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

 

[electricpete]

Since you quote rotor ratings, this appears to be a wound rotor ac induction motor. The purpose of leaving the resistors in the circuit may be to reduce the operating speed.

 

[DanDel]

The use of resistors in the rotor circuit of a wound-rotor motor is to limit torque while the motor is starting or running, which typically limits motor speed. This torque limitation is not typically for continuous operation, since the actual rated speed of the motor is unchanged(based on the number of poles, the frequency, and rated slip), just unavailable because of the limited torque(the slip is increased dramatically). Typically, these motors are rated for the extra amounts of heat generated for short time operation at low speeds/high slip. I have never seen resistors left in the rotor circuit for normal operation, though if the motor is reaching rated speed without running too hot, they would only limit the rated power and not cause any other problems that I can see. I have seen similar 'ballast' resistors located in the stator circuit, used for an old-fashioned 'soft-start'.

 

[acmotorengineer]

In AC motors the torque characteristic is a function of rotor resistance, among other design features. Generally high resistance is needed for high starting torque and high running slip.

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.

 

[Marke]

Hello redox

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

 

[mbe]

We have to find the particular application. As already stated above resistors in rotor circuit are normally provided for:
- 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.

 

[bmodrow]

The purpose of the resistors is to control motor speed.

 

[jbartos]

Suggestion: Since there are different motor types and kW ratings, the conveyor propulsion system was analyzed / modeled and the resistors were added there to have the all motor torque speed curves approximately same at the conveyor operating speed (either no load or full load). The original posting concern about removing resistors is somewhat justified, if the motor speed is going to be controlled more precisely than what the present resistors are approximately designed to accomplish. Supposing that the resistors were fine tuned for all motors no load condition on the conveyor, then the loading of conveyor will cause the motors to have different slips because of different motor kW ratings and motor torque speed characteristics curves. The proper way to control the motors will be over the motor shaft speed sensors that would synchronize all motors shaft speed. This can be accomplished more than one way.

 

[yinger]

The resistors that remain in the motor circuit are refered to as permanent slip resistors. The resistance is not intended to contain the maximum conveyor speed, this would have been done in the speed reducer ratio. I think this is a designed installation, not a modification to use surplus or mismatched drive components. Even if all the motors came out of the same factory built to the same specification there are slight differences, permenant slip resistors allow the multiple motors on one conveyor to evenly share the load. Marke's response is a great explaination.

 

[don01]

another vote for marke's explanation.
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

 

[Shortstub]

Gentlepersons:

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?

 

[jbartos]

Suggestion: The Variable Frequency Drives (VFD) are relatively new and provide speed control flexibility to the induction motors. Although, the induction motor with VFD may or may not be as precise as servo motor design for speed control or motion control, the speed control is feasible and used with the induction motors. Once, the motor shaft rpm are synchronized and controlled, the individual motors will contribute HP to the conveyor motion according to their ratings and characteristics.

 

[castera]

I agree with Marke, the slip resistance is so that the motor will load share. I look after a number of cranes which have dual hoist motors ( Some on the same shaft and some through epicyclic gearboxes ) they all have slip resistance left in circuit after the final rotor step ( tends to be around 4% ), whereas single hoist motor drives are shorted out completely.

 

[jraef]

Right on marke. Vote #2
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

 

[jbartos]

Suggestion to original posting: Please, would you describe the "smooth running". Also, how are those conveyors started considering the wound rotors? Is there any automation applied?

 

[redox]

Thanks for all the replies!

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

 

[jbartos]

Suggestion/comment to the previous posting. It is easy to see why you are not sure that I ask you the question. The current setup over the fine-tuning resistors is only approximate solution; however, it may be found satisfactory since there may be marginal shortcomings only, if they materialize. If the resistors were design or finetuned for optimally loaded conveyors, then the solution appears to be reasonable as it was indicated in the original posting "running smoothly." However, if the conveyors are empty, then the motor speed fine-tuning is somewhat off with respect to the assumed optimal resistor design for optimally loaded conveyors. The motor torque-speed curve varies from the motor HP size to motor HP size. Therefore, they can be designed or fine tuned for one operating point and its vicinity, only. I did not see this aspect addressed in Marke's postings. One just has to be very familiar with motor torque-speed curve to synthesize the system consisting of mechanically aligned motors.

 

[Shortstub]

I don't understand this hell-bent approach to high-tech solutions. Especially if the old-tech has withstood the test of time.

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.

 

[DougMSOE]

Having designed liquid rheostats for a number of applications (10,000 HP), Soda Ash should always be used. I do know that there are a number of different electrolytes that may be used byt the soda ash always works.
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?

 

[GusD]

We have approx. 100,000 hp of wound rotor motors.Yes,you read it right;100,000.Our HP ratings are anywhere from 7.5 hp to 1250 hp.
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