ME 2143-1 MOTOR CHARACTERISTICS(EA-03-06)

SEMESTER 42011-2012

Group Members: LAKSHMI BALASUBRAMANIAMA0074550J

NISHANT JALGAONKAR

SIDDHARTH RAJGOPALAN

Lab Date: 18th January 2012

NATIONAL UNIVERSITY OF SINGAPOREDEPARTMENT OF MECHANICAL ENGINEERING

ObjectiveTo be familiar with the wiring and basic characteristics of the following motors:i) D.C Series Motorii) D.C Shunt motoriii) A.C. 3 Phase Squirrel Cage Induction MotorTo examine the relationship between Torque, Speed, Voltage and Current for various types of motor connections in no-load and loaded configurations

Results:

DC Series Motor

DC Series Motor Test Results (Constant-Load)

Volts (V)Speed (rpm)Current (A)

180.426500.469

160.624000.462

140.421550.451

120.618600.447

10015000.442

79.911980.425

59.68550.422

DC Series Motor Test Results (Loaded)

Volts (V)Speed (rpm)Torque (Nm)Current (A)

180.826250.040.460

179.923870.100.518

179.221370.200.583

178.318750.300.667

176.916900.400.746

176.515370.500.818

175.514230.600.893

175.213390.700.953

174.612780.800.997

173.812040.901.082

4.3Questions

a)Why must a DC Series motor be always started under load and gives examples in your answer.

A no load condition causes load current to fall to a minimum, which reduces the amount of back EMF. This reduced back EMF on the armature in addition to a no load condition on the motor shaft, creates an increase in the armatures rotational speed. A higher rotational speed results in high centrifugal force which causes removal of armature windings and leads to mechanical failure.

An example of DC series motor is the starter motor used to start an automobile engine. In this case, the load is the engine.

b)What is the relationship between Torque and Current for a DC Series motor?

The equation gives the relationship between the torque developed by the motor and current in the armature . is the magnetic flux produced in the stator and K is a machine constant that depends on the winding and structural details of the motor.

Taking K(machine constant) and (flux strength) as constant, we observe that torque and current are directly proportional to each other.

c)Try your best to briefly explain the shape of the Torque vs Speed graph obtained in this experiment.

From the graph, we see that as the speed decreases the torque reduces showing that the Speed is inversely proportional to Torque. The rate of speed increases as Torque nears zero.

Examining the given DC motors equation further confirm our conclusion.

Given that:

- (1)

EA = Km - (2)

VT = RAIA + RFIF + EA - (3)

Substituting (2) in (3), we have

Km = VT - RAIA - RFIF - (4)

Rearranging (4) and taking field current(IF) and resistance(RF) into account,

- (5)

Combining (5) and (1), we get:

DC Shunt Motor

DC Shunt Motor Test Results (No Load)

Volts (V)Speed (rpm)Field Current, IF (A)Line Current, IL (A)

240.315060.1690.347

220.314470.1550.336

200.413920.1410.328

180.313300.1270.324

160.112700.1120.324

140.212030.0990.320

119.811290.0850.326

100.610450.0710.342

79.49350.0560.362

60.7797.10.0430.402

DC Shunt Motor Test Results (Loaded)

Volts (V)Speed (rpm)Torque (Nm)Field Current, If (A)Line Current, It (A)

241.415620.040.1700.341

240.215460.100.1680.384

23915060.200.1670.466

237.714840.300.1660.525

236.714430.400.1650.612

235.814150.500.1650.682

234.713820.600.1630.771

234.113610.700.1630.832

23313430.800.1620.899

231.513210.900.1610.984

230.813001.000.1601.055

5.3Questions

a)Explain how speed is regulated in a DC Shunt motor and give examplesThe speed of DC Shunt motor can be regulated in 2 ways. One method is to regulate the current to the field windings and the second, to regulate current into the armature windings.

In the first method, a field rheostat is connected in series with the field windings. When the field current is decreased, the armature rotated faster in order to maintain the back EMF essential to keep the load running. When the current is increased, the armature rotates slower to maintain the same amount of back EMF. This method causes a 10-20% change in rpm when it rotates at full speed.Examples of a shunt DC motor include the CNC Lathe machine. The Lathe machine will always maintain a certain rpm regardless of the load applied.

b)From no-load to full-load, explain in your own words why there is little speed variation over this range?

There is little speed variation of the motor as the load varies due to characteristics of the armature and shunt field. As the motor starts to rotate the armature produces a back EMF which is used to maintain its high speed (rpm). When the load increases slightly it will cause the armatures shaft to slow down thus lower back EMF is produced causing the difference between the back EMF and applied voltage to become larger hence increasing line current. This increase in current provides the motor with torque to regain its rpm when the load is increased. c)Try your best to briefly explain the shape of the Torque vs Speed graph obtained in this experiment.

The graph obtained shows that torque is inversely proportional to speed. When torque increase, speed will decrease and vice versa.

The only difference between the Shunt and Series motor is that the field winding is connected parallel to the armature winding, we can use the equation:

- (1)

EA = Km - (2)

VT = RAIA + EA - (3)

Substituting (2) in (3), we have

Km = VT - RAIA - (4)

Rearranging (4)

- (5)

Combining (5) and (1), we get:

AC Three Phase Induction Motor

AC Three Phase Motor @ 400VAC Results

Volts (V)Torque (Nm)Speed (rpm)Line Current (A)

4000.0314730.26

4000.1014650.27

3990.2014500.28

3990.3014360.29

3990.4014270.31

3990.5014150.33

3990.6014060.35

3990.7013940.37

3990.8013870.39

3990.9013700.41

4001.0013600.44

6