Starting Method for Induction Motor

Starting Method for Induction Motor

As theoretically we know that induction motor is self starting motor.Polyphase induction motor is worked similar to action of poly phase transformer with secondary short circuited with rotating secondary i.e. rotor.Therefore when applied of  supply voltage to motor terminal  at this condition rotor is in standstill just liked the short circuit secondary in Squirrel Cage Induction Motor.Therefore very large inrush current flow through the rotor  & consequently the stator also will draw a high current from supply line if full line voltage is applied to motor supply terminal at starting.Due this high starting current, there is drop in voltage at starting of motor.This high staring current is high but it momemtory & falls rapidly as motor pick up the speed . Magnitude of staring current is depends upon the motor design & however it also depend s upon the time required to accelerate motor. Generally the starting current of induction motor is about 5 to 7 times of rated full load current. If this starting current is not control, with in the time ,then there is drop in voltage & it will affect the line connected other equipment . Also it will  increase in cu loss ( I ^2. R loss )  & it will increase the  heat in motor , chance to failure of winding insulation & it will burn.Following are the starting method of induction motor :

1.     Reduce Voltage Starting

2.     Direct on line ( DOL ) Starting  or Line Starting

3.     Primary Resistance Starting

4.      Auto -Transformer Starting

5.     Star- Delta Stater

6.     Variable Frquency Drive

 

·        Reduce Voltage Starting : Reduce voltage is applied to motor terminal by use of variac  at the staring. The major drawback of this is that large reduction in starting torque , because the staring torque is proportional to the square of applied voltage.Therefore this method of starting of induction motor is applicabale where minimum starting torque is require.

·        Direct on line ( DOL ) Starting : This method involves direct switching of induction motor to the supply mains.In this the rate of temperature rise is high & motor may get damaged if the starting period is high as explain above.This is due to excessive load,insufficient rotor  resistance, excessive voltage drop in supply.Push button type DOL stater is commonly used. It involves start-stop push button , contactor (electromagnetic type ), overload & under voltage protection device.This stater is commonly used for small motor or star connected motor ,  because staring current & staring time is low as compare to large motor. The main advantage of this is that it cost is low & require minimum maintenance.

·        Primary Resistance or Reactor Starting : It is similar to reduce voltage starting as discuss earlier.This method of starting is very simple and provides smooth acceleration of motor.In this method motor started with applying reduce voltage by use of resistor which are connected  in series with stator. Due to resistance voltage drop is across resistance & it will reduces voltage drop across stator. When motor pick up speed the resistance is cut off .Smooth acceleration , high power factor during start , less expensive this is the advantages of this method. Disadvantages of this method is that very difficult to adjust resistance varying condition, if the motor is large then high resistance is require & motor is not started with minimum starting time then the resistance get overheated,chance to damage the resistance ,low torque during starting.

·        Auto - Transformer Starting : This method the reduce  voltage is applied to the motor by taking tapping at suitable point from three phase auto- transformer. This method is used both star & delta connected motor.At starting condition reduce voltage is applied to the motor terminal.When the motor is run,up to 80% of its normal speed , connection of auto- transformer are cut out & full supply voltage applied to motor terminal.The auto- transformer tapped at the 50 ,60 & 80 percent point ,so that adjustment at these voltage may be made proper starting torque requirements or tapped of auto transformer is design as per application of motor. Advantage of this method is that internal losses in the starter is small during starting ,availability of high torque per ampere supply current ,adjustment of starting voltage is easy as selection of proper tapping on the auto transformer ,suitable for long starting periods.

·        Star-Delta Starting : As the name indicate that the motor is connected in star at starting & then it will connected in Delta. When the motor is connected in star then the voltage across motor terminal is 57.7% of line to line voltage & when it is in delta full line voltage is applied to motor terminal.The stater is connects the stator terminal in star across the rated supply voltage at starting condition .After motor attains speed in same star winding ,through change over-switch are reconnect in delta connection of same supply.Disadvantage of this stater is that starting torque is also reduce to one-third of  staring torque obtained by direct switching.Also during change over from star to delta heavy current flow through the stator winding. This starter is useful only Delta connection motor.This method is simple,cheap ,effective & efficient since no power is loss in any auxiliary components.

·        Variable Frequency Drive (VFD ) Starting : VFD starting is the best staring method than the starting method discus in above. For this variable frequency drive is used. Motor can be start as per application ,stating time ,starting current ,torque by ues of VFD. Motor is accelerate smoothly therefore there is no any chance for jerk of motor during start. In VFD there is provision of over-voltage ,under-voltage protection ,over current , thermal protection etc. are available. Therefore not require to use external protective device. This is the most commonly use method for starting. The main disadvantage of this VFD starting is more costly than other starting method &  if fault  occur in VFD then it is difficult to find.