a 2 MW comisionning 1

[181273]

again sorry for my english.... I have to do a commisioning work on a asyncronous motor of 2 Mw, 6000 V, 425 rpm... pump application, and I was worried about the times I can start it up, cause the manufaturer tell me that I can start it three times on a hour, but I think it have to be a time to cool the motor rotor specially, and then they tell me that I´ll cheeck it on the RTD of the stator, but my idea is that the rotor is the more hot part and I couldn´t measure the heat...

Any of you know some rule, as NEMA or ISO to control the number of times for the start of the motor... and how it depends on the load of the motor... in example if the motor is without load, how many times i could start it.

The amps on the starting are 450 % of the full load amps...

 

[davidbeach]

Three starts per hour, and starting heating of the rotor will have only minor correlation to load. The longer it can run per start, the better for the rotor but more heat into the stator.

 

[181273]

thanks david, but you said three times, is there any place where it´s writed... on Nema, CEI..... ISO???

 

[dpc]

Standards don't cover a motor that large - if you want something in writing, you need to go by what the motor manufacturer tells you, especially if you expect him to honor the warranty when it fails. Is there some reason you don't believe the manufacturer?

If it were me, I would tell operations a maximum of two starts per hour.

Also, a good motor protection relay can block starting based on the relay's thermal model conditions.

 

[181273]

now I have a new question, how will the relay see the thermal model of the motor?? the relay signals are only RTD information.... and if the rotor is hot and the mechanical forces of the the starting breaks the bars, how does the relay knows it???
I´m not more into the relays for motor protection thing, sorry if i ask easy or stupid things....

Thanks again david

greats from Spain and maybe one day will drink a good beer under the sun and near the beach....

 

[oftenlost]

This motor has a relatively low inrush current and according to the manufacturer, can be started three times per hour. Unless the inrush time limit was very limited or the pump took too much time to start in relation to that limit, I would not worry too much about this machine. A 13 second locked rotor time is not unusual in motors of this type and pumps usually start in three seconds or so. The RTDs will not help you much in deciding how to set the time limits.

 

[dpc]

Well, I'd worry A LOT about a new 2 MW motor if you're already getting worried about how many times an hour you can start it. If the motor manufacturer says three starts on hour, I'd stick with that until someone overruled me - after they wrote it down and signed it.

The relay has a software-based thermal model of the motor stator and rotor that is based on the current it senses. In the most simplistic version, it would integrate the current to give a crude estimate of heating over time. But actual thermal models are much more sophisticated than this - but not perfect. But you're right, the relay does not know exactly what is going on in the motor.

I agree that you cannot depend entirely on the stator RTDs. But some motor relays will use the RTD temperature to bias the thermal model.

 

[LionelHutz]

Is the manufacturer certain about the 3 starts per hour or did someone or some datasheet just say that but when you talked to them they were not really sure?

If they say 3/hour, then they believe it can do 3 starts. There is no time limits to this. It means you can start to speed, immediate stop and then start again as soon as the motor has coasted to a stop until you have started 3 times.

If you don't trust what you have been told, then you need to decide on what you will allow. You could pick something like 2 starts per hour and 20 minutes between starts. Picking conservative numbers usually doesn't hurt anything. I'd expect you really don't need to start that often in normal operation.

 

[181273]

oh yeah, thanks for the replies to all of you... a new question related to this theme,... what´s the size for the standards for definition of the number of startings, and how the standard define it.... could somebody give some adress to see it...

on other hand, Lionel, the motor will not start more than one time per day, it´s a water pump.... but i´ll like to define a protection for the motor not to start more than two o three times per hour....

Thanks again