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Slip power recovery schemes

SLIP POWER RECOVERY SCHEMES

This system is mainly used for Induction motor speed control. The speed control in induction motor has poor efficiency due to wasting of slip power in the rotor circuit. By using recovery schemes the induction motor speed is controlled to avoid slip power loss.

The slip power is classified into two types
1. Scherbius system
2. Kramer system

1. Static scherbius drive system: 

                                    This system provides feedback path i.e. the wastage of slip power is again fed to AC mains supply. The static scherbius system is of two types
i) Conventional Scherbius system
ii) Static Scherbius system

i) Conventional scherbius system:   
                                In this system the recovery scheme is done by feedback path. The output of three phase Induction motor is connected to the DC motor by coupling them the mechanical power input of DC motor is converted into electrical power and fed to Induction generator and again back to mains.



ii) Static Scherbius drive system:   
                                 The phenomenon of this system is same as conventional type but the only difference is this system provides with diode bridge rectifier along with thyristor bridge inverter. This is also known as Sub-synchronous cascade drive.

               When Induction motor is operating at slip frequency the rotor slip power is rectified by the diode rectifier. The output of rectifier is fed to inverter three phase bridge again the output is fed back to supply lines with the help of transformer.



* Natural commutation proves involves across slip rings bus-bars. The induced emf frequency is made equal to rotor emf frequency by rectification of slip ring voltage to obtain speed control at injected voltage.

*In this circuit if commutation overlap is negligible the output voltage of uncontrolled three phase bridge rectifier is obtained as        
           



Advantages
Disadvantages
Instead of other speed control methods working range can be obtained at any speed
Motor turns ration is less than unity
If over excitation is occurred at rotor, it will take lead current which improves system performance
For reliable thyristor commutation inverter firing angle kept less than 180°.
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To improve power factor, capacitor is introduced into stator or rotor sides
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PWM technique is employed by replacing thyristor
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Slip is inversely proportional to power factor, hence if power factor decreases the slip increases.

Output power cascading:     The cascaded set with excessive excitation of auxiliary DC machine will run at high speed with low torque. The speed of main motor in this scheme is controlled by field regulation of auxiliary DC machine. This is called “Constant output power cascading”.

2. Static Kramer drive:

                      In this method the rotatory slip power is coverted into DC by a diode bridge. The DC power is fed to the DC motor which is mechanically coupled with the Induction motor. The speed control is done by varying the field current If.




                   From the characteristics you can easily observe the voltage and field current differences. The steady state operation is possible at Vd1 = Vd2


For large speed applications the diode bridge is replaced by using thyristor bridge, the speed can be controlled by varying the firing angle. Upto standstill condition the speed can be controlled.

Modification:

               The static Kramer drive system is modified by placing commutator less DC motor instead of DC machine. The DC motor consists of synchronous motor fed by load commutated inverter, the speed is controlled by field current




*If field current and inverter voltage reduced to zero then the drive runs at synchronous speed. This drive has better power factor and less harmonic content in line current compared to static Kramer drive. In this system the power is not fed back to the line.
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