DC Motor Control through Choppers – For controlling a dc motor operated from dc supply, the dc voltage level must be controlled. Examples can be cited of subway drives, trolley buses or battery-operated vehicles. Conventional methods of converting a fixed-voltage dc source to a variable-voltage dc source are:
A variable resistance is inserted between the load and the source. The method is highly wasteful of energy. Also, for a given output voltage, different values of resistance are needed for different values of load current. This method is still used for older traction installations.
Separate generator excitation gives a voltage which can be varied from zero to rated value with either polarity. The set-up is bulky, costly, less efficient and has a sluggish response because of the generator field time-constant.
The dc source could be converted to ac form by an inverter (see Sec. 11.9), transformed (by a transformer) to a suitable voltage level and then rectified to dc form. Because of two stages of conversion, the set-up is bulky, costly and less efficient.
As mentioned in Sec. 11.5, a chopper is essentially an electronic switch that turns on the fixed-voltage dc source for short time intervals and applies the source potential to motor terminals in series of pulses. This process controls the average voltage applied to the motor. The chopper can also be easily operated in the regenerative mode. The chopper provides efficient and stepless control of dc voltage and is less expensive and bulky compared to the three methods mentioned above. It has the added advantage of fast response and regeneration. The chopper is a relatively new technology and can be visualized as a dc-to-dc transformer with a continuously-variable ratio of conversion.
Principle of Operation
A chopper is essentially a thyristor switch in series with the load as shown in Fig. 11.27(a). A shunting diode is provided across the load for free-wheeling the load current when the thyristor is off. The thyristor shown enclosed by a dotted square can be turned-on and turned-off over a time period and the cycle is repeated.
The flow of dc current through the chopper during the on-period necessitates the use of a forced commutation circuit for turning off the thyristor. The average value of the chopper output voltage waveform shown in Fig. 11.27(b) is given by
It is therefore seen that the output voltage of a chopper is controlled by its duty cycle. It can be varied in the following ways:
In this system f = 1/T (fixed), i.e. the chopping frequency or period are fixed and tON is varied to control α. This is pulse–width modulation. Figure 11.28 illustrates this system of control.
Here the chopping period T is varied and either tON or tOFF is held fixed as illustrated in Fig. 11.29. This is frequency modulation. This method of control presents two difficulties: (i) Control of Va requires variation of chopping frequency over a wide range. Filter design for variable-frequency operation is difficult. (ii) At low output voltage, a large value. of tOFF makes the motor current discontinuous.
Chopping frequencies as high as several hundred cycles/second are used in practice.
The chopper configuration of Fig. 11.27(a) produces an output voltage less than the input voltage. An output voltage higher than the input voltage can be obtained by the chopper configuration of Fig. 11.30. When the chopper is on, the inductor is in the source circuit and energy from the source is stored in it. When the chopper is turned off, the inductor current is forced to flow through the diode and load. Decreasing current causes the inductor voltage to reverse which now adds up to the source voltage such that the load voltage is higher than the source voltage (va > V). The energy stored in the inductor is released into the load.
On the basis of average values, during the time the chopper is on, the energy stored in the inductor from the source is
During the time the chopper is off, the energy fed from the inductor to the load is
Assuming a lossless system, under steady-state the two energies will be equal, i.e
For α varying between 0 to 1, the theoretical load voltage varies from V to ∞
The step-up chopper can be employed for regenerative braking of a dc motor. In Fig. 11.30 if V represents the motor armature and Va the dc source, power is fed from the decreasing voltage motor to a higher but fixed voltage source.
Voltage and Current Waveforms
The voltage and current waveforms for the chopper configuration of Fig. 11.27(a) for a continuous current case are given in Fig. 11.31.
In this type of commutation, current through the thyristor is forced to become zero to turn it off. This can be accomplished in two ways.
- Voltage Commutation—A charged capacitor momentarily reverse-biases the conducting thyristor to turn it off.
- Current Commutation—A current pulse is forced in the reverse direction through the conducting thyristor. As the net current becomes zero, the thyristor is turned off.
The load current flowing through the thyristor either becomes zero (as in natural or line commutation employed in converters) or is transferred to another device from the conducting thyristor.
This commutation circuit comprises an auxiliary thyristor Th2, a diode D, inductor L and capacitor C as shown in Fig. 11.32 wherein the total chopper circuitry has been outlined with a dotted box.
