Failed motor overload relay

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Electron_Sam78

Senior Member
Location
Palm Bay, FL
Has anyone ever seen a motor overload relay such as a Schneider LCD 03 go bad and output under voltage? The resistance across it is 10 ohms and it's voltage on the load side is 203 volts (214 v on line ). This is with no load. I believe this failure caused a 230v motor to burn up.

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Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
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Electrical Engineer
All thermal type OL sensors have a voltage drop across them because they have resistance and that's why we call them "heaters". That resistance is what creates heat that moves the trip mechanism in the relay. On average, the heat they give off is about 9W per phase, regardless of size, because that's what the trip mechanism is designed to look for. So if you are talking about a 100HP motor, 27W out of 75,000W is insignificant. But if you are looking at a 1/4HP motor, 27W out of 170W might look like a significant enough amount to show up as a voltage drop.

It might be that your 230V motor burned up because it was a 230V motor that was getting less than the allowable -10%. Not all 230V equipment is rated to handle 208V services, ESPECIALLY if it was made to IEC specifications. Typically if the motor IS designed to be OK with a 208V service, it will say 208-230V on the nameplate. If it doesn't, you can't assume.
 

Electron_Sam78

Senior Member
Location
Palm Bay, FL
All thermal type OL sensors have a voltage drop across them because they have resistance and that's why we call them "heaters". That resistance is what creates heat that moves the trip mechanism in the relay. On average, the heat they give off is about 9W per phase, regardless of size, because that's what the trip mechanism is designed to look for. So if you are talking about a 100HP motor, 27W out of 75,000W is insignificant. But if you are looking at a 1/4HP motor, 27W out of 170W might look like a significant enough amount to show up as a voltage drop.

It might be that your 230V motor burned up because it was a 230V motor that was getting less than the allowable -10%. Not all 230V equipment is rated to handle 208V services, ESPECIALLY if it was made to IEC specifications. Typically if the motor IS designed to be OK with a 208V service, it will say 208-230V on the nameplate. If it doesn't, you can't assume.

The non-malfunctioning units right next to them have 0.4 ohms resistance and have zero voltage drop across them. Yes I'm assuming the motor went bad because of the bad OL which gave it about 12% less than the rated voltage as opposed to the 7-8% less (@ 214v) it was getting before.

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Electron_Sam78

Senior Member
Location
Palm Bay, FL
The non-malfunctioning units right next to them have 0.4 ohms resistance and have zero voltage drop across them. Yes I'm assuming the motor went bad because of the bad OL which gave it about 12% less than the rated voltage as opposed to the 7-8% less (@ 214v) it was getting before.

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Also a 230 volt motor will normally have a 240 volt nominal supply with an acceptable range down to 216 so a supply of 214v which would be just under that tolerance is just fine for a 230 rated motor with a tolerance of 10%.

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