Protection setting

[bob]

Hi,

 

What should be the ideal setting for overcurrent protection on a 250 k W 400 V motor. Actually my setting is 2,5 Ie 0.5 s.

 

Bob

[Guest]

Bob

What is the driven machine and starting method?

 

Fintan;q

[bob]

The starting method and driven load are immaterial as far as overcurrent protection is concerned. I am looking for any standard setting.

 

Bob

[marke]

Hi Bob

 

It depends on the protection that you are after. If you are looking for run time protection of the motor that comes into operation once the motor is up to speed, then that setting sounds pretty reasonable to me. That setting would almost definitely not alow you to start the motor however.

 

For motor protection, you need to have a thermal model that will operate in a shorter time than that required to damage the motor, but will enable the motor to start in the first place. This would give you protection that must operate at greater than 115% current, but the time can be very long.

Most motors will withstand longer than 10 seconds at 600%, with some being able to withstand 60 odd seconds at 600% current. A thermal model based on these figures could be hundreds of seconds at 250% current. Stall protection, or sever overload protection, commonly called shear pin protection will be operational only once the motor has reached full speed, and will typically operate at 250 - 300% current for around 0.5 seconds. This is in addition to the thermal model protection.

 

Is this the information that you were after??

 

Best regards,

[bob]

Hi Marke ,

 

Thanks for your always useful post. The motor is protected by a Rockwell CET4 Relay which basically caters for thermal overload through a model of the motor. But the motor used to trip, though not very often , on overcurrent actually set at 250 % 0.5 s. The thermal load on this motor rarely exceeds 50 %. I understand that the motor manufacturer will be in a better position to propose a convenient setting but this motor is quite old and it will be very difficult to trace the manufacturer.

Obviously , I am looking for a good setting to protect the motor and at the same time to prevent spurious tripping.

 

Kind regards.

 

Bob

[marke]

Hi Bob

 

Your CET4 will provide good thermal protection for the motor provided that it is set up correctly. Without good motor data, you could assume that the motor start time constant (Max Locked Rotor Time) is in the order of ten seconds. You can be prettey sure that it will be greater than or equal to 10 seconds, typically more like twenty seconds on older machines. This should prevent the motor from being overheated. The shorter, or fast overload protection is more for load protection than motor protection. Once the motor has reached 250% during run, it is well on the way to a stall condition and that would indicate that something has got jammed in the load and you are best to shut it down. This is a form of electronic shear pin protection. I would typically set the shear pin protection to between 250% and 300% for 0.5 to 1 second. This is usually enough to prevent niusance tripping, but also help to prevent major load problems.

You can get the odd niusnace tripping if there is a supply issue such as open transition switching, or major supply transients.

 

I do not see a major advantage to the motor from the use of this protection provided that the thermal model is correctly set.

 

Best regards,

[bob]

Hi Marke,

 

If the motor may withstand 7Ie during start , this means it should be able to withstand as high a current during transient period at normal operation without causing damage to the winding. If this holds true and as mentioned in your post, a good thermal model of the motor should provide adequate protection to the motor winding.

 

Bob

[marke]

Hi Bob

 

I agree, the thermal model should be adequate. The fast protection at 2.5 - 3 x In is primarily to protect against a stall condition unnecessarly heating the motor, and the high torque damaging the load. - electronic shearpin.

 

- That is my perspective anyway.

 

Best regards,

[Subhashish]

Hi bob

 

2.5 times to 3 times Full load current is absolutely a okay threshold in my opinion . The locked rotor withstand time of the motor will definately be something around ten /15 seconds . So you can safely increase the time setting slightly from the present value of 500ms as the motor winding shall not be damaged for 10 seconds at double the set overcurrent threshold ( i.e at 6 times FLC /LR Current) . However while increasing the timing care should be taken so that relay coordination is not disturbed ( relay should operate before the relay on the upstream breaker operates) . Generally you shall find convinient gaps to make small changes in setting of downstream relay.

Time setting increase shall help because I think that such overcurrent trips are due to transient happenings and therfore thermal relay that works using thermal imaging cannot appreciate .

But I still wish to know the type of load it drives as 2.5 times Flc is not generally drawn (even transients ) during normal operation .