Slip Ring Motor Resistor 1

[nightfox1925]

I am trying to model a slip ring motor in ETAP and the starter model requires both the acceleration time and resistor rating in Ohms. Is there a formula which I can use to estimate the resistor rating. I am assuming for a high inertia load that it will have around 15~20 second acceleration time.

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[motorspert]

Normally the resistor values changes, either by switching discrete values or by varying the distance between electrodes in a liguid resistor.

Changing the value of the resistor changes the torque delivers which affects the run up time.

I think the model has to be run with a range of resistances and keep repeating the analysis.

 

[Marke]

Hello nightfox1925

The value of resistance in the rotor determines the slip at which maximum torque occurs.
In a normal multistage slip ring starter, there are a number of resistors used with each one selected to give a reducing slip at which maximum torque occurs and the last resistors stage having a slip close to zero.
You can approximate the resistance by finding the reactance of the circuit at any given slip and making the resistance equal to that reactance.
Somewhere on this board, there is a long discussion about the resistor values etc if you need to look closer at the problem.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[nightfox1925]

Marke, the reactance you are refering to is the internal reactance of the rotor? stator or the equivalent of both?

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[Marke]

Hello nightfox1925

You can determine the impedance from the open circuit rotor voltage divided by the rotor current, both of which should be on the motor name plate. This gives you the impedance at a slip equal to the line frequency. As the speed is increased, the slip reduces and so the impedance also reduces.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[nightfox1925]

Thanks Marke.

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[electricpete]

Mark - which rotor current are you referring to?

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