Locked rotor definition
Locked rotor definition
(OP)
Hi,
I´m working with a motor that blew during commissioning.
this is a 1000cv@4000Vac motor with a locked rotor time = 3.5sec.
According the vendor recommendations, we can do 3 starts per hour: one start each 20 minutes.
According the relay records, this motor was started many times in a short period, less than 5min between starts.
It´s common that during commissioning operation guys do many starts and nothing happens.
My question is, is possible that in motors with a short rotor locked time, consecutive starts can blow the motor?
I´m working with a motor that blew during commissioning.
this is a 1000cv@4000Vac motor with a locked rotor time = 3.5sec.
According the vendor recommendations, we can do 3 starts per hour: one start each 20 minutes.
According the relay records, this motor was started many times in a short period, less than 5min between starts.
It´s common that during commissioning operation guys do many starts and nothing happens.
My question is, is possible that in motors with a short rotor locked time, consecutive starts can blow the motor?





RE: Locked rotor definition
"It´s common that during commissioning operation guys do many starts and nothing happens."
Well, now it has and hopefully your "commissioning guys" have burnt their fingers !!!
*Nothing exists except atoms and empty space; everything else is just an opinion*
RE: Locked rotor definition
RE: Locked rotor definition
However, from your question it sounds like the vendor said 20 min between starts with the attached load and this was exceeded. You now know why you were told not to do that.
Basically, the whole motor protection relay scheme is important. A good thermal model, time between starts protection, unbalance protection etc are all important. Skip one and you could get a damaged motor like you're seeing.
RE: Locked rotor definition
But that does not excuse their actions in my opinion. Most technicians and electricians with any amount of real-world experience will know that all motors have a starting condition specification and it only takes a moment out of their day to ask a question when they are not sure. What they did was a classic assumption, which gets it's name from making an ASS out of U and ME!
RE: Locked rotor definition
So that the combination of high start torque in a very short period (0.5sec)will explains the short locked rotor time.
Maybe for this motor the minimum time between consecutive starts might be more than 20 minutes.
RE: Locked rotor definition
The lock rotor time and acceleration time ( 2 different values) are based upon Nema WK^2 limitations or the actual load, and are basically advising the user the point where the motor can / will fail from overheated components; rotor or stator, typically rotor.
I agree with the other responders that your vendor was not educated in the proper start-up proceedures. Was this a manufacturers start-up engineer, or a vendors representative? In most cases on a machine above 1,000 hp it is adviseable to request a start-up proceedure from the manufacturers engineering department.
RE: Locked rotor definition
For smaller motors driving standard inertia's, limits are given by NEMA here:
ht
the situation is less standardized for larger motors. I think NEMA only requires one start hot or two starts cold.... not much definition of how much cooling off time is required beyond that.
Off the top of my head, if the start time is truly 0.5 sec and the safe locked rotor time is truly 3.5 seconds, then I think you should be able to do 6 starts in a row without violating any thermal limits (assuming your actual voltage is equal or lower to the voltage assumed by the 3.5 second limit). Does anyone agree/disagree with that?
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RE: Locked rotor definition
I have read something about the material used to build the rotor bars, when its needed a high torque and a high acceleration those might be made in aluminum cast
RE: Locked rotor definition
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Locked rotor definition
SKM white paper link
If the data is no longer available, ask another MV motor manufacturer of a similar motor for theirs. It will probably be close.
As to a solution, you should have a Motor Protection Relay on that motor. They are expensive (at least the good ones are), but you already know the cost of NOT having one!
JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> FAQ731-376
RE: Locked rotor definition
Regards,
GGOSS
RE: Locked rotor definition
NSV
RE: Locked rotor definition
The formula uses a factor of 2 or 3 to weigh the cooling time of a stopped rotor.
If the cycle is a running motor that is unloaded and is stopped and restarted, it will withstand more starts per hour than a motor that is started and then stopped, restarted and stopped again.
I have seen statements that rotor heat must be conducted away through the shaft, and that rotor heat must be disipated across the air gap to the stator. I suggest that rotor cooling in a running motor is primarily by air flow past the shorting rings on the ends of the rotor bars.
This is particularly true in the smaller motors where the squirrel cage and the cooling fan blades are cast in one piece of aluminum.
respectfully
RE: Locked rotor definition
http://www.selinc.com/techpprs/6164.pdf
http://www.selinc.com/techpprs/6024.pdf
http://www.selinc.com/techpprs/6023.pdf
RE: Locked rotor definition
Re: dissipation through the shaft and air gap etc.
I was referring to small NEMA frame motors and didn't make the distinction that it's only TENV motors. Those details were not germane to my point, that being that feeling the outer frame with your hand is not a good indicator that a motor is "cool", not to the subject of this thread, MV motors, so I didn't take the time to go into that detail. Maybe I should have..
RE: Locked rotor definition
I wasn't challenging you. I have seen such comments in other threads also.
The point that I was trying to make is that if one motor is running steadily and stopped and immediately restarted every 10 minutes, the motor may be expected to be cooler and less likely to fail than a similar motor that is started every 10 minutes and then immediately stopped.
I have re-read my reference and Mr. Cowern refers to the motors inability to cool itself when it is stopped, without particular reference to the rotor.
I apologise if I mis-spoke.
Here is a link to the Cowern Papers by Edward H. Cowern P.E., of Baldor Motors.
http://www.baldor.com/pdf/literature/PR2525.pdf
Starting on page 43 he discusses RMS loading of motors.
A point for discussion is the action of thermal modeling relays.