To start the circuit, capacitor C is initially charged by firing the thyristor Th2; charging is now from the source via C, Th2 and load with the charging thyristor turning off as the charging current decays to zero.
Voltage and current waveforms of a voltage-commutated chopper are shown in Fig. 11.33. The main thyristor Th1 is triggered at to. Current flows in two paths—load current ia through Th1 and commutation current ic through C, Th1, L and D. The commutation current ic is oscillatory of frequency determined by the LC series circuit. At the end of the first pulse of ic, C is charged negatively and the diode D blocks-off further flow of current.
To turn-off the main thyristor Thl, the auxiliary thyristor Th2 is triggered at t2. This causes the fully-charged capacitor C to reverse-bias Thl and causes it to turn off. The capacitor C itself gets charged positively from the source through Th2 and load; Th2 itself turns off at t3, the end of the charging process when the charging current decays to zero, i.e. it is load-commutated. The load current now freewheels through DFW. At t = T, the main thyristor is again triggered and the cycle repeats.
This simple chopper was extensively used at one time. It suffers from the fact that the capacitor has to be initially charged to begin the operation. Further, the voltage jumps to twice the value as soon as commutation is initiated by triggering Th2.
Figure 11.34 shows a current-commutated chopper. The main thyristor Th1 of the chopper is commutated by a current pulse generated in the commutation circuitry. The sequence of operation is given below, and various significant waveforms are shown in Fig. 11.35. It is to be noted that this charging of the capacitor C takes place from the source V through resistance R and the oscillation of the LC combination is set to be far faster than the RC charging transient.
The capacitor C is already charged via R through the source V. The main thyristor Th1 is fired at t = to. The load terminals are connected to the supply, and the load current flows through Th1. At t = t1, commutation of Th1 is initiated by firing the auxiliary thyristor Th2. An oscillatory current flows in the circuit consisting of C, Th2 and L. When ic reverses at t2, thyristor Th2 turns off (natural commutation), and the oscillatory current ic flows through D2 and Th1. At t3, ic = iTh1 and therefore zero current flows through Th1 and it turns off, and diode DI begins to conduct the current ic– ia and keeps the main thyristor Th1 reverse-biased.
At t4, ic = ia and iD1 = 0. At this point DFW is not forward-biased, the load current will flow through V, C, L, D2 and load.
At t5, the DFW becomes forward biased and starts to conduct the load current ia. The oscillatory circuit consisting of C and L is now subjected to new circuit conditions through V, C, L, D2 and DFW and ic decays to zero at t6 leaving νc. less than V. During t5 to t6, ia=ic+iFW and therefore, as ic decays, iFw builds up. From t6 onward, load current free-wheels through DFW and decays. The capacitor voltage νc also decays because of discharge through R.
At t = T, the main thyristor Th1 is fired again, and the cycle repeats.
This chopper has several advantages. Commutation is reliable as long as load current is less than the peak commutation current ic. The capacitor always remains Charged with right polarity.
In this method of commutation the load current flowing through the thyristor is made to become zero while the motor current is conducted by the free-wheeling diode. Figure 10.36(a) shows the circuit of a load-commutated chopper. The thyristor pairs Th1Th2 and Th3Th4 alternatively conduct the load current. The current conducted by any pair reduces to zero by the charging of capacitor C.
Voltage and current waveforms for the load-commutated chopper are shown in Fig. 11.36(b). It is assumed that in the cyclic operation, C is charged negatively (to that indicated in Fig. 11.32(a)) when at t = to, the thyristors Th1Th2 are fired. The voltage at motor terminals immediately jumps to 2 V. The motor circuit, because of being inductive, draws almost constant current. Capacitor C therefore gets charged positively at a uniform rate and the voltage at motor terminals falls linearly. The capacitor is fully charged positively at t = t1, the current through the conducting thyristors (Th1Th2) becomes zero and these go into the blocking mode. The motor current from this instant onwards is conducted by the free-wheeling diode DFW. At the end of one period (t =T), thyristors Th3Th4 are fired. This places the fully charged capacitor across Th1 Th2, reverse-biasing them and turning them off. The cycle now repeats.
The average value of the chopper output voltage is controlled by changing the firing frequency of the choppers. It is thus a frequency-modulated chopper. For a constant load current,
The output voltage is
where f= chopper frequency.
At maximum chopper frequency,
Then from Eq.(11.20)
The value of the capacitor is chosen for maximum load current Ia max
From Eq.(11.22a) this value is