First Volume

Openlab Laboratory

ROTATING ELECTRICAL MACHINESDL 10280

DL 10280

II Vers.e 2018/06/13

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DL 10280

Vers.e 2018/06/13 III

GENERAL CONTENTS DL 10280 FIRST VOLUME: ELECTRICAL MOTORS

1. GENERAL CHARACTERISTICS Page 1

1.1 Alternating current machines Page 2 1.1.1 Stator Page 2 1.1.2 Squirrel cage rotor Page 3 1.1.3 Ring rotor Page 4

1.2 Direct current machines Page 6

1.2.1 Stator Page 6 1.2.2 Rotor with segment commutator Page 7

2. BASIC CONCEPTS Page 9

2.1 Magnetic circuit Page 9 2.2 Induction law Page 12

2.2.1 Sinusoidal alternating voltage generator Page 13 2.2.2 Electromagnetic actions Page 15

2.3 Commutation Page 17

2.3.1 Armature reaction Page 18 2.4 Rotating magnetic field Page 20

2.4.1 Pulsating magnetic field Page 20 2.4.2 Three-phase rotating magnetic field Page 21

Experiment N°1: Flux produced by the poles Page 25

N°1.1: Main poles Page 26 N°1.2: Interpoles Page 28

Experiment N°2: Main magnetic field Page 29 Experiment N°3: Intensity of the magnetic field Page 33 Experiment N°4: Induced voltage Page 35 Experiment N°5: Interpole effect Page 39 Experiment N°6: No-load magnetic neutral axis Page 41 Experiment N°7: Rotating magnetic field Page 43

N°7.1: Three-phase rotating field Page 44 N°7.2: Single phase rotating field Page 46

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3. INDUCTION MOTOR Page 49 3.1 Starting of the induction motors Page 50

3.1.1 Three-phase squirrel cage motor Page 50 3.1.2 Three-phase motor with ring rotor Page 51 3.1.3 Star-delta starting Page 51 3.1.4 Single-phase motor Page 52

3.2 Control of the three-phase motor speed Page 53 3.3 Direction of rotation Page 54 3.4 Performances of the induction motor Page 54 3.5 Induction devices Page 54

3.5.1 Phase shifter Page 54 3.5.2 Voltage regulator Page 55

3.6 Induction synchronous motor Page 56

Experiment N°8: Three-phase squirrel cage motor, 2 poles, 24 V Page 57 Experiment N°9: Three-phase squirrel cage motor, 2 poles, 42 VY Page 63 Experiment N°10: Three-phase squirrel cage motor, 2 poles, 24 V Page 69 Experiment N°11: Three-phase squirrel cage motor, 2 poles, 42 VYY Page 75 Experiment N°12: Three-phase squirrel cage motor, 4 poles, 24 V Page 81 Experiment N°13: Three-phase squirrel cage motor ,4 poles, 42 VY Page 87 Experiment N°14: Three-phase Dahlander motor, 4/2 poles, 42 V/YY Page 93 Experiment N°15: Split phase motor Page 99 Experiment N°16: Capacitor start and run motor Page 105 Experiment N°17: Three-phase motor with wound rotor, 2 poles, 42 VYY Page 111 Experiment N°18: Phase shifter Page 117 Experiment N°19: Induction regulator Page 121 Experiment N°20: Three-phase synchronous induction motor, 2 poles, 24 V Page 125 Experiment N°21: Three-phase synchronous induction motor, 2 poles, 24 V Page 131

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4. DIRECT CURRENT MOTORS Page 137

4.1 Back-electromotive force Page 137 4.2 Excitation systems Page 138 4.3 Performances of the direct current motor Page 138 4.4 Rotation sense Page 138 4.5 Motor with separate excitation Page 139 4.6 Motor with shunt excitation Page 141 4.7 Motor with series excitation Page 142 4.8 Motor with compound excitation Page 144

4.8.1 Motor with cumulative compound excitation Page 144 4.8.2 Motor with differential compound excitation Page 145 4.8.3 Check of the excitation windings Page 146 4.8.4 Inversion of the rotation sense Page 146

