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Transcript
Electrical Machine (EET 306/4)
Laboratory Module 6
EXPERIMENT 6
SINGLE PHASE INDUCTION MOTORS – SPLIT PHASE MOTOR,
SPLIT CAPACITOR MOTOR AND SHADED POLE MOTOR
OBJECTIVE:
1. To construct the single-phase induction motors (split-phase, split capacitor and shaded
pole motors).
2. To investigate the starting behaviour of single-phase induction motors.
3. To analyze the behaviour of single-phase induction motors.
Equipment:
Power supply module (NO-5306) ……………………………………………….…. 1 set
Machine field frame module (NO-5310) …………………………………………… 1 set
AC Voltage/Ammeter module (NO-5307) ……………………………………….… 1 set
Split-phase motor graphic board (NO-5319) ………………………………….…… 1 set
Cage rotor (A06) ……………………………………………………………………… 1 set
Wide pole piece for field winding (A10) ………………………….…………………. 2 set
Field winding/300 turns (A13) ….……………………………………………………. 2 set
Narrow pole piece for field winding (A11) ………………………………………….. 2 set
Field winding/1700 turns (A15) …………………………………..…………………. 2 set
Polarization magnetic pole for field winding (A12) .……………………………….. 2 set
Rotor fixture (A20) ……………………………………………………………………. 1 set
Magnetic pole fixture (A21) ………………………………………………………….. 4 set
8 mm spanner (A24) …………………………………………………………………. 1 set
Fixing bolt (A22) ………………………………………………………………………. 4 set
Introduction:
A single-phase induction motor has no intrinsic starting torque. There are three
techniques commonly used to start these motors, and single-phase induction motors are
classified according to the methods used to produce their starting torque. The three major
starting techniques are split phase motor, capacitor-start motor and shaded-pole motor. All
three starting techniques are methods of making one of the two revolving magnetic fields in
the motor stronger than the other and so giving the motor an initial nudge in one direction or
the other.
Split-Phase Motor
A split-phase motor is a single-phase induction motor with two stator windings, main
stator winding (M) and auxiliary starting winding (A) (see figure 6-1). These two windings are
set 90° electrical apart along the stator of the motor and the auxiliary winding is designed to be
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Laboratory Module 6
switched out of the circuit at some set speed by a centrifugal switch. The auxiliary winding is
designed to have a higher resistance/reactance ratio than the main winding so that the current
in the auxiliary winding leads the current in the main winding. This higher R/X ratio is usually
accomplished by using smaller wire for the auxiliary winding.
Figure 6.1 (a) Split-Phase Induction Motor (b) The currents in the motor at starting
condition
To understand the function of the auxiliary winding, refer to Figure 6-2. Since the
current in the auxiliary winding leads the current in the main winding, the magnetic field B A
peaks before the main magnetic field BM. Since BA peaks first and then BM, there is a net
counter clockwise rotation in the magnetic field. In other words, the auxiliary winding makes
one of the oppositely rotating stator magnetic fields larger than the other one and provides a
net starting torque for the motor. A typical torque-speed characteristic is shown in Figure 6-2
(c).
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Laboratory Module 6
Figure 6.2 (a) Relationship of main and auxiliary magnetic fields. (b) IA peaks before IM ,
producing a net counter clockwise rotation of the magnetic fields (c) The resulting
torque-speed characteristic.
In a split-phase induction motor, the current in the auxiliary windings always peaks
before the current in the main winding and therefore the magnetic field from the auxiliary
winding always peaks before the magnetic field from the main winding. The direction of the
rotation of the motor is determined by whether the space angle of the magnetic field from the
auxiliary winding is 90° ahead or 90° behind the angle of the main winding. Since that angle
can be changed from 90°ahead to 90° behind just by switching the connections on the
auxiliary winding, the direction of the rotation of the motor can be reversed by switching the
connections of the auxiliary winding while leaving the main winding’s connection unchanged.
Capacitor-Start Motor
In a capacitor-start motor (see Figure 6.3), a capacitor is placed in series with the
auxiliary winding of the motor. By proper selection of capacitor size, the magneto motive force
of the starting current in the auxiliary winding can be adjusted to be equal to the magneto
motive force of the current in the main winding, and the phase angle of the current in the
auxiliary winding can be made to lead the current in the main winding by 90°.
Since the two windings are physically separated by 90°, a 90° phase difference in
current will yield a single uniform rotating stator magnetic field and the motor will behave just
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Laboratory Module 6
as thought it were starting from a three-phase power source. In this case, the starting torque of
the motor can be more than 300% of its rated value. (See Figure 6.4)
Figure 6.3 (a) Capacitor-Start Induction Motor. (b) Current angles at starting in this
motor
Figure 6.4 Torque-speed characteristic of a capacitor-start induction motor
Permanent Split-Capacitor and Capacitor-Start, Capacitor-Run Motors
If the capacitor’s value is chosen correctly, such a motor will have a perfectly uniform
rotating magnetic field at some specific load and it will behave just like a three-phase induction
motor at that point. The design is called permanent split-capacitor or capacitor-start-and-run
motor (Figure 6.5). Permanent split-capacitor motors are simpler than capacitor-start motors
since the starting switch is not needed.
However, permanent split-capacitor motors have a lower starting torque than capacitorstart motors since the capacitor must be sized to balance the currents in the main and auxiliary
windings at normal-load conditions. Since the starting current is much greater than the normal-
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Laboratory Module 6
load current, a capacitor that balances the phases under normal loads leaves them very
unbalanced under the starting conditions.
