Understanding Motors Question!!

[brother]

Theres this breaker in an mcc that is 50 amps instantaneous, it has settings on at the 'E' level that is about 350 amps. This motor keeps tripping on start up periodiocally but not all the time. Check everything else out but couldn't find any other problems.

It was decided to increase the setting to the next level, 'F'. I had some concerns about this being too much reading NEC 430.52 (3) exeception 1 max is 1300 percent. ALSO THE TABLE 430.52 under instantaneous trip breaker , but according to the manufacturers 'code' on the motor the code letter is 'G', which im told is to correspond to the breakers setting level which would be 2 levels above where it was originally set. they appear to go up at 50 amp increments from the 350 setting.

This is a 25 hp flc 29 amp motor , i think squirel cage. D series

Am I reading this wrong, is this what the settings are to let the motor start without tripping the breaker, ?? When I do the math 29 X 13= 377 amps . It just looks like that would be too much. Please help.
Thanks.

 

[tom baker]

Did you meg the motor?
Whats the driven load? The load controls the amperage of the motor. The motor is dumb and tries to put out as much torque as it can.

 

[jim dungar]

The Code Letter G on the motor has no direct relationship to the setting on the breaker.

Yes your math is correct, 377A is the maximum NEC allowable setting for the breaker feeding this motor.

You said tripping on start up. Is there too much load or bad bearings, or voltage drop?

 

[brother]

Did you meg the motor?
Whats the driven load? The load controls the amperage of the motor. The motor is dumb and tries to put out as much torque as it can.

Yes the motor was megged, and it was good. the load is fine, no change there, even did the running amps on it with an ampmeter.

 

[iMuse97]

With your instantaneous trip CB, and with a FLA of 29A, 377 is the maximum setting for your CB. Yes it may seem like too much, but I assume whoever designed it, also met all the other requirements of 430.52(C)(3)--the listed
combination motor controller having coordinated motor overload and short-circuit and ground-fault protection in each conductor. You could check that.

 

[quogueelectric]

If the motor is running without problems I would think the breaker is out of callibration. Amprobe the motor on startup to see the max it draws and see if it actually is drawing over scale I think you will find it is in range.

 

[Ranch]

A few comments if I may specific to the breaker.

1. Is this breaker thermal mag or mag only? (presuumably mag only from some of the comments)
2. If mag only is it an MPC or HMCP?

A few comments if I may specific the motor starter:

1. Is it a full voltage starter?
2. Assuming so, then you would have either solid state or bimetallic overload protection on the load side of your contactor - correct?
3. In addition, if these assumptions hold true, we may have solid state where attributes such as phase loss, phase imbalance and ground fault features are required – yes / no?

Second to last thought I have: I have experienced 20+ years of this incomplete statement:

“Meggered OK" But the statement is incorrect when the motor voltage and megger voltage is missing from the statement

Last, one of the easiest tests to do is measure across each phase of the breaker once we have current flowing. Amazing how conspicuous a loose connection is, look for a voltage drop or a dirty pole

 

[brother]

A few comments if I may specific to the breaker.

1. Is this breaker thermal mag or mag only? (presuumably mag only from some of the comments)
2. If mag only is it an MPC or HMCP?

It did say HMCP on the breaker, Im not sure about the rest of the question I will have to check again tommorrow.

A few comments if I may specific the motor starter:

1. Is it a full voltage starter?
2. Assuming so, then you would have either solid state or bimetallic overload protection on the load side of your contactor - correct?
3. In addition, if these assumptions hold true, we may have solid state where attributes such as phase loss, phase imbalance and ground fault features are required ? yes / no?

Yes its a full voltage starter. I will have to check back with your other questions.

Second to last thought I have: I have experienced 20+ years of this incomplete statement:

?Meggered OK" But the statement is incorrect when the motor voltage and megger voltage is missing from the statement

The megger voltage is 1000 volts. this is a 480 volt motor. I was always taught that you test/megger with 'double' the voltage you normally run the equipment with. ie 480 x 2 is almost a 1000. We test this to ground. Any other suggestions would be appreciated.

Last, one of the easiest tests to do is measure across each phase of the breaker once we have current flowing. Amazing how conspicuous a loose connection is, look for a voltage drop or a dirty pole

This was also done, no problem found.

Thanks for the info.

 

[Ranch]

Brother,

I think FVNR and FVR starters need to be illustrated in the best possible way to the Holtsters. I will eagerly await your findings and reply tomorrow.

To the "Megger" voltage. 480 times sqaure root 2 gives us a 680 VDC component (close enough). Now double that and we are over 1000V. And we add some more. Your megger may detect a completely failed motor but not necessarily a deteriorated motor. That is why we (me) recommend 1920V* for a megger phase to ground test on 460V 3P motor. (don't come after me Megger company - offer ideas on available surge testers if you have offerings available to test for in phase trouble)

*There is a formula, watched closely by NEMA, UL and (in my opinion) the ultimate governing party, IEEE

But to the task at hand - I am confident our public communication can help to determine if your breaker does in fact have a deficiency, we (me?) just wants to help.

