Losses in DC Machine

Losses in DC Machine

The DC machine is used to convert the electrical power into mechanical power or mechanical power into electrical power. During the conversion of energy, always there are some amounts of energy are lost. In this article, we will discuss different losses occurs in DC machine.

This energy converted into heat is called energy loss.

The energy loss produces heat and increases the temperature of machine. Hence, it is necessary to keep losses as low as possible to increase the efficiency and life of the machine.

The losses in DC machines are classified into two types;

  • Electrical Loss (Copper Loss)
  • Rotation Loss

The losses that occur due to the current flowing through the winding are known as electrical loss or copper loss. And the losses that occur due to the rotation of armature are known as rotational loss.

Electrical Loss

Due to the current flowing to the winding, some amount of energy is lost as heat. This type of loss occurs in the component that allows current to flow like the shunt field, series field, armature winding, interpole winding, compensating winding, brush contacts, etc.

These losses are produced due to the power consumed in forcing a current against the resistance of a winding.

The copper loss of winding depends on the effective resistance of a winding. The effective resistance of winding varies with different operating conditions of load and it is hard to determine the value of effective resistance for a particular operating condition. Hence, the resistance of winding is measured at 75˚C.

The copper loss is directly proportional to the effective resistance and directly proportional to the square of the current flowing through the winding.

In DC machines, there are two main windings where the loss is generated; Armature winding and Field winding.

Hence, the electrical loss or copper loss is further classified into two types;

  • Armature Copper Loss
  • Field Copper Loss

Armature Copper Loss

This loss occurs in the armature winding. The armature copper loss is directly proportional to the armature resistance and square of the armature current.

    \[ P_a = I_a^2 R_a \]

Where Ia = Armature current
Ra = Armature resistance

The armature copper loss is about 30-40% of the full-load loss.

For better accuracy, the resistance of brush contact also includes the armature resistance.

Field Copper Loss

The field copper loss is further divided into four parts;

  • Shunt field loss
  • Series field loss
  • Compensation winding field loss
  • Interpole field loss

Shunt field loss is given as;

    \[ P_{sh} = I_{sh}^2 R_{sh} \]

Where, Ish = shunt field current
Rsh = shunt field resistance

The shunt field copper loss only occurs in the shunt field DC machine and compound DC machine.

Practically, the shunt field copper loss remains constant if the supplied voltage remains constant.

Similarly, the series field copper loss is given as;

    \[ P_{se} = I_{se}^2 R_{se} \]

Where, Ise = series field current
Rse = series field resistance

The series field copper loss only occurs in the series DC machine and compound DC machine.

In some DC machines, the interpole and compensating windings are used.

The interpole loss is given as;

    \[ P_i = I_a^2 R_i \]

Where, Ia = armature current
Ri = interpole resistance

Similarly, the compensating loss is given as;

    \[ P_{com} = I_a^2 R_{com} \]

Where, Icom = armature current
Rcom = resistance of compensating winding

The current flowing through the armature depends on the loading condition. As the load increase, the armature current increases and it results in high field loss in interpole and compensating winding.

The filed copper loss is about 20-30% of full load loss.

Rotational Losses

The rotational losses depend on the speed and strength of the field of DC machine. These losses are further divided into two parts;

  • Iron Loss or Magnetic Loss or core loss
  • Mechanical Loss

Iron Loss (Magnetic Loss or core loss)

The iron loss is also known as core loss and it is a combination of two major losses;

  • Eddy Current Loss
  • Hysteresis Loss

Eddy Current Loss

When the armature core rotates in the magnetic field of the poles, an EMF is induced in it which circulates the eddy current in the armature core.

The losses that occur due to the eddy current are known as eddy current loss.

The eddy current loss occurs at the armature core, teeth, and pole faces. It depends on the variation of field density and frequency of the variation of flux. And also, it depends on the thickness of iron laminations.

The equation of eddy current loss is given as;

    \[ P_e = K_e B_{max} f^2 V t^2 \]

Where Ke is a constant and the value of this constant depends on the electrical resistance of the magnetic materials.

Bmax = maximum flux density
F = frequency of magnetic cycle per second
V = volume of magnetic material
T = thickness of magnetic material lamination

From the above equation, we can say that the eddy current loss does not depend on the quality of the magnetic material.

Eddy current is an electromagnetic character. But it is induced due to the rotation of an armature core. Hence, the eddy current loss is considered as a rotational loss.

If the speed and flux density remain constant throughout the operation, the eddy current loss also remains constant.

From the above equation, we can see that the eddy current loss depends on the square thickness of lamination. Hence, to reduce the eddy current, the thickness of lamination is kept as thin as possible.

Hysteresis Loss

The hysteresis loss occurs in the rotating armature core and teeth. The rotating armature is subjected to magnetic field reversals as it passes under successive poles.

The hysteresis loss is directly proportional to the number of reversals per second.

From the experiment, the variation of hysteresis loss with variation in the number of reversals per second and maximum flux density is given as;

    \[ P_h = \eta B_{max}^1.6 f V \]

Where, η = Steinmetz hysteresis coefficient
V = Volume of core
Bmax = Maximum value of flux density
f = Frequency of magnetic cycle

    \[ f = \frac{PN}{120} \]

P = Number of poles
N = Speed of rotation

The values of Steinmetz hysteresis coefficient for good dynamo sheet-steel and silicon steels are 502.4 and 191 J/m3.

The hysteresis loss does not depend on whether the core is laminated or not.

In DC generators, the speeds are constant, and in DC shunt motors that operate at substantially constant and DC shunt motors that operate at a constant speed, the hysteresis loss is constant because of the flux density is almost constant with very little with a load.

In DC series and compound motors, the hysteresis loss does vary with variation in load because both the speed and flux density are affected by variation in load.

The armature core is made of such material that has a low value of Steinmetz hysteresis coefficient to reduce the hysteresis loss.

Mechanical Loss

This type of loss occurs due to the friction of bearings, air friction or windage. The mechanical loss is caused by the motion of the moving parts through the surrounding medium.

The commutator is a moving part and the brush is fixed. In a DC machine, the power is transferred between the commutator and brush. But there are some friction losses occur between the brushes and commutator. This loss is also considered as a mechanical loss.

To reduce the energy loss, improved bearings are used to decrease friction. Also, in case of the roller bearing or ball bearing, the friction losses are so small compared to the windage losses that they are not computed separately but included in windage losses.

Proper lubrication decreases the friction loss. The friction loss is approximately proportional to the speed.

In machines where a commutator is used, the brush friction loss is very large. There are two types of brushes are used in DC machines; Hard brushes and soft brushes.

The coefficient of friction of hard brushes is greater than that of soft brushes. But the hard brush increases wear on the commutator and soft brush has less drag and wears faster.

The brush friction loss depends on the speed of the rotor.

Windage loss is the power loss occurs to move the air about the armature. In DC machines, the blades are mounted on the armature to pass more air throughout the winding and it makes the machine cooler.

The volume of air moved depends on the speed of machine. The windage loss varies as the cube of the speed.

The mechanical losses are constant in a machine operating at a constant speed and it is independent of the load. Generally, the mechanical losses are 10 to 20 % of full-load losses.

Stray-load Loss

The stray-load loss results from factors such as;

  • Distortion of the flux owing to armature reaction
  • Lack of uniform division of the current in the armature winding
  • Short-circuit currents in the coils undergoing commutation

It is difficult to determine the stray loss and it becomes necessary to assign it a reasonable value arbitrarily.

It is usually assumed to be 1 percent of the output of the machine when the rating is about 150 kW or more, for the smaller rating, the stray-load loss is generally neglected.

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