In this topic, you study Speed Control of DC Motor.
From the relationship between the back emf, speed, flux, etc. given by the Equation (2.3),
This is because for a particular machine Z, A and P are fixed Also from the Equation (2.2),
Resistance of the armature is normally very small. Therefore, the value of IaRa is also small in comparison with the voltage applied across the armature. If this voltage drop in the armature resistance is neglected, then,
Fig. 2.21 : Variation of speed with the armature voltage and flux per pole
The above expression shows that the speed of a dc motor is approximately proportional to the voltage applied across the armature and inversely proportional to the flux per pole as illustrated graphically in Fig. 2.21. This gives two basic methods for varying the speed of a dc motor, namely
(i) Armature voltage control,
(ii) Flux control.
These methods as applied to dc shunt and series motors are discussed below. These methods are also applicable for compound motors.
(a) Rheostatic Control : Since the supply voltage is normally constant, the voltage
applied across the armature is changed by simply connecting a variable resistance (R) called
controller in series with the armature as shown in Fig. 2.22 (a). As this external resistance is
increased, the voltage applied across the armature decreases and the speed falls. By changing
the value of the resistance, different motor speeds can be obtained (Fig. 2.22 b). This
rheostatic method of changing the armature voltage for speed control purposes has following
merits and demerits :
Fig. 2.22. Rheostatic control for a dc shunt motor
Merits: Simple method to obtain any speed below normal down to crawling. Normal speed is the speed which is obtained when rated voltage is applied across the armature and full excitation is provided to the field winding.
Demerits:
(i) This method cannot give speeds above normal as controller can only decrease the armature voltage.
(ii) Considerable amount of power is wasted in the controller which lowers the efficiency of the motor.
(iii) This method needs costly controller with proper heat dissipation arrangement.
(iv) With the controller in the circuit, speed of the motor varies greatly with the variationin load (Fig. 2.22 b). Thus the motor has poor speed regulation.
Fig. 2.23: Rheostatic control for a dc shunt motor with shunted armature
Because of the above drawbacks, this method is used in the applications where slow speeds are required for a short period only and where efficiency is of secondary importance.g. hoists, cranes, trams, printing machines, etc. If a diverter R’ (variable resistance) is placed across the armature in addition to the series resistance (Fig. 2.23), the rheostatic control gives more stable operation. This is because under this condition, a change in armature current (due to alteration in load torque) is not so effective in changing the voltage drop across the armature and hence the speed of the motor. This gives better speed regulation particularly in the low speed range.
Multiple Voltage Control:
In this method, the excitation is held constant by permanently connecting the shunt field of the motor across a definite voltage, but the armature is supplied with different voltages by means of suitable switch gear. The speeds are then approximately proportional to these voltages. Fig. 2.24 (a) illustrates the method when the motor is fed from a 3-wire system and Fig. 2.24 (b) shows the speed characteristics corresponding to two working voltages that are available. In large factories, special 3 or4 wire systems giving a number of graded values for the armature voltage are sometimes employed for the purpose of speed control. Since this method of speed control involves considerable investment on auxiliary equipments, it is used only where a wide range of speed is necessary, as in some machine-tool operations.
Fig. 2.24 : Multiple voltage control for a dc shunt motor fed from a 3-wire system
(c) Series-Parallel Control: Even though this method is more commonly used for dc series motors (in traction), it may also be used for controlling the speed of the dc shunt motors. In this method, two identical motors are mechanically coupled to a common load.
When the motor armatures are connected in series as shown in Fig. 2.25 (a), each motor armature receives one-half the normal supply voltage i.e. 2 . On the other hand, when they are connected in parallel as shown in Fig. 2.25 (b), the terminal voltage across each motor armature rises to the normal value of the supply voltage (i.e. V). Thus two operating characteristics are possible with this method (Fig. 2.25 c). The voltage applied across the field winding is maintained to the same value during both series and parallel working. For that the fields may be connected in parallel across the full voltage or in series as shown in the figure. So far as efficiency is concerned, this method is superior to the rheostatic control.
