OT - Slip Ring Resistor Calcs

Kidblue

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OT - but could someone please help/guide me on how to calculate the value of a Resistance bank for a 110kW Slip Ring motor?

It is to replace an existing unit (which i'm told is severely under-rated?!) - and has therefore been severely burnt and damaged whilst starting. At the moment, it consists of a 3 stage start-up via suitably rated shorting contactors.

All i need to try and provide is a new set of Resistor legs (the contactors are all suitably rated).

The application is for a 2000T flywheel press, Typical start-up currents are around 500A, settling down to about 150A idling (these values are for the STATOR, didn't get chance to check the ROTOR).


Anyone help??? :confused:
 
What does factory manual say??

Granted the old heaters burned up
- but was that because they were undersized
OR
they over heated due to lack of cleaning or ventilation?
Press implies to me a steel fab operation.
Lots of steel dust = lots of cleaning of electrical gear (or fans and filters)

Are all of each of the 3 stages of heaters burned up or just one stage ??

Dan Bentler
 
Thanks for the reply.

Granted the old heaters burned up
- but was that because they were undersized
OR
they over heated due to lack of cleaning or ventilation?

Definitely under-rated (never been right from day one - original supplier gone pop!).

Electrical cabinets mounted outside of forge so no dust etc from process.

All resistors burnt up not just one stage...................
 
That is called a Primary Resistance Starter, personally I would replace it but that isnt always an option.

The original resistance values may have been fine but the wattage rating may have been too low...ie they burned up. There will always be HEAT generated from the resistors so the WATT rating is very important.

The Primary Resistance starter will have one or more sets of resistors which, during start, are connected in series with the supply to the motor. The series resistors limit the starting current drawn by the motor, and thus reduce the starting torque of the motor.
The determining factor is start torque, calculate the necessary start torque and current needed to obtain it. Since this is using 3 sets I would assume they are timing and the resistors parallel each other in sequence, this would lower the series resistance and increase the current. The last contactor would bridge/bypass the resistors and allow full DOL current.

Addendum: The idea with the resistance is to take the motor to "almost" full speed (above 80%) before bridging to DOL.
 
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I've always found the resistor manufacturers the best people for sizing this kind of thing. Someone like Cressall will have a ton of data on different motors.

BTW the resistors in a slip ring motor starter are in the rotor circuit, not the primary side.
 
Slip Ring or Wound Rotor Motors

The slip ring or wound rotor motor is an induction machine where the rotor comprises a set of coils that are terminated in sliprings to which external impedances can be connected. The stator is the same as is used with a standard squirrel cage motor.

By changing the impedance connected to the rotor circuit, the speed/current and speed/torque curves can be altered.

The slip ring motor is used primarily to start a high inertia load or a load that requires a very high starting torque across the full speed range. By correctly selecting the resistors used in the secondary resistance or slip ring starter, the motor is able to produce maximum torque at a relatively low current from zero speed to full speed. A secondary use of the slip ring motor, is to provide a means of speed control. Because the torque curve of the motor is effectivley modified by the resistance connected to the rotor circuit, the speed of the motor can be altered. Increasing the value of resistance on the rotor circuit will move the speed of maximum torque down. It the resistance connected to the rotor is increased beyond the point where the maximum torque occurs at zero speed, the torque will be further reduced. When used with a load that has a torque curve that increases with speed, the motor will operate at the speed where the torque developed by the motor is equal to the load torque. Reducing the lad will cause the motor to speed up, and increasing the load will cause the motor to slow down until the load and motor torque are equal. Operated in this manner, the slip losses are dissipated in the secondary resistors and can be very significant. The speed regulation is also very poor.

Motor Characteristics.

The Slip Ring motor has two distinctly separate parts, the stator and the rotor. The stator circuit is rated as with a standard squirrel cage motor and the rotor is rated in frame voltage and short circuit current. The frame voltage is the open circuit voltage when the rotor is not rotating and gives a measure of the turns ratio between the rotor and the stator. The short circuit current is the current flowing when the motor is operating at full speed with the slip rings (rotor) shorted and full load is applied to the motor shaft.

Secondary Resistance Starters.

The secondary resistance starter comprises a contactor to switch the stator and a series of resistors that are applied to the rotor circuit and gradually reduced in value as the motor accelerates to full speed. The rotor would normally be shorted out once the motor is at full speed. The resistor values are selected to provide the torque profile required and are sized to dissipate the slip power during start. The secondary resistors can be metalic resistors such as wound resistors, plate resistors or cast resistors, or they can be liquid resistors made up of saline solution or caustic soda or similar, provided there is sufficient thermal mass to absorb the total slip loss during start.

To select the values of the resistors, you need to know the frame voltage and the short circuit current. The maximum torque occurs approximately at the point where the rotor reactance equals the termination resistance. The final stage of the resistance should always be designed for a maximum torque close to full speed to prevent a very large step in current when shorting the final stage of resistance. If a single stage was used and the maximum torque occured at 50% speed, then motor may accelerate to 60% speed, depending on the load. If the rotor was shorted at this speed, the motor would draw a very high current (typically around 1400% FLC) and produce very little torque, and would most probably stall!

Can I Bridge out the slip rings and use a soft starter on a slip ring motor?

