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Old 11-05-2009, 18:36   #1
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Electric Motor Temperature?

I'm trying to deduce if an electric motor is operating properly. At full load with an ambient temperature of 20c (68f), the Electric Motor Temperature is 70c (158f). It has Class B Insulation 130c (266f). The motor is spec'd to draw 1.85 Amps but is drawing approximately 1.95 Amp. The power it is putting out seams fine.
Is this within acceptable limits? Is there something else I should check?
If you know, let me know.

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Old 11-05-2009, 18:48   #2
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Waste heat correlates to the inefficiencies involved in converting electrical energy into mechanical energy. That can be measured in watts. If the increase in current drawn is only a 0.1 amp increase in current for a 1.85 amp motor, then that works out to a 5.4% increase in current. I would not worry too much about it. Just make sure the heat has a place to go.

If it ever "loses its smoke"...or starts making funny noises, then you clearly have a problem.
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Old 11-05-2009, 19:12   #3
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Quote:
Originally Posted by David M View Post
Waste heat correlates to the inefficiencies involved in converting electrical energy into mechanical energy. That can be measured in watts. If the increase in current drawn is only a 0.1 amp increase in current for a 1.85 amp motor, then that works out to a 5.4% increase in current. I would not worry too much about it. Just make sure the heat has a place to go.

If it ever "loses its smoke"...or starts making funny noises, then you clearly have a problem.
Thanks David.
Not sure if I remember this right, but I think I read that some small single phase motor are only about 25 to 35% efficient where 3 phase motors can be up to 90% plus efficient.
Any general comment regarding the 70c temperature versus the 130c rating with 20c ambient?

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Old 11-05-2009, 20:57   #4
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Generally, a closed motor should not get over 160 F depending on whether it's a class B, F or H. Your best bet would be to contact the manufacture.
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Old 11-05-2009, 22:24   #5
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Quote:
Originally Posted by Extemporaneous View Post
Thanks David.
Not sure if I remember this right, but I think I read that some small single phase motor are only about 25 to 35% efficient where 3 phase motors can be up to 90% plus efficient.
Any general comment regarding the 70c temperature versus the 130c rating with 20c ambient?

Regards,
Extemp.
No, sorry. That's beyond my level of knowledge of electric motors. I am certain there are others here who could help. I like delmarry's idea of calling the manufacturer.
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Old 11-05-2009, 23:28   #6
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Too hot to touch?

My test is that if it's too hot to touch then there is a problem.

Anyone else test this way?
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Old 12-05-2009, 02:10   #7
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Quote:
Originally Posted by Extemporaneous View Post
... the Electric Motor Temperature is 70c (158f). It has Class B Insulation 130c (266f).
Surface temperatures of 70̊C to 95̊C, are not uncommon on T frame motors, and do not necessarily indicate overload or impending motor failure. Surface temperatures may be 20̊C - 30̊C less than the internal winding temperatures.

The “too hot to touch” test is an “early”* indicator, because (for most of us), modern motors are designed to run hotter than our human hands can tolerate:

Tepid ≈ 34̊C (93F)
Warm/Hot ≈ 37̊C (100F)
Too Hot to Hold ≈ 42̊C (107F)
Too Hot to Touch ≈ 48̊C (120F)

*The “touch” test was more applicable to old “U” frame motors, with lower temperature rise.

A rule of thumb for a Class B motor (Rated 130̊C, for 20,000 Hr Winding Life @ 40̊C ambient) can operate satisfactorily for about 40,000 hours, at 120̊C.

For each 10̊ C that the maximum operating temperature is lower than the rated temperature, the average insulation life will double.

For each 10̊ C that the maximum operating temperature is higher than the rated temperature, the average insulation life will be halved.

Motor Temperature Explained:

motorsanddrives.com - Motor Temperature Ratings
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Old 12-05-2009, 05:52   #8
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Extemp.
When checking the current, the voltage should also be checked.
"The motor is spec'd to draw 1.85 Amps but is drawing approximately 1.95 Amp."
If the voltage is lower that the spec'd voltage then the current will be higher.
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Old 12-05-2009, 06:31   #9
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Quote:
Originally Posted by chala View Post
... If the voltage is lower that the spec'd voltage then the current will be higher.
Good point!

