Starting a large motor 2

[RRaghunath]

Starting a large motor such as 16MW, 11kV when the existing power system is not strong poses a problem. Reactor starting can reduce the starting current and thus limit the voltage drop in the system. I can see it is simplicity. Are there any better ways which are also economical as well as robust to stand desert conditions.

Thanks in anticipation.

 

[rbulsara]

Best and most robust method, in my poinion, is the autotransformer starting, but this looks like a capacity of the source problem.

I am sure you know that you need to perform a analysis on a motor staring transients to see if the resultant voltage dip is acceptable to rest of the system. Unless this is tied to a 11kV large generaotor bus, the motor alone will need a 20MVA or better transformer, rather too big for 11kV distribution system.

There may be medium voltage electronic (SCR) reduced voltage starters that some manufacuter can make upon custom order, but may not be suitable for desert conditions. (There are medium voltage static transfer switches made).

 

[jOmega]

The problem with the "robust" & "economical" criterion that rraghunath imposed as requirements for a solution is that you get what you pay for.

Are there better ways ? Certainly. Are they more expensive (less economical) ? Of course. More Robust ? Very likely.

But there is considerable cost associated with them.

Rbulsara's medium voltage electronic (SCR) reduced voltage starters is one...

Another would be a VFD ...transformer down to meet input voltage limitation of medium voltage VFD and transformer up at output to meet motor requirement of 11 kV.

Special system ... very costly ... but reduced stress not only on plant distribution supply, but also on motor and machine.

Could be considered as a one time investment with payback over time with elimination of maintenance costs and lost production ...

 

[tmahan]

Several companies make solid state starters in that size range that could be used. With any reduced voltage starting method the load's starting torque requirements need to be considered carefully.

 

[EdDanzer]

If the motor starts under low load, you could use a smaller motor with a viscous clutch to bring the large motor up to the point where across the line starting will not cause problems, otherwise use some form of reduced voltage starting.

 

[jbartos]

Suggestion: Starting resistors may be less expensive than the starting reactors, suitable for the desert conditions, and reliable.
The large VFDs or soft starter will experience heating problems, if not cooled properly. They will have to be custom made. The reliability may be lower than the simpler approaches mentioned.

 

[jbartos]

Suggestion: Visit

http://fie.engrng.pitt.edu/fie2002/papers/1499.pdf

for: soft start addressing resistors

http://webs.mn.catholic.edu.au/physics/emery/hsc_motors.htm

for: It is interesting to note that when the motor is first turned on and the coil begins to rotate, the back-emf is very small, since the rate of cutting flux is small. This means that the current passing through the coil in the forward direction is very large and could possibly burn out the motor. To ensure that this does not happen, adjustable starting resistors in series with the motor are often used, especially with large motors. Once the motor has reached its normal operating speed, these starting resistors can be switched out, since by then the back emf has reached a maximum and has thereby minimised the current in the coil.

 

[jOmega]

JB,
The staged resistor method of reduced current starting... is more ammenable to low voltage applications.

Can you imagine the special resistors and contactors required for this 11 kV motor....? If available, very special... and very expensive too.

 

[uno]

It may give us a more insight into the problem if we know the purpose of this motor ? Is it being used for process pumps , compressors, cement crusher, grinder etc. Some mechnical solutions for providing relief to power suppy network during start-up procedures and relief during run up can be provided with voith fluid couplings.

 

[TomG33]

HI, There are a few things to be considered in my opinion. As mentioned in other replies here knowledge of the application would help.
Also, What type of motor are you talking about, Synchronous or Induction?
Is the motor, to be started, under load ( such as a fan or compressor or conveyor, or is there some sort of unloading device such as a clutch?

If the motor is to be started unloaded and it simply has to accelerate it's own inertia then you could use a VFD to do the starting for you. the power required for the starting would be no where near the 11 MW. If it is a Synchronous motor, you can actively synchronize it to the line, before closing the motor breaker, Eliminating any surges at all. This type of synchronizing these days is a " No Brainer"

If the motor is an Induction Motor, Again VFD's can do the job. Synchronizing is also possible and again provides a smooth bumpless transfer to the line.

if your application requites the motor to be started fully loaded, then the VFD route becomes much more expensive.

Tom Grayson

 

[RRaghunath]

Thanks everybody. Some more information on the subject is the motor will be coupled to a fluid coupling and hence starts on no-load. The estimation is a starting torque of not more than 30% rated is the requirement.
Reactor starting is being considered but I am not sure that is the best thing to do. I heard of flux compensated magnetic amplification (FCMA) drives that can limit the starting current to about 200%. But, I am not sure of economics and relative merits.

Any help welcome. Thanks once again to every one.

 

[Laplacian]

Follow this link to read a GE presentation on medium voltage motor starting methods. It has some good information:

http://www.gehealthcaresolutions.com/MotorStarting.pdf

 

[dccap]

There is another softstarter also available in market. That is magnetic soft starter. This work on principle of flux compansating technique. I have seen this up to rating of 5.4MW. This is alos very robust in construction.

 

[edison123]

visit

http://www.magnadrive.com

 

[Difference]

Why dont you visit

http://www.mocotech.de

 

[gsimson]

Cetainly to study any motor starting the load characteristics is to be defined.
Generally Reactor / Auto transformer starting is used for pump/fan/compressor application. For such loads, a lot of energy can be saved if V/f drive is used, but depends on flow characteristic.Generally the payback period is 18 months.
For crushers/conveyors/kilns, reactor / auto transformer/soft starter starting is not suitable as the starting torque requirement is more.Necessarily starters like Vector drive which can provide higher starting torque and smooth acceleration should be used.
Simson

 

[jbartos]

Comment on:

http://www.magnadrive.com

Magnadrive requires electronic control of the magnetic coupling, a motor soft starter, at least; and a contactor bypass of the soft starter.
The Active Front End (AFE) drives, although still about 60% more expensive than the isolation (phase shifter) transformer drives, appear to be simpler solutions than the magnadrive.