However, only two operating speeds are possible.
Fig. 2.25. Series-parallel control for dc shunt motors
(d) Ward-Leonard System: Fig. 2.26 shows the complete scheme of connections used in Ward-Leonard system. Ml is the main motor whose speed is to be controlled. The field winding of this motor is permanently connected across the supply mains and its armature is provided with the variable voltage for the purpose of speed control using a separate generator this generator is driven at constant speed by a dc motor M2 as shown or any suitable a.c. motor can be used for this purpose. The output voltage of the generator (which is fed to the armature of the main motor) is varied from nearly zero to a certain maximum value by means of its field regulator. A reversing switch is also included in the field circuit of the generator to reverse the direction of its field current and thereby to change the direction of rotation of the main motor.
Fig. 2.26 : Ward-Leonard system
The principal merits and demerits of the Ward-Leonard system are listed below :
Merits :
(i) This is one of the most precise methods of speed control with a wide speed range in either direction of rotation.
(ii) No special starting gear is required and speed reversal can be carried out very smoothly and with ease. As such, the method is ideal for the applications requiring frequent starting, stopping, reversals and smooth acceleration.
Demerits :
(i) Two extra machines are necessary. This makes the system most expensive.
(ii) The absence of external resistance in the armature circuit of the motor under control considerably improves the efficiency. However, still the overall efficiency is low, particularly at light loads.Even though costly, this type of control is extensively used for rolling mill motors, colliery winding motors, elevators, hoists, large shears, paper mills, electric shovels, etc.Flux Control: A wide variation in speed can be obtained by changing the flux per pole of the dc motor. The flux per pole depends upon the value of the field current. Therefore,to change the field current, a variable resistance (R) termed as field regulator is connected in series with the field winding of a dc shunt motor as illustrated in Fig. 2.27 (a). When this resistance is increased, the field current and the flux are reduced. Consequently, the speed increases and vice versa (Fig. 2.27 b).
Fig. 2.27 : Flux control of a dc shunt motor
Some principal merits and demerits of this method of speed control are given below :
Merits :
(i) Speeds above normal can be obtained with ease.
(ii) Because of small current, power loss involved in the external resistor is also comparatively small. Hence, the method is more convenient, economical and efficient.
(iii) For any particular speed adjustment, speed regulation is excellent (Fig. 2.27 c).
Demerits :
(i) This method cannot give speeds below normal as field can only be weakened and can not be made stronger.
(ii) For motors requiring a wide range of speed control, weakening of flux for high speeds increases sparking at the brushes and hence puts a limit to the maximum speed obtainable with this method. Therefore with non-interpolar machines, speed range (the ratio of maximum to minimum speeds) is limited to about 2 : l, whereas in motors fitted with interpoles, it can be as great as 6 : 1.
In actual practice, by combining the above two methods i.e. armature voltage control and flux control, speed above and below the normal may be obtained.
Speed Control of Series Motors
Armature Voltage Control: As in the case of a dc shunt motor, following schemes are commonly used for changing the voltage applied across the armature terminals of a dc series motor for speed control purposes
(a) Rheostatic Method : In this method, a variable resistance R (called controller) is inserted in series with the motor circuit as shown in Fig. 2.28 (a). Increase in this external resistance reduces the voltage across the motor terminals (hence across the armature terminals) and consequently the speed drops. Fig. 2.28 (b) shows the operating characteristics of a motor with and without such series resistance in the circuit. The merits and demerits of this method are same as those mentioned earlier in the case of rheostatic control for a dc shunt motor. An improved performance can be obtained by using a diverter R’ across the armature as shown in Fig. 2.29. Since now the current taken from the mains is shared by the diverter and the armature, the current through the armature is reduced. But for a given load torque, when la reduces, (t) must increase T • la). Hence the motor draws more current from supply mains and speed falls. This scheme is, theref0R, very helpful in getting slow speeds at light loads and no-load speed of finite and reasonable value.
