How can I softstart a WRIM correctly ? 1

[BartholomeusNel]

We are about to install a WRIM onto a Ball Mill in a reduction Plant.The intended motor has a rating of 446A, 525V (Stator) and 320A, 600V (Rotor). A 22kV, 415kVA transformer is allocated to solely supply this motor.

We would like to start the motor as 'soft' as possible to reduce stresses on the supply network.

We thought of putting a softstarter (thyristor, phasecontrol) on the stator with 3-stage resitor set on the
Rotor ( Starting resistance, intermediate resistance and rotor shorted ).

If this is correct, how do we calculate the resistor banks ?

How does this method compare to a liquid starter on the rotor ? ( Is it softer and/or cheaper ? )

Please help.

 

[Skogsgurra]

I would say that it depends on a lot of parameters. If you are going to start the motor frequently, several times every day, then a frequency inverter and shorted rotor is just right (but then you could as well use a squirrel cage motor from the beginning and there might be a problem with harmonics if your feeder is weak). If you are going to start one time each day (or week) a liquid starter (a "bucket" with salt water and electrodes immersed in it) is a rather cheap but messy device. Starting resistors come somewhere in-between.

The ball mill usually needs a high starting torque and also has a rather high inertia, which a thyristor starter does not handle very well. I think that starting resistors in the rotor circuit is the next best solution (second to frequency inverter). You can then have the required high starting torque and at the same time minimise or control starting current.

A starting resistor (with time relays) is also a simple device and that might be important in places where special skills in electronics are rare.

For the design of the starter I think that you should go to a skilled consultant. He will need to know inertia, load profile, number of starts per hour (or day or week) environment (temperature, altitude, tropical/arctic climate and so on). The full electrical characteristics of the motor and the feeder and transformer will also be needed. I do not think that you should attempt this design yourself if you are not familiar with the technology.

 

[BartholomeusNel]

Thanks skoqsqurra for the comments. I will contact consultants in our area for inputs.

Regards

BartholomeusNel

 

[jraef]

If you go the route of using a solid state soft starter, you need only have one set of resistors selected to provide the requisite torque to accelerate the load at the lowest possible current. The short-cut formula for calculating that resistance is
R= (0.11)V/I where
R= calculated resistor ohms
0.11 = a constant
V = rotor nameplate voltage
I = rotor nameplate current

Do NOT assume that you can just short the rotor during startup. Many rotors are not designed to handle the starting current level that will flow when shorted. This by the way is sometimes true even if you use an inverter, depending upon the load profile. That is not to say it will not work, just don't ASS-U-ME that it will. It can be a very costly assumption.

You have another apparent problem if your information is correct. You state that you stator nameplate voltage is 525V, yet you have a transformer providing only 415V. This will never work.

Quando Omni Flunkus Moritati

 

[aolalde]

Certainly your transformer 415 kVA is so close to the motor demand 1.73*525*446/1000 = 405.56 kVA.

You will need a very soft start system to avoid damage of that transformer.

 

[jraef]

Oops, my bad. I read that as 415V, not 415kVA! Sorry. Need new glasses I guess


Quando Omni Flunkus Moritati

 

[Marke]

Hello BartholomeusNel

From your message, you are installing a wound rotor motor on a ball mill. The wound rotor motor is usually used on this application because of it's superior starting characteristics.
The Wound Rotor motor is able to develop a very high starting torque from zero speed to full speed at a relatively low start current.
You have total control over the start characteristics by the selection of the rotor resistors. If you wish to get the best start at the lowest current, then use a multi stage secondary resistance starter.
If you wish to simplify the starter, and can accept a higher start current, then select a single stage of resistors with a maximum torque at about 90% speed and apply a soft starter in the stator circuit. This could result in start currents of 450% as opposed to start currents of 200% for the multistage secondary resistance starter.
Adding a single stage of rotor resistance, is in effect converting the characteristics to emulate a standard cage motor, and if you are considering this option, then I would suggest to use a cage motor rather then a WRIM and save money and maintenance.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[BartholomeusNel]

Thanks to jraef, aolalde and Marke.

The idea was to use a multi-stage resistor. If the formula : R= (0.11)V/I applies to a single stage resistor, how do i calculate a resistor for eg. a) 0 - 30% speed, b) 31 - 60% speed and c) 61 - 100% Speed / Short circuit.

We have a speed monitoring sensor & PLC available to control the stages. Would you advise more stages ?

Regards,

BartholomeusNel

 

[jbartos]

Suggestion: The 415kVA transformer needs to be revisited for it suitability since the motor will draw about 405kVA. There is very little kVA margin left. It may be a good idea to simulate the motor starting by using software covering motor starting.
The soft starters tend to contaminate the input by a harmonic content. The transformer should have sufficient margin to withstand it or be k-rated.
See: M.G. Say "Alternating Current Machines" for calculations of the rotor resistances, steps, etc.

 

[Marke]

Hello BartholomeusNel

An approximate rule of thumb: The peak torque will occur when the rotor resistance is equal to the rotor reactance. The rotor reactance changes with slip frequency.
There is a thread elswhere on this forum going into a detailed discussion on the theory and forumula behind this and you could do a search to find the full discussion.

In order to avoid a large step in current, you need to select the final stage resistors at a slip of between 5 and 10% rather than apply the logic of equally spaced steps.

For a four stage unit, I would try the following values:
2.9 2.2 1.4 0.33 ohms with the resistors delta connected.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[electricpete]

This may be the thread that was referred to:

thread237-35410

My analysis of equivalent circuit seemed to show max starting torque occurs when the rotor resistance is equal in magnitude to the total series impedance. (X1+X2 in quadrature with R1).

Mark has shown me where his result is published in a textbook by Saye (spelling?). I never understoof what was wrong with my analysis.

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[BartholomeusNel]

Thanks to jbartos, Marke & electripete -

I appreciate all your input, and will definitely get a
"Alternating Current Machines" by M.G. Say.

The softstart stack is in the process, the cables are being laid and hopefully everything will be in place by the end of the month. I will defintely keep you posted.

BartholomeusNel