Operating a motor at the "outer limits", either high or low, of its Voltage requirements (Voltage tolerance band) reduces its efficiency and can cause premature failure. Don't think that you're okay, just because your supply voltage falls within these tolerance bands. The purpose of these bands is to accommodate the normal hour-to-hour swings in utility supply voltage. Operation on a continuous basis, at either the high or low extreme, will shorten the life of the motor. The best performance will always occur at the rated voltage*.

* Motors meeting the criteria contained in the NEMA standard will operate satisfactorily within plus-or- minus 10% of the rated nameplate voltage.

Low Voltage:
When you subject a motor to low voltage (below the nameplate rating), some of the motor's characteristics will change slightly, and others will change dramatically. Low voltage can lead to overheating, shortened life, reduced starting ability, and reduced pull-up and pullout torque.
To drive a fixed mechanical load connected to the shaft, a motor must draw a fixed amount of power from the line. In a single phase motor, Power in Watts* = Volts x Amps (conversely, Amps = Watts Volts). Thus, when voltage gets low, the current must increase to provide the same amount of power.
An increase in current is a danger to the motor only if that current exceeds the motor's nameplate current rating. When amps go above the nameplate rating, heat begins to build up in the motor. Without a timely correction, this heat may damage the motor. The more heat and the longer the exposure to it, the more damage to the motor.

High Voltage:
An assumption people often make is that since low voltage increases the amperage draw on motors, then high voltage must reduce the amperage draw and heating of the motor. This is not the case. High voltage on a motor tends to push the magnetic portion of the motor into saturation. This causes the motor to draw excessive current in an effort to magnetize the iron beyond the point where magnetizing is practical.
Motors will tolerate a certain change in voltage above the design voltage. However, extremes above the design voltage will cause the amperage to go up with a corresponding increase in heating and a shortening of motor life.

Such sensitivity to voltage is not unique to motors. In fact, voltage variations affect other magnetic devices in similar ways. The solenoids and coils you find in relays and starters tolerate low voltage better than they do high voltage. This is also true of ballasts in fluorescent, mercury, and high-pressure sodium light fixtures. And it's true of transformers of all types. Incandescent lights are especially susceptible to high voltage. A 5% increase in voltage results in a 50% reduction in the life of the lamp. A 10% increase in voltage above the rating reduces incandescent lamp life by 70%.

Rules of Thumb for High and Low Voltage:

• Small motors tend to be more sensitive to overvoltage and saturation than do large motors.

• Single-phase motors tend to be more sensitive to overvoltage than do 3-phase motors.

• U-frame motors are less sensitive to overvoltage than are T-frames.

• Premium efficiency Super-E motors are less sensitive to overvoltage than are standard efficiency motors.

• Two- and 4-pole motors tend to be less sensitive to high voltage than are 6- and 8-pole designs.

• Overvoltage can drive up amperage and temperature even on lightly loaded motors. Thus, high voltage can shorten motor life even on lightly loaded motors.

• Efficiency drops with either high or low voltage.

• Power factor improves with lower voltage and drops sharply with higher voltage.

• Inrush current goes up with higher voltage.

* Power in H.P. = (Volts x Amps x Motor Efficiency) 746
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Old 12-05-2009, 08:42   #10
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Valuable information Gord
Further more Extemp.
Some Standards accept 1.05 time the full load current as acceptable and the thermal protection should not trip. This is 1.85 Amps * 1.05 = 1.94A. Assuming that your Ampmeter read true, your reading is 1.95A.
I would assume that your motor is protected by a thermal overload, if not be careful; there is no need to burn the place down.
The above Standards also state that at 1.2 times the full load current, the thermal overload should trip in about 15 minutes.
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Old 12-05-2009, 11:53   #11
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Quote:
Originally Posted by chala View Post
Extemp.
When checking the current, the voltage should also be checked.
"The motor is spec'd to draw 1.85 Amps but is drawing approximately 1.95 Amp."
If the voltage is lower that the spec'd voltage then the current will be higher.
I'm on to this one and did check.
Thanks.
Quote:
Originally Posted by GordMay View Post
Good point!

Operating a motor at the "outer limits", either high or low, of its Voltage requirements (Voltage tolerance band) reduces its efficiency and can cause premature failure. Don't think that you're okay, just because your supply voltage falls within these tolerance bands. The purpose of these bands is to accommodate the normal hour-to-hour swings in utility supply voltage. Operation on a continuous basis, at either the high or low extreme, will shorten the life of the motor. The best performance will always occur at the rated voltage*.

