Large Induction motor startup

[SS7]

10-11 MW,13.8 KV motor needs to be installed in a green field project site. The existing electrical network does not permit to DOL start the motor, Is there any industry proven method of starting?
Is an auto transformer start method preferrable at this voltage/power level?

 

[DougMSOE]

Depending on many things such as required starting torque, allowable voltage drop (i.e. stiffness of the power supply), etc. Selection of the motor starting is critical.
Suggest the following:
Find out what the req'd starting torque (mechanical) is.
Look at the allowadle V drop is, not just volts but time as well.
From this pick any of the following
Starting in line inductors
Starting in line resistors
Auto transformer (not my choice)
Wound rotor induction motor
Electronic soft start
etc.
Caution on any electronic starting system.
What level of skill do you have in your repair people to fix whatever you buy?

 

[Marke]

Hello SS7

If the supply is too weak to start this machine DOL, you can use a reduced voltage starter (of which there are a number of options) provided that the motor is capable of generating sufficient shaft torque at the reduced starting current.
As the start voltage is reduced, (independent of starter type) the start torque will reduce by the square of the current reduction. i.e. at half voltage, you will have half current and a quarter of the torque.
With large machines, the typical starting torque is much lower than with small machines. The starting torque is a function of rotor design, and with a machine of this size ist must be a custom build and so a high start torque rotor should be specified.
The driven load determines the minimum starting torque required. The driven load should be looked at to see if there is any opportunity to reduce the start torque requirement, for example, a screw compressor can be pressure equalized during start, a pump can be started against a closed valve etc.
The motor determines the conversion from Amps start current into NM torque and the starter controls the actual current supplied.
If the load requires a high start torque and this can not be reduced, then a secondary resistance starter and motor may be required.
The big mistake that many people make is to ignore the motor starting characteristics when making the purchase, and then being faced with a less than ideal situation when reduced starting current is required.

If you can provide more information, I am sure that there is some good engineering advice available here.
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jraef]

Suggestion, since you don't know the starting parameters, I suggest using a Reduced Voltage Solid State starter, even at that voltage. They are infinitely adjustable to allow whatever it will take to start the load. ABB, Motortronics, Toshiba, all have 13.8kV starters. GE, Siemens and Schneider can do them on a case-by-case basis as well, you just need to ask the right person, i.e. one that will go beyond what is shown in their catalogs.

As far as fixing them, they are really quite simple. If it doesn't work, its either a PC board or an SCR. Modern diagnostics in the microprocessor control will tell you most problems. If the problem is a PC board, replace it. Gone are the days of needing to find the exact failed component on a PC board, it takes too long. If it is an SCR, replace it as well. They are now designed to be modular so that field replacement is about the same as changing a set of contacts on a contactor. The added benefit is getting built-in high-end motor protection, something that must be added to any electro-mechanical starter.

Quando Omni Flunkus Moritati

 

[SS7]

Thanks everybody for your valuable suggestions; I would like to present more facts as available on today.
1.I agree, The motor needs to be custom built.
2.Load characteristics are not known at this point of time as it is for a green field project.
3. Yes, the grid can not take up this motor on 4 KV level so this is going to be only one of it's kind nearby.
4. Repair skills- I must admit we do not have intelligent but they are consistant in what they do.
5.The load is going to be a multi stage centrifugal compressor (via speed increaser gear box)where suction valve is thought to be controlled(safely avoiding surge) to aid starting.

Hopeing the further help to narrow down the choice.

 

[DougMSOE]

Go back to the compressor people. They should have the speed torque information that you need to assist you in picking the right starting method.
Don't agree with the 'custom built' motor approach, but it is a thought. Although in your size range it will most likely be a 'custom built' motor.

 

[Warpspeed]

With that type of application the main problem is inertia at startup. So the starting torque required is not a fixed figure that you can easily specify. It is really a torque/time problem.

 

[jbartos]

Suggestion: The motor of this size may be built to be running on the higher voltage than 13.8kV. If there is the higher voltage available, e.g. a 23kV or 34.5kV transmission line, then, the DOL start might be feasible.