Undersized thermal protection for motors

[Tulsa Electrician]

Based on your advice I conclude that max overload would be F104C
Dialed down a bit. Starter has a small white dial (above red reset handle) that can be set between 90 to 110 (I assume percent value of thermals)

That is what I would do and set dial @.92 or 92%

 

[darekelec]

I understand now. Thank you

 

[topgone]

Based on your advice I conclude that max overload would be F104C
Dialed down a bit. Starter has a small white dial (above red reset handle) that can be set between 90 to 110 (I assume percent value of thermals)

No. Use F719B. Make sure your FLA is in the range of the nameplate motor full load current. You said your FLA is 56A F719B is for 55.9A to 59.7A. Your dial setting depends on what percent your motor FLA is from the lower rating of the heater. 56/55.9 ~ 100%.

 

[darekelec]

No. Use F719B. Make sure your FLA is in the range of the nameplate motor full load current. You said your FLA is 56A F719B is for 55.9A to 59.7A. Your dial setting depends on what percent your motor FLA is from the lower rating of the heater. 56/55.9 ~ 100%.

The 56 amp motor was the old motor that was replaced with 76 amp motor.

 

[darekelec]

I applied various PF and efficiency to the original motor and came up with 20HP! Imagine that. Eff lower with PF higher than replacement.

It would be interesting to learn more about your findings.

 

[ptonsparky]

It would be interesting to learn more about your findings.

HORSEPOWER				HORSEPOWER
1 PH	11.68​			1 PH	11.83​
3 PH	20.24​			3 PH	20.50​
volts	208.00​			volts	208.00​
amperes (avg of 3)	56.00​			amperes (avg of 3)	74.00​
efficiency	0.880​			efficiency	0.910​
power factor	0.850​			power factor	0.630​
kw (3PH)	15.090​			kw (3PH)	15.284​

I used the information available from your nameplates. Eff & PF were not available for the older motor so I just plugged in some values that I thought were close. Then adjusted them to get the HP.

Not sure now if the newer motor amps I used is correct, but you get the idea.

IIRC, I got the math via this forum and hopefully it is applied correctly, or at least close enough for government work.

 

[ptonsparky]

HORSEPOWER
1 PH	=(B4*B5*B6*B7)/745.7
3 PH	=(1.732*(B4*B5*B6*B7))/745.7
volts	208
amperes (avg of 3)	56
efficiency	0.88
power factor	0.85
kw (3PH)	=(B7*B5*B4*1.732*B6)/1000

Formulas. Enter known values in bold cells

 

[topgone]

The 56 amp motor was the old motor that was replaced with 76 amp motor.

There are two FLA columns, one for the standard heaters and another for "compensated" heaters. Standard, when the motor and controller are situated in the same location and "compensated" when the motor location is different from the controller location -> ambient temperatures are different.
What I saw is that you have chosen the "compensated" FLAs. Is that the actual situation of your equipment setup (motor and controller are not in the same location)? If they are in the same location, you have to choose the FLA under the "standard" column, which means choose F914B, not F104C.

 

[darekelec]

Controller and motor are in the same location. Table says something about red or black reset handle so I went with red handle because there is one (on pic) in previous threads.
Thank you for advice

 

[topgone]

Not a NEC violation to under protect the motor.

But let's just say that if OP's motors were loaded to 20 HP of output the one will need higher setting than the other if you don't want it to trip. This is all about efficiency and power factor differences even though mechanical output is the same. And the HP rating is the mechanical output rating.

Thank you for the clarifications. The marginal note re black or red reset buttons only tells you if the heater is a standard heater (with black reset button) or an ambient temperature-compensated (with red reset button) -> when you receive it.
BTW, regarding the dial setting, just interpolate the FLA range for the heater with the lower and higher settings of the heater dials. In your case your heater amp range is 71.6A to 78.2A while your dial available settings are 90% at the lowest and 110% at the highest. I interpolated the dial setting at a motor FLA of 76A, it's 103% = 76A FLA.

 

[kwired]

Thank you for the clarifications. The marginal note re black or red reset buttons only tells you if the heater is a standard heater (with black reset button) or an ambient temperature-compensated (with red reset button) -> when you receive it.
BTW, regarding the dial setting, just interpolate the FLA range for the heater with the lower and higher settings of the heater dials. In your case your heater amp range is 71.6A to 78.2A while your dial available settings are 90% at the lowest and 110% at the highest. I interpolated the dial setting at a motor FLA of 76A, it's 103% = 76A FLA.

So the dial is for "finer tuning" of the trip setting I take it?

I don't run into these GE overloads very often and have almost never had to make overload selection for the ones I have run into.

 

[topgone]

So the dial is for "finer tuning" of the trip setting I take it?

I don't run into these GE overloads very often and have almost never had to make overload selection for the ones I have run into.

Hmm. You can call it like that! The company that I have worked with in the past has this tester table intended for testing thermal overloads for proper setting and verification of trip times. That calculation method is used when setting the dial before current injection testing. The succeeding "fine tuning" adjustments are arbitrary, depending on the trip time the TOR in every check point required by the testing procedure. The idea back then was "install and forget".

 

[ptonsparky]

Max it out...you can always buy another motor, or keep a spare on hand.

 

[darekelec]

As a follow up:

This (and other motors) work well thru summer heatwave and we did not replace thermals.