DC Motor – It is a machine which convert electrical energy to mechanical energy. We use DC motors to run the machine such as lathe machines, milling machines, trains, cranes and elevator.
This was some information about DC motor. So let’s get more information about DC motor.
DC motor principle
A machine that converts DC (Direct Current) power into mechanical power is known as a DC motor. Its operation is based on the principle that when a current carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force. The direction of this force is given by Fleming’s left hand rule and the magnitude is given by
F = BIL newtons
Where, F = flux density
I = current
L = length of the conductor
Basically, there is no constructional difference between a DC motor and a DC generator. The same DC machine can be run as a generator or motor.
See also this – What is the DC generator? : Construction, Working principle, Types, EMF equation, Losses
What is the working of DC motor?
Consider a part of a multipolar DC motor as shown in figure above. When the terminals of the motor are connected to an external source of DC supply :
- Field magnets are excited to develop alternating N and S poles.
- Armature conductors carry currents. All the conductors under N-pole carry current in one direction whereas all the conductors under S-pole carry current in the opposite direction.
Suppose the conductors under N-pole carry current in the plane of paper and under S-pole carry current out of the plane of the paper. Since each armature conductor is carrying current and is placed in a magnetic field, a mechanical force acts on it.
Applying Fleming’s left hand rule, it is clear that force on each conductor is tending to rotate the armature in a anticlockwise direction. All these forces together produce a driving torque which sets the armature to rotate.
When the conductor moves from one side of the brush to the other, the current in that conductor reverse and at the same time it comes under the influence of the next pole which is of opposite polarity. As a result, the direction of force on the conductor remains the same.
See also this – What are the characteristics of DC generator?
What is back emf or counter EMF?
When the armature of a DC motor rotates under the influence of the driving torque, so the armature conductors move through the magnetic field and hence an EMF is induced in them like a generator.
The induced emf acts in the opposite direction to the applied voltage V (Lenz’s law) and is known as the back or counter emf Eb. Back e.m.f. (Eb = P Φ Z N / 60 A) is always less than the applied voltage V, although this difference is small when the motor is running under normal conditions.
Consider the shunt wound motor shown in the figure above. When DC voltage V is applied across the motor terminals, the field magnets are excited and current is supplied to the armature conductors. Hence, the driving torque acts on the armature which starts rotating.
As the armature rotates, a back emf Eb is induced which opposes the applied voltage V. The applied voltage V is to force the current through the armature against the back emf Eb.
The electric work done in overcoming and causing the current to flow against Eb is converted into mechanical energy developed in the armature.
Therefore, it follows that energy conversion in a DC motor is possible only due to the production of back emf Eb.
Net voltage in armature circuit = V – Eb
If Ra is the armature circuit resistance, then,
Ia = (V – Eb) / Ra
Since V and Ra are usually constant, the value of Eb will determine the current drawn by the motor. If the motor speed is high, then back emf (Eb = P Φ Z N / 60 A) is large and hence the motor will draw less armature current and vice versa.
See also this – What are the applications of DC generator?
Significance of back emf
The presence of back emf makes the DC motor a self-regulating machine i.e. it makes the motor to draw as much armature current as is sufficient to develop the torque required by the load.
Armature current, Ia = (V − Eb) / Ra
When motor is running at no-load, small torque is required to overcome friction and windage losses. Therefore, the armature current Ia is small and the back emf is almost equal to the applied voltage.
If the motor is suddenly loaded, the first effect is to slow down the armature. Therefore, the speed with which the armature conductors move through the field decreases and hence the back emf Eb falls. The decreased back emf allows a larger current to flow through the armature and a larger current means an increase in driving torque. Thus, the driving torque increases as the motor slows down. The motor will stop slowing down when the armature current is sufficient to produce the increased torque required by the load.
If the load on the motor is reduced, the driving torque is momentarily exceeds that required, causing the armature to accelerate. As the speed of the armature increases, the back emf Eb also increases and reduces the armature current Ia. Acceleration of the motor will stop when the armature current is sufficient to generate the low torque required by the load.
Hence, back emf in DC motor controls the flow of armature current i.e. it automatically changes the armature current to meet the load requirement.
See also this – What is a 3 phase induction motor?
Voltage equation of DC motor
Let in a DC motor
V = Applied voltage
Eb = Back emf
Ra = Armature resistance
Ia = Armature current
Since the back emf Eb acts in opposition to the applied voltage V, the net voltage in the armature circuit is V − Eb. The armature current Ia is given by
Ia = ( V − Eb) / Ra
V = Eb + Ia Ra
This is known as the voltage equation of a DC motor.
