Motor overload tripping question 4

[Jeff_Cameron]

Hi all,
I am having an issue with some motors on a piece of industrial machinery. The problem I am having is that the motor overloads keep tripping for all of my coolant supply pumps. This is really a 2 part question.

Part 1: I have a 480v 3 phase motor with a nameplate FLA of 28.0A (at 480v). the overload is tripping up to 12 times a day. Full disclosure, I did not design THIS system. the motor overload is a Siemens overload with a max rating of 32A. I did notice that although the overload is rated up to 32A, the max horsepower rating for the overload is 20HP. The motor is rated at 25HP. The ambient air inside the panel is approx. 98 degrees F. The ambient air outside of the panel is 73-75 degrees F. Thermal imaging of the panel shows that the overload is reaching temps of up to 150 degrees F. The wires going to the overloads are around the same and the cables on the outside of the machine running to the motors are also significantly warm. the motors have cycle time of about 1.5 hours, during which there are valves opening and closing periodically. The motors are definitely not short-cycling. all wire terminations have been checked for sufficient tightness. Does anyone have any idea why this could be happening? is it just as simple as the overloads need to be sized for a higher HP to account for the heat? Also, the service factor is the standard 1.15.

Part 2: In general I understand that an electrical horsepower is equivalent to roughly 746 watts. How does the horsepower figure into the amperage sizing of a motor? for instance, this motor is 25 HP. If i take 25 x 746, i get 18,650w. i then divide that by 480 and get 38.85A. Do i then need to divide that by 1.732? doing so gives me 22.43A. even with that, where do they get 28 from? is that just because that's the full load condition? Its been a long time since I went to school for this so I may have forgotten a few things. thank you for your help I sincerely appreciate it.

 

[che12345]

Dear Mr.waross
1. Thank you for your learned advice.
2. " (1) Overload devices ... the adjustable type, shall be set to trip at not more than the following:
(a) 125% of the full load current rating of a motor having a marked service factor of 1.15 or greater. "
2.1 I take it to mean that the OL shall be set (to a value that it [SHALL trip]), at not more that 1.25 x Ie, for a motor having SF of 1.15 or more. In this case 1.25 x 28A=35A.
3. When the OL is set at 35A, it [does NOT trip] when the motor is overloaded from 28A to 35A.
Attention: OL [does NOT trip] at (1.0 of setting).
4. Please enlighten me whether (in this case) setting the OL at 35A complied with the Code?
Che Kuan Yau (Singapore)

 

[waross]

Mr. Che
That setting is for a 1.15 SF motor.
For a 28 Amp, 1.15 SF motor the overload will not trip at 28 Amps x 1.15 SF = 32.2 Amps.
There will be an inverse time trip if the current exceeds 35 Amps.
This code does not consider the 28 Amp motor to be overloaded at 32.2 Amps.
That is allowed for a 1.15 SF motor.
A comment was made earlier that if a motor was run at the service factor load, that it would run hotter.
Any increase in load will cause a motor to run hotter.
Will it run too hot?
My code and the manufacturer say no.

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

 

[jraef]

All well and good, except you cannot make generalizations as to how to SET an overload relay. The ONLY valid procedure is as per the manufacturers instructions. This is because some of them built the pick-up factor into the design, some do not. You cannot tell, other than to read the instructions.

Siemens for example (I used to work for them) builds the 115% into their settings ALREADY, so if you have an IEC motor or one with a 1.0SF you are instructed to set it AT THE MOTOR FLA. But if you just reflexively set it at 1.25x FLA because it is a 1.15SF motor, the setting ends up having a pick-up point of 1.25 x 1.15, or 1.43, exceeding even the maximum exception. And if you choose to use the 140% exception, then the pick-up point ends up at 140% of 115% or 161% of motor FLA!

RTFM...


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

 

[waross]

Thanks Jeff.

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

 

[LittleInch]

Well going back to the OP.

The motor rating is 20hp shaft output.

FLA calculated at 27A once you include power factor and efficiency.

E.g. see

https://www.jcalc.net/motor-current-calculator

So maybe the motors are simply overloaded?

The OP needs to check actual running volts and current. Everything else is interesting but of little value if the motors ate indeed overloaded.

480V is the motor rating but 480V isn't the most common supply voltage I've ever seen. Lower volts = more amps.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[che12345]

Dear Mr. waross
Thank you for your learned advice.
1. " There will be an inverse time trip if the current exceeds 35 Amps."
Agreed. That is, there is NO trip at 35A.
2. See my post dated 9th instant:
** 3. When the OL is set at 35A, it [does NOT trip] when the motor is overloaded from 28A to 35A.
Attention: OL [does NOT trip] at (1.0 of setting).
4. Please enlighten me whether (in this case) setting the OL at 35A complied with the Code? **
3. Please advise me whether a motor with SF1.15 and Ie of 28A is (acceptable) to run it [continuously 24/7] at 35A (NOT 28A x 1.15 SF = 32.2A ); as it (would NOT generate heat) that would be [detrimental to its life expectancy]?

Che Kuan Yau (Singapore)

 

[waross]

Mr Che;
There is more to motor use, motor heat, motor burnout and motor protection than just the overload setting.
Ambient conditions; ie: hot, humid, dusty, oily etc.
I have protected motors that ran in ambient temperatures many degrees less than the ambient temperature of the overload relays.
I have protected motors that operated in ambient conditions above 100C/212F. (A max-min thermometer showed a peak of 230F)
Motors other than pump motors don't always see a steady load.
With pumps it may be possible in some installations to trim a valve and run the current at exactly 32.2 Amps or at 35 amps, but in many applications control of the load and current is either not possible or indirectly possible.
Some applications may subject the motor to cyclic overloads.
for cyclic loading, see
"The Cowern Papers", RMS loading of motors.
Operator abuse may be a factor.
You asked for justification in various standards and I have shared with you excerpts from the Canadian Electrical code.
Jraef has shared valuable insights as to setting overload relays in accordance with the manufactures instructions.
The answer to your question as to the current at which an overload relay will trim will be found in the manufacturers literature and time/current curves for specific overload relays.
The short answer is;
"It depends."
If you don't agree with me and with the Canadian Electrical Code, I will agree to disagree.

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

 

[davidbeach]

If the CEC is anything like the NEC it relies on its own internal logic and consistency. Don’t try to apply any external logic to it. A scientific review of the NEC would conclude two groups of errors. But the two groups cancel each other and the end result is that all is well. Gotta just take the whole thing as a whole. It is what it is. Can’t use any other standard or code to judge the requirements of just a part of it.

 

[edison123]

First question when OL trips is what is the motor current? OP yet to answer it.

Muthu

http://www.edison.co.in