When it is desired to have wide and very sensitive speed control then this system is more generally used. The system is as shown in the Fig. 1.
 |
| Fig. 1 Ward - Leonard speed control method |
As shown in the Fig. 1 M1 is the main motor whose speed control is required. The field winding of this motor is permanently connected to d.c. supply whereas armature is supplied with variable voltage so that motor can run at any desired speed. To provide this variable voltage, motor generator set is used. It consists of either a.c. or d.c. motor directly coupled to a generator. This motor runs at an approximately constant speed.
The output of generator G is fed to motor M1. The field circuit of this generator is separately excited from the available d.c. supply through a reversing switch and a potential divider so that its excitation can be varied from zero to maximum value. By reversing the direction of the field current of G with the help of reversing switch, polarity of the generated voltage will be reversed and thus change in direction of motor M1 also will be achieved.
As this method can give unlimited speed control in either directions, this system is commonly employed for elevators, hoists and main drive in steel mills. Also this system is ideal in applications where frequent starting, stopping and reversals are required. As the generator voltage can be raised gradually from zero, the motor starts up smoothly without any extra starting equipment. Although this system is advantageous as it is giving wide range of speeds it requires two extra machines which involves high capital cost.
In modern days SCRs are used for obtaining variable d.c. voltage which will take power from a.c. mains through a transformer. Though it is not less expensive, the arrangement is neat and free from maintenance problems. It will give automatic control of speed.
The modified Ward-Leonard system is called Ward - Leonard - IIgner system in which a flywheel is used in addition to motor generator set. It is used to reduce the fluctuations in power demand from the supply. When load on main motor is suddenly increased, the driving motor M2 from motor generator set slows down. Thus inertia of flywheel is used to supply part of the overload. However when load is suddenly decreased from motor M1 then motor M2 from set speeds up which allows energy to store in the flywheel.
Ratings of a D.C. Motor
Necessity of Starter for D.C. Motors
Hopkinson's Test
Related Articles :
Torque Equation of a D.c. MotorRatings of a D.C. Motor
Necessity of Starter for D.C. Motors
Hopkinson's Test
Sponsored links :