UNIT-1
SINGLE-PHASE INDUCTION MOTORS
1.1 INTRODUCTION:
There are two basic reasons for the use of single-phase motors rather than 3-phase motors.
1. For reason of economy, most houses, offices and also rural areas are supplied with single
phase a.c, as power requirements of individual load items are rather small.
2. The economics of the motor and its branch circuit.
Fixed loads requiring not more than 0.5KW can generally be served most economically
with single phase power and a single phase motor.
Single phase motors are simple in construction, reliable, easy to repair and comparatively
cheaper in cost and therefore, find wide use in fans, refrigerators, vacuum cleaners,
washing machines, other kitchen equipment, tools, blowers, centrifugal pumps, small
farming appliances etc.
Because of above reasons motors of comparatively small ratings (mostly in fractional
KW ratings) are manufactured in large number to operate on single phase ac at standard
frequencies. An indication of the number of such motors can be had from the fact that the sum of
total of all fractional kilowatt motors in use today far exceeds the total of integral kilowatt
motors of all types.
1.2 TYPES OF SINGLE-PHASE MOTOR:
The Single phase motors may be of the following types:
1. Single-phase Induction Motors:
A. Split-phase motors
(i) Resistance-start motor
(ii) Capacitor-start motor
(iii) Permanent-split (single-value) capacitor motor
(iv) Two-value capacitor motor.
B. Shaded-pole induction motor.
, C. Reluctance-start induction motor.
D. Repulsion-start induction motor.
2. Commutator-Type, Single-Phase Motors:
A. Repulsion motor.
B. Repulsion-induction motor.
C. A.C series motor.
D. Universal motor.
3. Single-phase Synchronous Motors:
A. Reluctance motor.
B. Hysteresis motor.
C. Sub-synchronous motor.
1.3 SINGLE-PHASE INDUCTION MOTORS
Applications and Disadvantages:
1.31 Applications:
Single phase induction motors are in very wide use in industry especially in fractional
horse-power field.
They are extensively used for electrical drive for low power constant speed apparatus
such as machine tools, domestic apparatus and agricultural machinery in circumstances
where a three-phase supply is not readily available.
Single phase induction motors sizes vary from 1/400 kw to 1/25 kw are used in toys, hair
dryers, vending machines etc.
Universal motor is widely used in portable tools, vacuum cleaners& kitchen equipment.
1.32 Disadvantages:
Though these machines are useful for small outputs, they are not used for large powers as
they suffer from many disadvantages and are never used in cases where three-phase machines
can be adopted.
The main disadvantages of single-phase induction motors are:
1. Their output is only 50% of the three-phase motor, for a given frame size and
temperature rise.
, 2. They have lower power factor.
3. Lower efficiency.
4. These motors do not have inherent starting torque.
5. More expensive than three-phase motors of the same output.
6. Low overload capacity.
1.4 CONSTRUCTION OF SINGLE PHASE INDUCTION MOTOR:
Single phase induction motor is very simple and robust in construction. The stator
carries a distributed winding in the slots cut around the inner periphery. The stator conductors
have low resistance and they are winding called Starting winding is also mounted on the stator.
This winding has high resistance and its embedded deep inside the stator slots, so that they have
considerable inductance. The rotor is invariably of the squirrel cage type. In practice, in order to
convert temporarily the single phase motor into two-phase motor, auxiliary conductors are
placed in the upper layers of stator slots. The auxiliary winding has a centrifugal switch in series
with it. The function of the switch is to cut off the starting winding, when the rotor has
accelerated to about 75% of its rated speed. In capacitor-start motors, an electrolytic capacitor of
suitable capacitance value is also incorporated in the starting winding circuit.
The main stator winding and auxiliary (or starting) winding are joined in parallel, and
there is an arrangement by which the polarity of only the starting winding can be reversed. This
is necessary for changing the direction of rotation of the rotor.
