LABORATORY MANUAL

ELEC 365

APPLIED ELECTRONICS AND ELECTRICAL

MACHINES

by

J.M.-S. Kim

Revised by Babak Manouchehrinia, 2015

University of Victoria

Department of Electrical and Computer Engineering

©University of Victoria, 1990

Revised July 2009

Revised July 2010

Revised July 2015

EXPERIMENT 2

DC MACHINES

2.1 Objective

To study the steady state operation and performance characteristics of a DC

machine both as a motor and generator running with the separate field excitation.

2.2 Introduction

The stator or field of a DC generator or motor (Fig. 2.1) consists of an even

number of magnetic poles (alternating N and S around the circumference) excited

by direct current flowing in the field windings. The rotor or armature consists of a

cylindrical iron core carrying the active conductors embedded in slots and

connected to the segments of the commutator. Stationary brushes riding on the

commutator carry the direct current to and from the armature winding. Switching

of the conductors is done automatically by the commutator so that the external

current from a generator or the torque from a motor is steady and unidirectional.

Thus the commutator acts as a rectifier as far as the external circuit is concerned.

Both the stator and rotor produce magnetic fields. The angle between these two

fields is called torque angle δ (Fig. 2.1a) and in the case of a DC machine δ = 90◦.

Section 2.2.1 describes the basic relation among the DC machine parameters at

steady state. The relation between the generated electromotive force (emf) and the

field current is explained by the magnetization curve in section 2.2.2. Different

modes of field excitation are discussed briefly in section 2.2.3. In section 2.2.4

and section 2.2.5 the operating characteristics of a DC machine as a motor and

generator are examined. The operation with only the separate field excitation is

discussed.

2.2.1 Basic Relations

Each conductor in the armature generates an emf e = Blu, where B is the flux

density; l is the length of conductor and u is the velocity of the conductor. The

total emf is determined by the number of conductors in series at any time. In the

steady state, the total emf generated is given by

𝐸𝑎 = 𝐾𝜑𝛺 (2.1)

where ϕ is the air gap flux per pole in webers, Ω is the angular velocity in

radians/second, and K is a constant for a given machine. The value of K is given

by

𝐾 = 𝑍⋅𝑃

2𝛱⋅𝑎 (2.2)

where

Z = Number of conductors in the armature winding.

P = Number of poles (an even number).

a = Number of parallel paths in the armature winding.

The torque developed in any conductor can be calculated from Td = Blir where i

is the current carried by the conductor and r is the radius of armature.

The total torque is the summation of the individual contributions. The steady state

torque is given by

𝑇𝑑 = 𝐾𝜑𝐼𝑎 (2.3)

where Ia is the armature current in amperes; K and ϕ have the same meaning as in

Eq. (2.1).

Eq. (2.1) and Eq. (2.3) are the basic relations of a DC machine. They apply at the

air gap; the terminal voltage differs from the emf by the armature resistance drop

and the shaft torque differs from the developed torque by the mechanical

resistance torque.

A DC machine is a bilateral energy converter so that at the air gap the developed

mechanical power is just equal to the generated electrical power or

𝑇𝑑𝛺 = 𝐸𝑎𝐼𝑎 (2.4)

The air gap power represents only the reversible portion of the electromechanical

energy conversion. It must be noted that in practical machines, all losses

occurring are irreversible forms of energy.

2.2.2 Magnetization Curve

In a DC machine, the relation between generated emf and field current is defined

by a magnetization curve (or open circuit characteristic)(Fig. 2.2). Eq. (2.1) shows

that Ea is proportional to ϕ. Also the field current IF is proportional to the mmf F.

This is similar to the ϕ-F curve for a magnetic circuit with an air gap. Running the

DC machine at a constant rated speed and measuring the no-load voltage for

different values of field currents gives the magnetization curve. A small emf is

generated when there is no field current and this is due to the residual magnetism.