Figure 6.5 (a) Permanent split-capacitor induction motor. (b) Torque-Speed
characteristic of this motor
Shaded-Pole Motors
A shaded-pole induction motor is an induction motor with only a main winding. Instead
of having an auxiliary winding, it has salient poles and one portion of each pole is surrounded
by a short-circuited coil called a shading coil (see Figure 6.6a). A time-varying flux is induced
in the poles by the main winding. When the pole flux varies, it induces a voltage and current in
the shading coil which opposes the original change in flux. This opposition retards the flux
changes under the shaded portions of the coils and therefore produces a slight imbalance
between the two oppositely rotating stator magnetic fields. The net rotation is in the direction
from the unshaded to the shaded portion of the pole face. The torque-speed characteristic of a
shaded-pole motor is shown in Figure 6.6b.
Shaded pole produces less starting torque than any other type of induction motor
starting system. They are much less efficient and have a much higher slip than other types of
single-phase induction motors.
Because shaded pole motors rely on a shading coil for their starting torque, there is no
easy way to reverse the direction of rotation of such a motor. To achieve reversal, it is
necessary to install two shading coils on each pole face and to selectively short one or the
other of them.
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Laboratory Module 6
Figure 6.6 (a) Basic Shaded-Pole induction motor. (b) The resulting torque-speed
characteristic.
Procedure:
1. Install the graphic board (NO-5319) for the split phase motor and 3 modules which are
Power Supply (NO-5306), AC Voltage/Ammeter (NO-5307) and AC/DC Machine Field
Frame (NO-5310) on the experimental board rack as shown in Figure 6.7.
Figure 6.7 Split-phase motor circuit
2. Install the field winding, magnetic pole and rotor on the machine field frame as shown
in Figure 6.8.
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Figure 6.8 Assembling diagram of split-phase motor
3. Fit the split-phase motor graphic board (NO-5319) in the terminal slot of the field
winding, AC/DC Machine Field Frame (NO-5310).
4. Configure the connection as shown in Figure 6.7.
5. After re-confirming the connection, turn ‘ON’ the power supply.
6. Remember to short the connecting code between T5 and T6 terminals of graphic board
(NO-5319) for split-phase motor. Which direction does the rotor spinning? Answer the
question in results section.
7. In practical motors, the starting winding part in circuit is open at about 75% of full
speed by centrifugal force switch. To experiment this, take out the connecting code
connected to T5, T6 terminals under operation. After taking out the connection, does it
keep spinning? Why?
8. To experiment the split capacitor method, connect the capacitor C1, 2.5µF to
terminals T5, T6 and then turn ‘ON’ the power supply. Is it started well as compared to
procedure 6? Answer the question in results section.
9. Replace C1, 2.5 µF with C2, 4.5 µF. Then, turn ‘ON’ the power supply. Is it started
even better or faster? Why? Answer the question in results section.
10. Turn ‘OFF’ the power supply. Take out the split-phase motor graphic board, exchange
the polarities of starting winding and connect the circuit for the experiment of rotary
direction conversion as described on Figure 6.9. It means that the polarities A, B of
starting winding are exchanged.
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Laboratory Module 6
Figure 6.9 Control the rotary direction of split-phase motor
11. After confirming the connection, turn ‘ON’ the power supply. Is the rotary direction
different from the previous one? Answer the question in results section.
12. When the experiment is completed, switch ‘OFF’ the power supply and disconnect all
the connections.
13. Then, install 3 modules which are Power Supply (NO-5306), AC Voltage/Ammeter
(NO-5307) and AC/DC machine Field Frame (NO-5310) on the experimental board
rack.
14. Assemble the polarization magnetic pole (A12), field winding (A13) on the machine
field frame (NO-5310) and put the cage rotor (A06) on the rotor fixture.
15. After the installation is completed, connect the circuit as shown in Figure 6.10.
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Figure 6.10 Shaded-pole induction motor circuit
16. Turn ‘ON’ the power supply, is the motor spined? Which direction does it spined?
Answer the question in results section.
17. Turn ‘OFF’ the power supply. Make connection as shown in Figure 6.11.
Figure 6.11 Polarity converting experiment circuit of field winding
18. Turn ‘ON’ the power supply. Is the rotary direction of motor converted? Answer the
question in results section.
19. When the experiment is completed, switch ‘OFF’ the power supply, disconnect all the
connections. Disassemble all the equipment and parts and store them at the
designated places.
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Name:______________________________ Matrix no.:_____________ Date:_________
RESULTS:
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Electrical Machine (EET 306/4)
Instructor Approval: …………………………….
Laboratory Module 6
Date: ………………
Name:______________________________ Matrix no.:_____________ Date:_________
DISCUSSION:
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CONCLUSION:
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Instructor Approval: …………………………….
Date: ………………
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Electrical Machine (EET 306/4)
Laboratory Module 6
Name:______________________________ Matrix no.:_____________ Date:_________
PROBLEMS:
1. Explain why a single-phase single-winding IM produces no starting torque
2. Explain the types of starting methods for single induction motor and their differences?
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Electrical Machine (EET 306/4)
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Instructor Approval: …………………………….
Date: ………………
Name:______________________________ Matrix no.:_____________ Date:_________
3. Explain the reason why the rotary direction is not converted on the shaded pole motor
experiment?
4. What is the used of shaded-pole induction motor?
Instructor Approval: …………………………….
Date: ………………
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