Experiment N°22: Dc motor with separate excitation Page 147 Experiment N°23: Dc motor with shunt excitation Page 153 Experiment N°24: Dc motor with series excitation Page 159 Experiment N°25: Dc motor with compound excitation, long shunt Page 165 Experiment N°26: Dc motor with compound excitation, short shunt Page 175 5. COMMUTATOR MOTORS FOR ALTERNATING CURRENT Page 185

5.1 Single phase series motor / universal motor Page 185 5.1.1 Inversion of the direction of rotation Page 186

5.2 Repulsion motor Page 186

5.2.1 Speed adjustment and rotation sense Page 188

5.3 Performances of the commutator motors Page 188 Experiment N°27: Single phase series motor Page 189 Experiment N°28: Repulsion motor Page 195

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SECOND VOLUME: ELECTRICAL GENERATORS AND APPENDICES

6. SYNCHRONOUS MACHINES 6.1 Three-phase alternator 6.2 Voltage variation 6.3 Alternator performances 6.4 Rotation sense 6.5 Parallel connection of the alternator with the mains 6.6 Synchronous motor Experiments N°29 ÷ N°37 7. DIRECT CURRENT GENERATORS 7.1 Excitation systems 7.2 Dynamo performances 7.3 Rotation sense 7.4 Separate excitation dynamo 7.5 Shunt excitation dynamo 7.6 Series excitation dynamo 7.7 Compound excitation dynamo Experiments N°38 ÷ N°45 APPENDICES A1. Component list A2. Magnetic probe A3. Test execution A4. Measurement of the mechanical power A5. Tolerances

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1. GENERAL CHARACTERISTICS

The modular system DL 10280 for the study of the rotating electrical machines includes the following components: Base plate DC stator Supports with bearing AC stator Coupling joints Rotor with commutator Flexible coupling Brush holder with 2 brushes Electronic speed transducer Cage rotor Assembling screws Ring rotor Wrenches Brush holder with 6 brushes Besides, a magnetic probe is provided to display the magnetic fields. Moreover, for the operator safety a transparent cover is included, that prevents from the accidental contact with the rotating parts.

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1.1 ALTERNATING CURRENT MACHINES The alternating current machines are divided into asynchronous machines (usually motors) and synchronous machines (generators and motors). 1.1.1 Stator For the asynchronous motors and for the fixed armature alternator the stator is composed of a metal frame supporting the laminated magnetic circuit, affected by a time variable flux, and the electrical winding.

The sheet pack is 60 mm long, with internal diameter of 80 mm and external diameter of 150 mm and it has 24 half-closed slots inside of which there is a double three-phase winding: the beginnings and the ends of the different phases are shown outside the stator on a suitable educational terminal board.

The winding is of the double layer, long coil lap type, with winding span 6 (1÷7). Every slot contains two coils of 19 turns each of enamelled wire of diameter 1.12 mm.

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1.1.2 Squirrel cage rotor The rotor is composed of a shaft to which a pack of magnetic sheets is fixed, where the slots suitable to contain the rotor winding are set.

The sheet pack is 60 mm long, with external diameter of about 78 mm. To avoid the phenomenon of the motor jamming in the starting phase and to reduce the noise, the rotor slots are inclined with respect to the stator slots. The rotor winding is composed of the squirrel cage.

The cage is carried out by setting in every rotor slot some conducting bars that are closed in short-circuit at both ends by means of conducting rings. Therefore, the rotor winding can be considered a multiphase winding, with a single conductor for pole-phase, so it does not have its own pole number, but it takes a pole number that is equal to that of the stator winding.

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1.1.3 Ring rotor The rotor is composed of a shaft to which the collector rings and a magnetic sheet pack are fixed: the pack has 21 semi-closed slots suitable to contain the winding.

The sheet pack is 60 mm long, with external diameter of about 78 mm. To avoid a noisy mechanical running, the rotor slots are inclined with respect to the stator slots. The rotor winding is composed of coils and it is a two pole, three-phase winding.

The winding is of the double layer, long coil lap type, with winding span 9 (1÷10). Each slot contains two coils of 8 turns each of enamelled wire of diameter 1.5 mm. The winding is star connected and ends at the collector rings while the star centre is internal. The terminals of the rotor winding are accessible by means of the collector rings to which the brushes, supported by a brush holder, are connected.

