Slipringmotor's grid resistance 3

[RalphChristie]

We have a 3ph,70hp slipringmotor been used on a bending machine.

Stator V - 400V
Stator I - 107A
Rotor V - 288V
Rotor I - 113A
Speed - 575rpm

The motor is conected via an old type drum controller (forward and reverse) to a grid. Normally, when the operator of the bending machine want to decrease the speed of the plate been bended, or to let the plate move very slowly, he just moves the handle of the drum controller in the first position. My question is how do you determine the resistance to use on the grid? (Maybe practical or with a formula.)

The reason why I ask this question is: after the motor was rewired, the speed on the first step is faster than normally, which result in a condition of no slow speed control for the operator.

 

[jbartos]

Suggestion: It appears that the motor has been re-wired approximately. To slow it down by inserting suitable resistor some information about the rewired motor will be needed; especially, the deviations from the above posted rotor parameters. It may be a good idea to bring it back to the motor rewinder and explain them your problem. They might help you effectively since they rewound the motor.

 

[Marke]

The speed of the motor with resistance in the rotor circuit is a function of the torque developped by the motor with that resistance in circuit, and the torque required by the driven load at that speed.
First, ensure that the load conditions are exactly the same as previously. If the load torque conditions have changed, then the speed will change.
The rotor resistance does not control the motor speed, but does control the speed torque curve of the motor. The speed is determined by the speed torque curve of the motor and the speed torque curve of the load.

If the load conditions are identical, then it is possible that the rewind has changed the characteristics of the motor, especially if the rotor has be rewound.

Best regards, Mark Empson

http://www.lmphotonics.com

 

[jbartos]

Suggestion to the previous posting: Strictly speaking, the rotor resistance does control the motor speed. If the rotor resistance is infinity or very high, then the motor may or may not even turn or start.

 

[Marke]

The rotor resistance determines the speed of maximum torque, it is the torque curves that determine the speed that the motor operates at.

An open shaft motor will spin at almost synchronous speed uless the resistance is so high that the torque produced is less than the frictional and windage losses at full speed.
Regards, Mark Empson

http://www.lmphotonics.com

 

[jbartos]

Suggestion to the already starred posting that make many statements without references or proofs. The intention of this posting is to supplement "some old lady said so" postings in this Forum. See for example:
Reference:
1. Gordon R. Slemon "Magnetoelectric Devices Transducers, Transformers, and Machines," John Wiley and Sons, Inc., 1966,
page 375 equation 5.104:
T=ms x Rr' x |Ee|**2 / {nys x s x [Re + (Rr'/s)]**2 + [Xe + XLr']**2}
For simplicity, consider T=constant, and other parameters equal to constants, respectively, except s (slip) and Rr' varying, which is approximately the case for the induction motor with wound rotor. Then clearly, if Rr' varies, the slip and the implied motor speed must vary too. Certainly, torque T affects the speed of motor; however, as a parameter that sets the family of Torque-speed curves.

 

[cbarn24050]

Hi Jbartos, this has been discussed at great length in the recent past, you are by no means alone in thinking that rotor resistance controls speed, quite why this opinion prevails so widely is beyond me as any text book will tell you otherwise.

 

[Marke]

In fairness to all, the use of a slip ring motor with variable rotor resistance has been used for many years as a means of machine speed control. By increasing the rotor resistance, you shift the torque curve, causing the machine to slow down, and provided that the machine torque does not vary significantly (except with speed) then you will get a useful means of machine speed control. If the load is variable, then the speed will also vary and so the speed regulation is very poor.
Best regards, Mark Empson

http://www.lmphotonics.com

 

[cbarn24050]

Hi Marke, thats another widely held myth.

 

[electricpete]

Mark's first post said:
"The rotor resistance does not control the motor speed, but does control the speed torque curve of the motor. The speed is determined by the [intersection of the] speed torque curve of the motor and the speed torque curve of the load."

If that statement is understood, it answers all of the questions which came afterwards, in my humble opinion. Good discussion, Mark.

I did previously post some textbook-type calculations of torque speed curves using the equivalent circuit. It gives a qualtitative idea of how the motor torque-speed curve changes as resistance changes.

http://www.geocities.com/pschimpf/R2.htm

The main points are shown in the graphs at the end. Specifically, increasing R2 has the following effect:
-shifts the location (speed) of the breakdown Torque towards lower speeds (higher slips)
- decreases speed for a constant torque, or for a centrifigal fan/pump with constant flow resistance.

I'm not sure that we addressed how to determine the actual value of resistance. From the attached graph you see a quantity s_Tmax which I define as the slip at which the peak of the torque speed curve occurs. It is given by s_Tmax =R2/sqrt(R1^2+X1^2+2*X1*X2+X2^2) = , ie maximum torque occurs at a value of slip which is equal to ratio of the total rotor resistance divided by the remaining total impedance of rotor and stator circuit. Still not very practical to work with... you would need to determine your motor stator parameters somehow. From another recent thread I think Mark posted another result which does not agree exactly with that above.

 

[RalphChristie]

Gentlemen:

Thanks for all the responses!

Like I've said in my previous post, (sorry if it was a little vague) just the motor changed in the whole machine-setup. The load conditions stayed the same, identical to what it was.

To electicpete, can you please insert a list of all your symbols + the meaning of each one in your R2.htm address. To a new guy in the field on floorlevel, like me, it looks a little bit greek.

Keep up all the excellent responses!
Ralph

 

[electricpete]

Thanks, I'll work on adding a list of symbol definitions to that file. Most of them are motor equivalent circuit symbols: R1 = stator resistance, R2 = rotor resistance, L1 = stator leakage reactance, L2 = rotor leakage reactance.

One obvious point on the disucsion of controlling speed. For effective speed control you need to keep the operating point to the left (higher speed/lower slip) of the breakdown torque on those curves.

 

[electricpete]

As the first step for starting, you would want max torque at starting, ie s_Tmax=1.

solve:
s_Tmax =R2/sqrt(R1^2+X1^2+2*X1*X2+X2^2) = 1

gives R2 = sqrt(R1^2+X1^2+2*X1*X2+X2^2)

This is the highest value of R2 which would be used at starting. Lower values of R2 would likely be used for speed control.

 

[jbartos]

Suggestion to ElectricPete posting with:

http://www.geocities.com/pschimpf/R2.htm

It appears that the calculations were performed on the rotor side of the motor electrical equivalent circuit. Although conceptually correct, practically, the calculations need to have the rotor parameters transferred to the stator side or the stator parameters transferred to the rotor side to be correct. Else, one would get somewhat lower grade than straight A in various educational institutions or somewhat different grade in Forums than red star(s).