Answer MET Question 40

Question: Write shorts notes on various types of DC Motors.

Answer: There are three type of DC motors

a. Separately excited

b. Permanent magnet and

c. Self-excited DC motor.

The  Self-excited DC motor is of three types:

a. Shunt excited

b. Series excited and

c. Compound excited DC motor.

The Compound excited DC motor is of two types:

a. Cumulative compound - (long and short shunt type)

b. Differential compound - (long and short shunt type)

Separately Excited DC Motor

  In case of a separately excited DC motor the supply is given separately to the field and armature windings. In these types of DC motors, the armature current does not flow through the field windings, as the field winding is energized from a separate external source of DC current.

  From the torque equation of DC motor we know $\displaystyle \small \mathrm{T\ \ \alpha \ \phi I_a}$

So the torque in this case can be varied by varying field flux ф, independent of the armature current Ia.

Permanent Magnet DC Motor

 The permanent magnet DC motor (also known as a PMDC motor) consists of an armature winding. A radially magnetized permanent magnets are mounted on the inner periphery of the stator core to produce the field flux. The rotor on the other hand has a conventional DC armature with commutator segments and brushes.

 The torque equation of DC motor suggests Here ф is always constant, as permanent magnets of required flux density are chosen at the time of construction and can’t be changed there after. In this case, the torque of DC Motor can only be changed by controlling the armature supply.

Self Excited DC Motor

 In case of self excited DC motor, the field winding is connected either in series or in parallel or partly in series, partly in parallel to the armature winding. Based on this, self excited DC Motors can be classified as:

Shunt wound DC motor

Series wound DC motor

Compound wound DC motor

Shunt Wound DC Motor

 In case of a shunt wound DC motor, the field windings are exposed to the entire terminal voltage as they are connected in parallel to the armature winding.

 The basic voltage equation given by,

$\displaystyle \small \mathrm{E_b = V -I_aR_a}$

[Where, V, Eb, Ia, Ra are the supply voltage, back emf, armature current and armature resistance respectively]

To calculate applied voltage, following realtion can be used:

$\displaystyle \small \mathrm{V = I_{sh}R_{sh} }$

Also

$\displaystyle \small \mathrm{E_b\ \ \alpha\ \ N\phi }$

or

$\displaystyle \small \mathrm{N\ \ \alpha \ \ \frac{E_b}{\phi } = \frac{V-I_aR_a}{\phi} }$

The shunt wound DC motor is a constant speed motor, as the speed does not vary here with the variation of mechanical load on the output.

This motor is also called constant flux motor so the torque will be:
$\displaystyle \small \mathrm{T\ \ \alpha \  I_a}$

Series Wound DC Motor

In case of a series wound self excited DC motor or simply series wound DC motor, the entire armature current flows through the field winding as its connected in series to the armature winding.

$\displaystyle \small \mathrm{N\ \ \alpha \ \ \frac{E_b}{\phi } = \frac{V-I_a(R_a+R_{se})}{\phi} }$

In a series wound DC motor, the speed varies with load. And operation wise this is its main difference from a shunt wound DC motor.

The flux in this motor is proportional to armature current :$\displaystyle \small \mathrm{\phi\ \alpha\ I_a}$

Thus, $\displaystyle \small \mathrm{T\ \alpha\ I_a^2}$

These motors must never run No-Load, the speed will rise very high and mechanical damage will occur.

Compound Wound DC Motor

The compound excitation characteristic in a DC motor can be obtained by combining the operational characteristic of both the shunt and series excited DC motor. The compound wound self excited DC motor or simply compound wound DC motor essentially contains the field winding connected both in series and in parallel to the armature winding.

The excitation of compound wound DC motor can be of two types depending on the nature of compounding.

Cumulative Compound DC Motor

When the shunt field flux assists the main field flux, produced by the main field connected in series to the armature winding then its called cumulative compound DC motor. $\displaystyle \small \mathrm{\phi\ =\ \phi _{sh} +\phi _{se} }$

Differential Compound DC Motor

In case of a differentially compounded self excited DC motor i.e. differential compound DC motor, the arrangement of shunt and series winding is such that the field flux produced by the shunt field winding diminishes the effect of flux by the main series field winding. The net flux produced in this case is lesser than the original flux and hence does not find much of a practical application.

$\displaystyle \small \mathrm{\phi\ =\ \phi _{se} -\phi _{sh} }$

 

Both the cumulative compound and differential compound DC motor can either be of short shunt or long shunt type depending on the nature of arrangement. Short Shunt DC Motor

If the shunt field winding is only parallel to the armature winding and not the series field winding then its known as short shunt DC motor or more specifically short shunt type compound wound DC motor.

$\displaystyle \small \mathrm{E_b=V-I_LR_{se}-I_aR_a}$

$\displaystyle \small \mathrm{I_{sh}R_{sh}=V-I_LR_{se}}$

Long Shunt DC Motor

If the shunt field winding is parallel to both the armature winding and the series field winding then it’s known as long shunt type compounded wound DC motor or simply long shunt DC motor.

$\displaystyle \small \mathrm{E_b =V-I_a(R_a+R{se})}$

$\displaystyle \small \mathrm{V=I_{sh}R_{sh}}$