speed control of dc motor

INTRODUCTION

Although a fan greater percentage of electric motors in service are a.c. motors, the d.c. motor is of considerable industrial importance. the principal advantage of d.c. motor is that its speed can be changed over a wide by a variety of simple methods. such a fine speed is generally not possible with a.c. motor. in fact, fine speed control is one the reasons for the strong competitive position of d.c. motors in the modern industrial applications. in this chapter, we shall discuss the various methods of speed control of d.c. motors.

Speed Control Of D.C. Shunt Motors

the speed of a shunt motor can be changed (1) Flux Control method (2) Armature Control Method (3) Voltage Control Method. the method ( i.e. flux control method ) is frequently used because is simple and inexpensive.

(1) Flux Control Method. it is based on the fact that by varying the flux Ø, the motor speed can be changed and hence the name control method. in this method, a variable resistance is placed in series with shunt winding as

the shunt field rheostat reduces the shunt field current I_sh and hence the flux Ø. therefore we can only raise the speed of the motor above the normal speed generally this method permits to increase the speed in the ratio 3:1. wider speed ranges tend to produce instability and poor communion.

Advantages

1. This is an easy and Convenient method
2. It is an inexpensive method since very little power is wasted in the shunt field rheostat due to relatively small value of   I_sh.
3. The speed control exercised by this method is independent of load the machine.

Disadvantages

1. Only speed higher than the normal speed can be obtained since the total field circuit resistance cannot be reduced below R_sh.- the shunt field winding resistance.
2. There is a limit to the maximum speed obtainable by this method. it is because if the flux is too much weaned, commutation becomes poorer. 

2. Armature Control method. This method is based on the fact by varying the voltage available across the armature, the back e.m.f. and hence the speed of the motor can be changed. this is done by inserting a variable resistance R_c (known as controller resistance) in series with the armature 

3. Voltage Control Method. In this method, the voltage source supplying the field current is different from that armature. this method avoids the disadvantages of poor speed regulation and low efficiency as in armature control method however it is quite expensive therefore this method of speed control is employed for large size motor where efficiency is of great importance 

• Multiple Voltage Control. In this method the shunt field of the motor is connected permanently across a field voltage source. the armature can be connected across several different voltage through a suitable switch gear. in this in this way, voltage applied across the armature can be changed the speed will be approximately proportional to the voltage applied across the armature. intermediate speed can be obtained by means of a shunt field regulator.
• Ward-Leonard System. In this method the adjustable voltage for the armature is obtained from an adjustable-voltage generator while the field circuit is supplied from a separate source. this is illustrated the armature of the shunt motor M (whose speed is to be controlled) is connected directly to a.c. generator G driver by a constant-speed a.c. motor A. the field of the shunt motor is supplied from a constant-voltage exciter E.the field of the generator G is also supplied from the exciter E. the voltage of the generator G can be varied by means of its field regulator. by reversing the field current of generator G by controller FC, the voltage applied to the motor may be reversed. sometimes a filed regulator is included in the field circuit of shunt motor M for additional speed adjustment. with this method the motor may be operated any speed upto its maximum speed 

Ward-Leonard  Speed-control System 

Advantages 

(a) The speed of the motor can be adjusted through a wide range without Resistance  losses which results in high efficiency. 

Speed Control Of D.C. Motors By Semi Conductor Devices- Semiconductor device are being used for speed control control of D.C. shunt motors as d.c. shunt motor shunt motors is operated by a.c. mains here the bridge rectifier circuit converts a.c. voltage into d.c. voltage the shunt field winding is connected directly across the d.c. line and the armature is connected in series with a thyristor (SCR)

Speed Control of D.C. Shunt Motor 

Filed Diverted Method- in this method a variable resistor is connected across the series field winding as here the variable resistor is called a diverter. by reducing the diverter resistance the field current is reduced and thus the flux is reduced which results in the reduction of speed 

Advantage. The speed of a motor can be increased above its normal speed by this method.

Field Diverter Method Of Series Motor

Armature Diverter Method- In this method a diverter is connected across the armature by decreasing the diverter resistance the armature current is reduction and thus the flux is reduced which results in the reduction of speed.

Armature Diverter Method For Series Motor

Disadvantage. The speed is lowered in comparison to the normal speed of the motor. 

Tapped Field Control Method. In this method many tapped are taken out from the series field winding as the tapping are adjusted by a handle by reducing the series field tuns the flux is reduced which results in an increase of speed.

Tapped Field Control Method For  Series Motor

Hand Starter Of a D.C. Motor. In the starting of a motor the back e.m.f. zero,it rises gradually with a rise in speed . in the absence of the back e.m.f. the motor will drew an excessive current which can burn the motor winding. therefore. it becomes necessary to control the excess flow of current. for this purpose, a variable is connected in series with the field As the motor speeds up the external resistance is gradually cut out of the circuit at full speed the whole of the variable resistor is cut out of the circuit and the induced back e.m.f. controls the armature and field current the current controlling device is termed as a hand starter 

Hand Starter With a Series  Motor- A single hand starter is connected in series with a series motor the resistance of the starter is sub-divided into a number of steps, which cut out of the circuit one after the other as the motor speed up in this way the whole of the starting resistance is cut out and the full supply voltage is applied to the motor but by this time the back e.m.f. is sufficiently high to prevent the excess flow of current. A no-volt coil. which is placed near ON position. it consists of an electromagnet which attracts the handle and holds it firmly till the supply is present. in the event of supply failure, the handle returns back to OFF position under the action of spring and the motor is switched off.

Hand Starter