Slipringmotor's grid resistance
Slipringmotor's grid resistance
(OP)
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.
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.





RE: Slipringmotor's grid resistance
RE: Slipringmotor's grid resistance
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
RE: Slipringmotor's grid resistance
RE: Slipringmotor's grid resistance
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
RE: Slipringmotor's grid resistance
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.
RE: Slipringmotor's grid resistance
RE: Slipringmotor's grid resistance
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Slipringmotor's grid resistance
RE: Slipringmotor's grid resistance
"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.
RE: Slipringmotor's grid resistance
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
RE: Slipringmotor's grid resistance
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.
RE: Slipringmotor's grid resistance
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.
RE: Slipringmotor's grid resistance
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).