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DC Motor

Energy can be transformed from one state to another. A motor is a device which transforms electrical energy into mechanical. It depends on the operation method. The conductor carrying current is known to be situated in the magnetic field.

Construction:

The configuration of a DC motor is similar to that of DC generator. When the field winding and armature winding of a DC generator is connected to a direct current source, any DC generator will function as a DC motor. The necessary magnetic field is produced by the field current, and the armature is rotated by the force produced when current flows through the armature windings.

Even though the inner configuration of the DC motor and generator are somewhat similar, the outer layout is different. For a DC generator the body is partially covered as, a generator is supposed to be kept in a clean environment and to be handled by skilled men. On the other hand, a DC motor works in a polluted or dusty environment and handled by unskilled men. So, a DC motor is mostly covered.

Applications of DC motors:

  • Paper industries.
  • Textile mills.
  • Steel factories.
  • Cranes and other construction machines.
  • Winches.
  • Printing press.
  • Excavators.

Due to precise and accurate speed control characteristics of DC motor, it is widely used in these fields.

Advantages of DC motors:

  • The starting torque of a DC motor is comparatively high.
  • High accuracy in speed control.
  • It is able to maintain a steeples speed with constant torque.
  • Quick responses for starting, accelerating, decelerating, reversing and stopping.

Disadvantages of DC motors:

Though it has more pros but the cons are also to be tolerated with the advantages. The initializing cost of a DC motor is very expensive. Many industries find it hard to afford the initial steps. The operating and maintenance costs of the motor are also high. So, it is difficult to carry on with regular service program.

Principles of operation of DC motor

Fleming’s left hand rule determines the direction of the force when, a conductor carrying  current is placed in a magnetic field. A DC motor works in that principle. When, a conductor carrying current is placed inside a magnetic field it experiences a force. This force drives the DC motor.

Suppose there are two poles set up with magnetic field, and a conductor carrying current. Now, it is inserted in the field, with the conductor’s axis at 90 degrees with magnetic flux.

  • The upper part of the conductor experiences an additive force of electric field and magnetic field.
  • The lower part of the conductor experiences a subtractive force of electric field and magnetic field.

Hence, the resultant field is stronger above the wire and weaker below the same. Now, the direction of the force is seen directed toward the weaker direction. But, when the direction of flow of current is changed, the direction of the force also changes.

The Force is given by the equation;

F=BIl Newton.

Where, B is the flux density, I is the current in the conductor and l is the length of the conductor.

Now, when an armature in introduced in a magnetic field and as the current starts flowing through the armature, the magnetic lines between the DC motor gets distorted. They are no more straight lines. This is the resultant of the torque and which helps in rotating the motor.

Counter E.M.F.

In a DC motor, an armature carrying current is introduced in the magnetic field. The armature rotates due to the resultant torque produced. The conductor in it cuts the lines of the magnetic force and produces an e.m.f. This induced e.m.f. acts opposite the current and the voltage. So, this induced voltage is named as back e.m.f or counter e.m.f.

The direction of the induced e.m.f can be determined from Lenz’s law. The law states that “the direction of the induced e.m.f. is opposite to the field creating it.” Hence, the direction of counter e.m.f. is opposite to the applied voltage.

The magnitude of back e.m.fis calculated by the formula;

E= k x magnetic flux x speed.

So, the magnitude is proportional with the magnetic flux and speed. It also depends on the number of armature windings.

The value of the counter e.m.f. is generally very less compared to the applied voltage. But, the difference between the two is very small when the motor runs in a normal condition.

The voltage is given by the formula;

V = E + (I x R)

E is the back e.m.f, I is the armature current flowing and R is the resistance.

Comparison between Motor and Generator Action

The action of a DC motor is determined by Left-Hand motor rule. The action of the DC generator is determined by Right-hand generator rule. In a generator the mechanical force moves the conductor in upward direction which is right angle with the induced e.m.f.

Due to the formation of the counter e.m.f in the conductor which is in the magnetic field, the conductor experiences motion created by the induced e.m.f.

Usually the principle of action in a DC motor and DC generator are similar. Hence, the same configuration can be used as either a motor or a generator.

In a motor;

  • The induced e.m.f is less than the applied or terminal voltage.
  • The e.m.f opposes the armature current.

In a generator;

  • The armature current is in the same direction of the counter e.m.f.
  • The magnitude of the induced e.m.f is more than the applied voltage.

The following equations show the formula for deducing induced e.m.f.

For the motor: V = E + (I x R)

For generator: E = V + (I x R)

V is the applied voltage, E in the counter e.m.f, I is the armature current and R is the resistance.

