Electrical Machines Lab Third Year M2: DC Machines

Faculty of Engineering Cairo University Electrical Power and Machines Dept.

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M2 DC Machines

I. Objectives This Experiment aims to observing the operation ad characteristics of the:

1. Separately excited DC generator. 2. Shunt excited DC generator. 3. Shunt excited DC motor. 4. Series excited DC motor.

II. Background

Small DC machines (fractional HP) are used as control devices such as tacho-generator and servomotors. Large DC machines are used in industry. A wide variety of volt-ampere or torque-speed characteristics can be obtained from various connections of the field windings. The DC machine is extensively used as a motor in industry. Its speed can be controlled over a wide range with relative ease. The most common speed control methods are adjustment of the flux usually be means of field current control, armature current control and adjustment of the armature terminal voltage.

III. Procedure Test 1: The external characteristic of a separately excited DC generator The experiment setup is as shown in Fig. (1) 1. Run the generator with the help of shunt motor at rated speed. 2. Switch on DC supply to the field winding of a separately excited DC generator and pass a

small current by changing the setting of the field rheostat. 3. Read the generator terminal voltage at different values of field current at constant speed. 4. Adjust the field rheostat to obtain rated generator voltage. 5. Close the resistive load switch and change the load of the generator with the help of

loading rheostat. 6. Read the load current and the terminal voltage. 7. Reduce the supply voltage to obtain another value of motor speed. 8. Repeat the above steps. Test 2: The external characteristic of a DC shunt generator 1. Connect as shown in Fig. (2). 2. Run the motor and bring it to its rated speed. 3. Adjust the field resistance of the generator so that the generator builds up its rated

voltage. 4. Close the load switch and change the load of the generator. 5. Check the speed, adjust it to rated value, and record the load current and terminal

voltage. 6. Reduce load and field current gradually. 7. Switch off the load. 8. Stop the motor.

Electrical Machines Lab Third Year M2: DC Machines

Faculty of Engineering Cairo University Electrical Power and Machines Dept.

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Test 3: The external characteristic of a DC shunt motor 1. Connect as shown in fig. (2). 2. At starting, a rated voltage is applied at the armature terminals. Increase the supply

voltage gradually until the motor builds up its rated speed. 3. Adjust the field rheostat of the generator to obtain rated voltage. 4. Load the generator, keeping its terminal voltage constant with the help of field rheostat. 5. Read the input voltage, current and speed. 6. Change the supply voltage and repeat the above steps.

Test 4: The external characteristic of a DC series motor 1. Connect as shown in fig. (3). 2. Set the generator field rheostat a lower value and keep suitable loading rheostat in the

circuit. 3. Switch on the DC supply and start the motor at reduced supply voltage. 4. At the time of starting, care should be taken, the generator builds up and is loaded to

some value so that the speed is within limit. 5. Adjust the field current of the generator to obtain rated voltage. 6. Change the load of the generator with the help of loading rheostat. 7. Read the input voltage, current and speed.

IV. Report

Test 1: - Plot generator O.C. terminal voltage versus the field current at rated speed. - Plot the external characteristic of the separately excited DC generator (VL – IL). Test 2:

- Plot the external characteristic of the DC shunt generator (VL – IL). Test 3:

- Plot the external characteristic of the DC shunt motor (n – IM) for several supply voltage values. - Plot the speed of the motor against the supply voltage.

Test 4 - Plot the external characteristic of the DC series motor (n – IM). Discuss the above results and comment on the main points

Electrical Machines Lab Third Year M2: DC Machines

Faculty of Engineering Cairo University Electrical Power and Machines Dept.

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Connection Diagrams

Electrical Machines Lab Third Year M2: DC Machines

Faculty of Engineering Cairo University Electrical Power and Machines Dept.

