equipped  

High resistance connection somewhere between power supply and motor could cause volt drop and high current

A high voltage or high frequency spike down the feed,  lightning, power glitch  etc       

A bunch of different things,  I can maybe get you closer if you want to send more details,  type of motor, type of load, voltage values, new or rely,  any other changes recently   if the relay was just changed  what was the failure mode of the existing prior to the new  if new of course,   

you can use my private email and it is much simpler to get back and forth  feel free   just put in subject line  from allexperts and remind me the issue    

If you have not  start with voltage at motor  measure that,  check caps if it has one or two or more  

Megger motor, check windings for continuity  check all connections for correct  and for tight,

Or write me back with more detail  might pick up something there    , check a  

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Three phase/ AC DC single phase motors, controls, any problems or failures, motor installation, performance issues, connections. All other electric motors/gearboxes/apparatus. Specialty repair concerns, obsolete motors and solutions. Other mechanical or specialty equipment. See my profile under Home/electrical at this site


30 plus years in the electrical motor and apparatus repair industry. VP level management of repair facilities, current owner of my own specialty repair and consulting firm.

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