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The brushes are two for each phase and they end at an external terminal board that shows the synoptic diagram of the rotor winding.

Notes 1) Three-phase asynchronous motor The starting of the motor with wound rotor is carried out by connecting the brushes to a rheostat

that temporarily increases the resistance of the rotor circuit.

2) Synchronous generator The magnetic field in the alternator is generated through the rotor poles by connecting the rotor

winding to a direct source in such a way that the current enters through one phase and leaves through the other two in parallel.

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1.2 DIRECT CURRENT MACHINES The machines operating in direct current are divided into dynamo (generators) and direct current motors. 1.2.1 Stator The stator is composed of a metal frame supporting the laminated magnetic circuit, with 2 main poles and 2 interpoles, and the electrical windings.

The sheet pack is 60 mm long, with internal diameter of 80 mm. The coils, whose terminals are shown on a suitable educational terminal board, are wound on the poles.

Terminals F1-F2/F5-F6 (separate or shunt excitation) Coil terminals (one for each pole) of 500 turns of enamelled wire of diameter 0.45 mm. Terminals D3-D4 (series excitation)

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Terminals of the series excitation winding: 28 turns for each pole carried out with two enamelled wires of diameter of 1.12 mm., parallel connected. Terminals D1-D2 (compensating excitation) Terminals of the series excitation winding to be used with compound excitation machine: 5 turns for each pole carried out with two enamelled wires of diameter of 1.12 mm., parallel connected. Terminals B1-B2 (interpoles excitation) Terminals of the winding carried out on the interpoles to be used to improve the commutation: 61 turns for each pole carried out with two enamelled wires of diameter of 1.12 mm., parallel connected. 1.2.2 Rotor with commutator The rotor is composed of a shaft to which the commutator is fixed and of a magnetic sheet pack where 20 semi-closed slots suitable to contain the electrical winding are set.

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The sheet pack is 60 mm long, with external diameter of about 80 mm.

The winding is of the double layer, long coil lap type, with winding span 9 (1÷10). Each slot contains two coils with two sections of 5+5 turns carried out with enamelled wire of diameter 1.12 mm. The winding ends at the 40 segments of the commutator on which two brushes, supported by a brush holder, are connected.

The brushes end at terminals set on two external boards that show the synoptic diagram of the rotor winding.

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2. BASIC CONCEPTS In this section some fundamental concepts are reminded for the correct comprehension of the operation of the rotating electrical machines. 2.1 MAGNETIC CIRCUIT The fundamental structure of a rotating electrical machine is composed of a fixed part (stator) and a moving part (rotor), carried out with magnetic material to support the passage of the magnetic flux , that can be generated by a permanent magnet or, better, by electromagnets. The flux lines flow through a closed circuit that includes both iron (stator yoke, poles and rotor) and air (air gap between stator and rotor).

Once known the value of the flux (Wb) that is necessary for the machine operation, it is possible to calculate the electromotive force F (Aturns), that is necessary for its sustenance, as the sum of the magnetomotive forces FM corresponding to the different sections of the magnetic circuit:

F = FM = H l where H (A/m) is the intensity of the magnetic field in a given section and l (m) is its length. Then, after having determined the magnetic induction B (T), defined as flux by surface unit and relevant to each section:

BS

the intensity of the magnetic field is determined by knowing that:

B = µ H where µ (H/m) is the magnetic permeability of the medium.

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For what concerns the air, the magnetic permeability is constant, µ = 1.2566370614…×10−6 H·m−1, while for the ferromagnetic materials we usually have to use the graphs that represent the magnetization curve in order to determine the intensity of the magnetic field, because their permeability is not constant.

Finally, since the total magnetomotive force F is applied to two poles, the ampere-turns for each pole are simply obtained by the relationship:

F = 2 N Ie where N is the number of turns per pole and Ie the excitation current. Note In the air gap and in the rotor teeth the useful flux is equal to , while in the poles we have to consider a higher flux p = 1.2 , to take into account the side leakage.