Torque Developed in a Motor

The field of the motor or generator is excited. A potential difference is applied on the terminals of the machine. The current flowing through the windings of the armature reacts with the magnetic flux in which it is induced. This reaction gives birth to a force or a rotation force known as torque. This torque revolves the armature.

When the armature is set in a neutral axis, the conductors of the armature placed on the north side or upper side carries current in the different direction than those placed on the South Pole side.

So, the conductors under the north pole carry inward flowing current. This current reacts with the magnetic flux and produces a force in the downward direction. This gives rise in a counter clock torque. Likewise, the conductors under south pole carry outward flowing currents. After reacting with magnetic field, an upward directed force.

The magnetic flux acts tangentially on the armature and from all direction, each force produces a turning moment. It is given by the formula;

Magnitude of torque = BIlr Nm, r is the radius from the centre to the shaft.

The magnitude increases with the number of conductors in the armature.

Magnitude of torque = BIlrZ Nm.

B is the magnetic flux gap, I represent the current flowing through the conductor, l in the length of the conductor, r in the radial distance and Z is the number of conductors.

Speed Regulation

The formula for regulating the speed of a DC motor is;

Mechanical Power Developed by Motor Armature

A counter e.m.f is induced when the terminals of the armature are excited by a potential difference of V. The induced e.m.f is very less than the applied voltage and is given by the formula;

V = E + (I x R)

Multiplying by armature current on both sides;

VI = EI + I x (I x R)

VI is the electrical input to the armature. EI is the power that is generated in the armature. IIR is the copper loss.

Mechanical power produced, is given by the following equation;

P = VI – IIR

Differentiating both sides with respect to I or armature current, it is proved that

E = V/2.

Mechanical power produced by the motor is the most when the magnitude of counter e.m.f is half of the applied terminal voltage. But in this condition, the amount of flow of current is more than normal. Even half of the input gets used up to compensate the losses, such as mechanical loss or magnetic loss. Hence the efficiency of the motor falls below fifty percent.

This is the only condition to gain maximum power theoretically.

DC motors and its types

Depending on the winding characteristics of the motor, the DC motor can be classified into three types;

  • Shunt Dc motor.
  • Series DC motor.
  • Compound Dc motor.

Shunt wound motor:

In this motor, the field winding is connected parallel with the armature winding. Due to this type of connection, the amount of voltage is constant for a constant current flow. As, the magnetic flux is constant, the induced e.m.f. is also constant, even the speed is also constant.

Hence shunt motor is also known as, constant speed machine. The speed can be varied with adding a variable resistor. It is used for the practice of new drivers, to maintain a constant speed.

Series motor:

In this motor the field winding is connected in series with the armature windings. For this type of connection, the amount of current passing through the field and armature is constant for a constant load. When the load is changed the amount of current has drawn also changes. But, the voltages across the windings are different.

All these result in the change in the magnetic flux. As, the magnetic flux increases, the speed reduces. With increase in load the speed decreases.  So, it is useful in many cases for many new drivers.

A compound motor contains two field windings. One is connected in series with the armature winding while the other is connected in parallel. Hence, it is combination of both shunt and series motor. The flux produced in this depends on current, number of turns and the winding direction of the shunt and series. When the two fluxes get added, it is called cumulative compound motor. When the fluxes oppose each other, it is called differential compound motor.

Motor Characteristics

The four fundamental characteristics of DC motors are as follows:

Starting:

The starting characteristics of a DC motor determine the operation of the motor from starting till it reaches the steady state of running. This phase includes the following steps;

  • The ratio of Starting current to running current determines the current required to start a motor.
  • The starting torque is calculated from the ratio of starting torque to running torque.
  • The time required to start a motor is considered.
  • The amount of energy that the motor considers, determines the economy required to start the operation.
  • The reliability of the motor.

Operating:

The speed, torque and efficiency of a machine determine the useful power and current passing through the armature. These factors are considered in operating characteristics.

Mechanical:

These characteristics are most important. The mechanisms operating on the function of torque and speed determines the mechanical characteristics of a motor. It is also known as braking characteristics.

Regulation:

When the speed of the machine is regulated the following characteristics are considered.

  • The regulation of speed is determined by the ratio of maximum speed to minimum.
  • The efficiency of the regulation depends on the cost required for setting up and maintaining the machine.
  • Type of regulation; either stepped or continuous.
  • The simplicity in controlling the machine.

The three characteristic curves are:

  • Electrical characteristics; armature torque and armature current curve.
  • Speed and armature current curve.
  • Mechanical curve; speed and armature torque curve.

