Static rotor-resistance control
In a slip-ring induction motor, 3 phase variable resistors R2 can be inserted in the rotor circuit as shown in figure. By wearing the rotor circuit resistance R2, the motor torque can be control as shown in figure. the starting torque and starting front can also be very by controlling the rotor circuit resistance as in figure both. The advantages of this method of speed control are:
- Reduced efficiency at lower speeds
- Speed changes vary widely with load variation.and balances in voltages and currents if the rotor circuit resistance are not equal. In in spite of these, this method of speed control is used when a speed drop is required for a short time, as for example in overhead cranes, In load equalization etc.
The three-phase resistors as shown in figure may be replaced by three- phase diode rectifier, chopper and one resistor as shown in figure. In this figure,the function of induction is the smooth and the chopper allows the effective rotor circuit resistance to be varied for the speed control of sleeping induction motor.Diode rectifier converts sleep frequency input power to DC at its output terminals.
When chopper a on ,Vdc=VD=O and resistance get short-circuited. When chopper is off ,Vdc=VD and resistance in the rotor circuit is R. This shown in figure. From this figure, effective external resistance Re.
Slip- power recovery schemes
In chopper method of speed control for slip ring induction motor, the slip power is dissipated Hindi external resistance and it leads to poor efficiency of the drive. However, instead of wasting the slip power in the rotor circuit resistance, it can be conveniently converted by various schemes for the speed control of slip ring induction motor. Two important slip power recovery schemes are static Kramer drive and static Scherbius drive. These are now discussed in what follows.
Static Kramer drive.
The circuit configuration for static Kramer drive is shown in figure.the sirf frequency power from the rotor circuit is converted to DC voltage which is then converted to line frequency and pumped back to the AC source. As a slip power can flow only in one direction, static Kramer drive offers speed control below synchronous speed only.
The slip power from the rotor is rectified to DC voltage by a diode bridge. Inductor smoothens the ripples in the rectified voltage Vd. This voltage Vdis then converted to AC voltage at LINE frequency by line commutated inverter. as the power flow is from rotor circuit to supply, static Kramer drive offers constant torque drive. As stated before, this is scheme offers speed control below synchronous speed only.
Static Scherbius drive.
In static Kramer drive, speed of slip ring induction motor can we control video synchronous speed only. For the speed control both below and above a synchronous speed, static scherbius drive is used. There are two possible configuration to obtain such a drive these are. (I) DC link static Scherbius drive (ii) cycloconverter static Scherbius drive. These are discussed briefly as under.
DC link Scherbius drive.
In sub synchronous speed control of WRIM,slip power is removed from the rotor circuit and is pumped back into the supply. In super synchronous speed control, the additional power is fed into the rotor circuit at sleep frequency. The circuit diagram of DC link as shown in figure below allows both subsynchronous and super synchronous speed control. It consists of WRIM , two phase control bridges, smoothing inductor and a transformer as shown.
For subsynchronous speed control, bridge one has firing angle less than 90 degree whereas bridge 2has firing angle more than 90 degree. and bridge 2 as arectifier with firing angle less than 90 degree. The power flow is now from the supply to transformer, bridge 2, bridge 1 and to the rotor circuit.
Cycloconverter scherbius drive.
The double controlled converter system used in DC link scherbus driveis replaced by 1 phase controlled line commutated cycloconverter as shown in figure. Sacchi schemes are used for very high power pumps and blower type drives. Cycloconverters permits the silip power flow in either direction and the machine can, therefore, be controlled in both sub synchronous and super synchronous rangers with motoring and regeneration features. As the slip power is either return to, are taken from, the supply mains, cycloconverter static Scherbius drive offers constant torque drive scheme.
Synchronous motor drives
Synchronous motors have two windings, 1 on the stator is three phase armature winding and the other on the rotor is the field winding. the three phase winding on its status is similar to the three phase winding on the stator of a three phase induction motor . Field winding is excited with DC and it produces its own MMF called field MMF. 3 phase stator winding carrying 3 phase balance current creates its own rotating armature MMF. The two MMF combine electromagnetic torque. A a synchronous motor runs always with zero slip that is is at synchronous speed given. power factor of synchronous motors can be controlled by varying its field current.
For the speed control of synchronous motors,both inverter and cycloconverters are employed. The various types of synchronous motors are:
- Cylindrical rotor motors
- Reluctance motor
- Salient- pole motors
- Permanent magnet motors
these are now described really in the following lines:
Cylindrical rotor motors.these motors have uniform air gap. the perfect equivalent circuit for silica rotor synchronous motor is given as shown in figure. Power factor of a synchronous motor depends on the field current. the variation of armature current with respect to field current for different loads on the synchronous motors is shown in figure. As the shape of these curves symbol the letter V, these are called V curves of a synchronous motor. Note that V-Curves are obtained for constant shaft load and for constant terminal voltage . Unity power factor curve is shown dotted. For low values of field current, synchronous motor operates at a lagging power factor. The field current is increased, it would start operating at unity power factor. If the field current is still increased beyond The unity power factor point, synchronous motor begins to operate at leading power factor.
As for power,the first and second component are called respectively the electromagnetic torque and the reluctance torque.
Reluctance motors.A salient- pole synchronous motor connected to a voltage source runs at a synchronous speed. If its field current is switched off, it continues running at a synchronous speed as a reluctance motor. Thus, machine design to operate as a reluctance motor is similar to a salient pole motor with no field winding on the rotor. 3 phase armature winding produces rotating magnetic field in the air. this rotating flux induces a field in the rotor which tends to align itself with the armature field, thus, producing a reluctance torque at a synchronous speed.
Reluctance motors are used for low power drives where constant speed operation is required and where more than one motor is needed for the job so that the number of motors can run in synchronism.
Permanent magnet motors. A permanent magnet synchronous motors is similar to the salient pole synchronous motors without the field windings on the poles. In permanent magnet synchronous motors the required field flux is produced by permanent magnets mounted on the stator. Indus motors, excitation EMF cannot be varied. These motors are used in robots and machine tools. a permanent magnet synchronous motor can be filled from a rectangular current source or sinusoidal current source.