 

[Ranch]

Brother,

To the "Megger" voltage. 480 times sqaure root 2 gives us a 680 VDC component. Now double that we are over 1000V. And we add some more. Your megger may detect a completely failed motor but not necessarily a deteriorated motor. That is why we typically see 1920V recommended for a megger phase to ground test on 460V 3P motor.

I'll check back tomorrow!

 

[Cow]

Do you have another MCC bucket you can test run this motor from? That's usually how I try and determine if it's a bad breaker or not.

 

[Besoeker]

Theres this breaker in an mcc that is 50 amps instantaneous, it has settings on at the 'E' level that is about 350 amps. This motor keeps tripping on start up periodiocally but not all the time. Check everything else out but couldn't find any other problems.

It was decided to increase the setting to the next level, 'F'. I had some concerns about this being too much reading NEC 430.52 (3) exeception 1 max is 1300 percent. ALSO THE TABLE 430.52 under instantaneous trip breaker , but according to the manufacturers 'code' on the motor the code letter is 'G', which im told is to correspond to the breakers setting level which would be 2 levels above where it was originally set. they appear to go up at 50 amp increments from the 350 setting.

This is a 25 hp flc 29 amp motor , i think squirel cage. D series

Am I reading this wrong, is this what the settings are to let the motor start without tripping the breaker, ?? When I do the math 29 X 13= 377 amps . It just looks like that would be too much. Please help.
Thanks.

I don't know about your breaker settings or what the difference is between the 50A instantaneous and the E setting.
For a three-phase cage motor, starting current would generally be in the six to eight times FLC range.
Even at eight times, this would below the 350A E setting but exceed the 50A instantaneous by some margin. Is the instaneous trip possibly the problem?

 

[don_resqcapt19]

The full load current from Table 430.250 for that motor is 34 amps. Per 430.6 this value is used for the application of section 430.52. This would permit the trip to be set at 442 amps (13x34).

 

[brother]

The full load current from Table 430.250 for that motor is 34 amps. Per 430.6 this value is used for the application of section 430.52. This would permit the trip to be set at 442 amps (13x34).

Thanks for the clarification. I actually went back and read the code and double checked it and thats what I came up with.

I still have a another question about the 'code letter G' on the motor since it DOES mean the 'lock rotor current' or starting current.

Would this not mean the same as what they are talking about in table 430.7(B) and when you use that formula you get a higher amp rating?? and when exactly do you use the lock rotor current in determining conductor or breaker size/settings?? Is it in fire pumps only??

 

[brother]

bump to the top, hope someone answers my question.

 

[ATSman]

Theres this breaker in an mcc that is 50 amps instantaneous, it has settings on at the 'E' level that is about 350 amps. This motor keeps tripping on start up periodiocally but not all the time. Check everything else out but couldn't find any other problems.

It was decided to increase the setting to the next level, 'F'. I had some concerns about this being too much reading NEC 430.52 (3) exeception 1 max is 1300 percent. ALSO THE TABLE 430.52 under instantaneous trip breaker , but according to the manufacturers 'code' on the motor the code letter is 'G', which im told is to correspond to the breakers setting level which would be 2 levels above where it was originally set. they appear to go up at 50 amp increments from the 350 setting.

This is a 25 hp flc 29 amp motor , i think squirel cage. D series

Am I reading this wrong, is this what the settings are to let the motor start without tripping the breaker, ?? When I do the math 29 X 13= 377 amps . It just looks like that would be too much. Please help.
Thanks.

Back in the old days when Westinghouse was still around, and assuming the wiring, motor, etc was up to snuff, they recommended the following procedure for setting the inst trip on their MCP (motor circuit protector, instantaneous only) circuit breakers in MCC buckets: turn the dial (1-10) up to max and start the motor. If the breaker does not trip turn the dial down 1 notch and start the motor again. Continue this test until the breaker trips. Then turn the dial up 1 notch and start the motor. If it holds then this is the correct setting for proper motor protection. This procedure accounts for field variations and inconsistancies but should be consistent with the calculated values. It has served me well in the past and hope it helps.

Tony

 

[zog]

ANSI/NETA minimum is 1000VDC, nothing wrong with going above that.

Brother, what were the megger test results? How long was the test? Did you temperature correct your readings? What were the DAR/PI results. "Good" dosent tell us much.

I am guessing you simply have a bad breaker. Test it and replace it if necessary.

 

[brother]

Well, it looks like this breaker is holding, no more trips. They did adjust the settings on the breaker raised it to "F" which is about 400 amps, but it just seems like its too high when you do the calculations if i understand the code correctly. Not too mention that its RPM's are less than 1200 , as I read 430.6.

ANSI/NETA minimum is 1000VDC, nothing wrong with going above that.

Brother, what were the megger test results? How long was the test? Did you temperature correct your readings? What were the DAR/PI results. "Good" dosent tell us much.

I am guessing you simply have a bad breaker. Test it and replace it if necessary.

The results of my megger test was 'infinity' above 500 megohms. I believe the test suppose to be 30 seconds, but thats about how long it was. There was no, 'temperture' correction done. The way its been done, (the way we do it now) has worked fine in the past in finding problems.

I do not know what 'DAR/PI results' are.