Three- phase double converter drives
The schematic diagram for a 3- phaseDC drive as shown in figure. Converter 1 allows motor control in 1&4 quadrant whereas with converter2, the operation in in first and third quadrant is obtained. applications of double converter are limited to about to megawatt drives will stop for reversing the polarity of motor generated EMF for regeneration purposes, field circuit must be energized from single- phase and three- phase full converter. When converter one or two to operation, average output voltage is with a 3 phase full converter in the field circuit.
when variable DC voltage is to be obtained from a fixed DC voltage, DC chopper which is the ideal choice. Use of chopper in traction system is now accepted all over the world. Open is inserted in between a fixed voltage DC source and the DC motor armature for its speed control below base speed. In addition,chopper is easily adaptable for regenerative braking of DC motors and the kinetic energy of the tribe can be returned to the DC source. This results in overall energy saving which is the most welcome feature in in transportation systems requiring frequent stops, for examples in rapid transit systems. Chopper drives are also used in battery operated vehicles where energy-saving is of prime importance .
Tough choppers can be used for dynamic braking and for combined regenerative and dynamic control of DC drive, only the following two control modes are described in what follows:
- Power control or motoring control
- Regenerative braking control
Both the chopper control method describe. In addition, two quadrants and four quadrant chopper are also described.
Power control or motoring control
As shown in figure the basic arrangement of a DC chopper feeding power to a DC series motor. The choppers is shown to consist of a fos commutated thyristor, it could equally well to be a transistor switch. It offers one quadrant drive as shown in figure (B) armature current continuous and ripple free. The waveforms for the source voltage Vs , armature terminal voltage Vt= Vv , armature current ia , DC source current is and freewheeling diode current iFd are shown in figure.
In regenerative braking control, the motor acts as a generator and the kinetic energy of the motor and connected load is returned to the supply. The motor is then working as a generator in the regenerative braking mode.
The principle of regenerative braking mode explain with the help of figure where a separately excited DC motor and chopper are Shown. For active loads, such as a train going down the hill or a descending hoist , let it be assume that motor counter EMF Ea is more than the source voltage Vs . When chopper CH is on , current a through inductance La rises as the armature terminals get short circuited through CH. When Chopper is it turned off , Ea being more than source voltage Vs , diode D conducts. On the assumption of continuous and ripple free armature current,the relevant voltages and current waveforms are shown in figure.
With respect to first quadrant operation Raj offered by motoring control figure.regenerative braking control offers 2nd quadrant operations terminal voltage has the same polarity but the direction of armature current is reversed as shown in figure waveforms in the figure the following relations can we cleared.
Two -quadrant chopper drives
Motoring control circuit for chopper drives offers only first quadrant drive, because armature voltage and armature current remain positive over the entire range of speed control. In regenerative braking,second quadrant drive is obtained as a armature terminal voltage remains positive direction of armature current is reversed.
In two- quadrant DC motor drive, both motoring mode regenerative braking mode are carried out by 1 chopper configuration. One such circuit is shown in figure. Which consists of two choppers CH1,&CH2 anda separately excited DC motor.
Motoring mode. When chopper CH1 is turned on, the supply voltage Vs gets connected to armature terminal and therefore armature current ia rises. When CH1 is turned off ,ia freewheels through D1 and therefore ia decays. This shows that with CH1 and D1, motor control in first quadrant is obtained.
Regenerative mode: when CH2 is turned on , the motor acts as a generator and the armature current rises and therefore energy stored in the armature inductance La. When CH2 is turned off, D2 gets turned on therefore direction of armature current is reversed. Now the energy stored in La is returned to DC source and second quadrant operation of well as shown in figure. In the figure, first -quadrant operation of DC motor is sometimes called forward motoring mode and second- quadrant operation and forward regenerative braking mode.
Four quadrant chopper drives: in four -quadrant DC chopper drives, a motor can be made to work in forward motoring mode (first quadrant), forward reagebarative breaking mode(second quadrant), reverse motoring mode (third quadrant) and rivers regenerative braking mode (fourth quadrant). This circuit consists of 4 choppers, four diodes and a separately excited DC motor. Its operation in the four quadrant can be explained as under:
Forward motoring mode.during this mode for first quadrant operation, choppers CH2, CH3 are are kept off, CH4 is kept on whereas CH1 is operated. When CH1, CH4 are on, motor voltage is positive and positive armature current rise . When CH1 is turned off , positive armature current freewheels and decreases as its flow through CH4, D2. In this manner, the control motor operation in first quadrant is obtained.
Forward regenerating braking mode.A DC motor can work in the regenerative braking mode only. If motor generated is made to to exceed the DC source voltage. For obtaining this mode, CH1,CH3 and CH4 are kept off whereas CH2 is operated. When CH2 is turned off , diodes D1,did you are turned on and the motor acting as a generator returns energy to the DC source. This result in forward regenerative braking mode in the second quadrant.
Reverse motoring mode. This operating mode is opposite to forward motoring mode choppers CH1 ,CH4 are kept off , CH2 is kept on whereas CH3 is operated. When CH3 and armature current ia are negative.as Armature current is reversed, motor torque is reversed and consequently motoring mode in third quadrant is obtained. When CH3 is turned off , negative armature current freewheel through CH2,D4.armature current decreases and the speed control is obtained in third quadrant. Note that during this mode, the polarity of Ea opposite to the as shown in figure.
Reverse- regenerating braking mode . as in forward braking mode,reverse regenerative braking mode is visible only if motor generated EMF is made to exceeds the DC source voltage. Price for this operating mode, CH1 ,CH2 and CH3 are kept off where as CH4 is operated. When CH4 , is it turned on, positive armature current ia,rises through CH4, D2, ra ,La,Ea,. When CH4 is turned off, diode D2,D3 begins to conduct and motor acting as a generator returns energy to the DC source. This leads to reverse regenerative braking operation of the DC separately excited motor in four quadrant.
Note that as shown in figure (a)the numbering of chopper is done to agree with the quadrants in which these are operated.for example ,CH1 is operated for first quadrant,CH4 for fourth quadrant etc.