Resistors in rotor-circuit
Resistors in rotor-circuit
(OP)
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
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





RE: Resistors in rotor-circuit
RE: Resistors in rotor-circuit
RE: Resistors in rotor-circuit
In AC Induction motors with squirrel cage rotors the resistance and consequently torque is fixed and a compromise between starting and running torque.
With wound rotors with external resistors, the torque characteristic can be tailored and changed for starting and running. A useful characteristic for long belt conveyors.
Whereas wound rotors may operate without external resistors, the torque characteristic is generally unsuitable for normal applications, particularly starting.
Keeping resistors in the circuit for running is normal.
RE: Resistors in rotor-circuit
The primary reason for the resistors remaining in circuit is to increase the slip to a uniform value for all motors. You are using multiple motors to drive a single conveyor. This could result in one motor providing almost all the torque if the full load slip was different for each motor.
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. If the load required 100% torque, then the first motor would be overloaded and the second motor would still not be at full load.
Adding the resistors ensures that the slip is determined by the external resistance rather than the internal motor characteristics.
During start, these resistors are so small relative to the vapourmatic resistors that they will have no influence at all.
The resistors are for load sharing and can be removed form single motor applications unless they are required to run at the same speed as the other conveyors.
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Resistors in rotor-circuit
- Limiting starting current
- Adjusting the starting torque (and the slip).
(By connecting a suitable resistance in rotor circuit starting torque and current can be varied over a wide range (any value upto the pullout torque))
- Speed variation
- and in some cases a relatively smooth start
Conveyors can be horizontal, inclined (up)
or downhill conveyors. There are chances of belt slippage during starting of long belt conveyors (loaded or unloaded).
By rotor resistance assisted start we can have a smoother start and avoid slippage.
In case of downhill conveyors, where the motors may run in generating mode, braking becomes a problem. Some people do it by plugging (bring the motor to near zero rpm by plugging and then apply mechanical brakes). A relatively large resistance is added for this purpose to have a smoother transition for reversal
Sometimes two motors (each capable of taking full load)are provided for long conveyors for standby purposes. Normally both the motors share the load. in case of failure of any one of these the other can run the conveyor. As so correctly suggested by Marke the values of resistances can be adjusted to attain a desirable load sharing.
RE: Resistors in rotor-circuit
RE: Resistors in rotor-circuit
RE: Resistors in rotor-circuit
RE: Resistors in rotor-circuit
I recall seeing a similiar set up on a large mill drive where the 2 motors were on a "common" shaft nose to tail and physically coupled together. 2 motors were used to get a lower profile to fit the plant and still do the job. I thought there was a booboo in the control schematic and thats exactly how it was explained to me.
This was pre super dooper electronics but it would be hard to beat the cost of 2 cast iron resistors in an existing plant.
regards
Don
RE: Resistors in rotor-circuit
Induction motors cannot be mechanically coupled, or even tandem coupled (series-windings excluded), to the same load without more slip than would be provided by the rotor alone. Not even if they were of identical design.
I learned this the hard way during the second week on my first job, a looong time ago. It was also the second-most embarrassing job-related story of my career.
Is there a "Job-related humor" forum?
RE: Resistors in rotor-circuit
RE: Resistors in rotor-circuit
RE: Resistors in rotor-circuit
He has hit the nail on the head. The arrangement you have is VERY typical for long overland conveyors. If the motors are not tuned for load sharing, the uphill-most motor will usually take the brunt of the load and burn up. Long conveyors can act like rubber bands when varying loads are applied, such as material handling like crushed rock or sand. Having the variable resistance control on WR motors allows torque control and thus proper load sharing. My recomendation: do not mess with it unless you are willing to buy 3 vector drives, the only other technology that can do this job. (OK, maybe DC as well, but that means changing the motors too)
Quando Omni Flunkus Moritati
RE: Resistors in rotor-circuit
RE: Resistors in rotor-circuit
Due to unforseen circuimstances,I haven't been able to reply to your responces.
Marke: I'll give you a red star for your comments.
jraef: You are right, this is a overland conveyor system (iron ore)
jbartos: I am not sure what you asked, but like I said, there is no problems with the system, it is running normally. (During start and running times.) It is started with an automatic liquid rotor starter (vapourmatic starter) with statormatic probes, and use three steps to bring the motor to speed.
Principle of starting operation: With certain electrolytes such as sodium carbonate,sodium hydroxide etc, 2 actions take place when an electric current is passed through the starter -
a) The electrolyte heats up and its resistance decreases
b) The electrolyte vaporises and its resistance increase (±50 times the value of the cold liquid electrolyte)
c) Range of vapo cold resistance 0.06 ohms to 19,5 ohms. With this low starting resistance and taking into account that slipring open rotor volts can reach up to 750 volts or more, very heavy rotor currents flow at the instant of the start. These currents cause an immediate formation of vapour bubbles on the electrode blades (±10 m.sec) increasing the resistance and decreasing the current proportionately. This decreased current now cannot maintain the vapour condition and the vapour recombines with the liquid and the resistance decreases to a low level giving a smooth accelerating torque of the motor.
Hope this help understand the starting principle.
Thanks again for all the comments/tips.
RCC
RE: Resistors in rotor-circuit
RE: Resistors in rotor-circuit
Remember this is not a robot application for, for example, brain-surgery. It's dirt or rock that's being moved! However, I do agree with the collaterial benefits!
BTW, returning to my earlier post on a unique marine-propulsion system. An added benefit is the ship was de-gaussed every time the system operated. The cabling was placed on the ship's periphery. Talk about old tech, and the test of time... it was implemented over 70 years ago.
RE: Resistors in rotor-circuit
As temp goes up, Resistance goes down.
(In motors of 10HP and up, rotor resistance (per phase) is rare to go above 1 ohm.
As the electrolyte temp goes up some evaporation will happen but the effect on resistance is very low in all but a few cases.
If you need to measure electrolyte concentration use density and correct for temperature.
BUT MOST IMPORTANT IN ANY APPLICATION... Who has to repair the device when it fails? Are you willing to spend time and money to train people in the operation of some VFD with high tech equipment? Or will it take a few simple tools and easy tests and it runs again.
All I ask is that before applying high tech stuff think... Who will be able to repair it?
RE: Resistors in rotor-circuit
All the conveyors that have multiple drives have what we call slip resistors or permanent resistors.Their mission is to balance load/slip in between 2 drives on a common shaft.
Their small ohmic value has negligible impact on either speed or torque.For Example . Our resistor grids are about
8 ohm /phase.In 16 steps we bring the resistance value down to the slip value of approx. (.125 ohms)A very small value comparing to the 8 ohms for starting.
GusD
RE: Resistors in rotor-circuit
RE: Resistors in rotor-circuit
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
RE: Resistors in rotor-circuit
RE: Resistors in rotor-circuit
RE: Resistors in rotor-circuit
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.\\\