* Motors meeting the criteria contained in the NEMA standard will operate satisfactorily within plus-or- minus 10% of the rated nameplate voltage.

Low Voltage:
When you subject a motor to low voltage (below the nameplate rating), some of the motor's characteristics will change slightly, and others will change dramatically. Low voltage can lead to overheating, shortened life, reduced starting ability, and reduced pull-up and pullout torque.
To drive a fixed mechanical load connected to the shaft, a motor must draw a fixed amount of power from the line. In a single phase motor, Power in Watts* = Volts x Amps (conversely, Amps = Watts Volts). Thus, when voltage gets low, the current must increase to provide the same amount of power.
An increase in current is a danger to the motor only if that current exceeds the motor's nameplate current rating. When amps go above the nameplate rating, heat begins to build up in the motor. Without a timely correction, this heat may damage the motor. The more heat and the longer the exposure to it, the more damage to the motor.

High Voltage:
An assumption people often make is that since low voltage increases the amperage draw on motors, then high voltage must reduce the amperage draw and heating of the motor. This is not the case. High voltage on a motor tends to push the magnetic portion of the motor into saturation. This causes the motor to draw excessive current in an effort to magnetize the iron beyond the point where magnetizing is practical.
Motors will tolerate a certain change in voltage above the design voltage. However, extremes above the design voltage will cause the amperage to go up with a corresponding increase in heating and a shortening of motor life.

Such sensitivity to voltage is not unique to motors. In fact, voltage variations affect other magnetic devices in similar ways. The solenoids and coils you find in relays and starters tolerate low voltage better than they do high voltage. This is also true of ballasts in fluorescent, mercury, and high-pressure sodium light fixtures. And it's true of transformers of all types. Incandescent lights are especially susceptible to high voltage. A 5% increase in voltage results in a 50% reduction in the life of the lamp. A 10% increase in voltage above the rating reduces incandescent lamp life by 70%.

Rules of Thumb for High and Low Voltage:

Small motors tend to be more sensitive to overvoltage and saturation than do large motors.

Single-phase motors tend to be more sensitive to overvoltage than do 3-phase motors.

U-frame motors are less sensitive to overvoltage than are T-frames.

Premium efficiency Super-E motors are less sensitive to overvoltage than are standard efficiency motors.

Two- and 4-pole motors tend to be less sensitive to high voltage than are 6- and 8-pole designs.

Overvoltage can drive up amperage and temperature even on lightly loaded motors. Thus, high voltage can shorten motor life even on lightly loaded motors.

Efficiency drops with either high or low voltage.

Power factor improves with lower voltage and drops sharply with higher voltage.

Inrush current goes up with higher voltage.

* Power in H.P. = (Volts x Amps x Motor Efficiency) 746
Excellent!
I love this kind of info (when I stand a chance of understanding it).
The over voltage info is enlightening.
Thanks Gord.
Quote:
Originally Posted by chala View Post
Valuable information Gord
Further more Extemp.
Some Standards accept 1.05 time the full load current as acceptable and the thermal protection should not trip. This is 1.85 Amps * 1.05 = 1.94A. Assuming that your Ampmeter read true, your reading is 1.95A.
I would assume that your motor is protected by a thermal overload, if not be careful; there is no need to burn the place down.
The above Standards also state that at 1.2 times the full load current, the thermal overload should trip in about 15 minutes.
It is thermally protected but is a fairly old motor and hadn't run for some time. I'll be keeping an eye on it.

Thanks all.
Extemp.
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Old 04-08-2012, 02:07   #12
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Re: Electric Motor Temperature?

Small 12v computer fans are often usuful to cool and extend the life of 12v motors. They only draw a small current. Spectra use this on the feed pumps for their watermakers to change an intermittently rated motor to one suitable for continous use.
I recently wired up a fan to my new windlass motor. It certainly reduces the motor temperature for very little extra current draw.

BTW thanks for the great information Gord.
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Old 04-08-2012, 02:37   #13
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Re: Electric Motor Temperature?

I have also read that 150 is the magic number for an electric motor. I think superfast is talking about a niro engine. 250 would melt an enbell on an electric motor. My motor is usually in the 140 range after a 5min race, outside temp 90*. I havnt had any brushes burn up or blue so Id say 150 is a good limit.
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