Embedded Over Temperature Protection 2 2

[buzzp]

I have taken over a project in the US where a 200HP, 460VAC motor burned up. The only protection, which was in place, was the 'Over Temp Protection 2', per the MG standard. This is essentially a thermostat wired into a NC contact. After reading an older version of the MG standard, this thermostat has a fixed trip point of 190C (correct me if I am wrong).

The motor in question is rated for 40C ambient with an insulation class of F (max temp of 155C).

It would seem to me if the motor did reach this temperature and was taken off line, the motor would still continue to heat up beyond the trip temperature of 190C, probably well passed the 195C rating of the insulation. This is also a TEFC motor. I am looking to those with more experience with this type of protection to provide some feedback based on personal experience.

 

[edison123]

The maximum permissible winding temperature for class F machine is 140 deg C. The thermistor should have been set for 140 deg C to trip the motor contactor.

Muthu

http://www.edison.co.in

 

[che12345]

Dear Messrs buzzp ,edison123

1. I refer to NEMA publication MG1 (2010) based on:
- Service factor of 1.0,
- with maximum ambient temperature of 40 deg C,
- winding temperature rise by resistance method,
- as per sub-clause 20.8.1; the temperature rise for Class F is 105 deg C.
1.1 The total temperature rise would be (40+105)= 145 deg C.
1.2 There is NO mention of "hot spot" temperature of 10deg C.
2. It is desirable as pointed out by learned Mr.edison123 to set the thermostat at 140 deg C.
3. I would strongly propose [to add a three phase thermal over load relay with single phasing detection capability] , if it is not installed/omitted or damaged in the existing circuitry.
4. Suggestion: set the thermal over load current value to the [measured full load current] but [NOT exceeding the motor name plate full load current value].

Che Kuan Yau (Singapore)

 

[buzzp]

Class F can be operated at max of 155C or if you word it another way, 105C + 40C ambient + 10C for hot spot. I won't argue that.
Based on a very unreadable version of the MG standard I found online, it appeared to show 190C. Obviously, this is not correct.
Also, anyone care to address the additional heat buildup that could occur once the motor is taken offline or is this negligible?
I fully understand the need for an overload relay and how to set it, no instruction needed there. Thanks

 

[electricpete]

I'm not really familiar with that protection approach. All our motors in that size range would have some kind of current based overload protection. Some rambling thoughts fwiw

I agree with Buzz about the the hotspot allowance. You use a different temperature limit if you are reading by resistance (average temperature) than by RTD (near the hottest spot).

Also I recall a higher temperatuare rating is called out for service factor rating, about 10C higher.

The rated temperatures for insulation systems are for continuos operation for the life of the motor. You have to get a little philosophical when you think about overload protection but I think it is geared more towards an excursion into an overload condition rather than steady state operation just below the overload setpoint. In the older organic insulation world, Arhennius equation applied. At a higher temperature the motor didn't fail immediately, but it aged faster. (I realize modern insulation systems don't follow those organic rules... and there are other components to consider like the bearings).

These factors may explain part of the difference in temperatures. Or not.


=====================================
(2B)+(2B)' ?

 

[jraef]

Interesting, I see where that comes from, but I have never noticed this discrepancy:

12.56 THERMAL PROTECTION OF MEDIUM MOTORS

The protector in a thermally protected motor shall limit the winding temperature and the ultimate trip
current as follows:
12.56.1 Winding Temperature
12.56.1.1 Running Load
When a motor marked “Thermally Protected” is running at the maximum continuous load which it can
carry without causing the protector to open the circuit, the temperature of the windings shall not exceed
the temperature shown in Table 12-8.

Table 12-8
WINDING TEMPERATURES
Insulation System Class
Maximum Winding Temperature,
Degrees C
A 140
B 165
F 190
H 215
Tests shall be conducted at any ambient temperature within the range of 10°C to 40°C.

This does not jive with the winding insulation rating data.

Even if the motor has a 1.15 Service Factor, Class F is 115C rise over 40C ambient + 10C hot spot = 165C max. Embedded detection limits are higher than resistance method limits, but that's actually the "10C hot spot" allowance anyway; you can't have both.

The discrepancy here is that if the motor is "thermally protected", the protection can have a trip point of 190C, but the insulation limit is never more than 165C???




" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[che12345]

Dear All,

I received the following informative views of individual manufacturers,from NEMA :
" NEMA MG 1-2016, Part 20.8.1 applies to large machines and specifies the allowable temperature rise by insulation class. NEMA MG 1-2016, Table 12-8 applies to small and medium machines and specifies the maximum allowable winding temperature without causing the protector to open the circuit. Comparing temperatures in 20.8.1 to Table 12-8 is not relevant because NEMA MG 1 does not define maximum allowable temperature limits for thermal protection of large motors.

In reference to small and medium motors, the temperatures listed in Table 12-8 are different than the temperatures listed in NEMA 12.42.1 for small motors and 12.43.1 for medium motors because they apply to a completely different set of operating conditions.

Industry recognizes that overloading a motor results in increased insulation temperature, reduced insulation life and makes a statement to these effects in Part 12.48 and 20.37.

Please be advised that the above statements shall not be considered an official position of the National Electrical Manufacturers Association, but rather, they represent the views of individual manufacturers".

Che Kuan Yau (Singapore)

 

[edison123]

Thanks, Che. That last graf is some weasally word salad from nema though.

Muthu

http://www.edison.co.in

 

[electricpete]

To me the part that makes sense is "Industry recognizes that overloading a motor results in increased insulation temperature, reduced insulation life and makes a statement to these effects in Part 12.48 and 20.37."

It's the same point I made. Operating continuously at the insulation temperature rating gives you "normal life" (whatever that is, presumably 20+ years expected). You can go to higher temperatures for short times but you will get some reduction in that expected life. If you have a temporary overload 10C or 20C above rated temperature, no big deal. It is not anticipated that you would operate continuously just below the overload setpoint (in which case significant life reduction would be expected).

=====================================
(2B)+(2B)' ?

 

[che12345]

Dear All,

I will wait for another say five or more learned advice/comment/opinion and consolidate them before reporting as feedback to NEMA. I may request for [NEMA official stance] on this matter, instead of "weaselly salad" . Apologies, this is the [text] that I received.

All advice/comment/opinion are welcome. It shows we "care".

Che Kuan Yau (Singapore)

 

[buzzp]

I think clarification in the MG standard is in order.
Sorry, I have been out of commission for a while.

 

[waross]

In an internal combustion engine, the heat is generated in the combustion chamber and conducted through the casting to the water.
The casting temperature is above the water temperature or there would be no increase of water temperature when the circulation stops.
In a motor the heat is produced in the windings and is conducted away. As the source of heat, windings are the hottest part of the motor.
When the motor is disconnected the windings cease to develop heat. There is not hotter components to bleed heat into the windings.
The surface of the motor may increase in temperature but that is heat conducted from the hotter windings.
The windings do not get hotter after shutdown.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Excellent point, Bill. LPS for you.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[buzzp]

Thanks for addressing the last paragraph Waross.
If you are monitoring the hottest part of the winding, it is apparent you won't see an increase in temp. If your not, you may see an increase in temp at the monitored locations, which I guess is what I was thinking when I posted that last paragraph.