UNIT4

E3106/04/12

                         

UNIT 4

 

 

DIRECT CURRENT (DC) MACHINERY (PART IV)

 

             

                            OBJECTIVES

 

 

 

General Objective

 

To understand and interpret the concept of DC motor

 

 

Specific Objectives

 

By the end of this unit, you would be able to:

 

• sketch the circuit and characteristic curve
• explain the relation between speed motor and the change of flux and armature resistance

 

 

 

 

                            INPUT

 

 

 

1. CIRCUIT AND CHARACTERISTIC CURVE

 

motors are, of course driven from a dc power supply. The specified the voltage input to a dc motor is assumed to be constant because that

assumption would simplify the analysis and comparison between different types of motors. There are three basic types of motors:

 

1. series motors
2. shunt motors
3. compound motors

 

Each of these types is shown on the Table 4.I below.

 

 

 

 

 

 

 

 

 

 

 

 

Table 4.1: Types of motor, equivalent circuit and characteristics curve

TYPES OF MOTOR	EQUIVALENT CIRCUIT	CHARACTERISTICS CURVE
THE SERIES DC MOTOR A series DC motor is a DC motor whose field windings consist of relatively few turns connected in series with the armature circuit. In a series motor, the armature current, field current, and line current are all the same.	Figure 4.1: Equivalent      circuit series DC motor	Figure 4.2: Typical speed-torque and current-torque characteristics
THE SHUNT DC MOTOR A shunt dc motor is a motor whose field circuit gets its power directly across the armature terminals of the motor.	Figure 4.3: Equivalent      circuit shunt DC motor	Figure 4.4: Torque-speed and torque-current characteristics of a shunt motor
THE COMPOUND DC MOTOR A compound dc motor is a motor with both a shunt and a series field.	Figure 4.5 : Equivalent      circuit compound DC motor	Figure 4.6: Typical speed versus torque characteristics of various motor

(Source: Electrical Machines, Drives and Power System 5th edition; Wildi Theodore)

 

 

 

Test your UNDERSTANDING before you continue to the next input

ACTIVITY 4A
 

 

 

 

 

 

1. Explain the difference between the series and  shunt DC motor from the following aspects :

 

1. definition
2. equivalent circuit

 

2. Fill in the blanks with the correct answers

 

i.              …………………. is a motor with the both shunt and series field.

 

ii.               The three basic types of DC motors are:

1. …………………..
2. …………………..
3. …………………..

 

 

 

 

 

 

 

FEEDBACK TO ACTIVITY 4A

 

 

1. i. Definition
• The series DC motor is DC motor whose field windings consist of relatively few turns connected in series with the armature circuit.
• The shunt DC motor is a motor whose field circuit gets its power directly across the armature terminals of the motor.

 

2. Equivalent circuit
2. The series DC motor

                                

 

• The shunt DC motor

                                                 

 

2.  

i.                The compound DC motor

ii.

a The series DC motor

b The shunt DC motor

c The compound DC motor

 

                            INPUT

 

 

 

4.2               RELATION BETWEEN SPEED MOTOR AND THE CHANGE OF FLUX AND ARMATURE RESISTANCE

 

 

When a DC motor drives a load between no-load and full-load, the IR will drop due to armature resistance which is always small compared to the voltage supply. This means that the counter-emf (EO) is very nearly equal to the supply voltage (ES). The counter-emf (EO) is expressed by . That means;

…..(i)

 

From this equitation, the motor speed is directly proportional to the armature supply voltage, and inversely proportional to the flux per pole. From this statement, we will discuss the armature speed control, and field speed control. At the end of this section, we will understand the relation between speed, flux and armature resistor.

 

4.2.1               Armature speed control

 

Figure 4.7: Ward-Leonard speed control system

According to equation (i), if the flux per pole (f) is kept constant (permanent magnet field or field with excitation), the speed depends only upon the armature voltage ES. By raising or lowering ES, the motor

speed will rise and fall in proportion.

To control the speed of a DC motor,

we will use either a Ward-Leonard

Figure 4.8: Armature speed control using resistor

(Source: Electrical Machines, Drives and Power System 5th edition; Wildi Theodore)

 

 

system or place a resistor variable in series with the armature (armature resistor).Ward-Leonard system (Figure 4.7) is more than just a simple way of applying a variable DC voltage to the armature of a DC motor. It can actually force the motor to develop the torque and speed required by the load.

