During the nineteenth century, when power supply was dc, dc motors were used extensively to draw power direct from the dc source. The motor speed could be varied by adjusting field current by a rheostat. That was an open loop control. Most of the drives were constant speed and the characteristics could not be matched with a job requirement. A vast development in the dc drives system took place when the ward Leonard Control System was introduced in the 1980s. The system was motor-generator system to deliver power to the drive motor.
The supply power available was still dc and dc motor was used to drive the dc generator set at a more or less constant speed. Afterwards when the ac power system came into existence and became popular, ac motors were developed and became attractive owing to their constructional simplicity, ruggedness and lower initial as well as maintenance cost. Machine requiring variable speed drives use the ward Leonard System employing ac motors driving dc motors at a constant speed. In the 1950s electronic came into existence and brought about remarkable improvement in the speed control system.
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The open- loop manual control system was replaced by close loop feedback control, which resulted in improved response and better accuracy. Initially, gas diodes and ignitrons were developed and ac to dc converters were used to control dc motors. The advent of thyristors capable of handling large current has revolutionized the field of electric power control. Thyratrons, ignitrons, mercury arc rectifiers, magnetic amplifiers and motor generator sets have all been replaced by solid state circuits employing semi-conductor diodes and thyristors.
Thyristor controlled drives employing both ac and dc motors find wide applications in industry as variable speed drives. In the 1960s ac power was converted into dc power for direct control of drive motors with solid state devices (high power silicon diodes and silicon controlled rectifiers). Initially saturable reactors were employed in conjunction with power silicon rectifiers for dc drives. Of late solid state circuits using semi- conductor diodes and thyristors are becoming popular for controlling the speed of ac and dc otors and are progressively replacing the traditional electric power control circuit based on thyratrons, ignitrons, mercury arc rectifiers, magnetic amplifiers, motor-generator sets, etc as compared to the electric and electro-mechanical systems of speed control. The electronic system has higher accuracy, greater reliability, and quick response and also has higher efficiency as there is no I2R losses and moving parts. Moreover four-quadrant speed control is possible to meet precise high standards. All electronic circuits control the speed of the motor by controlling either:
- The voltage applied to the motor armature or
- The field current or
- Both of the above DC motors can be run from dc supply if available or from ac supply, after it has been converted to dc supply with the help of rectifiers which can be either half wave or full wave and either controlled ( by varying the conduction angle of the thyristors used) or uncontrolled.
AC motors can be run on the ac supply or from dc supply, after it has been converted into ac supply with the help of inverters (opposite of rectifiers).
As stated above, the average output voltage of a thyristors controlled rectifiers by changing its conduction angle and hence the armature voltage can be adjusted to control its speed. When run on a dc supply, the armature dc voltage can be changed with the help of uncontrolled rectifiers (using only diodes and not thyristors). The dc voltages so obtained can be then chopped with the help of a thyristors chopper circuit. In this method of speed control of a dc motor, available ac supply is first rectified into dc supply using uncontrolled rectifiers. The supply is then filtered and smooth ended dc output is supplied to the thyristors chopper.
It allows dc to flow through for the time Ton and then disconnects for the time Toff. This cycle is repeated. During supply-on period (i. e. for the time period Ton) the dc motor gets supply and accelerates. During the supply off period Toff (i. e. for the time period Toff) there is no supply to the motor and the motor decelerates till the next on cycle begins. If the cycles repeated continuously at a definite frequency and the elements of the cycle are maintained in a fixed relationship, the motor will then operate at a constant voltage across the motor will be.
The dc voltage across the motor can be control by varying the Time Ratio Control (TRC) which may be accomplished by:
- Varying the duration of the on-time, Ton keeping the total time period, T or frequency, f constant
- Keeping the on- time, Ton constant and varying the frequency, f.
- Varying both.
The Variable dc voltage below the supply dc voltage is made available to the dc motor and therefore, the motor speed available is below base speed.
For automatic control of speed, both current feedback and speed feedback is used.
Brief Discussion On Chopper
A dc chopper is a static device used to obtain variable dc voltage from a source of constant dc voltage. The dc chopper offers great efficiency, faster response, smooth control, lower maintenance, small size, etc. Solid state chopper due to various advantages are widely used in the battery operated vehicles, traction motor control, control of a large number of dc motors from a common dc bus with a considerable improvement of power factor.
Principle Of Chopper Operation
A chopper is a thyristors on/ off switch that connects load to and disconnects it from the supply and produces a chopped load voltage from a constant input voltage. The chopper is represented by a thyristors (SCR). It is triggered periodically and is kept conducting for a period Ton and is blocked for a period Toff. During the period Ton, when the chopper is on, the supply terminals is connected to the load terminals. And during the interval Toff when the chopper is off, load current flows from the freewheeling diode Df.
So, the load terminals are short circuited by Df and load voltage is therefore zero during Toff. Hence the chopper dc voltage is produced at the load terminals. Now, the average load voltage, Eo is given by Eo = Edc*? [? =Duty Cycle=(Ton/Toff)] Or, Eo = Edc*(Ton/T) [T=Ton + Toff] So the voltage can be varied by varying the duty cycle, ? of the chopper. CLASSIFICATION OF CHOPPER: Power semiconductor devices are used in chopper circuits are uni-directional device. A chopper can however operate in any of the four quadrants by an appropriate arrangement of semiconductor devices.