The magnetization curve is almost linear over a wide range. However, at higher

values of IF, saturation of the iron core occurs and hence the curve deviates from

linearity above certain values of IF.

2.2.3 Field Excitation

The magnetic field of the DC machine is normally supplied by means of a set of

coils placed on each pole piece and collectively called as the field winding,

although there are examples of DC machines where the field is produced by a set

of permanent magnets. Depending on the arrangement of supplying current to the

field winding, the DC machine can be classified into one of four major categories:

1. separately excited

2. shunt excited

3. series excited

4. compound excited

The conventional symbols and connection diagrams for the above types of

excitation are shown in Fig. 2.3. In the case of a separately-excited DC machine,

the field winding is supplied by an independent DC supply through a rheostat.

The separately-excited machine has well-defined operating characteristics and is

easy to control. For a shunt machine, the field winding is connected directly

across the armature. The operating characteristics of shunt machines are very

similar to those of separately-excited machines. In both cases, the field winding

consists of many turns of fine wire, as the field current is a few percent of the

armature current. These two modes of field excitation are the most commonly

used.

A field winding for a series connected machine consists of a few turns of heavy

wire since it carries the entire armature current. This is essential in order to

minimize the losses. Series connected generators are unsatisfactory for most

applications and series motors are mainly used in transportation systems, such as

street cars and subway systems.

In a compound machine, all field poles have both shunt and series windings.

Depending on the type of connection used, the total mmf is either the sum or

difference of ampere turns produced by the two windings.

2.2.4 DC Generators

A DC machine acts as a generator when driven by a prime mover (external

rotating device). The field of the dc generator can be excited by one of the

methods suggested in section 2.2.3. The circuit models and the characteristics of

dc generators with different field excitation are presented below.

Separate Excitation

Since the field current is independent of the terminal voltage of the armature, in

an ideal machine, changes in armature current have no effect on the field current.

Both the armature winding and field winding have inductances and winding

resistances. Under steady-state conditions only the resistances are to be taken into

account. Hence the circuit model for a separately excited generator is shown in

Fig. 2.4. The terminal voltage is given by,

𝑉𝑡 = 𝐸𝑎 − 𝑅𝑎𝐼𝑎 (2.5)

In practice Vt will be further reduced due to reduction in Ea with increases in Ia

and this effect is referred to as armature reaction. A plot of terminal voltage

versus load current (Ia) is the external characteristic of the DC generator (Fig.

2.5).

Shunt Excitation

This type of excitation is also called self-excitation. In shunt excited DC

generators, the field winding is connected directly across the armature winding

(Fig. 2.6). Any change in armature current will cause a change in the resistive

drop (IaRa). Hence, both the terminal voltage and the field current must also

change and thus the induced voltage (Ea) is dependent on the armature current.

However, the armature terminal voltage is still given by Eq.(2.5), but Ea is no

longer constant as in the case of separate excitation, even when the effect of

armature reaction is neglected.

The armature current I a must supply both load current (IL) and the field current

(IF). Therefore

𝐼𝑎 = 𝐼𝐿 + 𝐼𝐹 (2.6)

As load current is increased, Vt decreases due to an IaRa drop. A further increase

in Ia (i.e. load resistance RL is reduced) results in a condition wherein Ea has

greater effect on the value of the load current than a decrease in the load RL and

consequently IL begins to decrease or the characteristic ‘turns back’. Fig. 2.7

shows the external characteristic of a shunt excited dc generator.

The external characteristic of a shunt excited DC generator can be obtained from

the open-circuit characteristic and the RF -line as shown in Fig. 2.8.

The build-up of voltage in a self-excited generator can be explained referring to

Fig. 2.6. When the generator is rotating, a small voltage exists due to the residual

magnetism. This voltage gives rise to a field current which increases the flux

which in turn increases the voltage, etc. This cumulative process continues until a

stable operating point is reached. No further increase in voltage is possible, except

by lowering the resistance of the field circuit (Note: The minimum value of the

resistance of the field circuit is the winding resistance).

