DC MACHINES

• INTRODUCTION
• DC machine is a highly versatile and flexible machine.
• It can satisfy the demands of load requiring high starting, accelerating and retarding torques.
• If the conversion is from mechanical to electrical energy, the machine is called as Generator.
• If the conversion is from electrical to mechanical energy, the machine is called as Motor.

Types of DC Machines:

• The types of DC machine depends upon the excitation of DC machine.
• The production of magnetic flux in the machine by circulating current in the field winding is called excitation.
• DC Machines can be classified according to the electrical connections of the armature winding and the field windings.

There are two methods of excitation namely, separate excitation and self-excitation.

• In separate excitation, the field coils are energised by a separate DC source. The terminals of the winding can be connected across the input voltage terminals or fed from a separate voltage source.
• In self-excitation, the current flowing through the field winding is supplied by the machine itself. The field winding can be connected either in series or in parallel with the armature winding

Separately Excited DC Machine

As the name implies, the field coils are energised by a separate DC source. The armature and field winding are electrically separate from each other. 

(a) Separately excited DC generator, and (b) Separately excited DC motor

Here, I_a = I_L , and R_a = Armature resistance.

• For Generator

E_a = V + I_aR_a

Or 

• For Motor

Armature power = P_a = E_aI_a, Output = VI_L, and Armature copper loss = I²_a R_a

Shunt Wound DC Machine

• The armature and field winding are connected in parallel.
• A machine in which the field coils are connected in parallel with the armature is called a shunt machine.
• The armature voltage and field voltage are the same.

(a) Shunt wound DC generator, and (b) Shunt wound DC motor

Characteristics Equations:

where, P_a = E_aI_a = Armature power (developed power),  I²_sh R_sh= shunt field Cu loss, I²_a R_sh = Armature Cu loss, and VI_L= Power delivered.

Series Wound DC Machine

• A DC machine in which the field coils are connected in series with the armature is called a series machine.

• The field winding carries the same current as the armature winding.

(a) DC series generator (b) DC series motor

• A series wound motor is also called a universal motor. It is universal in the sense that it will run equally well using either an ac or a dc voltage source.

Characteristics Equations:

where, P_a = E_aI_a = Armature power (developed power), VI_L = Power delivered, and  I²_se R_se= Series field Cu loss.

Compound Wound DC Machine

• A DC machine, having both shunt and series fields is called a compound machine.
• In a compound machine, the series field winding is connected in series with the armature, and the shunt field winding is connected in parallel.

Short-shunt compound DC Machine:

Here, Figure (a) is Short-shunt compound DC generator, and (b) is Short-shunt compound DC motor.

• For generator

or 

• For motor

Similarly in motor,

where, P_a = Power developed, VI_L = Power delivered, I²_se R_se= Series field Cu loss, and I²_sh R_sh= Shunt field Cu loss.

Two types of arrangements are possible in compound motors:

• Cumulative compounding : If the magnetic fluxes produced by both series and shunt field windings are in the same direction (i.e., additive), the machine is called cumulative compound.
• Differential compounding : If the two fluxes are in opposition, the machine is differential compound.

In both these types, the connection can be either short shunt or long shunt.

Long-shunt compound DC Machine:

(a) Long-shunt DC generator (b) Long-shunt DC motor

• For generator

or 

• Similarly, in motor

where, P_a = Developed power, and VI_L = Delivered power.

Speed Control of DC Motors

Speed of a DC motor can be varied by varying flux, armature resistance or applied voltage. Different speed control methods for different DC shunt and series methods are there.

Or

Speed Control of Shunt Motors:

• Flux control method
• Armature and Rheostatic control method
• Voltage control method
  1. Multiple voltage control
  2. Ward Leonard system

Speed Control of Series Motors:

• Flux control method
  1. Field diverter
  2. Armature diverter
  3. Trapped field control
  4. Paralleling field coils
• Variable Resistance in series with motor
• Series -parallel control method

Flux Control Method:

• In this flux control method, speed of the motor is inversely proportional to the flux. Thus, by decreasing flux and speed can be increased vice versa.
• To control the flux , he rheostat is added in series with the field winding will increase the speed (N), because of this flux will decrease.
• The field current is relatively small and hence I²R loss is decreased. This method is quite efficient.

Armature Control Method:

• In the armature control method, the speed of the DC motor is directly proportional to the back emf (E_b) and E_b = V- I_aR_a.
• When supply voltage (V) and armature resistance Ra are kept constant, the Speed is directly proportional to armature current (I_a).
• If we add resistance in series with the armature, the armature current (I_a) decreases and hence speed decreases.

Voltage control method:

• Multiple voltage control: In this method shunt field of motor is connected to a fixed exciting voltage, but the armature is supplied with different voltages by connecting it across one of the several voltages with the help of a switch. The intermediate speeds can be obtained by adjusting the field regulator. This method is very rarely used.
• Ward-Leonard system: This system is used where very large variation in speed is required. In this method axillary machines along with DC motor whose speed is to be varied. The motor is supplied by a generator which is driven by a motor. Very sensitive and smooth speed control can be obtained by this system. Thus this method can be used in colliery winders, electric excavators, elevators and the main drives in steel mills.