Finally, in the rotor section and in the stator yokes only half of the useful flux will have to be considered.

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NUMERICAL EXAMPLE N° 1 Determine the value of the excitation current of a direct current machine in such a way to obtain a flux = 1.4 mWb, knowing that coils of N = 500 turns are wound on the poles. The geometrical data of the machine are shown in the calculation table. Calculation table

Section Section S (mm2)

Section S (mm2)

Induction B (T)

Intensity H (A/cm)

Portion l (cm)

M.m.f. FM (A)

Yoke

1.2x0.7 780 1.1 3 21 63

Poles

1.2x1.4 3600 0.5 0.8 2x1.7 2.72

Air gap

1.4 5000 0.3 2400 2x0.1 480

Teeth

1.4 2600 0.5 0.8 2x1.5 2.4

Rotor

0.7 600 1.1 3 6 18

Total m.m.f. F. 566

The intensity of the magnetic field for the air gap is calculated with the relationship:

H (A/cm) = 8000 B while for the remaining sections of the magnetic circuit the magnetization characteristic FeSi (1.3 W/kg) has been used. The intensity of the excitation current is:

IeF

2

566

2 5000566

N (A).

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EXPERIMENT N°8 THREE-PHASE SQUIRREL CAGE MOTOR, 2 POLES, 24 V Purposes: Connect the stator winding in delta Start the motor with direct connection to the mains Record the operation characteristics of the motor Components: 1 Ac machine stator and squirrel cage rotor 1 DL 10281 Supply module 1 DL 10282N Measurement module 1 DL 10284 Adapter support 1 DL 10300A Electromagnetic brake with arms

(Weight G = 3.5 N, balance weight g = 1.5 N) 1 MY60 Digital multimeter Note For an accurate speed measurement we advise: 1 DL 2026 Digital tachometer Electrical diagram

3M

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EXPERIMENT N°8: THREE-PHASE SQUIRREL CAGE MOTOR, 2 POLES, 24 V

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Procedure Assemble the group asynchronous motor-brake, by using the stator of the alternating current machine completed with squirrel cage rotor. Carry out the circuit shown in the previous topographical diagram. Set the supply module DL 10281 for a fixed alternating voltage 24V/14A: (selector "a0b" to position "a" and switch L1/L2/L3 to position "0") and for a variable direct voltage 0÷40V/5A (selector “c0d” to position "c" and control knob to 0%). Use first and second devices of the measurement module DL 10282N to read the values of voltages, currents and power to the stator terminals of the motor for alternating current. For the active power press the “Function” button on the right of the appropriate measuring device. Use the measurement module DL 10282N for speed measurements on third device. Use the MY60 Digital multimeter for current measurement in excitation circuit of the brake Activate the supply module and start the motor by setting the switch L1/L2/L3 to position "1": the motor turns clockwise. Once the motor has been started provide to balance the brake: the system is balanced by setting the weight G = (2+1.5)N in correspondence of the zero of the graduated scale and by moving the balance weight g = 1.5 N until the water level shows the horizontal position. Start measuring the supply voltage U, the absorbed current I and the powers P13 and P23. Record the speed n of the motor. The motor is therefore loaded by steps by means of the brake: the load is carried out by moving the weight G to a distance b from the zero position and therefore, by means of the variable direct voltage 0 ÷ 40 V, its excitation current is adjusted until balancing the system again. Carry out the measurements previously shown for every value of arm b shown in the following table, where the measurement indications are therefore written. f= 50 Hz

U (V)

I (A)

P13 (W)

P23 (W)

Pin (W)

cos G (N)

b (m)

M (Nm)

n (min-1)

P (W)

(%)

3.5 0 0 0 0 3.5 0.025 0.0875 3.5 0.05 0.175 3.5 0.075 0.2625 3.5 0.1 0.35 3.5 0.125 0.4373 3.5 0.15 0.525

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Stop the group by setting the switch L1/L2/L3 to position “0” and deenergize the brake. Complete the table with the calculated values of: - absorbed power Pin = P13 + P23 - power factor

cos Pin

3 U I

- output power P = 0.1047 n M - efficiency

% P

Pin

100

Draw on the same diagram the absorbed current I, the power factor cos, the speed n and the efficiency as a function of the output power P.