Shunt motor:

Torque current curve:

  • No load – A small amount of armature current keeps flowing to compensate the losses and friction and producing the field.
  • Adding load – As the load is increased; the torque rises due to the rise in current flow. Though the flux is assumed to be constant. But the flux actually decreases a little due to armature reaction.
  • The torque at the start determines the resistance during the initial stage. At the first stage the speed is zero, so the counter e.m.f is also zero.

Speed current characteristics:

  • Neglecting the conditions of temperature rise and falling in the initial current, it is seen that as load increases, the speed of the shunt machine decreases. The fall is proportional to the voltage drop. Even the flux falls due to the increase in load. So, the reduction in flux mitigates a small part of the falling speed.
  • The speed of the shunt motor can be raised by adding a resistance. The addition of a field regulator weakens the field and helps the machine to run faster.

Speed torque characteristics:

  • As, load increases the speed falls, but it is able to maintain a constant speed. So, shunt machine is used in this characteristic to maintain a constant speed from no load to full load phase.
  • When different speeds are required but almost constant.

Series motor:

Torque current characteristics:

  • The torque increases with armature current. As, the flux increases with the current flow through the armature, the torque increases. Torque is directly proportional with the square of the current.
  • Huge amount of flowing current gives rise to heavy torque. It is helpful during starting the machine.
  • The torque is maintained even if the applied voltage is removed. As, the current stays constant the torque does not fall.

Speed current characteristics:

  • The flux does not remain constant with increase in load. At first the flux increases with the load, but after saturation the increase is not uniform and rapid.
  • The counter e.m.f decreases as armature current increases. The speed is proportional to the division of back e.m.f with flux.
  • At very low current the machine tens to run at a very high speed.

Speed torque characteristics:

  • With increase in the torque of the load the speed falls rapidly.
  • Runs at high speed in low current.

Industrial applications for the motor:

Shunt motor:

  • Boring mills
  • Grinders and shapers
  • Wood working machines
  • Drills and milling machines

Series motor:

  • Vacuum cleaners
  • hair driers
  • sewing machines
  • Tram cars and railway cars
  • Fans and air compressors

Compound motors:

  • Lifts, haulage gears and mine hoists
  • Battery boosters
  • Rolling mills, stamping presses and large printing presses
  • Pumps and power fans

Comparison of D.C. Motor Characteristics

The comparisons can be easily determined from the curves. The following can be deduced from the curves about the different characteristics of the different motors.

  • Divergence in the curve between no load and full load.
  • Shunt motor will draw least amount of current during mid load phase, while series motor will draw the most.
  • The series motor can form a torques of high magnitude, almost double of the rated by drawing the least amount of current.

Summary

  • Separately excited D.C. motors: Ability to obtain a very accurate speed. Suitable where variation of speed is required, from low value to high.
  • Shunt motors: Two types of motor functions are available; constant speed or adjustable speed. The magnitude of torque is medium for constant speed type. Speed can be decreased by armature voltage. The speed can be adjusted in the later type. Usually starts with a medium speed.
  • Series motor: Variable speed. Starting torque is of high magnitude. Speed regulation is widely variable and produces high torque during no load.
  • Compound motors: Two types are there; cumulative and differential. Adjusting and variable speed. Torque and speed are constant for suitable conditions of load.

Starting of D.C. Motors

A starter voltage is required to start a DC motor. Every motor requires a starter voltage. Otherwise the initial resistance being high, if the machine is started without any initial voltage, the motor may get damaged.

Starter for Shunt motor:

  • The main function of the starter in shunt and compound motors are to limit the increase in armature current during the initial stage.
  • The output power and output voltage determines the rate of the motor starters.
  • There are two types of starter; three point types and four point types.

Three point starter: This starter has three points, L, F and A. The point L is connected to either the positive or negative side of the DC source. The F is connected to the field winding and A is connected the armature winding of the motor. The speed of the motor can be adjusted by adding a rheostat to the motor circuit.

Cons of three point starter: One well-known disadvantage is its unscheduled stoppages. The machine can stop if the arm of the starter is released when the magnetic field weakens due the reduced current through the coil for the rheostat can stop the machine any time. So it is not widely used.

Four point starter: The con of the three point starter is eliminated in the four point starter. A new point is added to this new one, name L2. The coil holding the magnet in connected with the L2 point when the arm of the motor is moved from OFF state.

This starter does not protect the motor from voltage removal. Hence, when there is a power cut, the arm goes back to the OFF state and the pointer L2 gets disconnected. So, fluctuating voltage cannot harm the machine.