How can I softstart a WRIM correctly ?
How can I softstart a WRIM correctly ?
(OP)
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.
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.





RE: How can I softstart a WRIM correctly ?
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.
RE: How can I softstart a WRIM correctly ?
Regards
BartholomeusNel
RE: How can I softstart a WRIM correctly ?
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
RE: How can I softstart a WRIM correctly ?
You will need a very soft start system to avoid damage of that transformer.
RE: How can I softstart a WRIM correctly ?
Quando Omni Flunkus Moritati
RE: How can I softstart a WRIM correctly ?
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
RE: How can I softstart a WRIM correctly ?
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
RE: How can I softstart a WRIM correctly ?
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.
RE: How can I softstart a WRIM correctly ?
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
RE: How can I softstart a WRIM correctly ?
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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RE: How can I softstart a WRIM correctly ?
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