11

UNIVERSITI TUNKU ABDUL RAHMAN

Faculty : Engineering and Science Unit Code :

Course : Unit Title : Electrical Machines

Lecture group : Experiment : Load test on DC shunt motor

Tutorial group : Date :

Name : ……………………………………………………………

Student ID: ………………………………………….……………

UEEA2433: Electrical MachinesLab-1

Load test on DC shunt motor

Table no. : --------

33

E

V

E

Load test on DC shunt motor

Objectives To understand the load characteristics of dc shunt motors. To understand the performance such as torque, speed, current and efficiency of dc shunt

motors.

Apparatus/machines required

Background theoryA DC motor consists of a stator, a rotor, and other mechanical parts, such as the bearings, shaft,and the housing. The stator contains the field windings (or permanent magnets) that establish the magnetic field. The rotor (also called as Armature) is the rotating part inside the stator. The rotor has its own windings.A voltage

A is induced in the armature due to the motion of its conductors relative to themagnetic field. This voltage is usually referred to as back emf and is given by

A K�ffim (1)where:

K is a machine constant that depends on the winding and structural details of the motor,� is the magnetic flux produced in the stator (Webers),and ffim is the motor angular speed (rad/s)

Another important relationship for the D.C. motors involves the terminal voltage, the back EMF generated by the rotation of the armature, the resistance of the armature circuit and the armature current. By Kirchoff's Voltage Law, we have

T RA I A

E A

(2)

where:VT is the terminal voltage applied to the motor terminals,RA is the resistance of the armature circuit in ohms which, in the case of a series motor, includes the resistance of the series field coils andIA is the current in the armature in amperes.

T

44

Combining the two equations gives:K�ffim

VT

RA I A

(3)

Re-arranging this equation gives:V I Rffim T A A

K� (4)

The torque developed by a D.C. motor can be calculated from the equationdev K�I

A

(5)

where:Tdev is the torque developed by the motor (Nm),IA is the current in the armature (A).

or, in general, P T ffi T ( Nm) 2π N (rpm) I 60.....W

60 P t 2π N

60 P for output I load torque ,t o o 2π N

60 P for mechanical torque ,t m m

Efficiency , η P

2π N

out Pin

In a shunt motor, as the field windings and the armature are in parallel connection, the armature current IA is different from the field current IF. While the armature current IA is dependent on the load, the field current is independent of the load conditions. As a result, the flux for a shunt motor can be considered to remain constant. You will learn in theory that the field produced by the armature current in the armature conductors can affect the field strength but the shunt field flux can be considered to remain unaffected by the load. As the flux for a shunt connected motor is independent of thearmature current the only effect the load can have on the speed is to increase the effect of the voltage drop in the armature.

One of the outcomes of this experiment will be experimental proof of the curves showing the relationship between speed, torque and current in D.C. shunt motors and an understanding of these curves and what they tell you about the motor. Comparing these curves for the

55

various D.C. motor configurations will help you select the correct motor for a given purpose.

66

Procedure1. Tabulate below the specifications of the given machine.

2. Fig. 1 shows the connection diagram of the motor. Establish the connections according to the diagram of Fig.1.

3. Make sure the starting resistance is at the MAXIMUM resistance position (WSM6-02).4. Make sure the field resistance is in the MINIMUM resistance position (WSM6-03).5. The circuit connections are to be checked by the lab supervisor.6. Turn ON the power switches of all modules (WSM6-07, WSM6-14, WSM6-53, WSM6-54).

Connect the brake of DC motor strong, and put the brake exciting resistor RB of speed and torque measurement module (control knob of the electrodynamometer, WSM6-14) to'0' position (i.e., minimum load).

7. Adjust the voltage to DC 50V by increasing the power of WSM6-07 and turn the starting resistor to anti-clockwise. You can find the motor is started.

8. Adjust the voltage to DC 200V by adjusting the knob of WSM6-07. Turn the field resistor in the clockwise direction until the machine rotates at 1500 rpm. Re-adjust the voltage to 200V (if required).

9. Read all the minimum load data (speed, line current, field current and terminal voltage)from the respective meters and enter into table 1.

10. Adjust the control knob of WSM6-14 to increase the brake exciting resistor RB (the load applied to the motor) in steps of 0.03kg/m u n t i l a m ax i m u m of 1.0 5 % of t h e r at e d c u r r e n t f lo w s o r , a to r qu e of 0.24 k g / m is re a ch e d. Re-adjust the voltage to 200V (if required). At each step record the speed, line current, field current and terminal voltage. Enter the data into table 1.

11. When data collection is completed, turn the control knob of the electrodynamometer (WSM6-14) slowly in CCW direction until it is fully CCW [Turning the knob too fast would cause the load to start rocking, which is undesirable] and then reduce the motor supply voltage (power of WSM6-07) to minimum (0V).

77

12. Adjust the knob of WSM6-02 to MAXIMUM resistance position and the knob ofWSM6-03 to its MINIMUM resistance position.

13. Switch OFF all relevant power supplies.14. Measure the armature resistance (Ra) with a multimeter I ohmmeter. Complete table-1.15. Plot the characteristic curves such as torque, speed, efficiency, input power and output

power as a function of the load current (in a single graph sheet).

Fig.1

200 0.24

VoltageV1[V]

Line current

11[A]

Field current

1F[A]

SpeedN[rpm]

TorqueT[kg/m]

Arm. Current

1a[A]

Pin[W] V111

Pd[W] Eb1a

Efficiency Pd/Pin

%Pout[W]

TωPout/Pin

%

200 0.4 0.25 1498 0.00 0.15 80 29.74 37.18 0.00 0.00200 0.57 0.24 1475 0.03 0.33 114 66.00 57.89 45.43 39.85200 0.68 0.24 1463 0.06 0.44 136 88.00 64.71 90.12 66.27200 0.78 0.24 1450 0.09 0.54 156 108.00 69.23 133.98 85.88200 0.89 0.24 1432 0.12 0.65 178 130.00 73.03 176.42 99.11200 1.00 0.24 1422 0.15 0.76 200 152.00 76.00 218.99 109.49200 1.10 0.24 1406 0.18 0.86 220 172.00 78.18 259.83 118.10200 1.23 0.24 1391 0.21 0.99 246 198.00 80.49 299.90 121.91

9.8

P

88

Ra = 11.37 Ohm

Table 1

Formulae:T TNm kg I m

I a I1 I FV Iin 1 1

P E I V d b a(

1 I

a R

a ) I

a

P Tffi T 2πN

0 60

99

Sample Calculation

TNM=9.8x0 =0NmIA=IL-IF

=0.4-0.25=0.15A

Pin=VlIl = 200x0.4

=80w

Pd=EbIa=(V1-IaRa)Ia

=(200-0.15x11.37)x11.37=29.74w

Po=Tw

=T

0.15 0.33 0.44 0.54 0.65 0.76 0.86 0.990

50

100

150

200

250

300

350

0

0.05

0.1

0.15

0.2

0.25

Chart Title

speedefficiencyinput powerpower outputtorqueSp

eed

Efficie

nct

Inpu

t pow

erO

utpu

t Pow

erTorque

Torque