A shunt-excited dc generator cannot build-up if

1. there is no residual magnetism,

2. if the field is connected in a wrong-way around, opposing the permanent

magnetism, and

3. the value of the field resistance is greater than a value called critical

resistance.

Series Excitation

Fig. 2.9 shows the circuit model. The external characteristic (Fig. 2.10) is

obtained directly from the magnetization curve by noting that the armature and

field currents are the same in this case.

Compound Excitation

Using a set of coils (series field and shunt field) in an appropriate connection, the

characteristics of a shunt generator can be modified. Depending on the fields

aiding or opposing, they are respectively called cumulative compound generators

or differential compound generators.

Load Regulation of DC Generators

The load regulation of a dc generator is defined as

𝐿𝑜𝑎𝑑𝑅𝑒𝑔𝑢𝑙𝑎𝑡𝑖𝑜𝑛 = (𝑁𝑜𝐿𝑜𝑎𝑑𝑉𝑜𝑙𝑡𝑎𝑔𝑒)−(𝐹𝑢𝑙𝑙𝐿𝑜𝑎𝑑𝑉𝑜𝑙𝑡𝑎𝑔𝑒)

𝑁𝑜𝐿𝑜𝑎𝑑𝑉𝑜𝑙𝑡𝑎𝑔𝑒

2.2.5 DC Motors

A DC machine operates as a motor when electric power is supplied. In the case of

a dc motor the direction of current, and hence torque, is opposite to that of a

generator. DC motors are classified in the same way as DC generators. The

generated voltage in the case of a motor opposes the flow of current, and is called

back emf. The convention used is that of a passive circuit since electric energy is

absorbed. Only shunt and series motors are explained below.

Shunt Excited

When the same supply voltage is used for both the armature and the field, then the

separately excited case also belongs to this case. The terminal voltage is given by

(Fig. 2.11)

𝑉𝑡 = 𝐸𝑎 + 𝐼𝑎𝑅𝑎 (2.7)

where

Vt = supply voltage and

Ea = armature generated voltage (or back emf )

= KϕΩ.

Therefore

𝛺 = 𝑉𝑡+𝐼𝑎𝑅𝑎

𝐾𝜑rad/sec (2.8)

Since IaRa is usually less than 5% of Vt

𝛺 ≃ 𝑉𝑡

𝐾𝜑rad/sec (2.9)

Equation (2.8) can be written as

𝛺 =𝑉𝑡

𝐾𝜑−

𝐼𝑎𝑅𝑎

𝐾𝜑 (2.10)

𝛺 =𝑉𝑡

𝐾𝜑−

𝑅𝑎𝑇𝑑

(𝐾𝜑)2using eqn. (2.3)

𝛺 = 𝛺𝑛𝑙 − 𝑚𝑇𝑑rad/sec (2.11)

where Ω nl is the no-load speed and

𝑚 =𝑅𝑎

(𝐾𝜑)2 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡

Hence, the speed-torque characteristic of a shunt motor is a straight line as shown

in Fig. 2.12.

𝑉𝑡𝐼𝑎 = 𝐸𝑎𝐼𝑎 + (𝐼𝑎)2𝑅𝑎watts (2.12)

where

VtIa = electrical power supplied to the armature, (2.13)

EaIa = gross mechanical power developed by the armature,

some of which is absorbed in friction and core losses,

= Ωtd

From Eq. (2.3), for constant ϕ, Td is proportional to Ia. For a shunt machine, ϕ

is constant, for a constant Vt. Hence the torque varies linearly with armature

current (Fig. 2.13). The net torque is obtained by subtracting the frictional torque.

