Lecture notes

OF

DC Machines

Department of Electrical engineering

MBM Engineering College, Jodhpur

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 UNIT-V
Testing of dc machines

SWINBURNE’S TEST
Swinburne’s Test is an indirect method of testing of DC machines. In this method the losses are
measured separately and the efficiency at any desired load is predetermined. Machines are tested for
finding out losses, efficiency and temperature rise. For small machines direct loading test is performed.
For large shunt machines, indirect methods are used like Swinburne’s or Hopkinson’s test.

The machine is running as a motor at rated voltage and speed. The connection diagram for DC shunt
machine is shown in the figure below.

Fig: Swinburne’s Test

Let

V be the supply voltage

I0 is the no-load current

Ish is the shunt field current

Therefore, no load armature current is given by the equation shown below.

No-load input = VI0

The no-load power input to the machine supplies the following, as given below.

• Iron loss in the core

 • Friction losses in the bearings and commutators.

• Windage loss

• Armature copper loss at no load.

When the machine is loaded, the temperature of the armature winding and the field winding
increases due to I2R losses. For calculating I2R losses hot resistances should be used. A
stationary measurement of resistances at room temperature of t degree Celsius is made by
passing current through the armature and then field from a low voltage DC supply. Then the
heated resistance, allowing a temperature rise of 50 C is found. The equations are as
follows:-

Where, α0 is the temperature coefficient of resistance at

0C Therefore,

Stray loss = iron loss + friction loss + windage loss = input at no load – field copper loss – no load
armature copper loss

Also, constant losses

If the constant losses of the machine are known, its efficiency at any other load can be determined
as follows.

Let I be the load current at which efficiency is required.

Efficiency when the machine is running as a Motor.

Therefore, total losses is given as

The efficiency of the motor is given below.

Efficiency when the machine is running as a Generator.

Therefore, total losses is given as

The efficiency of the generator is given below.

Advantages of Swinburne’s Test:

The main advantages of the Swinburne’s test are as follows:-

• The power required to test a large machine is small. Thus, this method is an economical and
convenient method of testing of DC machines.
 • As the constant loss is known the efficiency can be predetermined at any load.

Disadvantages of Swinburne’s Test:

• Change in iron loss is not considered at full load from no load. Due to armature reaction flux
is distorted at full load and, as a result, iron loss is increased.

• As the Swinburne’s test is performed at no load. Commutation on full load cannot be

determined whether it is satisfactory or not and whether the temperature rise is within the
specified limits or not.

Limitations of Swinburne’s Test:

• Machines having a constant flux are only eligible for Swinburne’s test. For examples – shunt
machines and level compound generators.

• Series machines cannot run on light loads, and the value of speed and flux varies greatly.
Thus, the Swinburne’s Test are not applicable for series machines

BRAKE TEST ON DC SHUNT MOTOR:

Brake test is a method of finding efficiency of dc motors. We took dc shunt motor as
running machine. Brake test also called as direct loading test of testing the motor because
loading will be applied directly on shaft of the motor by means of a belt and pulley
arrangement.
Test Requirements:

1. DC shunt motor
2. Water-cooled pulley
3. Spring balance

Procedure of Brake Test on DC Shunt Motor:

1. By adjusting the handle of the pulley take different readings of the spring balance.
2. The tension in the belt can be adjusted using the handle. The tension in kg can be obtained
from the spring balance readings.
3. Adjusting the load step by step till full load, number of readings can be obtained. By
increasing the load is slowly, adjust to get rated load current.
4. The power developed gets wasted against the friction between belt and shaft. Due to the
braking action of belt the test is called brake test.
5. The speed can be measured by tachometer. Thus all the motor characteristics can be
plotted.
Calculation of Brake Test on DC Shunt Motor

Let R (or) r= Radius of pulley in meters

N = Speed in R.P.M.

W1 = spring balance reading on tight side in kg

W2 = spring balance reading on slack side in kg

So, net pull on the belt due to friction at the pulley is the difference between the two spring
balance readings.

Net pull on the rope = (W - S) kg = (W - S) X 9.81 newtons ......(1)

As radius R and speed N are known, the shaft torque developed can be obtained as,

Tsh = Net pull X R = (W - S) X 9.81 X R .....(2)

Now let, V = Voltage applied in volts

I = Total line current drawn in amps.

