Indiction motors

[johngerhart]

We have a 9000 hp induction motor on a turbo comppressor . The question has been raised on frequency of allowable starts . per hour, day ,life . We protect the unit with a multi-lin which requires it to get to speed in less than 16 seconds or trip and holds the unit from starting again until the temperature inside is cool . We also start with the compressor suction and discharge valves closed .
Can someone help me with the number of allowable starts and the NEMA std driving this limit . We are planning a 8 hours on at night , 16 hours off day operation for possibly 3-4 months . Thank you

 

[waross]

The safest action is to query the manufacturer. 9000 HP is a lot to trust to free advice.

Bill
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"Why not the best?"
Jimmy Carter

 

[dpc]

What Bill said. This is way above the size where any NEMA standards apply.

Allowable starts will be a function of the load inertia, acceleration time, rotor construction, and a bunch of other variables. There is NO standard for this size motor other than "fewer is better".

Dave

 

[johngerhart]

thank you , but the supplier has been quite evasive even when I asked for an engineering judgement

 

[jraef]

NEMA standards stop at roughly 250HP and NEVER applied to medium voltage motors anyway. So once you get into this realm, only the motor manufacturer can help you. Too bad they are being uncooperative.

Without that information, the method you are taking now of measuring the internal temperature is about as close as you are going to come. Hopefully you are using embedded RTDs as your temperature sensing method. Unfortunately it may not help in determining the dissipation rates at some critical areas.

It might be beneficial for others to know who this manufacturer is so they can be avoided in the future. It also may be that some of us know them or work for them and can get you some deeper help.

 

[itsmoked]

There is no manual for this motor?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ScottyUK]

Are you refering to the compressor supplier or the motor OEM as being unhelpful?



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If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

Pretty amazing as they are opening themselves to some liability by NOT answering one of the 4 top questions about a large motor. Unless, as you suspect ScottyUK, its the compressor people.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Gianoli]

I assume we are talking full voltage starts at 6.9, 11, or 13.2 kV. I agree completely with comments made by the others who have replied and would like to add a few points:

• Motors in this horsepower range are almost always custom built. A manufacturer may have a standard frame size for a certain HP range but may of the design details are only finalized after consultation with the customer. When the motor was purchased the manufacturer should have queried the customer concerning the starting duty and designed the motor accordingly. We have several 12,000 HP, 13.2 kV, 4-pole, induction motors that are started several times a day across the line. The motors were designed for this duty. We also have a 7000 HP Induction motor that has a warning label on the motor stating something like this “THE NUMBER OF FULL VOLTAGE STATRS THIS MOTOR EXPERIENCES MUST BE KEPT TO AN ABSOLUTE MINIMUM”.

• One of the primary concerns is the affect these full voltage starts have on the mechanical integrity of the stator winding and the rotor bars. If the motor was not designed for frequent starting duty starting it frequently will shorten its life significantly. It has been my experience that if there are starting limitations (other than thermal) it is usually stated on the motor nameplate or on a separate nameplate on the motor. If you have visual access to the motor stator winding you may be able to draw some conclusions about the design intent based on the bracing of the stator end coils. Motor of this size designed for frequent starting will have the end coils braced rigidly which should be quit evident.

 

[Skogsgurra]

In a recent (well, it has been going on for a decade now) case, we have an issue with a 10 kV 6 MW synchronous motor that is using the excitation winding (shorted by a thyristor device) as the starting winding.

There is some agreement that this motor can take one start easily. If that fails, you are allowed one more start. If that also fails, you have to wait six hours before you try again.

Never got this confirmed from the manufacturer. But I know for sure that this motor has been rewound (rotor) three times because someone didn't observe the starting limitations. I checked the thyristor devices after each rewind, that's why I know. I am now in a rather tough dialogue with the operators. I have told them to either respect the limitations or install a device that makes starting impossible if rotor temperature is too high. No sucess so far. Now waiting for fourth repair...

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Laplacian]

You need to speak with the motor manufacturer, not the supplier.

Without knowing anything but the horsepower and application, I can only speculate this is a 2 pole motor?

Who manufactured the machine and how long has it been in service?

What is the learned acceleration time in the Multilin; you say 16 second limit?

This motor will probably be rotor limited on startup, especially if the compressor manufacturer supplied/specified the motor. It will most likely have the lowest cost rotor construction and aluminum rotor bars.

What Multilin do you have? The 469 has a good thermal model protection feature.

The duty cycle you mention of 1 cold start per day should not be a problem. Repeated start attempts will quickly overheat the rotor and it takes a LONG time to cool that much mass without air circulation.

The motor "should" have been designed for two hot starts within a 1 hour period, but you can only determine that by consulting with the manufacturer.

The best advise is to make sure the operators take extreme care to start the machine unloaded so that each start attempt is successfull.

 

[7anoter4]

From ABB Manual for Induction Motors and Generators [chapter 6] quotation:
"For information on the allowed consecutive or annual starts, please see the performance data sheet or consult the manufacturer. The load characteristics of the application are needed for determining the starting frequency. As a guideline, the maximum number of starts in a typical application is 1000 starts per year."
see:

http://library.abb.com/GLOBAL/SCOT/...2F1/$File/3BFP 000 050 R0101 revE ENGLISH.pdf