Download Pre-Lab

RIT
•
•
Rochester Institute of Technology
Electrical Engineering Department
Technical Assistants' Office
Office# 09-3248
(716) 475-7092
EEEE 531 – Mechatronics
Experiment #2
Analysis of D.C. Electric Motors
Purpose
The purpose of this experiment is to study the operation of direct current machines. The
mathematical models of the motors will be used to compare and examine the theoretical
performance with real electrical machines.
Pre-Lab
Conventional separately exited DC electric machines have stator and rotor windings
excited by direct currents. The rotor has a commutator, and the dc voltage to the armature
winding is supplied through the brushes, which establish electric contact with the commutator.
The brushes are fixed with respect to the stator, and they are placed in the specified angular
displacement to produce the maximum electromagnetic torque. In particular, to maximize the
electromagnetic torque, the stator and rotor magnetic axes are displaced by 90 electrical degrees
using commutator. Expression (5.3) for the electromagnetic torque Te   Lsr isir sin r indicates
that Te can be developed only if there exists a displacement between the armature (rotor) and
fields (stator) magnetic axes.
An electric machine can be either a motor or a generator depending on whether it drives a
load or it is driven by a prime mover. The direction of the armature current is reversed when an
electric machine changes from motor to generator operation. However, there is no change of the
line voltage polarity, and the direction of rotation and field current are the same. These facts are
essential, and Kirchhoff’s voltage law can be applied to study the steady-state operation. In
particular, by using the Kirchhoff law, one obtains
ia 
ua  Ea
.
ra
If ua is greater than E a , the armature current is positive, and the machine operates as a
motor. An electric machine operates as a generator if E a is greater than ua , and ia is negative.
Schematic diagrams of separately excited motors and generators are shown in Figures 1 and 2.
Experiment #1
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•
Rochester Institute of Technology
•
Electrical Engineering Department
Technical Assistants' Office
Office# 09-3248

r fr
rar
ra
rf
(716) 475-7092

ia
if
La
Load
TL
 r , Te
uf

Lf
quadrature axis
Armature
Ea  Laf i f  r
Field
ua
Voltage
supply
direct
axis



Figure 1: Equivalent circuit for separately excited DC motors

r fr
rar
ra
rf

ia
if
La
Tpm
Prime
Mover
 r , pm
uf

Lf
Ea  Laf i f  r
quadrature axis
Armature
Field
ua
Load
direct
axis



Figure 2: Equivalent circuit for separately excited DC generators
The motors that are going to be studied in this experiment are DC series and shuntconnected machines. The diagrams for these two types of motors are shown in Figures 3 and 4.
Experiment #1
Page 2
RIT
•
Rochester Institute of Technology
•
Electrical Engineering Department
Technical Assistants' Office
Office# 09-3248
rar
ra
rf
(716) 475-7092

ia
uf
if
La
Load
TL
 r , Te
r fr
ua

Lf
quadrature axis
Armature
Ea  Laf i f r
Field
direct
axis


Figure 3: Schematic diagrams of Shunt connected DC electric machines
rar
ra

ia  i f
La
 r , Te
rf

Lf
Ea  Laf ia r
ua
Load
TL
quadrature axis
Armature
Field
direct
axis


Experiment #1
Page 3
RIT
•
Rochester Institute of Technology
•
Electrical Engineering Department
Technical Assistants' Office
Office# 09-3248
(716) 475-7092
Figure 4: Schematic diagram of series-connected DC electric machines
The assignment for the pre-lab is to plot the theoretical values of the torque / speed
characteristics for both the DC shunt and series connected electric machines using the DC motor
parameters given below.
Shunt – Connected DC motors
For the shunt motor, use ra = 8, Laf = 3.2H, and if = .2A. Plot the angular velocity
(rad/s) vs Te values of 0, .25, .5, .75, 1, 1.25, and 1.5 (Nm), for the following values of ua (20, 40,
60, 80, 100, 120) Volts. Use the following equation from page 303 in the book:
r 
ua
ra