Power equation of DC motor
V = Eb + Ia Ra
Multiplying both the side of the above equation by Ia , we get
V Ia = Eb Ia + I2a Ra
This is known as the power equation of a DC motor.
V Ia = Electric power supplied to armature (armature input)
Eb Ia = Power developed by armature (armature output)
I2a Ra = Electric power wasted in armature (armature Cu loss)
Thus a small part (about 5%) of the armature input, is wasted as I2a Ra and the remaining part Eb Ia is converted into mechanical power within the armature.
See also this – What is alternator in electrical
Condition for maximum mechanical power in DC motor
The mechanical power developed by the motor is
Pm = Eb Ia
Pm = V Ia − I2a Ra
Since V and Ra are fixed, the power developed by the motor depends on the armature current. For maximum power, dPm / dIa should be zero.
Therefore, dPm / dIa = V − 2 Ia Ra = 0
Ia Ra = V / 2
V = Eb + Ia Ra = Eb + V / 2 (Because Ia Ra = V / 2)
Therefore, Eb = V / 2
Hence the mechanical power developed by the motor is maximum when the back emf is equal to half of the applied voltage.
Limitations : In practice we never aim for maximum power for the following reasons :
- Under this condition the armature current is very large–much higher than the rated current of the machine.
- Half the input power is wasted in the armature circuit. In fact, if we take into account other losses (iron and mechanical), the efficiency will be much less than 50%.
Types of DC motor with diagram
The classification of various types of DC motor and their circuit diagrams are discuss below.
Like generators, there are three types of DC motors characterized by the connection of the field winding with respect to the armature.
1. Shunt -wound motor in which the field winding is connected in parallel with the armature (See Fig-1 above). The field winding (shunt) has more number of turns with thin wires.
2. Series-wound motor in which the field winding is connected in series with the armature (See Fig-2 above). It has less number of turns with thick wires.
3. Compound-wound motor having two field windings ; One connected in parallel with the armature and the other in series with it. There are two types of compound motor connections.
- Short shunt connection
- Long shunt connection
When the shunt field winding is connected directly to the armature terminals (See figure-1 below), it is called a short shunt connection.
When the shunt field winding is connected in such a way that is shunts the series combination of armature and series field (See Fig-2 below), it is called a long shunt connection.
What is the brake horsepower? (BHP)
Horsepower developed by shaft torque is known as brake horse power (BHP). If the motor is running at N r.p.m. and the shaft torque is Tsh newton meters, then,
W. D. / revolution = force × distance moved in 1 revolution
= F × 2πr = 2π × Tsh J
W.D. / minute = 2π N Tsh J
W.D. / sec. =
=
Therefore, useful output power =
BHP =
What is the speed of DC motor?
Eb = V − Ia Ra
But, Eb = PΦZN / 60A
Therefore, PΦZN / 60A = V − Ia Ra
N =
N = (where, K = 60 A / PZ)
But, V − Ia Ra = Eb
Therefore, N =
N ∝
Therefore, in a DC motor, speed is directly proportional to back e.m.f. Eb and inversely proportional to flux per pole Φ.
What is the speed regulation of a DC motor?
The term speed regulation refers to the change in speed of a motor with change in applied load torque, other conditions remaining constant.
By change in speed here is meant the change that is due to the inherent properties of the motor under these conditions, not the changes which are affected by manipulation of rheostats or speed-controlling devices.
Speed regulation is defined as the change in speed when the load on the motor is reduced from rated value to zero, it is expressed as a percentage of the rated load speed.
% Speed regulation =
=
Where, N0 = no-load speed
N = full-load speed
What are the losses in DC motor?
The losses in a DC motor are similar to those of a DC generator. They are :
- Copper losses
- Mechanical losses
- Iron losses
As a generator, these losses cause (a) an increase in temperature and (b) a decrease in the efficiency of the DC motor.
What is the efficiency of a DC motor?
Like a DC generator, the efficiency of a DC motor is the ratio of output power to input power, i.e.
Efficiency, η =
For a generator, the efficiency of a DC motor will be maximum when : Variable losses = Constant losses.
What is the power stages of DC motor?
The various stages of energy transformation in a motor and the various losses occurring in it are show in the flow diagram of figure below.
It is seen that A−B = Copper losses and B−C = Iron and friction losses.
Overall or commercial efficiency, ηc = C/A
Electrical efficiency, ηe = B/A
Mechanical efficiency =, ηm = C/B