Rheostatic Control

Speed Control of D.C. Series Motor (Part2)      

2. Rheostatic Control
       In this method, a variable resistance (Rx) is inserted in series with the motor circuit. As this resistance is inserted, the voltage drop across this resistance (Ia Rx) occurs. This reduces the voltage across the armature. As speed is directly proportional to the voltage across the armature, the speed reduces. The arrangement is shown in the Fig 1(a). As entire current passes through Rx, there is large power loss. The speed-armature current characteristics with changes in Rx are shown in the Fig 1(b).
Fig. 1
Related Articles :

Speed Control of D.C. Series Motor : Flux Control

Speed Control of D.C. Series Motor (Part1)      
       The flux produced by the winding depends on the m.m.f. i.e. magnetomotive force which is the product of current and the number of turns of the winding through which current is passing. So flux can be changed either by changing the current by adding a resistance or by changing the number of turns of the winding. Let us study the various methods based on this principle.

1. Flux Control
       The various methods of flux control in a d.c. series motor are explained below :
1.1 Field Divertor Mehtod
       In this method the series field winding is shunted by a variable resistance () known as field divertor. The arrangement is shown in the Fig. 1(a).
       Due to the parallel path of Rx , by adjusting the value of Rx, any amount of current can be diverted through the divertor. Hence current through the field winding can be adjusted as per the requirement. Due to this, the flux gets controlled and hence the speed of the motor gets controlled.
       By this method the speed of the motor can be controlled above rated value. The speed armature current characteristics with range in Rx is shown in the Fig. 1(b).
Fig. 1
1.2 Armature Divertor Method
       This method is used for the motor which require constant load torque. An armature of the motor is shunted with an external variable resistance (Rx) as shown in the Fig. 2. This resistance Rx is called armature divertor.
Fig. 2  Armature divertor
       Any amount of armature current can be diverted through the divertor. Due to this , armature current reduces. But as T α Φ Ia and load torque is constant, the flux is to be increased. So current through field winding increases, so flux increases and speed of the motor reduces. The method is used to control the speed below the normal value.
1.3 Tapped Field Method
       In this method, flux change is achieved by changing the number of turns of the field winding. The field winding is provided with the taps as shown in the Fig. 3.
Fig. 3  Tapped field
       The selector switch 'S' is provided to select the number of turns (taps) as per the requirement. When the switch 'S' is in position 1 the entire filed winding is in the circuit and motor runs with normal speed. As switch is moved from position 1 to 2 and onwards, the number of turns of the field winding in the circuit decreases. Due to this m.m.f. require to produce the flux, decreases. Due to this flux produced decreases, increasing the speed of the motor above rated value. The method is often used in electric traction.
1.4 Series - Parallel Connection of Field
       In this method, the field coil is divided into various parts. These parts can then be connected in series or parallel as per the requirement. The Fig. 4(a) and (b) show the two parts of field coil connected in series and parallel.
Fig. 4  Series parallel grouping of field coils
        For the same torque, if the field coil is arranged in series or parallel, m.m.f. produced by the coils changes, hence the flux produced also changes. Hence speed can be controlled. Some fixed speeds only can be obtained by parallel grouping, the m.m.f. produced decreases, hence higher speed can be obtained by parallel grouping. The method is generally used in case of fan motors.

Applied Voltage Control method of dc motor

Speed Control of D.C. Shunt Motor (Part3)

 3. Applied Voltage Control
        Multiple voltage control : In this technique the shunt field of the motor is permanently connected to a fixed voltage supply, while the armature is supplied with various voltages by means of suitable switch gear arrangements.
       The Fig. 1 shows a control of motor by tow different working voltages which can be applied to it with the help of switch gear. 
Fig. 1  Multiple voltage control
       In large factories, various values of armature voltages and corresponding arrangement can be used to obtain the speed control.
1.1 Advantages of Applied Voltage Control
1. Gives wide range of speed control.
2. Speed control in both directions can be achieved very easily.
3. Uniform acceleration can be obtained.
1.2 Disadvantages of Applied Voltage Control
1. Arrangement is expensive as provision of various auxiliary equipments is necessary.
2. Overall efficiency is low.
General steps to solve problems on speed control
1. Identify the method of speed control i.e. in which of the motor, the external resistance is to be inserted.
2. Use the torque equation,  T α  Φ Ia to determine the new armature current according to the condition of the torque given. Load condition indicates the condition of the torque.
3. Use the speed equation N α  Eb/Φ to find the unknown back e.m.f. or field current.
4. From the term calculated above and using voltage current relationship of the motor, the value of extra resistance to be added, can be determined. The above steps may vary little bit according to the nature of the problem but are always the base of any speed control problem

Electrical Units

Unit
Symbol
General Quantities
m/s2  (metre/second/second)
a
Acceleration, linear
m2  (square metre)
A
Area
J (joule)
W
Energy or work
N (newton)
F
Force
m (meter)
l
Length
kg (kilogram)
m
Mass
W (watt)
P
Power
Pa (pascal)
p
Pressure
K or °C (Kelvin or degree Celsius)
θ
Temperature value
s (second)
t
Time
Nm (newton metre)
T
Torque
rad/s (radian/second)
ω
Velocity, angular
m/s (metre/second)
v or u
Velocity, linear
m3  (cubic metre)
V
Volume
m meter
λ
Wavelength
Unit
Symbol
Electrical Quantities
Ω(ohm)
Y
Admittance
C (coulomb)
Q
Charge (quantity)
S (siemen)
G
Conductance
A (ampere)
I
Current
A/m2  (ampere/square metre)
J
Current density
V (volts)
E
Electromotive force (emf)
Hz (hertz)
f
Frequency
Ω (ohm)
Z
Impedance
s (second)
T
Period
V (volt)
V
Potential difference (p.d.)
W (watt)
P
Power, active
VA (volt ampere)
S
Power, apparent
VAr (volt ampere reactive)
Q
Power, reactive
Ω (ohm)
X
Reactance
Ω (ohm)
R
Resistance
Ωm (ohm metre)
ρ
Resistivity
s (second)
t
Time constant
Unit
Symbol
Electrostatic Quantities
F (farad)
C
Capacitance
V/m (volt/metre)
E
Field strength
C (coulomb)
Ψ
Flux
C/m2  (coulomb/square metre)
D
Flux density
F/m (farad/metre)
ε
Permittivity, absolute
Unit
Symbol
General Quantities
 no units
εr  
Permittivity, relative
F/m (farad/metre)
   ε0  
Field strength

Unit
Symbol
Electromagnetic Quantities
At/m (ampere turn/metre)
H
Field strength
Wb (weber)
Φ
Flux
T (tesla)
B
Flux density
H (henry)
M
Inductance, mutual
H (henry)
L
Inductance, self
A/t (ampere-turn)
F
Magnetomotive force (mmf)
H/m (henry/metre)
μ
Permeability, absolute
no units
μr
Permeability, relative
H/m (henry/metre)
μ0
Permeability, of free space
At/Wb (ampere turn/weber)
S
Reluctance