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°.
|
------
|
To improve power factor, capacitor is
introduced into stator or rotor sides
|
-------
|
PWM technique is employed by replacing
thyristor
|
-------
|
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.
No comments:
Post a Comment