Compare Starting Torque and Full load Torque in Starter

Relation between starting torque and full load torque

                ( T_st / T_f )  = ( I_st / I_f )² × S_f …… ( 1 )

Where

        T_st  = Starting Torque

        T_f  = Full load Torque

        I_st  = Starting Current

Compare DOL Starter & Star Delta Starter

The comparison between Direct ON Line Starter and Star Delta Starter is given as below.

Sr.No	Direct ON Line Starter	Star Delta Starter
1	The control circuit consists of only one contactor.

Rotor Parameter in the Three Phase Induction Motor

Rotor emf

• When the three phase supply is given to the three phase induction motor, the rotating magnetic field is produced which rotates at synchronous speed.

Rotating Magnetic Filed by Two Phase Winding

Rotating Magnetic Field

• When three phase supply is given to three phase winding, it produces rotating magnetic field which rotates and synchronous speed. 
• Similarly, when two phase supply is given to the two-phase winding, the rotating magnetic field produced by the winding is given here.

Rotating Magnetic Field: Two Phase Winding

• Let us consider that the two windings P and Q are placed at 90^o with respect to each other. 
• We assume that when the two phase supply is given to the winding, flux produces in each winding is purely sinusoidal. 
• The waveform of the flux is shown in the figure B. 
• The direction of the flux is assumed as positive whereas its opposite sides indicate negative values. The instantaneous value of flux can be given by

F_P = F_m Sin θ

F_Q = F_m Sin ( θ – 90^o )

• The voltage of the winding P is taken as reference or zero degree and winding Q is taken at 90 degree with respect to winding P.

Rotating Magnetic Field: Point 1

The voltage of winding P is zero whereas winding Q is negative as shown in the figure. The voltage of the winding Q negative sign is taken because we assume that the direction of voltage for winding P and winding Q is positive in the first quadrant.

F_P = 0 and

F_Q = –  F_m

Resultant flux F = √ P² + Q² – 2PQCos θ

                           = √ 0 + ( –  F_m )² – 0

                           =  F_m 

Rotating Magnetic Field: Point 2

The voltage of the winding P and winding Q is 45 degrees but both are in the opposite direction.

F_P = F_m / √ 2 and

F_Q = –  F_m / √ 2

Resultant flux F = √ P² + Q² – 2PQCos θ

                           = √ F_m² / 2 + F_m² / 2 + 0

                           =  F_m

Rotating Magnetic Field: Point 3

The voltage of the winding P is at 90 degree but voltage of the winding Q is at and winding Q is 180 degree.

F_P =  F_m and

F_Q = 0

Resultant flux F = √ P² + Q² – 2PQCos θ

                           = √ ( F_m )² + 0

                           =  F_m 

Rotating Magnetic Field: Point 4

The voltage of the winding P is at 135 degree but voltage of the winding Q is at and winding Q is 45 degree but both are in the opposite direction.

F_P = – F_m / √ 2 and

F_Q = F_m / √ 2

Resultant flux F = √ P² + Q² – 2PQCos θ

                           = √ F_m² / 2 + F_m² / 2 + 0

                           =  F_m

                                 

Conclusion

Position of winding	Voltage of winding P	Voltage of winding Q	Phase Difference between two winding
Point 1	0 degree	– 90 degree	90 degree
Point 2	45 degree	–  45 degree	90 degree
Point 3	90 degree	0 degree	90 degree
Point 4	135 degree	45 degree	90 degree
Point 5	180 degree	90 degree	90 degree
Point 6	–  225 degree	135 degree	90 degree

 

We can conclude that the rotating magnetic field produced by two phase winding is constant in magnitude. It rotates at constant synchronous speed in the clockwise direction.

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Torque - Slip Characteristic of the Induction Motor

The torque in the three phase induction motor is given by

• T = KFSE₂R₂ / [ R₂² + ( SX₂ )² ]

Torque at standstill condition ( S = 1 )

• The starting torque does not become zero at starting condition or at

Compare Squirrel Cage Induction Motor and Synchronous Motor

Three Phase Induction Motor

• It requires only three phase alternating supply. It is single excited motor.
• The speed of the Three phase induction motor decreases as the load on the motor increases.
• The induction motor runs at lagging power factor.

Squirrel Cage Induction Motor & Slip Ring Induction Motor

Parameters	Squirrel Cage Induction Motor	Slip Ring Induction Motor
Rotor	Copper bars are slotted in the rotor and these bars are short circuited from both ends by end rings	Three phase winding is slotted in the rotor similar to stator winding
End rings	Two end rings are used on both side of rotor	End rings are not used
External Resistance	As the rotor is short circuited by end rings, extra resistances are not connected.	External resistance can be added from rotor side
Slip rings and brushes	Not used	Three slip rings and brushes are used as per current capacity at rotor side
Starting torque	Moderate starting torque. The starting torque cannot be increased.	High starting torque is achieved by adding external resistances to rotor side.
Speed control	Speed control from rotor side is not possible because the rotor itself a closed by end rings.	Speed control is possible from stator and rotor side. The speed control from rotor side is done by rotor resistance starter.
Copper losses	Better space factor for rotor slots, shorted over hang and smaller copper loss	Large over hang resulting higher copper loss
Cooling condition	Cooling condition is better because it has bare end rings thus large space available for fan	Cooling condition is affected by some times sparking between slip rings and brushes
Overload capacity	Better overload capacity	Low overload capacity as compared to squirrel cage induction motor
Power factor	Better power factor due to smaller rotor overhang	Comparatively high power factor
Pull out torque	High	Low
Maintenance	Low	High
Construction	Simple and robust	Complicated
Cost	Low	High
Efficiency	High	Low as compared to Squirrel cage Induction Motor
Applications	Fan, printing machine, lathes, drill machine, blower etc	Lift, compressor, crane, hoist etc.

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