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Start by
Willian Fernandez Conde
09-16-2013 10:07 PM

Starter of SAG Mills with rotor resistance

Hi everybody,
For now I am working on a mining project which involves starting two SAG mills, the method of starting these mills is by rotor resistance and likewise we are using an energy recovery system (SER), could someone tell me how this system works SER?
each mills have two motors of 8000 kW at 13.8 kV.

thank you for all.
09-17-2013 12:20 AM
Top #2
Enrique Gallegos
09-17-2013 12:20 AM
wound rotor motor is not the best option for SAG mills becuse the slip rings always are a problem even with liquid resistors.Best option is synchronuos motor in combination with variable frequency drive. However if you already have the wound rotor motor the ERS operation is simple.

Instead of dissipating the rotor power in a resistor we can use it to power a 3 phase inverter that returns the power to the AC source. The line commutated inverter is connected to the same feeder that suplies power to the stator, usually a transformer is used so that the effective voltage input at primary side be 80-90 % of voltage at inverter input this ensures that the firing angle is reasonably close to the permissible limit while reducing the reactive power absorbed by the inverter.

Hope this helps.

09-17-2013 02:48 AM
Top #3
Ing. Ole Knudsen
09-17-2013 02:48 AM
Primarily, make sure that the SER is installed VERY close to the motor, otherwise there will be no slip energy to recover!
If variable speed is not an absolute requirement, I would normally prefer a simple rotor-resistance starter, and a shorting arrangement for the slip rings for full speed operation. That could save both capital and maintenance cost.
09-17-2013 05:01 AM
Top #4
Praveen Sharma
09-17-2013 05:01 AM
slip energy recovery now a days obsolute in the market. first we have to start the motor by conventional vlrs/lrs/grr and then after 60% speed, the rpm is controlled with sprs. I would prefer to go with medium voltage drive for better control, reliability and energy saving. the slip ring terminals may be shorted and drive available equivallent to 36 pulse, so no issue of abnormal heating due to harmonics or bearing failures.
09-17-2013 07:17 AM
Top #5
Terry Robson
09-17-2013 07:17 AM
For large mills requiring variable speed, the wound rotor motor and SER drive are economical for a total rating of approximately 2MW to 16MW. Above 16MW, the gearless drive (cyclo-converter) is typically used because gearboxes and pinion gears reach their present limit in size. Around 2MW and below, the squirrel cage/VVVF drive is simple and cost effective.
Advantages of the wound rotor/SER drive are:
1. If the SER converter drive fails, the drive can be switched to fixed speed bypass - starting the usual way with the LRS.
2. The converter only needs to be sized for 15-20% of the total motor rating with associated reduction in floor space, air-conditioning etc. The converter is only sized for the feedback energy which is proportional to the speed difference from synchronous speed. The drives are typically set up to run between about 85% to 110% of synchronous speed for an optimised arrangement.
3. Relatively low capital cost when all things considered - including spare motor cost etc.

Brush/slip ring maintenance is one issue. However, when the brushes are specified correctly for the load, the wear is manageable. Once the maintenance program is set up for shutdowns, it is not a major issue.
I expect that this type of drive would be the most common large mill variable speed mill drive in the world's minerals processing industry for the range mentioned above for the last 15 years (approximately).
09-17-2013 09:57 AM
Top #6
Terry Robson
09-17-2013 09:57 AM
In answer to the original question:

The SER drive converter controls the voltage in the rotor. Motor speed is proportional to rotor voltage. Resistance in the rotor indirectly achieves the same thing (with a different torque curve shape), but energy is lost in the resistors which is very inefficient. The SER drive via a feedback transformer feeds energy back into the power supply. This returned energy is proportional to the speed difference from synchronous speed. So at say 85% speed, 15% of the motor rated power is returned from the rotor to the supply. At a hypersynchronous speed, the SER drive feeds power into the motor rotor allowing it to run faster than synchronous speed. So for a fixed torque and higher speed, the power obtained from the motor is higher than the motor nameplate rating.

Gearbox ratio is best set up to allow the speed range to be covered using the SER drive's hyper-synchronous capability.
09-17-2013 12:15 PM
Top #7
Omer A.Osman
09-17-2013 12:15 PM
Dear, although I am very happy with all previous valuable comments, I would like to reflect the following points that I believe will simplify the issue.

As per Torque / Slip characteristic for AC Motor, the value of the Max. Torque can be developed is constant while the Starting Torque occurs @ S=.1, (T proportional to r2 and S also proportional to r2 where r2 is the rotor resistance, the ratio r2/x2 when equal to 1 gives the max. Torque w.r.t Slip at Starting. Wound rotor motors are suitable and recommended for application for MV drive where it is required to be started on load such as ID. Fans, S.D Fans, Drill, etc.

As you aware the torque is directly proportional to the rotor resistance "r2" & varies with slip "S", hence injection of resistance into the rotor via Slip Rings, High Starting Torque can be got while the Speed, efficiency and Starting current will be reduced. Therefore resistance is the most practical method of changing the torque (i.e. wound rotor Slip ring Motors). Moreover, the Max torque can be achieved at starting when rotor Resistance "r2" = The Stator impedance, at starting S=1.

On the otherhand, the slip of the Induction Motor (speed) can be changed by “extracting” electrical power from rotor circuit, more extraction increases the slip. By using thyristorized Slip-Recovery Scheme “ i.e Kramer Scheme” feedback of Power from rotor circuit to supply circuit whih also known as "the slip Power recovery scheme". The scheme is simply consists of rectifier and an inverter connected between slip-rings and the A.C Supply circuit. The Slip Rings voltage is rectified by the rectifier and again inverted to AC by the inverter and feedback to supply via a suitable Transformer. Such arrangement gives good efficiency with high cost due to Rectifier and Invertor.
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