These characteristics of their operation in any of the four quadrants form the basis of their classification as:
- Type-A or First Quadrant Chopper
- Type-B or Second Quadrant Chopper.
- Type-C or Two Quadrant Type-A Chopper
- Type-D or Two Quadrant or Type-B Chopper
- Type-E or Four Quadrant Chopper.
Closed Loop Control Of Chopper Fed DC Motor
For practical purposes motors are required to operate at desired speed with low losses to meet the desired load –torque characteristics which depends on the armature current. Suppose a motor is operating at a particular speed an suddenly a load is applied, the speed falls and the motors takes time to come up to the desired speed . but a speed feed back with an inner current loop provides faster response to any disturbance in speed command ,load torque and supply voltage.
Another reason for the requirement of feedback loop in dc drives is that, the armature of a large motor represents very small impedance which when supplied with nominal voltage would result in an excessive current of up to 10 times the nominal value. Under normal conditions, this is prevented by the induced armature voltage, E which cancels most of the applied voltage, Va so that only the difference is driving the armature current, Ia. But under transient conditions or steady state over load of the motor, there is always a danger of excessive currents due to sudden torque demand and rapidly changing armature voltage or speed . t is therefore important to provide a fast current or torque limit to protect the motor, the power supply and the load. This is best realized by feedback control establishing an effective safe guard against electrical and mechanical stresses. In it the output of the speed controller, Ec is applied to the current limiter which sets the reference current, Ia (reference) for the current loop. the armature current is sensed by a current sensor, after being filtered by an active filter to remove ripples which is then compared with the reference current, Ia (ref. the error current is processed through a current controller whose output, Vc adjusts the firing angle of the chopper and brings the motor speed to the desired value. Any positive speed error caused by an increased in either speed command or load torque demand can produce a high reference current, Ia (ref) the motor accelerates to correct the speed error and finally settles down at any reference current, Ia(ref) which makes the motor torque equal to the load torque resultant in a speed error closed to zero.
For any large positive speed error, current limiter saturates and limits the reference currents, Ia (ref) to a maximum value, Ia (max) the speed error is then corrected at the maximum permissible armature current ,Ia(max)until the speed error becomes small and the current limiters comes out of the saturation . normally ,the speed error is corrected with the Ia less than the maximum permissible armature current, Ia max. For speeds below the base speeds, the field error, Ef is large and the field controller saturates thereby applying the maximum ield voltage and current. The speed control from zero to base speed is normally done at the maximum field by armature voltage control. When the speed is closed to the base speed, Va is almost near the rated value and field controller comes out of saturation. The speed control above base speed is generally done by field weakening at the rated armature voltage. In the field control loop, the back emf Eb is compared with a reference voltage , Eb (ref) the value of which is generally between 0. 85 to 0. 95 of the rated armature voltage.
For a speed command above the base speed, the speed error causes a higher value of Va then motor accelerates, back emf , Eb increases and field error, Ef decreases. The field current when decreases and the motor speed continue to increase until it reaches the desired speed. In this mode of operation, the drive responds slowly due to large field time constant. A full converter is generally used in the field because it has the ability to reverse the voltage thereby reducing the field current much faster as compared to the semi converter.
Industrial Applications DC
Drives are highly versatile energy conversion devices. It can meet the demand of loads requiring high starting, accelerating and decelerating torques. At the same time dc drives are easily adaptable for wide range of speed control and quick reversal. So, in industrial application where accurate control of speed and / or torque is required chopper controlled dc drives are unrivalled.
Therefore, chopper controlled dc motors are universally employed in steel and aluminum mills, power shovels, electric elevators, railway locomotives and large earth moving equipments.
Scope Of Modifications
Chopper controlled dc drives are widely used in hoists, cranes, elevators, shears, crushers, conveyor, blending mills, punch presses, air compressors, ice making machines, tractions, etc.
So these drives should be modified in the following ways to make them more efficient and accurate:
- The chopper controlled dc motors should be made with large diameter armatures and large pole size of reduced height.
- The yoke as well as the main and commutating poles should be well laminated to reduce the eddy current effect and to improve the commutation.
- Large numbers of commutator bars should be used to reduce the voltage between the commutator segments and to improve the commutation.
- The commutator should be made larger in order to provide extra insulation to withstand large and rapid voltage fluctuations.
- Compensating windings should be used in large motors to reduce the armature reactions effects.
- The current densities used for the armature and Interpol windings should be reduced as compared to the conventional dc motors of the same frame size and rating in order to reduce the effect of heating of armature and Interpol.
- Low inertia armature should be employed for improving the response.
- Split brushes of good commutating quality should be used for reducing the effect of transformer voltage in the coil undergoing commutations.
- Better class of insulation should be used to allow higher temperature rise and dissipation of more losses from a given frame.
- Now a days chopper controlled dc drives are widely used in the automobile industries. So, it should have high efficiency and accuracy, light weight, low maintenance cost.
- Electric Drives – Ramakrishnan, Prentice Hall India.
- Power Electronics – P. S. Bimbhra, Khanna Publishers.
- Software: MATLAB 6. 5 and PSIM.
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