Do thermal models differentiate between the cooling of an unloaded running motor and the slower cooling of a stopped motor. If not, must extra care be taken when commisioning to avoid multiple short starts that may "fool" the thermal model and overheat the motor without the condition being reflected in the thermal model?
comments?
respectfully
RE: Locked rotor definition
Yes, that is absolutely true and not so obvious to non-motor folks. If you think only about the heat input-side and not the heat removal mechanisms, you would come to the opposite (wrong) conclusion. I have explained it to some folks that do machine balancing and they were very surprised.
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RE: Locked rotor definition
But I will say that most erred on the side of being conservative, i.e. basing their cooling time constant on a stopped motor, not one that had been running longer. The problem with that then would only be that the motor may be off-line longer than necessary if it tripped on OL or that it may trip on OL sooner than necessary with repeated starts. There were a few however that had what I considered to be very short cooling time constants for the motors they were intended to protect, shorter than many large motors have published on their data sheets. It's something that a prudent engineer would want to investigate if truly concerned about proper motor protection.
RE: Locked rotor definition
Respectfully
RE: Locked rotor definition
The SEL motor relays are supposed to take into account differences between stopped, starting and running conditions in the thermal model. At least that is what they claim.
RE: Locked rotor definition
"Whenever you are asked if you can do a job, tell them, certainly I can! Then get busy and find out how to do it." Theodore Roosevelt.
RE: Locked rotor definition
RE: Locked rotor definition
RE: Locked rotor definition
RE: Locked rotor definition
Why didn't they just say "Minimum time between starts is 20 minutes.."
Sigh.
Three starts per hour is one every 20 minutes.. PERIOD!
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Locked rotor definition
RE: Locked rotor definition
That really leaves things in a state... But I see what you mean. So three starts in in the first three minutes is fine? <Just kicking sand.>
Still if you stick with a start every 20 minutes you can do this forever and not actually look at the 'clock'.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Locked rotor definition
RE: Locked rotor definition
NEMA MG-1 is pretty clear about the 1 start hot or two starts cold... anything beyond that depends on the user spec and the manufacturer. Our large motor spec requires a labelplate defining motor starting duty for large motors and the terminology varies widely.
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RE: Locked rotor definition
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RE: Locked rotor definition
It always amazes me how a VCR can come with three pages of warnings,(do not operate while in a bath, etc.), and 5 pages of installation instructions, followed by 8 pages of instructions.
Then a motor that consumes 1000X the power, can be applied in dozens of ways, costs 100X more, often comes down to a little crummy plate - always missing a few critical values.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Locked rotor definition
In practice, I've never seen the limitiation come into play in a motor's planned duty cycle, but rather when successive starts were required due to trip, power failure, etc. This is one very good reason why electricians should be reprimanded for routinely resetting breakers and restarting equipment multiple times after a trip.
As far as I'm concerned a motor that will start and stop more than every hour on an ongoing basis ought to have some sort of soft starting method.
Regards,
JB
RE: Locked rotor definition
I have seen datasheets listing such as 3 starts per hour from a cold motor or 2 hot starts per hour and 10 minutes between starts.
RE: Locked rotor definition
The load inertia Wk^2 equal or less than values on tables 12-6 or 20-1.
Applied voltage and frequency in accordance with tolerances.
Load torque equal or less than a torque, which varies as the square of the speed and is equal to 100% of rated-load torque at rated speed.
Then, the motor is capable of: “Two starts in succession (coasting to rest between starts) with the motor initially at the ambient temperature or one start with the motor initially at a temperature not exceeding its rated load operating temperature.”
20.43.2 If additional starts are required, it is recommended that NONE BE MADE until all conditions affecting operation have been thoroughly investigated and the apparatus has been examined for evidence of excessive heating. It should be recognized that the number of starts should be kept to a minimum since the number of starts affects the life of the motor.
RE: Locked rotor definition
The terminology manufacturers use varies widely and you have to pay attention to whether the motor is specified running between starts since as was mentioned this makes a BIG difference.
To assume that a vendor requirement for "3 starts per hour" equates to starting three times in a row and then waiting 58 minutes amounts to a nonconservative interpretation of an ambiguous requirement IMO.
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RE: Locked rotor definition
Have you analysed the failure mode ie. what exactly failed on the motor? I am interested to see if it was the insulation and if so, exactly where it failed.
RE: Locked rotor definition
JB - sure, you can install a soft-starter but it will not lower the thermal impact on the motor. It lowers mechanical stresses in the motor and load as well as lowering electrical stress on your power system.
Pete - it may be non-conservative but if someone only tells me 3 starts per hour I assume there's no time between starts limitation. If there is, it should be specified.
RE: Locked rotor definition
Lionel – I believe you are mistaken. The requirement that aolalde and I mentioned is in section 20 of NEM\A MG-1 which applies to large motors.
That is most definitely a nonconservative assumption IMO. Look at that NEMA MG-10 repetitive starting limits for small motors... even though they don't apply here, they give us a glimpse of the nature of limits applicable to induction motors. There is one limit for number of starts per hour and another limit for minimum time between starts. The minimum time between starts is less than the inverse of the starts/hour, but it's definitely not 0. If it was 0, they wouldn't have to tabulate it.
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RE: Locked rotor definition
Check out page 19 here:
h
Look at a 4 pole 250 hp motor. Check out Column A for that motor. They allow 3.7 starts per hour which is more than the 3 starts per hour we've been talking about.
Does that mean that you can go out and start it 3 times in a row 1 minute apart, and then let it cool off for 58 minutes before the next start? No!!!
Now the weird thing. The label for column C is "Minimum rest or off time in seconds between starts."..... doesn't exactly make sense considering as we said before that the motor disippates that heat from a start a helluva lot faster while running than while shut down. In my mind the minimum time between starts should be much lower if the motor is running most of that time between starts than if it is off most of that time between starts.
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