As we know V and I are input parameters of dc motors in brake test.

Then,

Pin=V.I Watts..... (3)

We have output and input. Then why late go and find the efficiency of dc shunt motor.

Efficiency (η)=Output/Input [No units]

From equation (2) & (3)

Advantages of Brake Test on DC Shunt Motor:
1. Actual efficiency of the motor under working conditions can be found out.
2. Brake test is simple and easy to perform.
3. It is not only for dc shunt motor, also can be performed on any type of D.C. motor.

Disadvantages of Brake Test on DC Shunt Motor:

1. In brake test due the belt friction lot of heat will be generated and hence there is large
dissipation of energy.
2. Cooling arrangement is necessary to minimize the heat. Mostly in our laboratories we use
water as cooling liquid.
3. Convenient only for small rated machines due to limitations regarding heat dissipation
arrangements.
4. Power developed gets wasted hence brake test method is little expensive.
5. The efficiency observed is on lower side.

HOPKINSON’S TEST

Hopkinson’s Test is also known as Regenerative Test, Back to Back test and Heat Run Test.
In Hopkinson Test, two identical shunt machines are required which are coupled both
mechanically and electrically in parallel. One is acting as a motor and another one as a
generator. The input to the motor is given by the supply mains.

The mechanical output of motor drives the generator, and the electrical output of the
generator is used in supplying the input to the motor. Thus, the output of each machine acts
as an input to the other machine. When both the machines are running on the full load, the
supply input is equal to the total losses of the machines. Hence, the power input from the
supply is very small.

The Circuit Diagram of the Hopkinson’s Test is shown in the figure below.

Supply is given and with the help of a starter, the machine M starts and work as a motor. The
switch S is kept open. The field current of M is adjusted with the help of rheostat field R M,
which enables the motor to run at rated speed. Machine G acts as a generator. Since the
generator is mechanically coupled to the motor, it runs at the rated speed of the motor.
 Fig: Hopkinson’s Test

The excitation of the generator G is so adjusted with the help of its field rheostat RG that the
voltage across the armature of the generator is slightly higher than the supply voltage. In
actual the terminal voltage of the generator is kept 1 or 2 volts higher than the supply voltage.

When the voltage of the generator is equal and of the same polarity as the of the busbar
supply voltage, the main switch S is closed, and the generator is connected to the busbars.
Thus, both the machines are now in parallel across the supply. Under this condition, when
the machines are running parallel, the generator are said to float. This means that the
generator is neither taking any current nor giving any current to the supply.

Now with the help of a field rheostat, any required load can be thrown on the machines by adjusting
the excitation of the machines with the help of field rheostats.

Let,

• V be the supply voltage

• IL is the line current

• Im is the input current to the motor

• Ig is the input current to the generator

• Iam is the motor armature current

• Ishm is the motor shunt field current

• Ishg is the generator shunt field current

• Ra is the armature resistance of each machine

• Rshm is the motor shunt field resistance

 • Rshg is the generator shunt field resistance

• Eg is the generator induced voltage

• Em is the motor induced voltage or back emf

Since the field flux is directly proportional to the field current.

Thus, the excitation of the generator shall always be greater than that of the motor.

Calculation of the Efficiency of the Machine by Hopkinson’s Test

• Power input from the supply = VIL = total losses of both the machines
• Armature copper loss of the motor = I2am Ra
• Field copper loss of the motor = I2shm Rshm
• Armature copper loss of the generator = I2ag Ra
• Field copper loss of the generator = = I2shg Rshg
The constant losses Pc like iron, friction and windage losses are assumed to be equal and is written
as given below.
Constant losses of both the machines = Power drawn from the supply – Armature and shunt
copper losses of both the machines.
Assuming that the constant losses known as stray losses are divided equally between the two machines.

Total stray loss per machine = ½ PC

Efficiency of the Generator

• Output = VIag
• Constant losses for generator is given as PC/2
• Armature copper loss = I2ag Ra
• Field copper loss = I2shg Rshg
The Efficiency of the generator is given by the equation shown below

Efficiency of the Motor

• Constant losses of the motor is given as PC/2

• Armature copper loss = I2am Ra
• Field copper loss = I2shm Rshm
The Efficiency of the motor is given by the equation shown below

Advantages of Hopkinson’s Test

The main advantages of using Hopkinson’s test are as follows:-

 • This method is very economical.
• The temperature rise and the commutation conditions can be checked under rated load
conditions.
• Stray losses are considered, as both the machines are operated under rated load conditions.
• Large machines can be tested at rated load without consuming much power from the supply.
• Efficiency at different loads can be determined.

