R I T• •Rochester Institute of Technology

Electrical Engineering Department

Technical Assistants' Office

Office# 09-3248 (716) 475-7092

EEEE 531 – MechatronicsExperiment #2

Analysis of D.C. Electric Motors

PurposeThe 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-LabConventional 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 indicates that 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

.

If is greater than , the armature current is positive, and the machine operates as a motor. An electric machine operates as a generator if is greater than , and is negative. Schematic diagrams of separately excited motors and generators are shown in Figures 1 and 2.

Experiment #1 Page 1

R I T• •Rochester Institute of Technology

Electrical Engineering Department

Technical Assistants' Office

Office# 09-3248 (716) 475-7092

Figure 1: Equivalent circuit for separately excited DC motors

Figure 2: Equivalent circuit for separately excited DC generators

The motors that are going to be studied in this experiment are DC series and shunt-connected machines. The diagrams for these two types of motors are shown in Figures 3 and 4.

Experiment #1 Page 2

R I T• •Rochester Institute of Technology

Electrical Engineering Department

Technical Assistants' Office

Office# 09-3248 (716) 475-7092

Figure 3: Schematic diagrams of Shunt connected DC electric machines

Experiment #1 Page 3

R I T• •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 motorsFor 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:

* Page 304 in the book has an example of this

Series – Connected DC MotorsFor 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:

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) 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.

Experiment #1 Page 4

R I T• •Rochester Institute of Technology

Electrical Engineering Department

Technical Assistants' Office

Office# 09-3248 (716) 475-7092

(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

21

4

DC Machine

3

Series

Shunt

5 687

N S

Electro-Dynamometer

120 V60 Hz

0 5ADC

0 120VDC

120 VAC

0 0.5ADC

7

N3N

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.

(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.

Experiment #1 Page 5

R I T• •Rochester Institute of Technology

Electrical Engineering Department

Technical Assistants' Office

Office# 09-3248 (716) 475-7092

(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 re-approach that data point.

Adjust the dynamometer control to 0lbf·in, return the voltage to zero and turn off the power supply.

Series-Motor Load Characteristics

21

4

DC Machine

3

Series

Shunt

5 687

N S

Electro-Dynamometer

120 V60 Hz

0 5ADC

0 120VDC

120 VAC

7

N3N

Figure 6: D.C. Series Motor Connection

Experiment #1 Page 6

R I T• •Rochester Institute of Technology

Electrical Engineering Department

Technical Assistants' Office

Office# 09-3248 (716) 475-7092

(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.

ReportThe 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 7

R I T• •Rochester Institute of Technology

Electrical Engineering Department

Technical Assistants' Office

Office# 09-3248 (716) 475-7092

Table 1 – Equivalent Circuit Parameters

ra   rfshunt   rfseries  V V V V V VI A I A I Ara rfsh rfs

Table 2 – Shunt – Motor Load Characteristics

Torque Torque ua it if ia N N Laf

LBf/in Nm Volts Amps Amps Amps RPM rad/s Henry0                2                4                6                8                10                12                

Table 3 – Series – Motor Load Characteristics

Torque Torque ua it N N LafLBf/in Nm Volts Amps RPM rad/s Henry12            10            8            6            4            2            0   120 .65 5270    

TA Signature _____________________________ Date __________________

Experiment #1 Page 8