Types of DC Motors
- DC shunt motor
- DC series motor
- Compound motors
- Separately excited DC motors
- Permanent magnet DC motors
DC Shunt Motor
In the DC shunt motor, the armature and field winding are connected in parallel as shown in the figure.
The field winding consists of a large number of turns of fine wire. The cross-sectional area of the wire used for field winding of shunt motor is always smaller than that of the wire used for the armature winding. Therefore, the resistance of field winding is more than that of the armature winding.
Voltage and Current Relations for DC Shunt Motor
Total current drawn from the voltage source I = Ia + Ish
Where Ia = armature current
Ish (field current) = V/Rsh
Since the applied voltage (V) and the field resistance (Rsh) are almost constant, therefore field current (Ish) remains constant. As the field current is responsible for flux generation so the flux produced in the shunt motor also remains constant. This is why shunt motor is also known as constant flux motors.
Therefore flux, φ α Ish (constant)
Supply voltage V = Eb + IaRa
Where Eb = back EMF
Ra = armature resistance.
DC Series Motor
In the DC series motor, the armature and field windings are connected in series with each other. The field winding of DC series motor consists of few turns of thick wire. Therefore, the resistance of the series field winding (Rs) is much smaller as compared to that of the armature resistance.
Voltage and Current Relations
As armature and the field winding are in series, therefore,
I = Ia = Is
Where I = total current drawn by the motor
Ia = armature current
Is = series field current
Total supply voltage V is given by,
V = Eb + Ia(Ra + Rs)
Eb = back EMF
Ia = armature current
Ra = armature resistance
Rs = series field resistance.
In the DC motors flux produced is proportional to the field current. But in the series motor, the field current is same as the armature current. Thus the armature current (Ia) and hence field current is load dependent. So with the increase in load flux also increases. Therefore, DC series motor is a variable flux motor.
DC Compound Motors
Shunt and series, both the field windings are present in compound motors. In these motors, a part of the field winding is connected across the armature and remaining field winding is connected in series with the armature. These compound motors are further subdivided into two types, namely, cumulative compound and differential compound.
Cumulative Compound Motor
In the cumulative compound motor, shunt and series field winding is connected in such a way that the direction of flow of current is same in both the field windings i.e. series field flux strengthens the field due to shunt field winding.
Differential Compound Motor
In the differential compound motor, shunt and series field winding is connected in such a way that the direction of flow of current is opposite in both the field windings i.e. series field flux weakens the field due to shunt field winding.
Separately Excited DC Motor
In these motors, the armature and field coils are fed from different supply sources. Very accurate speed control can be obtained by these motors. Moreover, these motors are best suited for the applications where speed variation is required from very low value to high value.
Permanent Magnet DC Motor
In these motors, the magnetic field is produced by small magnets. These motors are made in very small sizes and ratings. These motors are used where very small driving torque is required like toys.
Thanks for reading about types of dc motors.
DC Motors — 3 | Objective Type Question Answers
#1 For constant torque drive which speed control method is preferred ?
Armature voltage control
#2 In Ward-Leonard control the lower limit of speed is imposed by
residual magnetism of the generator
#3 The main disadvantage of the Ward Leonard control method is
all of the above
#4 The losses occurring in a D.C. generator are given below. Which loss is likely to have the least proportion ?
#5 In a D.C. generator all of the following could be the effects of iron losses except
Increase in terminal voltage
#6 The losses occurring in a D.C. generator are given below. Which loss is likely to have highest proportion at rated load of the generator ?
armature copper loss
#7 If ‘t’ be the thickness of the laminations, then eddy current loss in a generator will vary as
#8 If Bmax is the maximum flux density, then eddy current Ioss will vary as
#9 The hysteresis loss in a D.C. generator varies with the frequency of magnetic reversals as
#10 Which of the following methods of braking is used in rolling mills ?
#11 Regenerative method of braking is based on that
back e.m.f. of rotor is more than the applied voltage
#12 The retardation test is applicable to shunt motors and generators and is used to find
the stray losses
#13 Compensating winding in a D.C. machine is placed
on yoke in the pole faces
#14 Torque developed by a D.C. motor depends upon
all above factors
#15 In a manual shunt motor starter
over load relay is connected in series and no-volt relay in parallel with the load
#16 Which of the following steps is likely to result in reduction of hysteresis loss in a D.C. generator ?
Using material of low hysteresis co-efficient for armature core material
#17 Which loss in a D.C. generator does not vary with load as well as flux density ?
#18 The total losses in a well designed D.C. generator of 10 kW will be nearly
#19 The condition for maximum efficiency for a D.C. generator is
variable losses = constant losses
#20 D.C. generators are normally designed for maximum efficiency around
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