Disadvantage of Hopkinson’s Test

The main disadvantage of this method is the necessity of two practically identical machines for
performing the Hopkinson’s test. Hence, this test is suitable for large DC machines.

FIELD’S TEST:

This is one of the methods of testing the D.C. series motors. Unlike shunt motors, the
series motor cannot be tested by the methods which area available for shunt motors as it is
impossible to run the motor on no-load. It may run at dangerously high speed on no load. In
case of small series motors brake test may be employed.

The series motors are usually tested in pairs. The field test is applied to two similar series
motors which are coupled mechanically. The connection diagram for the test is shown in the Fig.
1.

Fig. 1 Field test

As shown in the Fig. 1 one machine is made to run as a motor while the other as a
generator which is separately excited. The fields of the two machines are connected in series
so that both the machines are equally excited. This will make iron losses same for the two
machines. The two machines are running at the same speed. The generator output is given
to the variable resistance R.

The resistance R is changed until the current taken by motor reaches full load value. This will be
indicated by ammeter A1. The other readings of different meters are then recorded.

Let V = Supply voltage

 I1 = Current taken by motor

I2 = Load current

V2 = Terminal p.d. of generator

Ra, Rse = Armature and series field resistance of each machine

Power taken from supply = VI1

Output obtained from generator = V2 I2

Total losses in both the machines, WT = VI1 - V2 I2

Armature copper and field losses, WCU = ( Ra + 2 Rse ) I12

+ I22 Ra Total stray losses = WT - WCU

Since the two machines are equally excited and are running at same speed the stray loses are
equally divided.

For Motor;

Input to motor = V1 I1

Total losses = Aramture Cu loss + Field Cu loss + Stray loss

= I12 ( Ra + Rse) + Ws

Output of motor = Input - Total losses = V1 I1 - [ I12 ( Ra + Rse) + Ws ]

For Generator:

Efficiency of generator is of little importance because it is running under conditions of separate

excitation. Still it can be found as follows.

Output of generator = V2 I2

Field Cu loss = I12 Rse

 Armature Cu loss = I22 Ra

Total losses = Armature Cu loss + Field Cu loss + Stray loss

= I22 Ra + I12 Rse + Ws

Input to generator = Output + Total losses = V2 I2 + [ I22 Ra + I12 Rse + Ws ]

The important point to be noted is that this is not regenerative method though the two
machines are mechanically coupled because the generator output is not fed back to the motor
as in case of Hopkinson's test but it is wasted in load resistance.

RETARDATION TEST OR RUNNING DOWN TEST

This method is generally employed to shunt generators and shunt motors. From this
method we can get stary losses. Thus if armature and shunt copper losses at any given load
current are known then efficiency of a machine can be easily estimated.

The machine whose test is to be taken is run at a speed which is slightly above its normal
speed. The supply to the motor is cut off while the field is kept excited. The armature
consequently slows down and its kinetic energy is used in supplying the rotational or stray
losses which includes iron, friction and winding loss.

If I is the amount of inertia of the armature ans is the angular velocity.

Kinetic energy of armature = 0.5 Iω2

.
. . Rotational losses, W = Rate of change of kinetic energy
 Fig. 2 Retardation test

Angular velocity, ω = (2 πN)/60

Thus, to find the rotational losses, the moment of inertia I and dN/dt must be
known. These quantities can be found as follows;

1.1 Determination of dN/dt

The voltmeter V1 which is connected across the armature will read the back e.m.f. of the
motor. We know that back e.m.f. is proportional to speed so that voltmeter is calibrated to read
the speed directly.

When motor is cut off from the supply, the speed decrease in speed is noted with the
help of stop watch. A curve showing variation between time and speed which is obtained
from voltmeter which is suitably calibrated is shown in the Fig. 3.

Fig. 3 Determination of dN/dt

 At any point C corresponding to normal speed, a tangent AB is drawn. Then

The value obtained from above can be substituted in the expression for W which can give
the rotational looses.