 

Another way to control the speed of

a DC motor is to insert a series variable

armature resistor (see Figure 4.8).

The current in the variable resistor

produces a voltage drop which

subtracts from the fixed source

voltage (ES), yielding a smaller supply voltage across the armature. This method allow us to reduce the speed below its nominal speed. In conclusion, armature resistor will control the speed of DC motor by reducing a supply voltage across the armature.

 

 

 

 

1. Ward-Leonard system is very efficient.

 

2. Field speed control is frequently used because it is simple and inexpensive

Information Gallery

 

             

4.2.2              Field speed control

    

According to equation (i), we can also vary the speed of a DC motor by varying the field flux (f). Let us now keep the armature voltage (ES) constant so that the numerator in Figure 4.9, is constant. Consequently, the motor speed now changes in the speed will drop and vice versa.

 

 

 

 

 

 

Figure 4.9: Schematic diagram of a shunt

motor including the field      rheostat

(Source: Electrical Machines, Drives and Power System 5th edition; Wildi Theodore)

 

 

 

 

To understand the method of speed control,

supposed that the motor in Figure 4.9 is initially running

at constant speed. The counter-emf (EO) is slightly less

than the armature supply voltage (ES), owing to the IR drop

in the armature. If we suddenly increase the resistance of

the resistor field (Rf) both exciting current (IX), and

the flux (f) will diminish. This immediately reduces

the counter-emf (EO), causing the armature current (IA) to jump to a much higher value. The current changes dramatically because its value depends upon the very small difference between (ES) and (EO). Despite the weaker field, the motor develops a greater torque than before. It will accelerate until EO is again almost equal to ES. Clearly, to develop the same EO, with a weaker flux, the motor must turn faster. We can therefore raise the motor speed above its nominal value by introducing a resistance in series with the field.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Test your UNDERSTANDING before you continue to the next input

ACTIVITY 4B
 

 

 

 

 

(1)
L
				(2)
(1)				T
		(2)			E

 

 

4.3              Complete this puzzle below

Horizontal

1. ……………. resistor a resistor variable

              in series with the armature.

 

2. Field ……….. control is one of the

              method to control a DC motor.

 

Vertical

1. …………can actually force the motor

              to develop the torque and speed required by the load.

 

2. The current in the variable resistor produces a

              ………… drop.

 

4.4              Explain the relation between speed and flux.

 

 

 

FEEDBACK TO ACTIVITY 4B
 

4.3

(1) W
A
R
D
L
E				(2)V
O				O
N				L
(1) A	R	M	A	T	U	R	E
R				A
D				G
		(2)S	P	E	E	D

 

 

Figure 4.9: Schematic diagram of a shunt

motor including the field      rheostat

 

4.4

To understand this method of speed control,

supposed that the motor in Figure 4.9  is initially running

at constant speed. The counter-emf (EO) is slightly less

than the armature supply voltage (ES), owing to the IR drop

in the armature. If we suddenly increase the resistance of

the resistor field (Rf) both exciting current (IX), and

the flux (f) will diminish. This immediately reduces

the counter-emf (EO), causing the armature current (IA) to jump to a much higher value. The current changes dramatically because its value depends upon the very small difference between (ES) and (EO). Despite the weaker field, the motor develops a greater torque than before. It will accelerate until and EO is again almost equal to ES. Clearly, to develop the same EO, with a weaker flux, the motor must turn faster. We can therefore raise the motor speed above its nominal value by introducing a resistance in series with the field.

SELF-ASSESMENT

 

 

 

If you face any problem, discuss it with your lecturer

You are approaching success. TRY all the questions ini this self-assessment section and check your answers with those given in the feedback on Self-Assessment given on the next page.

 

 

Question 4-1

 

A.              Name two techniques that are used to vary the speed of a DC motor

 

B.              A   …………….  DC motor is a motor whose field circuit gets its power directly across the armature terminals of the motor

 

 

Question 4-2

 

A.              What is the method used to speed up the control in armature speed control ?

 

B.              From the equation below counter-emf, Eo, give the name of parameters used in this equation.

FEEDBACK TO SELF-ASSESMENT

 

 

 

Question 4-1

 

A.              a) armature speed control

b) field speed control

 

B.              shunt

 

 

Question 4-2

 

A.              Ward-Leornard system

 

B.              Z – total number of armature conductors

n – speed of rotation (r/min)

f - flux per pole (Wb)