Resistors in rotor-circuit 7

[redox]

On our conveyors we have slipringmotors. Some of the conveyors have just one motor, the longer belts have up to three motors. On all our motors we have vapourmatic starters, but with statormatic probes. (Each motorstater have three steps to bring the drive on speed)

The motor sizes varies from:
200kW (3.3kV, rotor V - 390V, rotor I - 305Amps)
500kW (3.3kV, rotor V - 855V, rotor I - 347Amps)

My question is:

Between the rotor of each motor and its vapourmatic starter there is a set of very low value resistors (1 resistor per phase) The resistors stay the hole time in the circuit - meaning that the final starpoint of the rotor, when the motor is full speed, is after the resistors.

1. Why is there resistors in the circuit? (Maybe to limit
the starting current to a more lower level?)
2. Wouldn't it be better to take all the resistors out of
the circuit when the motor runs full speed? (Star rotor
in front of resistors)

There is no problem with the system - it is running smoothly, I am just curious to know why it is connected on this way - it is a old setup and no one can answer my question.

Thanks for any replies/tips
RCC

 

[jbartos]

Question to the previous posting: Since you have that much of HP in wound motors with "slip or permanent" resistors, do you happen to know how they are engineered and designed? Are they fine-tuned for loaded conveyors or empty ones? No one seems to be addressing this aspect which is the essence of this application. It should not be overlooked that the wound rotor induction motor still has very similar electrical equivalent circuit to the squirrel cage induction motor with R2/s where the R2 is the rotor resistance that includes the so-called very small slip resistor with respect to very small slip, e.g. 0.02 per unit. Therefore, this small slip resistor counts very much. Where are the debaters on this aspect? I guess "gone with the stars" or direction of the "Forum wind"?

 

[GusD]

Hi Jbartos
I don't pretend to know the specifics of the engineering that goes into designing these systems.
I can only say in answer to one of your questions, that
resistors would have to be rated (wattage) for maximum
loads that may be encountered.
Normal operation dictates that we start these systems with full loads from time to time, at -30C emperature.Basically we are trying to start a block of frozen material that may be a mile long.If resistors were not designed for maximum loads,they would not stand up.
GusD

 

[jbartos]

Comment to the previous posting. The motors starting and running fully loaded conveyors probably had the "slip or permanent" resistors designed for this mode of operation to balance motor speeds and slips. When motors are unloaded, conveyors are empty, then the motor speeds are higher due to lower slips that could potentially converge to the "almost" same slip values and the conveyor(s) accelerate to the maximum speed. Then, of course, the meaning/purpose of the "slip or permanent" resistor is at a question, it means it is unneeded. This has been my point right from the beginning of this thread; meaning that the "slip or permanent" resistor approach to ballace the motor speeds for all motor load is only approximate solution; however, it is working and it is used frequently.

 

[induction]

Lots of good info here I have work on large slip ring motors for cranes. The bank of resistors helps the motor start smoothly. At the shop with out the resistor bank they start poorly and sometimes even cog where the shaft jogs clockwise then counter clockwish then take off. A gentle spin will get them going in the right direction. when testing a rewound rotor we leave the connect from the rings open and test the voltage the rotor does not move it is becuase the circuit is open. Much like a gridmotor the bank heats up as current increase under load keeping a constant speed. Resitance goes up in the bank and more power indelivered be the rotor. Some very large sliprings have a limited duty. It also helps with reversing do to the fact that there is very high currents when reversed.

 

[jbartos]

Suggestion to redox (Electrical) Jan 7, 2003 marked ///\\My question is:
1. Why is there resistors in the circuit? (Maybe to limit
the starting current to a more lower level?)
///I agreed with Marke's posting except the statement in it:
"For example, if one motor had a slip of 1% full speed, and another had a slip of 2% full speed at full load, then at a speed of 1% slip, the first motor would be fully loaded and the second motor would be half loaded."
The mentioned proportions would hold true for exactly same motors with exactly same torque speed characteristics and HP ratings. Then, the resistors would not be needed. Supposing that motor #1 has horsepower rating HP1 greater than motor #2 with horsepower rating HP2. Then, motor #2 is not necessarily half loaded at slip %1 since the torque-speed curve of HP1, greater than HP2, has a steeper slope than HP2. To achieve the half proportion, the motors would have to have their parameters, including torque speed curves, exactly the same. The motor HP2 would be noticeable less than ½ loaded.\\\

2. Wouldn't it be better to take all the resistors out of
the circuit when the motor runs full speed? (Star rotor
in front of resistors)
///It depends. If unloaded, then they could be removed and the rotor might simply be shorted instead. If at the full load, they serve for speed balancing and should not be removed.\\