I have a drawing of a wound rotor induction motor for a crane. This crane motor has 5 forward speeds & 5 reverse speeds implemented by switching in differing amounts of resistance for each speed setting selected. What has got me scratching around is the fact that 8 of the 10 speed selections switch in uneven (unbalanced) amounts of resistances on each of the 3 legs of the rotor.

Can this be possible? I would have assumed this would produce unbalanced rotor currents and therefore vibration in the motor. Is this assumption correct? All the external resistive speed controller examples I can find all have even amounts of resistance switched on to each leg for the variation of speed hence the confusion. I guess in summary will the uneven amounts of resistance cause any great issue in operation?

I would greatly appreciate anyone's comments on this.
Many thanks,
ben.

 

Ben... please provide operating data:

a) Line-currents.

b) Supply phase-phase voltages.

c) Supply phase-ground (or earth) voltages.

d) Rotor secondary-currents (per step if possible.)

e) Rotor ring-ring voltages. Make sure volt-meter is able to "read" lower slip-frequency.

f) Rotor phae-phase resistances (with external resistances open-circuited.

h) External resistance-bank resistances (per step if possible.)

Regards, Phil Corso

 

Phil,

Really appreciate your response. Unfortunately all i've got is a really poor wiring / schematic diagram from the contractor. This crane is an old school Russian design must easy be 30 years of age and has just received a couple of modifications. I'm trying to prove whether the drawing is correct or not as I've never seen uneven resistances being used.

I guess at this point after digging around for information for so many hours now I'm really just interested to know if uneven resistance would theoretically work else what effect would be seen.

Once again any thoughts on this would be great to hear.

Thanks,
ben

 

Ben... this sort of problem can be solved using Symmetrical Components Analysis!

Say the resistances of one phase is X ohms!Then the other two phases can be expressed in percent, or per-unit of X. Using Symm Comp, the positive- and negative-sequence values of rotor-current can be determined! Search the Control.com Archive for the two-motor theory, i.e., a large and a small motor coupled to the same shaft. The large one’s torque rotates the motor in one direction; the smaller one produces a counter-torque!

The question you raised about phase-resistance deviations for crane control was discussed in another thread! Frankly, I can’t find it, but I do recall that a Website, LM Photonics, had a resistance calculator for WRIMs!

Regards,
Phil

 

Phil,

Again appreciate your valued comments. It's people like yourself that keep the world spinning in a good way.

Keep up the good work.

Cheers,
ben

 

Ben, using the procedure I suggested illustrates how the number of speed-torque curve increments can be increased! Thus, materially improving raise/lower speed-control of the crane.

In the interest of increasing knowledge, can you provide the resistance measurements obtained with the old speed-controller. With those values, it might be possible to determine the number of speed-torque increments provided!

Regards, Phil

 

Hi Ben and Phil,

We are also just doing an overhaul of a crane that has rotor resistance drives installed for both hoist and travel. This was completely new to me and I enjoy the challenge and the gaining of knowledge. We too have very substandard hand drawn 50 year old schematics which we can't understand. We have replace all the control gear and rewired the resistor bank as it was but have been trying to gain a better understanding so we can commission successfully and also hand in the as built drawings.

Today we tested with the rotor disconnected what resistance we have on the 3 legs and between 2 of the legs the resistance was a few ohms higher which I didn't think would happen. My thoughts were the same as yours, that all 3 would be equal. I have found this document (https://www.i-gard.com/site/assets/files/1120/resistor_control_of_wound_rotor_motors.pdf) which was quite enlightening and this bit which helps to explain that our thoughts are not the case

UNBALANCED SECONDARIES - DRUM CONTROLS
Most controls for wound rotor motors use balanced (or symmetrical) secondary resistors which means that the same ohmic value is used in each phase. Some older controls may have steps that are not balanced. If we take a balanced set of secondary resistors that would normally allow 100% starting torque and open up one phase (break its connection to the neutral point of the wye connected resistors), then we would get 50% torque. I know of one crane builder who takes advantage of this. They use class 164 resistors with only three steps of resistance. Class 164 resistors allow 100% starting torque.

They start with the open secondary to get the normally required 50% starting torque and then switch to balanced phases for the 100% torque. This eliminates the first step of resistance but is not necessarily good for the motor. The Manual Drum Control is a heavy-duty rotary switch used to manually switch resistors in and out of the secondary circuit. The way these drum controls usually work is to have an off point (completely open secondary) for the first point, the open secondary like above, then all three phases in. The resistance in each phase is cut out one phase at a time. There are sometimes balanced points on these controls but for the most part they are unbalanced. Usually the resistance values are staggered so that the imbalance is never too great. The nice thing about the drum control is that is usually has 11 or 13 speed points. They are often used for accurate speed regulating duty on machines such as wire drawers or other older equipment where slight changes in speed may be necessary. The disadvantage is that they are subject to operator abuse for starting duty. The operator may switch the resistors out too quickly. These controls may be recognized by their unusual terminal markings (Rl to R3, R3 to R6, etc instead of Rl to R2, R2 to R3, etc.).

Cheers