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Anonymous Poster #1

Setting of Bi-Metal Relay in DOL

07/19/2011 1:31 PM

Sir, what should be the settings to be made in Bi-metal relay. say for a 2.2 KW 415V 3 phase 50Hz motor with 4.1 A motor full load current. What should be the setting of the bi-metal relay. i.e. if i set at 4.1A and during overload of say exactly at 4.3A the relay may take some time delay for tripping due to thermal action. whether the motor will withstand or winding will get damaged.
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Paulusgnome
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#1

Re: Setting of Bi-Metal Relay in DOL

07/19/2011 2:41 PM

Set the overload relay to the FLA of the motor, 4.1A.

And yes, the tripping will be delayed, that's how a thermal overload relay is designed to work - the greater the current the shorter the delay. How else do you suppose that the motor could briefly draw 6x FLA on startup without the OLR tripping?

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Anonymous Poster #1
#3
In reply to #1

Re: Setting of Bi-Metal Relay in DOL

07/20/2011 1:01 PM

Thanks for your valuable comments.

I agree the tripping time will follow thermal curve of the relay based on class 10 or class 20 or Class10a or so on. My point is that by setting the relay at exactly the full load current when will the tripping occur when load current is fluctuating near to full load current.

In a GE relay it is return as tripping will be at 120% of setting.

In other words any thumb rule is there for setting the OLR at 90% or 95% of FLA.

I also agree that during starting the motor drawing 6 x FLA is taken care of .
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PWSlack
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#6
In reply to #3

Re: Setting of Bi-Metal Relay in DOL

07/21/2011 11:24 AM

If the overload is set higher than the motor FLA, then there is a risk the motor will burn out before the trip goes.

Come on!

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JRaef
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#2

Re: Setting of Bi-Metal Relay in DOL

07/19/2011 5:26 PM

The curve at which the overload relay trips at any level is designed to be faster than the thermal damage curve of the motor.

As was said so eloquently by kvsridhar in another thread, it is the raison d'être of the overload relay.

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Anonymous Poster #1
#4
In reply to #2

Re: Setting of Bi-Metal Relay in DOL

07/20/2011 1:04 PM

I fully agree with you sir.

My question was that it is told time again and again that the setting of the OLR is set at FLA. We have a band for the tripping time since it is not definite time and follows thermal curve.

Can you give me the link for viewing KVSridhar thread on the above subject.

Thank you sir
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electricalexpert65
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#7
In reply to #4

Re: Setting of Bi-Metal Relay in DOL

07/26/2011 11:18 AM

You have to plot the motor damage curve and the OLR curve in the same graph sheet and should ensure that the upper band value of the OLR curve for any given multiple of current is always lying lower than the motor damage curve. This has to be repeated both for the Cold Withstand Curve and for the Hot Withstand Curve of the motor and the appropriate HOT and COLD charectersitics of the OLR respectively.
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Paulusgnome
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#5

Re: Setting of Bi-Metal Relay in DOL

07/20/2011 3:32 PM

Here is a copy of the trip curves for your GE Thermal Overload Relay.

I think you should be able to draw your own conclusions from these.

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Anonymous Poster (2); electricalexpert65 (1); JRaef (1); Paulusgnome (2); PWSlack (1)

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