Te
Laf i f ( Laf i f ) 2
* Page 304 in the book has an example of this
Series – Connected DC Motors
For the series motor, use rt = 10 and Laf = 3.2H. Plot the angular velocity (rad/s) vs Te
values of 0.01, .25, .5, .75, 1, 1.25, and 1.5 (Nm), for the following values of ua (20, 40, 60, 80,
100, 120) Volts. Use the following equation from page 308 in the book:
Te 
Laf
( Laf  r  rt )
2
u a2
For the series motor, rt (rtotal) is used since ra and rf are in series. It is simply the sum of the two.
Make sure that the calculated values of r and the formula used to calculate it are handed in with
the prelab. Just a graph will not be acceptable.
Procedure
Determination of DC Motor Parameters
(a) Place the variable 120 VDC supply across the armature (#1 and #2 of the DC
Machine Stator). Place an ampermeter in series with the input supply. Be sure that
the ampermeter rating is greater than the Stator Current Rating. Have the lab
instructor verify the circuit construction.
(b)
Experiment #1
Increase the input voltage until the ampermeter is at the rated current value for the
armature (3A). Record the input voltage and current. From these values calculate
ra.
Page 4
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Rochester Institute of Technology
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Electrical Engineering Department
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Office# 09-3248
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(c)
Place the previous supply and ammeter across the series winding. After the TA has
verified the circuit apply the rated current and calculate the rfsr.
(d)
Place the previous supply and ammeter across the shunt winding. A smaller range
ammeter will be beneficial. After the TA has verified the circuit apply the rated
current and calculate the rfsh.
Shunt-Motor Load Characteristics
DC Machine
0 5
A DC
7
1
Electro-Dynamometer
2
S
N
Series
0  120
V DC
4
3
A DC
120 V
60 H z
Shunt
5
0  0.5
6
7
8
N
3
N
120 V AC
Figure 5: D.C. Shunt Motor Connection
(a) Connect the DC motor for shunt field operation as shown in Figure 5. Connect the
dynamometer to the fixed 120 VAC output. Connect the belt between the DC
motor and the Dynamometer.
(b)
Set the shunt field rheostat control at its full clockwise position (for maximum shunt
field excitation). Make sure the brushes are in their neutral position.
(c)
Set the dynamometer control at its full counterclockwise position (to provide a
minimum starting load for the dc motor). (0lbf·in.)
(d)
Have the lab instructor inspect the circuit before continuing.
Experiment #1
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RIT
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•
Rochester Institute of Technology
Electrical Engineering Department
Technical Assistants' Office
Office# 09-3248
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(e)
Turn on the power supply. Adjust the variable output voltage to 120VDC as indicated
by the meter. Note the direction of rotation; if it is not clockwise, turn the power off
and interchange the shunt field connections.
(f)
Adjust the field rheostat for a no-load motor speed of 1800rpm as indicated by the
hand-held tachometer. Make sure that the voltmeter, connected across the input of
the circuit, indicates exactly 120VDC . NOTE: DO NOT change the field rheostat
for the remainder of the experiment.
(g)
Measure the line current and the field current, as indicated by the ammeters, for a
motor speed of 1800rpm .
(h)
Apply a load to the dc motor by varying the dynamometer control knob until the
dynamometer reads 2lbf·in. Readjust the power supply, if necessary, to maintain
exactly 120VDC .
(i)
Measure and record the input current, field current and motor speed. Repeat this
step for each of the following torque values: 4, 6, 8, 10 and 12lbf·in. Be sure to
readjust the power supply to 120VDC if necessary. Be very careful when adjusting
the dynamometer. The Dynamometer will enter into oscillation very easily! If this
occurs and the TA is not immediately available, turn off the power supply and reapproach that data point.
Adjust the dynamometer control to 0lbf·in, return the voltage to zero and turn off the power
supply.
Experiment #1
Page 6
RIT
•
Rochester Institute of Technology
•
Electrical Engineering Department
Technical Assistants' Office
Office# 09-3248
(716) 475-7092
Series-Motor Load Characteristics
DC Machine
0 5
A DC
7
1
Electro-Dynamometer
2
S
N
Series
0  120
VDC
4
3
120 V
60 H z
Shunt
5
6
7
8
N
3
N
120 V AC
Figure 6: D.C. Series Motor Connection
(a)
Connect the motor for series field operation as shown in Figure 3, with the belt
connecting the DC machine and the dynamometer.. Connect the dynamometer to the
fixed 120VAC output. Set the dynamometer control at its mid-range position to
provide a starting load for the series dc motor.
(b)
Turn on the power supply and gradually increase the DC voltage until the motor
starts to turn. Note the direction of rotation. If it is not clockwise, turn off the power
supply and interchange the series field connections. Adjust the variable voltage for
exactly 120VDC as indicated by the meter.
(c)
Adjust the loading of the motor by varying the dynamometer control knob until the
dynamometer indicates a torque of 12lbf·in. Readjust the power supply, if necessary,
to maintain exactly 120VDC .
(d)
Measure and record the input current indicated by the ammeter and the motor speed
using the hand-held tachometer. Repeat this procedure for the following torques:
10, 8, 6, 4, and 2lbf·in. Be sure to readjust the power supply to 120VDC if necessary.
At 120VDC with 0 lbf·in, the speed will be 5270rpm and the current will be 0.65
amperes. DO NOT BRING THE MOTOR TO 0 lbf-in!
(e)
Return the voltage to zero and turn off the power supply.
Experiment #1
Page 7
RIT
•
•
Rochester Institute of Technology
Electrical Engineering Department
Technical Assistants' Office
Office# 09-3248
(716) 475-7092
Report
The report will concentrate on the study of the torque – speed characteristics. This means
that all theoretical equations that were used in the pre-lab must be included.
Convert the Torque and velocity data to SI units. (Nm and rad/s respectively) To convert
the torque from (Lbf/in) to (Nm), multiply it by 0.112. To convert the velocity from (RPM) to
(rad/s), multiply it by 0.1047.
Calculate the Laf values for both the series and shunt connected motors using the
equations from the pre-lab. Do this by using the values found in lab and the equations given in
the pre-lab.
Plot the theoretical and measured torque – speed characteristics for both the series and
shunt connected motors. Discuss the accuracy of the plots.
Calculate the efficiencies of the motors at different torques. The efficiency is the power
in divided by the power out. The power out is the velocity times the torque (e.g., Pout = Tw).
Discuss the efficiency.
Experiment #1
Page 8
RIT
•
Rochester Institute of Technology
•
Electrical Engineering Department
Technical Assistants' Office
Office# 09-3248
(716) 475-7092
Table 1 – Equivalent Circuit Parameters
ra
V
I
ra
V
A

rfshunt
V
I
rfsh
V
A

rfseries
V
I
rfs
V
A

Table 2 – Shunt – Motor Load Characteristics
Torque
LBf/in
0
2
4
6
8
10
12
Torque
Nm
ua
Volts
it
Amps
if
Amps
ia
Amps
N
RPM
N
rad/s
ua
Volts
it
Amps
N
RPM
N
rad/s
Laf
Henry
120
.65
5270
Laf
Henry
Table 3 – Series – Motor Load Characteristics
Torque
LBf/in
12
10
8
6
4
2
0
Torque
Nm
TA Signature _____________________________
Experiment #1
Date __________________
Page 9