Also, note that

𝐼𝑎 = 𝑉𝑡−𝐸𝑎

𝑅𝑎 =

1

𝑅𝑎(𝑉𝑡 − 𝐾𝜑𝛺) (2.14)

Therefore

𝑇𝑑 = 𝐾𝜑

𝑅𝑎(𝑉𝑡 − 𝐾𝜑𝛺)Nm (2.15)

If ϕ can be assumed constant, Td varies linearly with speed, and maximum (brake)

torque occurs when Ω = 0 (Fig. 2.14). For E a > V t i.e. in the generating region,

Td reverses its sign. The effect on the characteristics with variations in IF is also

shown in Fig. 2.14.

One most important result from Eq. (2.8) is that if ϕ → 0, Ω → ∞. Hence, the

field of a shunt DC motor must never be open circuited.

Series Excited DC Motor

The terminal voltage is given by

𝑉𝑡 = 𝐸𝑎 + 𝐼𝑎(𝑅𝑎 + 𝑅𝐹) (2.16)

and

𝐼𝐹 = 𝐼𝑎 (2.17)

As was shown earlier on

𝛺 = 𝑉𝑡−𝐼𝑎(𝑅𝑎+𝑅𝐹)

𝐾𝜑rad/sec (2.18)

Since the field winding carries Ia

𝜑 ∝ 𝐼𝑎(assuming linear relation) (2.18)

and therefore

𝛺 ∝ 1

𝐼𝑎 (2.19)

Now, since𝑇𝑑 = 𝐾𝜑𝐼𝑎 (2.20)

𝑇𝑑 ∝ 𝐼𝑎2(using (eqn. 2.19)) (2.21)

However, when IF (= Ia) causes magnetic saturation, it produces a constant ϕ and

Td ∝ Ia.

The characteristics are shown in Fig. 2.15. Neglecting the resistance drop

𝛺 ≃ 𝑉𝑡

𝐾𝜑 (2.22)

Then

𝐼𝑎 = 𝑉𝑡

𝐾𝛺 (2.23)

where ϕ = kIa and K1 = Kk

Therefore

𝑇𝑑 = 𝑉𝑡

2

𝐾1𝛺2

or

𝑇𝑑 ∝ (𝑉𝑡

𝛺)

2

(2.24)

Hence, the torque/speed characteristic is as shown in Fig. 2.16. Note that

speed becomes dangerously high on light load and hence a series dc motor

must never run uncoupled to a load.

Starting and Speed Control

From Eq. (2.14) it can be observed that when normal supply voltage is applied to

a dc motor before the speed builds up, the value of Ia will be limited only by Ra,

resulting in an extremely high current. In order to limit this, starting resistances

are added until the motor picks up speed.

In a DC motor starter, series resistances are added to the armature at the

beginning and as the motor speeds up, resistances are cut in steps. Finally all the

external resistors are shorted.

From Eq. (2.8)

𝛺 = 𝑉𝑡+𝐼𝑎𝑅𝑎

𝐾𝜑rad/sec (2.25)

it can be seen that the speed of a dc motor can be controlled using one of

the following methods:

1. The field flux can be varied to vary the speed, i.e. the value of the field

current IF controls the speed. IF can be varied by supplying the field

voltage separately and controlling this voltage, or by inserting a variable

resistance in series with the field winding.

2. The value of Vt, the supply voltage, can be varied to control the speed.

This value of Vt can be varied by generating Vt, using a separate generator.

This is an old method and the present day method is to use solid state

controllers to vary Vt. A variable supply can be obtained from an ac supply

using phase-controlled converters, or from a dc supply using choppers.

These power electronic converters are very efficient. With such

converters, the power flow can even be reversed. Soft-starting of the

motor is possible by slowly increasing the voltage at the start.

3. Additional resistance in series with the armature. This method is very

inefficient, since the additional resistance carries the full armature current

and, therefore, leads to increased power losses.

Efficiency

The efficiency of a dc motor is defined as

𝜂 = 𝑠ℎ𝑎𝑓𝑡𝑜𝑢𝑡𝑝𝑢𝑡𝑝𝑜𝑤𝑒𝑟

𝑖𝑛𝑝𝑢𝑡𝑝𝑜𝑤𝑒𝑟

Where

Shaft Output Power = TΩ watts

= Input power − losses

The different losses incurred are shown in the power flow diagram of Fig. 2.17.