Fig. 2.28 : Rheostatic control for a dc series motor
Fig. 2.29 : Rheostatic control for a dc series motor with shunted armature
(b) Series-Parallel Control: Figs. 2.30 (a) and (b) show the scheme of connections for series-parallel control for two identical mechanically coupled dc series motors. With the series connection of the motors, speed is lower because the voltage applied across the terminals of each motor is one-half the supply voltage i.e.. On the other hand, parallel connection of motors gives higher speed as voltage applied across each motor gets doubled. Thus two operating characteristics are possible using this method (Fig. 2.30 c). This method of speed control is widely used in electric traction.
Fig. 2.30 : Series-parallel control for dc series motors
(c) Variable-Voltage Control : A modified form of the conventional Ward-Leonard system of speed control as shown in Fig. 2.31 may be used for a series motor. Variation of the speed of the series motor is achieved with the help of a field-diverter of a series generator. This generator is driven at constant speed with the help of a suitable a.c. or dc motor. The usual range of speed with this arrangement is about 10: l.
Fig. 231 : Variable-voltage control for a series motor
Flux Control : In a series motor, variation in flux for speed control purposes can beachieved by using any one of the following methods :
(a) Diverter Control : A change in field current and hence in the flux can be achieved by placing a variable resistance R called diverter in parallel with the field winding of a series motor as shown in Fig. 2.32 (a). By adjusting this resistance, any desired portion of the current can be diverted from the field and thereby the speed can be increased above normal
(Fig. 2.32 b).
Fig. 2.32 : Diverter field control
(b) Tapped-Field Control : In this method, variation in flux is achieved by changing the number of turns of the field winding. For that number of taps are provided on the field winding as shown in Fig. 2.33 (a). With full field winding in the circuit, the motor runs at its minimum speed. For higher speeds, some of the series turns are cut out in steps. Fig. 2.33 (b)shows the operating characteristics with different number of series turns in the circuit. This method is often used in electric traction.
Fig. 2.33 : Tapped-field control
(c) Series-Parallel Connections of Field Coils: In this method, field m.m.f. and hence the flux is varied by series-parallel grouping of field coils and thereby several fixed speeds are obtained. If the field winding is divided into two equal halves, then by connecting these halves in series or in parallel as illustrated in Figs. 2.34 (a) and (b), two operating characteristics can be obtained (Fig. 2.34 c). Obviously, parallel field connection gives the higher speed for the same torque. This method is usually employed in the case of fan motors.
The various limitations of flux control method discussed earlier for shunt motors also stand in all the above cases of flux control for series motors. In actual practice, as said earlier, combination of armature voltage control and flux control is normally employed for speed variation over a wide range.
Fig. 2.34 : Flux control by series-parallel connections of field coils
REVERSAL OF ROTATION IN DC MOTORS
We know that in a dc motor, every current carrying conductor of the armature being in the magnetic field produced by the stationary field poles, experiences a mechanical force which ultimately results into rotation of the armature in a particular direction (Fig. 2.6). Now, we know that the direction of the force experienced by the current carrying conductor placed in the magnetic field depends upon the direction of field and the directionof the current flow in the conductor. From this, it follows that the direction of the force experienced by each conductor of the armature and therefore the direction of rotation of a motor depends on the direction of the field and the direction of the current flow in the armature.
Fig. 2.19 : Reversal of direction of rotation of a dc shunt motor by
(a) Reversal of field connections, (b) Reversal of armature connections
Fig. 2.20 : Reversal of direction of rotation of a dc series motor by
(a) Reversal of field connections, (b) Reversal of armature connections. The direction of the field in turn is decided by the direction of the field current. Therefore, if either the direction of the field current (If) or the direction of armature current (la) is reversed, the rotation of the motor will reverse. However, if both of the above two factors are reversed at the same time, there will be no effect on the direction of rotation. Thus, whenever it becomes necessary to change the direction of rotation of a dc motor, this can be achieved simply by reversing the connections of either the armature or the field(but not both) as illustrated in Fig. 2.19 and Fig. 2.20 for dc shunt and series motors respectively.