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Starting andcontrol of slip-ring 5

5 Starting andcontrol of slip -ringinduction features of a slip - ring motor 5 of slip - ring motors 5 of rotor resistance 5 external resistance and time of start 5 of steps 5 cycle and duty rating of resistance units 5 rise limits 5 procedure to calculate the rotor resistance 5 of time between each step 5 control of slip - ring motors 5 for speed control 5 electrode electrolyte starters and controllers 5 a rotor resistance for slip - ring motors 5 speed control of slip - ring motors 5/110 Relevant Standards 5/110 List of formulae used 5/1105/95 Starting and control of slip - ring induction motors5/97 The use of wound motors is on the wane, for reasons ofinherent advantages of a squirrel cage motor over a woundmotor and the availability of static drives which canmake a squirrel cage mot

5 Starting andcontrol of slip-ring induction motors Contents 5.1 Important features of a slip-ring motor 5/97 5.2 Starting of slip-ring motors 5/97 5.2.1 Selection of rotor

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Transcription of Starting andcontrol of slip-ring 5

1 5 Starting andcontrol of slip -ringinduction features of a slip - ring motor 5 of slip - ring motors 5 of rotor resistance 5 external resistance and time of start 5 of steps 5 cycle and duty rating of resistance units 5 rise limits 5 procedure to calculate the rotor resistance 5 of time between each step 5 control of slip - ring motors 5 for speed control 5 electrode electrolyte starters and controllers 5 a rotor resistance for slip - ring motors 5 speed control of slip - ring motors 5/110 Relevant Standards 5/110 List of formulae used 5/1105/95 Starting and control of slip - ring induction motors5/97 The use of wound motors is on the wane, for reasons ofinherent advantages of a squirrel cage motor over a woundmotor and the availability of static drives which canmake a squirrel cage motor perform the same duties as awound motor and even better.

2 Nevertheless, the use ofwound motors exists and will continue worldwide foryears. Where slip - ring motors are employed whose onlypurpose is limiting the Starting current or a limited speedcontrol, the resistance or electrolyte method of startingis most commonly adopted, for reasons of cost. Staticdrives generate harmonics and distort the supplyvoltage, and call for larger sizes of cables. It is alsocumbersome and cost-inhibiting to provide filter circuitsto suppress harmonics, particularly when the installationis where accurate speed control is the processrequirement, static controllers, termed slip recoverysystems (Section ) are recommended, which inaddition to exercising extremely accurate speed control,also conserve slip losses.

3 Static drives are discussed inChapter 6. Below we describe a procedure to determinethe value of resistance, its steps and switching and controlschemes for a rotor resistance electrolyte starter is almost a standard product likea motor and the manufacturer, depending upon the numberof starts and the speed control requirement, can adjustthe quantity of electrolyte, size and depth of electrodes Important features of a slip -ringmotorThese motors are switched through their rotor circuit byinserting suitable resistances and then removing themgradually.

4 In view of their varying characteristics throughtheir rotor circuit, they can provide the following features:1 The external resistance adds up to the total impedanceof the motor windings and limits the Starting also improves the Starting power the performance of an induction motor can bevaried by altering the rotor parameters, a slip -ringmotor, through its rotor circuit, can be made to suitany specific torque and speed speed of a slip - ring motor can be varied throughan external resistance. Therefore the torque can bemaintained at any value up to the pull-out torque inthe entire speed range by suitably varying the externalresistance.

5 (See circle diagram in Figure andSection ). At lower speeds, however, theefficiency of the motor will be poor, as the output isproportional to the speed. The efficiency would beroughly in the ratio of the two speeds, = NNIn fact, it would be even worse as a result of theequally reduced cooling effect of the fan at lowerspeeds. Since kW Nr T, kW would vary withspeed, the torque remaining almost the samethroughout the speed range. The motor would drawthe same power from the supply as before, which,proportional to speed variation, would appear as sliploss in the rotor circuit.

6 For instance, at 25% slip , thepower output will be 75% minus the cooling effect andthis 25% will appear as a slip loss in the rotor in Starting current and a requirement forhigh Starting torque to accelerate heavy rotating massessometimes limit the use of a squirrel cage motor. Forsuch applications a slip - ring motor provides a discussed in Section , during start-up, therotor is more vulnerable to damage due to excessiveheat in the rotor compared to the stator. But in slip - ring motors a major portion of this heat is shared bythe external resistance, in proportion to its resistivevalue.

7 Therefore, a slip - ring motor can be switchedON and OFF more frequently, compared to a squirrelcage motor. It can also withstand a prolonged startingtime, while accelerating heavy loads. Now the externalresistance will have to be suitable for such lossFrom Equation ( ), slip loss = S Ps. If the full-loadslip is S and the speed is varied to slip S1 the additionalslip loss due to the increased slip Ps(S1 S) kW (S1 S) (ignoring rotor losses).Example the speed of a 125 kW, 1480 motor is varied atconstant torque to 750 , then the additional slip loss 125 ( ) 125 61 kWwhereSS = 1500 14801500 = and = slip - ring motor is expensive, as are its controls,compared to a squirrel cage motor.

8 It also requiresmeticulous and periodic maintenance of the brushes, brushgear, slip -rings, external rotor resistances etc. A squirrelcage motor is thus preferred to a slip - ring motor. A slip - ring motor also requires a larger space for the motor andits Starting of slip - ring motorsThese can be started by adopting either a current limiting method or a definite time control method. In a currentlimiting method the closing of contactors at each step isgoverned by the current limiting relays which permit theaccelerating contactor of each step to close when themotor current has fallen from its first peak value to thesecond pre-set lower value.

9 The relays determine the5/98 Electrical Power Engineering Reference & Applications Handbookclosing time by sensing the motor data between eachstep and close only when the current has fallen to apredetermined value of the current relays. The closingsequence is automatic and adjusts against varying Starting time may be shorter or longer dependingupon the load. The disadvantage is that if for any reason,say, as a result of excessive load due to friction ormomentary obstructions the motor takes a longer time topick up, the second contactor will not close until themotor has picked up to a certain speed, and may thuscreate a false stalling condition and allow persistence ofa higher Starting current.

10 In a definite time start theother contactor will close after a pre-set definite time,cutting a piece of external resistance and hence increasingthe torque, giving the motor a chance to pick up. Thus,only this method is normally employed for modern slip - ring motor starts and controls. Below we describe onlythis type of Selection of rotor resistanceThe choice of the external resistance to be introduced inthe rotor circuit during start-up will depend upon thetorque requirement or limitation in the stator current,without jeopardizing the minimum torque Tst and Ist are interrelated (Section ) limitationin one will determine the magnitude of the other.


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