Overload relay for compressor

[kallileo]

Hi,

I think that most compressors have internal thermal protection and can shut them itselves off in case of overheating.
Is it nesessary to use additional thermal overload relay for the protection of the compressor from overcurrent?

Thanks

[frank]

Here in the UK all motors larger than 0.37kW have to be fitted with overload protection (Regulation 552.1.2 - BS7671-2011).
It is always good practice and it is cheaper to fit one than to buy a new compressor.

[glenn1340]

An internal relay on a compressor will reset itself when the relay cools down resulting in the compressor trying to restsart then trip, restart trip etc until the motor burns out. Fitting an external o/load relay with a manual reset will trip the compressor and will only restart when you find the find the fault and reset it.

[chemi-cool]

compressors should have currant over load relay set to manufacturer recommendations.
Internal cutout protects only when the motor exceed certain temp.
there is no connection between the two.

[Peter_1]

Agree with Chemi, internal is overload and certainly temperature protection. Your national regulation will tell it you exactly.
@Frank, do you have to fit in the UK an OL relay on a 1HP hermetic then?

[frank]

@Frank, do you have to fit in the UK an OL relay on a 1HP hermetic then?

If it's a seperate motor then to comply with 'the bible' BS7671-2011, then yes you need to incorporate a motor overload (protection by automatic disconnection of supply)

If the motor is part of a 'Unit' with BS certification, then no you don't need to fit an additional O/L.

One thing to note though is that BS7671 is not a legal document unless it is incorporated into the contractural documents. It is purely a guide.


But, as it's such a comprehensive document it is considered to be the standard for all electrical installations in the UK.
There is no defence in Law for electrocuting someone, so, if you can prove that you design/installation was in accordance with BS7671, you have a defence in Law. The person being electrocuted must have been doing something outside of your design/installation

[moideen]

Hi,I think that most compressors have internal thermal protection and can shut them itselves off in case of overheating.Is it nesessary to use additional thermal overload relay for the protection of the compressor from overcurrent?Thanks

internal TH has dual function.when occur higher amps comp.will be tripped on this same controller.

[kallileo]

Thank you for your replies.

There is something a little bit confusing about the current ratings of the compressors.
For example for L'Unite TAG4573Z the nominal current is RLA 11,4A as stated on the compressor's plate. In the documentation of the compressor the maximum current is 18A.
For Maneurop MTZ72 compressor the Maximum Continuous Current (MCC) is 15,5A as stated on the plate but it does not mention the nominal current.

Should I adjust the overload relay according to Max or RLA current?

[chemi-cool]

You should measure with clamp ampere meter the drown currant and adjust the the over load setting some 10% above.

[kallileo]

I want to cool the milk in a cooling tank from 35 to 4 in 3 hours.
Isn't the current of the compressor much dependent on the ambient temperature or evaporating temperature?
I think that the current does not remain constant during this 3 hours cooling time.

[nike123]

internal TH has dual function.when occur higher amps comp.will be tripped on this same controller.

I agree with you.
To answer to the original poster:
Internal temperature protection is best way to protect motor, but we should have also external overload (current) protection, since heat rise could be inert as oppose to current rise. Therefore, it is always better to have both protections in place.

[nike123]

Overload relay should be set to FLA rating of motor!
Overload relay has in itself accounted value for overload (usually 125%). Therefore, it should not be set more than FLA.
If properly selected, type and size, it will protect motor only if over-current is happened.

[moideen]

I agree with you.
To answer to the original poster:
Internal temperature protection is best way to protect motor, but we should have also external overload (current) protection, since heat rise could be inert as oppose to current rise. Therefore, it is always better to have both protections in place.

Ok.nike. I always agree to fix an additional external overload to ensure the reliability. But I have seen some manufactures don’t install the proper compressor overload safeties. Good examples that carrier chiller 30 series reciprocating chillers, not equipped with internal and external safeties. It has only MCB, I never seen it works properly. It will trip after compressor grounded.