Loading a DC Motor

In the laboratory, a DC motor is loaded using a dynamometer or a synchronous

generator. A dynamometer is a DC machine with its armature having the

provision for limited angle of rotation and a provision for reading the torque. The

DC motor under test is coupled to a dynamometer (or synchronous generator) and

the armature of dynamometer (or stator of synchronous generator) is loaded using

appropriate resistors. By loading the dynamometer (or synchronous generator)

appropriately, the armature of a dc motor can be made to draw the required

current.

2.2.6 Armature Reaction

Armature reaction is the secondary (undesirable) magnetic field set-up in a DC

machine due to the armature current. In a large DC machine they are nullified in

the polar region by a compensating winding (Fig. 2.18), which is mounted on the

main pole faces and connected in series with the armature.

2.2.7 Commutation Poles or Interpoles

When coils come in contact with brushes, they enter ideally a zero flux region so

that the current in them can change smoothly. This process is called commutation

of a coil. However due to armature reaction, the flux in the interpolar region is not

zero. Hence arcing and sparking takes place during commutation, damaging

brushes and commutator segments. To prevent this, small poles (Figs. 2.18 and

2.19) are placed between the main poles. They are called interpoles or commutation

poles. Since the flux to be zeroed in the interpolar region is proportional to armature

reaction, they are connected in series with the armature.

However, interpoles cannot replace compensating windings and vice versa.

2.3 Apparatus/Instruments

A separately excited DC machine is used as the test object in this experiment.

A synchronous machine is used as a source or a sink of mechanical energy. It is

coupled to the DC machine to be tested. A tachometer is attached to the shaft of

the machine to indicate the rotational speed.

The motor torque can be read on the torque meter mounted on the DC machine.

Devices Required

DC Motor Generator, model 8501 1

Wiring Module DC Motor/Generator, model 8502 1

Synchronous Motor/Generator, model 8507 1

Wiring Module for Synchronous Motor/Generator, model 8508 1

Variable Resistance, model 8509 6

DC Volt-Ammeter, model 8513 1

Field Rheostat Module with built-in 125V, 2A power supply 8524 x 1

Synchronous Motor Starter, model 8520 1

Field Rheostat, model 8524 2

Three-Phase Power Supply, model 8525-10 1

Precision Hand Tachometer, model 8920 1

Electrical Tachometer, model 8930 1

Coupler, model 8943 1

2.4 Preparation

1. Study the relevant parts of your lecture notes, the recommended text and the

laboratory manual section 2.2.

2. Prepare complete connection diagrams for all of the tests called for in the

Procedure using standard electrical symbols and showing clearly all

instrumentation.

3. Inspect the machines on which you will be working with and make sure that

you understand the information on the name plates. (Do this at the beginning

of the lab period).

4. Prepare tables for the measurements specified in the Procedure.

5. Sketch the characteristics you expect to measure for the generator and motor

operation and discuss them.

6. Try to answer the questions provided at the end of section 2.6. Include the final

answers in the report.

2.5 Procedure

2.5.1 Construction of DC Machine

1. Examine the construction of DC machine through the transparent insulation

material, and identify the following:

armature winding, stator poles, commutators shunt-field winding, series-

field winding, brushes, commutating poles or interpoles with winding.

2. While making an observation, answer the following questions:

- How many stator poles are there?

- Approximately how many commutator segments are there?

- How many brushes are there?

- How is the shunt-field winding different from the series-field windings,

and what is the reason for the difference?

- Why are commutator poles or interpoles used?

15

2.5.2 Operation of a Separately Excited Motor

1. Set up the DC machine to be operated as a separately excited motor. Use a

synchronous generator as the load to the DC motor. (Fig. 2.20 shows the

required connection diagram for the synchronous generator).

