Undersized thermal protection for motors
- Thread starter darekelec
- Start date
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Max size of heaters is per nameplate amps. No issue going smaller.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
You just accept the risk of nuisance trips. The motor is protected either way.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
The bigger issue is that if we assume the motor leads were sized for a 50HP motor, and now you have a 60HP motor, regardless of the actual current you are using, the conductors are required to be sized for the 60HP motor. So per the NEC charts, that means 77A x 1.25 = 96.5A rating for the 60HP, vs what you likely had was 65A x 1.25 = 81.25A for the 50HP. So that means you likely had 4AWG conductors, now you are required to have 3AWG.
ptonsparky
Tom
- Occupation
- EC - retired
Dadgum details. It was a simple project.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
I know, right?Dadgum details. It was a simple project.
kwired
Electron manager
- Location
- NE Nebraska
No way you have 56 and 76 amp motor (assuming same speed and voltage rating) and both are 20 HP.
56 amps is slightly higher than NEC FLA ratings for 20 HP @ 208 volts though, usually table ratings is for worst case efficiency and power factor and actual motors you find are lesser rated on nameplate.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
One minute is a short time in the world of small percentage motor overloads, especially thermal types. Holding up for one minute would not give me any confidence in the workability of the 56A nominal overload with that motor.
ptonsparky
Tom
- Occupation
- EC - retired
Is this a stand alone motor or does it feed others? As in augers or conveyors?
We worked with augers often enough to find that an oversized motor is very likely to overwhelm subsequent augers even if the overloads are dialed down to what the smaller motor would have drawn at designed capacity.
We worked with augers often enough to find that an oversized motor is very likely to overwhelm subsequent augers even if the overloads are dialed down to what the smaller motor would have drawn at designed capacity.
Thermal overloads have an inherent +/- 1% repeat accuracy, that's why. If we do a quick math there, 56A setting would mean the tripping amps will be 1.15 X 56 = 64.4. But because of the inherent reproducibility of 1%, the thermal overload needs 64.4A X 1.01 = 65.044A ->thermal overload doesn't trip at just 65A! If your current meter also has an indication error of +/- 10%. 65A could be just 60A, it couldn't be 71.5A since the thermal overload didn't trip. As said by the person above me, small things matter, we're in a real world. If you want a tighter protection, set your thermal overload lower than the expected overload.
Question:
If it reads on an adjustable trip dial of overload protection f.eg. 10 amps
Does it mean it’s meant to be used on a 10 amp motor?
Or 8.75 amp motor?
You advised to set the thermal overload protection at 100% of the FLA of the motor. There is a built-in fudge factor of 1.15 in most TOR manufacturers. Say you have set 10A on the TOR, the TOR trips if the motor current exceeds 1.15 X 10 = 11.5A. The TOR will start waiting if the overload is just momentary or longer, it trips if the overload persists for a longer time, the I2Rt energy builds heat in the heaters ultimately separating the NO contact of the TOR.
To add, for most thermal overload relays, when subjected to about 700% of the setting amps, depending on the trip class (5, 10, 20, 30 sec), the TOR trips in those time ranges.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
I would expect a 56A overload to carry 65A for 10s of minutes to hours or even longer. These things have trip curves which extend to infinity for slight overloads.
I am surprised that the new motor draws higher current then the old motor. All other things being equal I'd expect higher efficiency and better power factor, but the nameplate pictures show a different story.
The old motor nameplate doesn't quite make sense. The current rating is IMHO a bit low, and implies both a high efficiency and power factor in an old 8 pole motor. Did you get an actual current measurement on the old motor before taking it out of service?
The new motor has a slightly higher rated rpm which will mean more real power consumption moving a bit more air.
Jonathan
I am surprised that the new motor draws higher current then the old motor. All other things being equal I'd expect higher efficiency and better power factor, but the nameplate pictures show a different story.
The old motor nameplate doesn't quite make sense. The current rating is IMHO a bit low, and implies both a high efficiency and power factor in an old 8 pole motor. Did you get an actual current measurement on the old motor before taking it out of service?
The new motor has a slightly higher rated rpm which will mean more real power consumption moving a bit more air.
Jonathan
Last edited:
ptonsparky
Tom
- Occupation
- EC - retired
That sure makes it code legal, as odd as it is to have such a wide variation in current for the "same" motor.
ptonsparky
Tom
- Occupation
- EC - retired
The nameplate FLA would come from the mfg prior to any outside influences such as load pulley ratios.
eta: The mfg has no idea how the end user will apply it.