Locked rotor definition 2

[RaulChavez]

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?

 

[dpc]

waross,

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.

 

[powerjunx]

Areva MICOM P2xx series provides start/ stop inhibit, supervision and lock out persitent start up beyond specified setting. Yet, like GE MULTILIN, still its thermal-time model characteristic were approximated.

"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.

 

[Hoxton]

I know it's a little while since the last post, but three starts per hour is one every 29 minutes, not one every 20 minutes. I used to refuse warranty claims on failed motors over this very point!

 

[davidbeach]

Strange math there. A start every 20 minutes would be 72 starts in a 24 hour period, or 3 starts per hour. A start every 29 minutes would be 49 starts in a 24 hour period, or just more than 2 starts per hour, no where near three starts per hour.

 

[jraef]

I think what he means is that if I start at 12:00, start again at 12:29 and start again 29 minutes later at 12:58, that is technically 3 starts per hour. The problem with that logic is that it only works for that first hour, because if I started a 4th time at 1:00 then I only had 2 minutes between the last 2 starts. If I waited to start again at 1:25, then again at 1:53, I am only getting 2 starts per hour from there on.

 

[itsmoked]

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

 

[davidbeach]

Keith, because with a maximum of three starts per hour you could start it at the beginning of the hour, a few minutes later, and once again a few minutes later. Then you have to wait for the beginning of the next hour for the next start. The motor can apparently absorb the heating due to three starts without ill effect but it takes an hour to recover that heat capacity.

 

[itsmoked]

Hi David.

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

 

[davidbeach]

Keith, that's how I take it.

 

[electricpete]

As far as what is meant by 3 starts per hour... you really have to check with the manufacturer that stated it. (in this particular case, the original post did say something about one every 20 minutes)

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.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

also I didn't mean to imply the starting duty is a characteristic of motor alone... motor manufacturer in developing the limit would have to know the load and the voltage assumption as we all know

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[itsmoked]

electricpete; Nice to see you guys really labeling the motors correctly.

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

 

[JBinCA]

Davidbeach's heat capacity explanation is in line with how I've always understood the X starts per hour limitation.

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

 

[LionelHutz]

I agree with Davidbeach - Starts per hour is not a limit on the time between starts. I'd say the people denied warranty based on that assumption were wrongfully denied. First, 3 starts per hour if spaced out evenly is 20 minutes between starts and secondly just stating 3 starts per hour is not the same as specifying 20 minutes between starts.

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.

 

[aolalde]

Standard motors are address by NEMA MG1 under “Normal Starting Conditions”

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.

 

[electricpete]

That's the "1 start hot or two starts cold" I mentioned and is the only standard mentioned here.

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.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[SandS34]

RaulChavez,

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.

 

[LionelHutz]

It's nice to quote NEMA MG-1 but it DOES NOT apply to a large motor such as this. Jeff has already posted this. You must use the manufacturer's data.

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.

 

[electricpete]

It's nice to quote NEMA MG-1 but it DOES NOT apply to a large motor such as this. Jeff has already posted this.

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.

NEMA MG-1-2003
20.1 SCOPE
The standards in this Part 20 of Section III cover induction machines having (1) a continuous rating greater than given in the table below and (2) all ratings of 450 rpm and slower speeds.


20.12 NUMBER OF STARTS
20.12.1 Starting Capabiltity
Squirrel-cage induction motors (or induction generators specified to start and accelerate a connected load) shall be capable of making the following starts, providing the Wk2 of the load, the load torque during acceleration, the applied voltage, and the method of starting are those for which the motor was designed.
a. Two starts in succession, coasting to rest between starts, with the motor initially at ambient
temperature.
b. One start with the motor initially at a temperature not exceeding its rated load operating
temperature.

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.

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.


=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

Just to follow up a little more on the small motor info.

Check out page 19 here:

http://www.joliet-equipment.com/pdf/techinfo/EASAHandbook.pdf

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!!! Column C (for the 4 pole 250 hp motor) tells you that there is a minimum time of 500 seconds or about 8.3 minutes between starts.

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.


=====================================
Eng-tips forums: The best place on the web for engineering discussions.