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Motor Protection Relays

Thermal overload relays are supposed to be hot, that's how they work.Simply said they are designed to emulate the temperature characteristics of the motor they protect.
For a three phase motor all three phases should have:
    + A similar temperature at the relay terminations     
    + Similar current measurements.

When InfraScan Thermal imaging discovers a thermal anomaly you should act as follows:
Where there is no current Imbalance, or overload on the Overload Relay:    
    +  Tighten ALL terminations across the relay. Motor temperature varies due to loading, 
        duty cycle etc. Consequently; Relay terminations often work themselves loose Check 
        if the problem is resolved.
    + Check if similar motor protection relays feeding a similar load have a similar thermal 
        pattern, and if not contact the manufacturer to ensure the relay is 
        within specifications. 
    + Alternately where the unit is old or data is not available - replace the relay. 
       Check to see if the matter is resolved.
       Check the motor terminations are within the standards either 60ºC or 75ºC.

Where a considerable difference of temperature between phases is evident you must investigate further. In short:
     + Check the current at the motor terminations. (We sometimes see control 
        circuitry erroneously placed on one of the (protected) phases. This creates
        false imbalances).
     +Check the motor bearings.
     + De-commission the motor or you may experience production problems within a 
        short period.
     + Check the resistance of the 3 phases.
    + If necessary - Recondition or replace the motor accordingly.

For single phase motors protected by a 3 phase relay: All three terminations should have similar temperatures at the contactor / relay terminations. This is most often NOT the case, despite the same current running through the 3 poles of the relay. I when testing, assume this imbalance is due to a number of reasons: 
     + The relay is faulty.
     + The relay is set very low and operates inefficiently at low currents.

I have approached suppliers on occasion and to date have received no concrete answer.

Note
Ensure your plant equipment operates within the recommended and statutory standards at all times. All Safety Guidelines should be observed

In the relay:
Overload relays recognise balanced overload events as well as unbalanced currents. These relays heat up (and ultimately trip) on:
    • Termination faults
    • Stator winding faults
    • Earth faults
    • Single Phasing events
    • Overload and locked rotor events

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Very good reports , thank you. I’m going to have a bit of work following on from them! Thanks again
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