Speed control Of DC Machines
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The document discusses three methods for controlling the speed of a DC motor: 1) varying the armature circuit resistance, 2) varying the field flux, and 3) varying the terminal voltage using the Ward-Leonard method. It provides details on how each method works, including equations showing how speed is affected. The Ward-Leonard method uses a motor-generator set to convert AC power to DC power at a controlled voltage to regulate motor speed. Advantages and disadvantages of each method are also summarized.
Speed control Of DC Machines
- 2. The term ‘speed control’ stands for intentional speed variation, carried out manually or automatically. The speed of a DC motor is given by:- (Eb= Ka.Φ.ωm1) From the above eqn. we can make out that there are basically three methods of speed control and these are:- Φ − = a aat m K RIV 1ω
- 3. The various methods for controlling the speed of DC motor are:- a)Speed control by varying armature circuit resistance. b)Speed control by varying the field flux. c)Speed control by varying the terminal voltage (“Ward-Leonard Method”).
- 4. Speed control by varying armature ckt resistance:- The scheme for the connection of a shunt motor is illustrated in the fig shown, where ‘Rg’ is the controlling resistance and is connected in series with the armature circuit. When ‘Rg’ is not present then the Armature current Ia is given by:- -----Eqn (1) (Eb=KaΦωm1) a mat R KV Ia 1 1 φω− =
- 5. When Rg is inserted in the armature circuit and if it is assumed that there is no change in speed for the time being, then:- -----Eqn(2) In shunt motor field flux Φ remains unchanged therefore with the reduction in armature current, torque,T(α Φ .Ia) decreases and hence the back emf also decreases . From the above eqn(1). ga a ga mat RR R Ia RR KV Ia + = + − = 1 1 2 φω φφ ω a a a t m K E K RaIaV 11 1 = − =
- 6. When new steady state condition is reached, with Rg in the armature circuit then from Eqn(2):- Therefore --Eqn(a) From the above equation (a) we can easily make out that the speed has decreased from its previous value. φφ ω a a a gat m K E K RRIaV 21 2 )( = +− = aat gaat a a m m RIV RRIV E E N N 1 1 1 2 2 1 1 2 )( − +− === ω ω
- 7. Hence we find that as we go on increasing the external resistance the speed-torque curve will vary as shown.
- 8. Advantages:- The principal advantage of this method is that speed below base speed down to the creeping speed are easily obtainable. Disadvantages:- Lower Efficiency and higher operational cost at reduced speeds. Poor speed regulation with fixed controller resistance ‘Rg’ in the armature circuit.
- 9. Speed Control by Varying the field flux:- The field flux and hence the speed can be controlled easily by varying the field regulating resistance. This is one of the simplest and economical methods and is used extensively in modern electric drives.
- 10. Under steady running conditions, if field circuit resistance is increased, the field current If & the field flux Φ are reduced. Decrease in the field flux causes a reduction of counter emf. As a result more current flows through the armature. In view of this the electromagnetic torque is increased & became more then the load torque hence the motor gets accelerated.
- 11. If armature current is Ia1 for flux Φ1 and Ia2 for flux Φ2, then for a constant load torque:- The above two eqn’s describe the change in speed and armature current as the field flux is varied. 2 2 2 2 2 1 1 1 1 1 )( )( φ ω φ φ ω φ a aat a Le a aat a Le K RIV K TT Ia K RIV K TT Ia − =⇒ = = − =⇒ = =
- 12. WARD-LEONARD SYSTEM In the fig. shown, ‘M’ is a separately excited DC Motor whose speed is to be controlled and G is separately excited generator driven by a 3-phase (Induction Motor). The combination of AC motor & DC generator is called Motor-Generator set & It converts AC to DC which is fed to main motor ‘M’.
- 13. For starting the motor ‘M’, its field winding is first energized and then the generator output voltage is adjusted to a low value by decreasing its field excitation. This is done in order to limit the starting current to a safe value but it should be ensured that enough starting torque is produced to accelerate the Motor & load.
- 14. In view of this, no starting rheostats are necessary and therefore, considerable amount of energy is saved during starting. A change in the generator field current varies the voltage applied to the motor armature and therefore the motor speed is changed. Thus the motor speed control is obtained merely by changing the generator field current.
- 15. Speed from the lowest possible speed up to the base speed are obtained by increasing the generator output voltage with constant motor flux. Since speed control is carried out at rated current ‘Ia’ and with constant motor field flux ‘Φ’, a constant torque (Φ. Ia) up to the base speed is obtained. Power(Torque*Speed) increases in proportion to speed. Thus a constant torque and variable power drive is obtained up to the base speed with “armature voltage control method”.
- 16. Speed above base speed are obtained by decreasing the motor field flux with constant generator voltage. As before the armature current ‘Ia’ is kept equal to its rated value, under these conditions, Vt.Ia or Ea.Ia remains constant and electromagnetic torque proportional to (Φ. Ia) decreases as the flux is decreased Thus weakening of the motor field flux results in constant power.
- 17. The combined graph of both the methods of speed control is shown as below:-
- 18. Advantages:- It provides wide range and smooth speed control. The direction of main motor rotation can be changed merely by reversing the generator field current. The efficiency at low speeds is higher than that obtained by other methods of speed control. Disadvantage:- The only disadvantage of this method is its higher initial cost, because 3 machines of equal ratings are required.