When the speed is controlled by regulating the motor terminal voltage while maintaining constant field current, it is called voltage control.
With voltage control, the change in speed is almost proportional to the change in voltage as shown in Fig. 78. The output varies directly with speed and the torque remains constant. Since the voltage has to be regulated without affecting the field, the application of voltage control is limited to separately excited motors (Fig. 931 only.
Fig. 93. Voltage control method.
- For D.C. motors of fractional and relatively low power rating, the variable D.C. voltage source may be a D.C. vacuum tube, a gas or thyraton tube, or a semi-conductor (silicon controlled rectifier) amplifier, operating from a three-phase or single phase A.C. supply.
- Motors of moderate rating up to 75 kW may be controlled by this method using Rototrol or Regulex or magnetic amplifiers as the adjustable D.C. voltage source.
- ·Large D.C. motors are controlled in this manner by means of rotary amplifiers such as the amplidyne or the Ward-Leonard control system.
1.Speed control over a wide range is possible.
2.This method eliminates the need for series armature starting resistance.
3.Uniform acceleration can be obtained.
4.Speed regulation is good.
1.Arrangement is costly as two extra machines are required.
2.The overall efficiency of the system is low, especially at light loads.
Inspite of the high capital cost, this method finds wide applications in :
(i) Steel mills for reversing the rolling mills.
(ii) Seamless tube mills and shears.
(iii) High and medium speed elevators in tall buildings, mine hoists, paper machine drives and electric shovels.
Ward-Leonard System. This method of control not only gives a wide range of operating speeds, but reduces to the very minimum the wastage of energy that may take place at staring and stopping.
Fig. 94. Ward-Leonard method .
M = main motor whose speed is to be controlled
G = separately excited generator which feeds the armature of the motor M
E = an exciter (a small shunt generator) which provides field excitation to the generator G and motor M
M’ = driving motor-a constant speed motor which drives G and E.
[If the system is to work on A.C. supply, the driving motor M’ is a 3-phase induction motor. If the system is to work on D.C. supply, the motor M’ is a shunt motor. In the latter case the exciter E is not necessary because the excitation for the generator G and motor M can be obtained from D.C. mains. A diesel engine can also be used in place of motor M
R = a potentiometer rheostat
S = a double throw switch.
The working of this system is as follows:
- The motor M’ drives the generator G and excitor E at constant speed. The voltage fed to motor M can be controlled by varying the setting of R. A change in voltage applied to motor M changes its speed. The speed can be adjusted to any value from zero to maximum in either direction by means of a rheostat R and switch S.
- When the sliding contact of R is at extreme right, the motor is running at full speed in one direction. To decrease the speed the sliding contact is moved to the left. When the sliding contact is at the extreme left position, the speed of motor M is zero. In order to reverse the speed of the motor, the sliding contact is shifted to the extreme left, the switch S is reversed and the sliding contact shifted to right again.
- A modification of the Ward-Leonard system is known as Ward-Leonard Ilgner system, which uses a small motor-generator set with the addition of a flywheel whose function is to reduce fluctuations in the power demand from the supply circuit. When the main motor M becomes suddenly overloaded, the driving motor M’ slows down, thus allowing the inertia of the flywheel to supply a part of the overload. However, when the load is suddenly thrown off the main motor M, then M’ speeds up thereby again storing energy in the flywheel. When Ilgner system is driven by means of an A.C. motor (whether induction or synchronous) another refinement in the form of a ‘slip regulator’ can be usefully employed thus giving an additional control.
One important feature of the Ward-leonard system is its regenerative action. When a locomotive, fitted with this system, is descending a slope, it speeds up due to the action of gravity. The speed of motor M increases until its back e.m.f. exceeds the applied voltage. Motor M then runs as generator and feeds the machine G which now works as a generator and feeds electrical energy back into the trolley wire. This results in salvaging of considerable amount of energy and a superior and smooth braking action. Such an action is known as regenerative braking.
Advantages of Ward-Leonard system:
1.A wide range of speed from standstill to high speeds in either direction.
2.Rapid and instant reversal without excessively high armature currents.
3.Starting without the necessity of series armature resistances.
4.Stepless control from standstill to maximum speed in either direction.
5.Larger units employing generator field reversal eliminate the need for heavy armature conductors for reversing, and at the same time prevent motor runway since the motor field is always excited.
6.The method lends itself to adaptation of intermediate electronic, semi·conductor, and magnetic amplifiers to provide stages of amplification for an extremely large motor. Thus the power in the control circuit may be extremely small.
7.Extremely good speed regulation at any speed.
1.High initial cost.
2.Since the efficiency, neglecting the exciter efficiency, is essentially the product of the individual efficiencies of the two larger machines, the efficiency of this method is not as high as rheostat speed control or the field control method.
Example 42. A series motor drives a fan for which the torque varies as square of the speed. Its resistance between terminals is 1.2 ohm. On 220 V, it runs at 350 r.p.m. and takes 30 A. The speed is to be raised to 450 r.p.m. by increasing the voltage. Find the voltage.
Assume that flux varies directly as current.
Solution. Resistance between terminals = 1.2 ohm