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M2: DC Machines Lab results

1. Magnetization Curve

n =

If (A) 0

E (V)

2. Separately excited DC generator n =

IL ( Ia ) (A) 0

VL (V) 200

3. Shunt excited DC generator n =

IL (A) 0

VL (V) 200

4. Shunt excited DC Motor At Vs = 200 V

Ia (A)

n (rpm)

At Vs = 150 V

Ia (A)

n (rpm)

5. Series DC Motor At Vs = 150 V

Ia (A)

n (rpm)

Electrical Machines Lab Third Year M2: DC Machines

Faculty of Engineering Cairo University Electrical Power and Machines Dept.

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Magnetization curve

Electrical Machines Lab Third Year M2: DC Machines

Faculty of Engineering Cairo University Electrical Power and Machines Dept.

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External characteristics of separately excited DC generator and shunt DC generator

Electrical Machines Lab Third Year M2: DC Machines

Faculty of Engineering Cairo University Electrical Power and Machines Dept.

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External characteristics of DC shunt motor and DC series motor

Electrical Machines Lab Third Year M2: DC Machines

Faculty of Engineering Cairo University Electrical Power and Machines Dept.

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Appendix of DC Machine Lab (M2)

A) Sample of rating plate of a DC motor (Siemens)

Where,

Electrical Machines Lab Third Year M2: DC Machines

Faculty of Engineering Cairo University Electrical Power and Machines Dept.

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B) Sample of faults and its countermeasures

1. Faults in operation

Electrical Machines Lab Third Year M2: DC Machines

Faculty of Engineering Cairo University Electrical Power and Machines Dept.

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2. Commutator faults

C) Datasheet of a DC motor (Buhler Motor, Type: 1.13.063.xxx)

STOCK SERVICE

DC Motor Ø 64 1.13.063.XXX

1.13.063.XXX

Design

Commutator Copper/12-segments

RFI Protection 2 chokes

Insulation class Winding H, otherwise A

Protection class IP40

Commutation carbon brushes

Armature straight slot

Magnet system Permanent magnets, 2-pole

Bearings 2 preloaded ball bearings

Housing Steel, corrosion protected

End shields zinc die-cast on both sides

Type 1.13.063.XXX 220 221 407 408

Characteristics*

Rated voltage V V 12 24 12 24

Rated power PN

W 115 115 150 150

Rated torque TN

mNm 350 350 400 400

Rated speed nN

rpm 3150 3150 3400 3400

Rated current IN

A 15 7.5 17 8.5

No load characteristics*

No load speed nO

rpm 3700 3700 3900 3900

No load current IO

A 2.6 1.3 2.0 1.0

Starting characteristics*

Starting torque TS

mNm 2500 2500 3400 3400

Starting current IS

A 95 47 128 64

Performance characteristics*

max. Output power Pmax

W 230 230 340 340

max. Constant torque Tmax

mNm 350 350 400 400

Motor parameters*

Weight G g 1300 1300 1600 1600

Rotor inertia J gcm2 850 850 1050 1050

Terminal resistance R Ohm 0.125 0.5 0.1 0.4

Mech. time constant τm

ms 15 15 11 11

Electr. time constant τe

ms 2.0 2.0 2.5 2.5

Speed regulation constant Rm

rpm/mNm 1.5 1.5 1.02 0.98

Torque constant kM

mNm/A 27 54 27 54

Thermal resistance Rth1

K/W 2.8 2.8 2.5 2.5

Thermal resistance Rth2

K/W 3.3 3.3 3.0 3.0

Axial play < 0.1 < 0.1 < 0.1 < 0.1

Direction of rotation bidirectional

1.13.063.XXX

Operational conditions

Temperature range T °C -10 - +70

Axial force FA

N 50

Radial force, 15 mm from mounting surface FR

N 200

* at 25 °C

Customized versions

The following modifi cations are available upon request: 3 Encoder possible

3 Internal chokes and/or capacitors

3 Speed adjustment by winding change

3 Modifi cation of shaft length on both ends

3 Modifi cation of shaft confi guration (fl at, groove, etc.)

3 Assembly of gears, pinions, worms, etc.

3 Assembly of adapters and mounting plates

3 Reduced cogging torque possible