slip ring motor grr

[Guest]

sir,

we have got a slip ring motor of siemens make 2650kw,11kv,174amps, 1000rpm synchronous and rated rpm 994 ,rotor voltage/rotor current:1425/1125amps used for fan application.

 

we are varying the speed of the motorfrom 60 to 100% with grid resistance with five resistances(0.09,0.18,0.36,0.72,1.44 ohms)and 32 step control.The grr panel is designed for 2300kw and 980 amps.

the problem we have is we dont have linear speed variation in the upper speed range and the jump of rpm is around 50rpm fromthe last step to short circuiting stage.that is at the last step the rpm is around 940.

Earlier the motor power was around 2300kw and now due to process variatons the power has gone up to 2650kw(full load).

 

My question is can the existing grr bank (rated for 2300kw) can be modified with additional resistances to provided a linearity of 10 rpm in the final stages and the same bank(rated 2300kw) can be used ..

what will the rotor current at the last but onestep(ie before short circuiting)

can we still reduce the base speed to 500rpm(process demand) and what will the grr losses

thanks

ram

[marke]

Hello ram

 

When you vary the secondary resistance, you alter the speed at which maximum torque occurs, but do not have direct control over the speed that the motor and load spin. By varying the rotor resistance, you are affectively changing the shape of the speed torque curve. The speed of the motor and load will be the speed where the speed torque curve of the driven load and the motor cross. i.e. the torques are equal.

Any change in load will result in a change in speed.

It sounds to me like you need to add one more low resistance stage in the rotor circuit with a resistance of about half of the existing lowest resistance, i.e. about 0.05 ohms.

 

Best regards,

[Guest]

sir,

thanks for your reply. kindly clarify my doubt. the grr panel and the resistance are rated for 2300kw /980 amps .Now since the loading has gone up to 2650 kw will the resistance be overloaded at speeds close to sychronism ie at 970 rpm.

 

the rotor resistance is 0.00641 ohms at 20 deg.i dont have the value of rotor reactance ( this may be neglected in comparision with resistance values when rpm is close to synchronism and slip is very less).

 

At the rpm of 970 what will be circulating current in the rotor.

Rotor rated current is 1125 amps .

The above application is for fan drive.

 

will the panel resistances have to be redesigned for 2650 kw.

 

At present at 940 rpm(one step below short ckt) the kw is around 2400.

 

Rotor voltage 1425 volts.

kindly clarify

Thanks

ram

[marke]

Hi ram

 

There is more to the secondary resistance than just the KW rating. If the resistances were selected for a smaller motor, or a different motor, then the resistances will not be optimised for the new motor. This will mean that the speed torque curves will not be ideal, plus there could be an issue with power dissipation.

At 970rpm, you will have a slip of 3%. This means that the slip losses will equal about 3% of the power deliverd to the load.

 

If the frame voltage and rotor current do not change, there is no need to change resistors. If the frame voltage and/or rotor current change, then the resistors may need to be adjusted.

 

Best regards,

[Guest]

thanks sir for my clarification and still i have little bit confusion in the above case regarding the resistance..

earlier when our grid frequency crosses 50.3hz(rated freq 50 hz) the kw rating of the 2650kw motor exceeded its limit(up to 2700kw) and the protection relay acted.To avoid this situation we reduced the speed by reducing one step below and the motor ran at 2300kw with 0.09 ohms resistance(delta connected) in circuit and the rpm went down up to 950.

Now beacause of this 950 rpm our process got affected(fan application) and we are in the process of bringing down the speed by 10 rpm only(around 980, full load rpm 994).

MY question is since the at this reduced 10 rpm the motor will be taking around full load(2650kw) . will the resistance be overloaded(1125 amps against desing 980) to deliver the full load. or the motor slip will increase....

 

 

kindly clarify my another doubt

We got one application where twin drives of rating 11kv,2100 kw slip ringmotor driven by common Lrs are used for driving a ball mill application.

The load of these two motors are 1750 kw to 1800kw (fullload).

 

We have got a motor of 1900kw ,11 kv ,134 amps stator current and rotor voltage 1425volt and 980 amps rotor current.

 

Can one of the above 2100 kw motors be replaced by the above 1900kw motor.

 

The rotor open circuit voltage of 1900 and 2100kw are not same.

 

thanks

ramkumar

[marke]

Hello ramkumar

 

I can not determine whether the existing resistor bank will be overloaded whithout knowing the designe criteria applied when the resistor bank whas designed and built. You need to get advise from the manufacturers, or from a consultant who can make the appropriate measurements and calculations.

 

For your secod question, you could threoretically replace one motor with a smaller one provided that the rotor resistance controller was also replaced with a controller that accurately matches the original motor.

You need the services of a skilled engineer on site to look at this. I suggest you get a local consultant.

 

Best regards,

[Guest]

sir,

kindly explain me in brief about gd2 of rotor. i want to know what is the ratio of load to motor gd2(which one should be greater and to what extent).what is significance of gd2.

 

in comparision of motor(while replacing) if a rotor has gd2 lower compared to other what will be its effect for driving the same load

[marke]

I presume that you are refering in the inertia of the rotor.

There are two figures given by some manufacturers. The first is the actual inertia of the rotor itself and the second is the maximum load inertia that can be controlled by the motor.

 

There is no fixed or required relationship between the actual inertia of the rotor and the actual inertia of the driven load.

 

The normalised inertia of the driven load (normalised to the motor shaft speed) must not exceed the rated maximum load inertia if you are using the motor to start the load.

 

Best regards,

[Guest]

thanks sir regarding your reply for my question regarding inertia.

regarding my earlier query of slip ring grr i had a consultation with my manufacturer and he has suggested to replace the resistance with rating equal to the full load rotor current.

 

i have got another query..

what is magnetic centering in motors... how it will affect the perfomance of the motor. whether it will cause any vibration effects..

 

What is the max set temp limit for bearings and windings(class f) in motors...

[marke]

Hi

These are questions that would be best answered by the motor manufacturer as the temperatures are dependant on the design criteria and materials used and can change between machines.

 

Best regards,

[Guest]

sir,

what is the protection available for rotor ground fault and how to protect rotor stalling