Fig: 1.41
SINGLE-PHASE INDUCTION MOTORS
1.1 INTRODUCTION:
There are two basic reasons for the use of single-phase motors rather than 3-phase motors.
1. For reason of economy, most houses, offices and also rural areas are supplied with single
phase a.c, as power requirements of individual load items are rather small.
2. The economics of the motor and its branch circuit.
Fixed loads requiring not more than 0.5KW can generally be served most economically
with single phase power and a single phase motor.
Single phase motors are simple in construction, reliable, easy to repair and comparatively
cheaper in cost and therefore, find wide use in fans, refrigerators, vacuum cleaners,
washing machines, other kitchen equipment, tools, blowers, centrifugal pumps, small
farming appliances etc.
Because of above reasons motors of comparatively small ratings (mostly in fractional
KW ratings) are manufactured in large number to operate on single phase ac at standard
frequencies. An indication of the number of such motors can be had from the fact that the sum of
total of all fractional kilowatt motors in use today far exceeds the total of integral kilowatt
motors of all types.
1.2 TYPES OF SINGLE-PHASE MOTOR:
The Single phase motors may be of the following types:
1. Single-phase Induction Motors:
A. Split-phase motors
(i) Resistance-start motor
(ii) Capacitor-start motor
(iii) Permanent-split (single-value) capacitor motor
(iv) Two-value capacitor motor.
B. Shaded-pole induction motor.
, C. Reluctance-start induction motor.
D. Repulsion-start induction motor.
2. Commutator-Type, Single-Phase Motors:
A. Repulsion motor.
B. Repulsion-induction motor.
C. A.C series motor.
D. Universal motor.
3. Single-phase Synchronous Motors:
A. Reluctance motor.
B. Hysteresis motor.
C. Sub-synchronous motor.
1.3 SINGLE-PHASE INDUCTION MOTORS
Applications and Disadvantages:
1.31 Applications:
Single phase induction motors are in very wide use in industry especially in fractional
horse-power field.
They are extensively used for electrical drive for low power constant speed apparatus
such as machine tools, domestic apparatus and agricultural machinery in circumstances
where a three-phase supply is not readily available.
Single phase induction motors sizes vary from 1/400 kw to 1/25 kw are used in toys, hair
dryers, vending machines etc.
Universal motor is widely used in portable tools, vacuum cleaners& kitchen equipment.
1.32 Disadvantages:
Though these machines are useful for small outputs, they are not used for large powers as
they suffer from many disadvantages and are never used in cases where three-phase machines
can be adopted.
The main disadvantages of single-phase induction motors are:
1. Their output is only 50% of the three-phase motor, for a given frame size and
temperature rise.
, 2. They have lower power factor.
3. Lower efficiency.
4. These motors do not have inherent starting torque.
5. More expensive than three-phase motors of the same output.
6. Low overload capacity.
1.4 CONSTRUCTION OF SINGLE PHASE INDUCTION MOTOR:
Single phase induction motor is very simple and robust in construction. The stator
carries a distributed winding in the slots cut around the inner periphery. The stator conductors
have low resistance and they are winding called Starting winding is also mounted on the stator.
This winding has high resistance and its embedded deep inside the stator slots, so that they have
considerable inductance. The rotor is invariably of the squirrel cage type. In practice, in order to
convert temporarily the single phase motor into two-phase motor, auxiliary conductors are
placed in the upper layers of stator slots. The auxiliary winding has a centrifugal switch in series
with it. The function of the switch is to cut off the starting winding, when the rotor has
accelerated to about 75% of its rated speed. In capacitor-start motors, an electrolytic capacitor of
suitable capacitance value is also incorporated in the starting winding circuit.
The main stator winding and auxiliary (or starting) winding are joined in parallel, and
there is an arrangement by which the polarity of only the starting winding can be reversed. This
is necessary for changing the direction of rotation of the rotor.
Fig: 1.41