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Draw on a diagram the mechanical characteristic M = f(n).

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EXPERIMENT N°22 DC MOTOR WITH SEPARATE EXCITATION Purposes: Start the dc motor with separate excitation Invert the rotation sense of the motor Record the operation characteristics of the motor Components: 1 dc machine stator with assembled commutator rotor and brushes 1 DL 10281 Supply module 1 DL 10282N Measurement module 1 DL 10283 Loads and rheostats 1 DL 10284 Adapter support 1 DL 10300A Electromagnetic brake with arms (Weight G = 3.5 N, balance weight g = 1.5 N) Electrical diagram

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EXPERIMENT N°22: DC MOTOR WITH SEPARATE EXCITATION

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Procedure Assemble the group dc motor-brake, by using the stator of the direct current machine completed with commutator rotor and brushes. Carry out the circuit shown in the previous topographical diagram. Set the supply module DL 10281 for a fixed direct voltage 42V/10A (selector "a0b" to position "b" and switch L+/L- to position "0") and for a variable direct voltage 0÷40V/5A (selector “c0d” to position "c" and control knob to 0%). Use the measurement module DL 10282N and read the values of the voltmeter and the ammeter for direct current measurements on first and second devices, and observe the polarities (+ at red terminal). Use the measurement module DL 10282N for speed measurements on third device. Set on the module DL 10283 the starting rheostat RA = (1 + 2) for the maximum resistance (control knob to position "b") and the excitation rheostat RF = 80 with the minimum resistance (control knob to position "a"). Activate the supply module and supply the motor by setting the switch L+/L- to position "1": the motor starts and it turns clockwise. Stop the group by setting the switch L+/L- to position "0". Invert the connection F1 with F6 and start the group again by setting the switch L+/L- to position "1": the motor turns now counter clockwise. Stop the group by setting the switch L+/L- to position “0” and restore the initial connection of the excitation winding. Start then the group again: the motor has to turn clockwise. Switch off gradually and completely the starting rheostat RA (control knob to position "a": only the resistance 1 remains connected): adjust the excitation rheostat RF in such a way the separate excitation current is Ie = 0.9A, that has to remain constant during all the test. With the rotating group provide to balance the brake: the system is balanced by setting the weight G = (2+1.5)N in correspondence of the zero of the graduated scale and by moving the balance weight g = 1.5 N until the water level shows the horizontal position. After the group has reached the thermal stability, control the excitation current and measure the excitation voltage Ue, the voltage U and the supply current I of the motor and the speed n. The motor is therefore loaded by steps by means of the brake: the load is carried out by moving the weight G at a distance b from the zero position and therefore, by means of the variable direct voltage 0÷40V, its excitation current is adjusted until balancing the system again.

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By checking that the excitation current is the pre-established one (eventual adjustments can be carried out by means of the rheostat RF) perform the measurements previously shown for every value of arm b shown in the following table, where also the measurement indications are therefore written. Ie = 0.9 A Ue = ....... (V)

Stop the group by setting the switch L+/L- to position “0” and deenergize the brake. If we foresee a further starting of the motor set the starting rheostat RA to position "b" again (maximum resistance) and the excitation one RF to position "a" (minimum resistance). Complete the table with the calculated values of: - absorbed power Pin = U I - output power P = 0.1047 n M - efficiency

P

Pin

100

U (V)

I (A)

Pin

(W) G

(N) b

(m) M

(Nm) n

(min-1) P

(W)

(%) 3.5 0 0 0 0 3.5 0.02 0.07 3.5 0.04 0.14 3.5 0.06 0.21 3.5 0.08 0.28 3.5 0.10 0.35 3.5 0.12 0.42 3.5 0.14 0.49 3.5 0.16 0.56 3.5 0.18 0.63 3.5 0.20 0.7 3.5 0.22 0.77

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Draw on the same diagram the output power P, the speed n, the torque M and the efficiency as a function of the absorbed current I.

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Draw on a diagram the mechanical characteristic M = f(n).