no need of thermal over load relay for 3ph AC ind motor 1

[khanengr]

Hi dear all,

I am talking about Circuits for 3 phase induction AC motor. What we usually have is a Crcuit Breaker, magnetic Contactor and thermal over load.

My question is that if I use Crcuit Breaker which has thermal over load tripping and immediate short circuit tripping capabilites then why I use thermal over load relay?????

Why don't I let my Circuit Breaker to take care of thermal overload???

Kindly give your valuable feedback with if any attachment of technical literature explaining answer to my question

Thanks to all

 

[rcw retired EE]

Thermal overload relays can be set to closely mimic the motor's thermal characterisitics to protect the motor from thermal damage. The circuit breaker's characteristic cannot be fine tuned to match the motor. Both the breaker or overload will evenutally trip for an overloaded motor, but the breaker will probably wait until the motor is destroyed.

 

[LionelHutz]

You use one because most electrical codes require it.

You use one because a properly sized thermal overload will protect the motor where the breaker likely won't.

You use one because the breaker, contactor and overload is series listed and the high fault rating only applies when the overload is provided.

You can take your pick from the above.

 

[jraef]

It should be noted:
If you are not from / in North America, you may not be aware that the term "Circuit Breaker" here generally means a FIXED TRIP thermal-magnetic circuit breaker. But technically, IF the thermal trip curve of the Circuit Breaker happens to fall into a range that is acceptable to protect the motor, and the CONSEQUENCE of that size selectrion does not end up allowing nuisance tripping on magnetic inrush, then there is nothing saying you cannot do that. The same holds true for fuses by the way.

The REALITY of that however is that this is highly unlikely to work. If you have a CB thermal trip that falls within the motor protection rules, most likely the magnetic trips will end up too low to allow it to even remain energized when the contactor closes.

Elsewhere in the world where an MCCB may have adjustable trip settings, you can indeed use the breaker as the OL protection device.

Second point: Most CBs are not rated to be used very often, so that must me taken into consideration when deciding to try.

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