2. Connect 125 V (2A) DC source available on field rheostat module. Adjust

the field current, by varying the rheostat to If=0.9 A.

3. Provide the armature current of the DC motor from a 0-120 V DC variable

supply. Also connect the commutating poles in series with the armature.

Make sure that the variac reads zero before switching on/off the power

supply.

4. Observe the motor characteristics by taking at least five readings between

no load and 100% of rate torque (rated amature current of 23 A and

maximum torque 11 Nm). Record speed, armature current, armature

voltage and torque. The load to the motor can be varied by changing the

resistors connected across stator terminals of the synchronous generator and

also varying its field rheostat. Before taking any measurement, try to

change the resistors and the field rheostat, and observe how the torque

changes. Do not increase the armature current higher than the rated value.

Take one measurement at rated Ia and calculate the efficiency of the motor.

5. Draw the motor characteristics in the plot of speed vs load torque.

2.5.3 Operation of a Separately-Excited Generator

To achieve this operation, the synchronous machine and the DC machine have to

interchange their functions. The synchronous machine is set up as a motor to drive

the DC generator at a constant speed of 1800 rpm.

1. Set up the synchronous motor as shown in Fig. 2.22. Adjust the field

rheostat for max. Resistance and ensure the field circuit toggle switch is

closed (up position).

2. Provide the field current to the DC generator from 125 V (2A) DC source

available on field rheostat module

3. With the DC machine armature terminals open, turn on the power supply

and start the synchronous motor through a three phase variac. Once the

speed reaches near rated speed after the variac ac output reaches the rated

voltage of the motor, the DC excitation of the synchronous motor is

switched on to lock the motor into synchronism.

4. Increase the DC field current of the synchronous motor by varying the

rheostat until the stator currents are at their minimum.

5. Measure the terminal voltage of the DC generator by DMM and adjust the

field current of the DC generator by varying the supply voltage until the

terminal voltage is the rated 120 V DC.

6. Turn off the power supply.

7. Load the DC generator by connecting a variable resistor (made up of two

variable resistance modules connected in parallel) to the armature terminals

of the DC generator as a load. Arrange instruments to measure the load

current and generator terminal voltage, Vt. Adjust the load resistance to the

maximum. Connect the commutating poles in series with the armature.

8. Turn on the power supply and start the synchronous motor.

9. Take at least six readings with armature current varying between 0 to 90%

of the rated value and record the corresponding terminal voltage. Plot the

external characteristics Vt vs Ia.

2.5.4 Measurement of Steady-State Machine Parameters

1. Measure the resistance of the DC machine shunt field by means of a DMM.

2. The armature resistance, Ra, can be measured as follows:

With the machine at a standstill, lock the rotor and disconnect the field.

Connect a variable resistor (made up of three variable resistance modules

connected in parallel) to the armature and supply 120 V DC to this circuit.

Adjust the resistor to vary the armature current between 0 to 110% of rated

value. Take readings and plot armature voltage vs armature current. Obtain

the brush drop and armature circuit resistance from this graph.

3. Use the Ra measured in the previous step and the name plate data to

determine the constant KØ of the DC generator at the rated condition.

Verify the value of KØ by comparing it to the measurement at no load in

Procedure 2.5.2(5).

2.6 Report

The report should include

1. Objectives

2. Preparations (of all group members)

3. Name-plate data

4. Experimental Results

(a) Answer the questions included in procedure 2.5.1.

(b) The plot of speed-torque characteristics of DC motor (Procedure

2.5.2(5)).

(c) Calculate the efficiency of the separately-excited motor at rated

condition (Procedure 2.5.2(5)).

(d) The plot of external characteristics vt vs ia (Procedure 2.5.3(9)).

(e) Calculate the voltage regulation of the separately excited DC generator

(Procedure 2.5.3(9)).

(f) Draw an equivalent circuit of the DC machine using the parameters

measured in Procedure 2.5.4.

5. Conclusions