A slip ring motor uses resistors in the rotor circuit to modify the starting characteristics of the slip ring motor. Increasing the resistance in the rotor circuit has two effects:
1. It reduces the start current
2. It increases the slip at which maximum torque occurs.
If the slip ring motor has been employed to provide a very high starting torque across the entire speed range during start, then the slip ring or secondary resistance starter can not be replaced. In this case, the first stage of the resistors would be selected to provide a high torque at 100% slip (zero speed) and a number of stages are then employed, each with reducing resistance to move the Slip point in steps from 100% towards 0%. The effect of this is to provide maximum torque at all speeds and at a reduced start current. (typically 200 - 300%)
Shorting out the slip rings and attempting any form of reduced voltage start in the stator supply, will result in a much reduced start torque at a much higher start current. Effectively, the motor could exhibit a Locked Rotor Current in excess of 1000% and a Locked Rotor Current less than 100%. If we reduce the start current down to say 400%, then the start torque would be less than 100 x (400/1000) x (400/1000) or less than 16%!
If the driven load does not require a high start torque, then the slip ring motor can be set up to emulate a standard cage motor by applying rotor resistance that will cause a full voltage start current of about 550%. A reduced voltage starter can now be applied, and the rings should be shorted out once the machine reaches full speed. If you do not short the rings at full speed, the slip will be higher than ideal and the motro efficiency will be reduced. There will be a high power dissipation in the resistors.
 
thoughts

Kidblue you may have other problems causing the resistors to over heat also. A few weeks ago we had an old motor like this on a metal roller acting up come to find out the old pnuematic timing relays were not working every time. The timers were timing out but the contacts didn`t shift leaving the resistance in the circuit. So be sure and check this out. If this starter is old as i think it is. If it were under sized it wouldn`t have lasted this long, but there must be some problem because you should be able to use the three stages as ruff speed control. For the new resistors you`ll need to know the voltage in the rotor circuit. To find this lift all the brushes off the slip rings and put a volt meter between any two of your slip rings then energize the stator long enough to read the voltage between the rings. If i have not understood your question and it is a primary resistance starter the resistors need to be able to disapate probably 2 times the kw of the motor. (AWG)and not American Wire Gauge either. Be careful the voltage from the rotor may be Higher than the voltage going to the stator!
Definitely under-rated (never been right from day one - original supplier gone pop!).
How long has it been in service.

:site:
 
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Thanks for the replies, very helpful.


Thomas, the unit has been in service for over 2 years, BUT the resistance bank has been an issue from day one (apparently). The resistance unit has been a constant source of problem and repair (overheating, insulation failure, flashover's etc). It has now reached the stage where it can be patched up no more (it's pretty well BBQ'd).

The timing of the shorting contactors is done by the PLC, and all contactors are weel rated and in good working order (checked contacts/springs etc).

I don't doubt that the resistance values are correct, but the power rating is extremely questionable.

I will attempt to measure the Rotor volts (production allowing!!).

Thank you.......(y)
 
I have a few technical questoions for thoes in the know.

If this project is 2 years old then:

1) Why a slip ring motor? What the advantage here.
2) Why cant the starters & resistance bank be replaced with a Soft Starter, or Drive of some type.
3) cant you just size the grid (for watts) based on the locked rotor apms?
 
Try these people, I have found them very helpful and cheaper than cressell.

Or you might consider an inductor starting system. This works by using the fact that the rotor frequency decreases as the rotor speed increases, so an inductor in series with the rotor windings will reduce in impedance as the frequency decreases, thus controlling the rotor current. The advantage is you don't need a series of sequentially operated contactors, just one to switch out the inductor when up to speed.

Or I have just found these people on the net http://www.wilsondencol.co.uk/switch.html
 
The slip ring motor is used primarily to start a high inertia load or a load that requires a very high starting torque across the full speed range

They have their place. Its possible to match load/speed requirements in many applications like long heavy load conveyors etc.

The other issue involved with replacing is its 110KW which probably means it is not a small motor...possibly 150HP range
 
#3

3) cant you just size the grid (for watts) based on the locked rotor apms?
Mike you can check the rotor the current, but you still have to know the rotor voltage and it`s usually not the same as the primary voltage. I`ve fooled with several over the years never seen one where the two were the same. This doesn`t mean they can`t be just never seen them the same.

:site:
 
elevmike,
The reason why I have chose to cut and paste is because people have tendency not to read the linked info.


2) Why cant the starters & resistance bank be replaced with a Soft Starter, or Drive of some type.

Shorting out the slip rings and attempting any form of reduced voltage start in the stator supply, will result in a much reduced start torque at a much higher start current. Effectively, the motor could exhibit a Locked Rotor Current in excess of 1000% and a Locked Rotor Current less than 100%.

Can I Bridge out the slip rings and use a soft starter on a slip ring motor?

A slip ring motor uses resistors in the rotor circuit to modify the starting characteristics of the slip ring motor. Increasing the resistance in the rotor circuit has two effects:
1. It reduces the start current
2. It increases the slip at which maximum torque occurs.
If the slip ring motor has been employed to provide a very high starting torque across the entire speed range during start, then the slip ring or secondary resistance starter can not be replaced. In this case, the first stage of the resistors would be selected to provide a high torque at 100% slip (zero speed) and a number of stages are then employed, each with reducing resistance to move the Slip point in steps from 100% towards 0%. The effect of this is to provide maximum torque at all speeds and at a reduced start current. (typically 200 - 300%)
Shorting out the slip rings and attempting any form of reduced voltage start in the stator supply, will result in a much reduced start torque at a much higher start current. Effectively, the motor could exhibit a Locked Rotor Current in excess of 1000% and a Locked Rotor Current less than 100%. If we reduce the start current down to say 400%, then the start torque would be less than 100 x (400/1000) x (400/1000) or less than 16%!
If the driven load does not require a high start torque, then the slip ring motor can be set up to emulate a standard cage motor by applying rotor resistance that will cause a full voltage start current of about 550%. A reduced voltage starter can now be applied, and the rings should be shorted out once the machine reaches full speed. If you do not short the rings at full speed, the slip will be higher than ideal and the motro efficiency will be reduced. There will be a high power dissipation in the resistors.
Yesterday 05:59 PM
 

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