Single Phase Induction Motor
Single Phase Induction Motor
Double revolving field theory
• A single-phase ac current supplies the main winding that produces a pulsating magnetic field.
• Mathematically, the pulsating field could be divided into two fields, which are rotating in opposite directions.
• The interaction between the fields and the current induced in the rotor b
ars generates opposing torque
- The single-phase induction machine is the most frequently used motor for refrigerators, washing machines, clocks, drills, compressors, pumps, and so forth.
- The single-phase motor stator has a laminated iron core with two windings arranged perpendicularly.
- One is the main and The other is the auxiliary winding or starting winding
This “single-phase” motors are truly two phase machines.
The motor uses a squirrel cage rotor, which has a laminated iron core with slots.
Aluminum bars are molded on the slots and short-circuited at both ends with a ring.
Operation
• The single-phase induction motor operation
can be described by two methods:
– Double revolving field theory; and
– Cross-field theory.
• Double revolving theory is perhaps the
easier of the two explanations to understand
• Learn the double revolving theory only
Double revolving field theory
• A single-phase ac current supplies the main winding that produces a pulsating magnetic field.
• Mathematically, the pulsating field could be divided into two fields, which are rotating in opposite directions.
• The interaction between the fields and the current induced in the rotor bars generates opposing torque
• The interaction between the fields and the current induced in the rotor bars generates opposing torque.
• Under these conditions, with only the main field
energized the motor will not start
• However, if an external torque moves the motor in any direction, the motor will begin to rotate
Double revolving field theory
• The pulsating filed is divided a forward and reverse rotating field
• Motor is started in the direction of forward rotating field this generates small (5%) positive slip
Spos= nsy - nm
nsy
• Reverse rotating field generates a larger (1.95%) negative slip pos
Spos= nsy + nm
nsy
• The three-phase induction motor starting torque inversely depends on the slip
• This implies that a small positive slip (0.01–0.03) generates larger torque than a larger negative slip (1.95–1.99)
• This torque difference drives the motor continues to rotate in a forward direction without any external torque.
• Each of the rotating fields induces a voltage in the rotor, which drives current and produces torque.
• An equivalent circuit, similar to the equivalent circuit of a three phase motor, can represent each field
• The parameters of the two circuits are the same with the exception of the slip
• The two equivalent circuits are connected in series.
• The current, power and torque can be calculated
from the combined equivalent circuit using the Ohm Law
• The calculations are demonstrated on a numerical example
Starting torque
• The single-phase motor starting torque is zero because of the pulsating single-phase magnetic flux.
• The starting of the motor requires the generation of a rotating magnetic flux similar to the rotating flux in a three-phase motor.
• Two perpendicular coils that have currents 90° outof- phase can generate the necessary rotating magnetic fields which start the motor.
• Therefore, single-phase motors are built with two perpendicular windings.
• The phase shift is achieved by connecting
– a resistance,
– an inductance, or
– a capacitance in series with the starting winding.
• Most frequently used is a capacitor to generate the starting torque.
• Figure shows the connection diagram of a motor using a capacitor to generate the starting torque.
• When the motor reaches the operating speed, a centrifugal switch turns off the starting winding.
• A less effective but more economical method using shaded pole motors
• The motor has two salient poles excited by ac current.
• Each pole includes a small portion that has a short circuited winding. This part of the pole is called the shaded pole.
• The main winding produces a pulsating flux that links with the squirrel cage rotor.
• This flux induces a voltage in the shorted winding.
• The induced voltage produces a current in the shorted winding.
• This current generates a flux that opposes the main flux in the shaded pole (the part of the pole that carries the shorted winding).
• The result is that the flux in the unshaded and shaded parts of the pole will be unequal.
• Both the amplitude and the phase angle will be different.
• These two fluxes generate an unbalanced rotating field. The field amplitude changes as it rotates.
• Nevertheless this rotating field produces a torque, which starts the motor in the direction of the shaded pole.
• The starting torque is small but sufficient for fans and other household equipment requiring small starting torque.
• The motor efficiency is poor but it is cheap
• The motor has two salient poles excited by ac current.
• Each pole includes a small portion that has a short-circuited winding.
• This part of the pole is called the shaded pole
• Under these conditions, with only the main field
energized the motor will not start
• However, if an external torque moves the motor in any direction, the motor will begin to rotate
Double revolving field theory
• The pulsating filed is divided a forward and reverse rotating field
• Motor is started in the direction of forward rotating field this generates small (5%) positive slip
Spos= nsy - nm
nsy
• Reverse rotating field generates a larger (1.95%) negative slip pos
Spos= nsy + nm
nsy
• The three-phase induction motor starting torque inversely depends on the slip
• This implies that a small positive slip (0.01–0.03) generates larger torque than a larger negative slip (1.95–1.99)
• This torque difference drives the motor continues to rotate in a forward direction without any external torque.
• Each of the rotating fields induces a voltage in the rotor, which drives current and produces torque.
• An equivalent circuit, similar to the equivalent circuit of a three phase motor, can represent each field
• The parameters of the two circuits are the same with the exception of the slip
• The two equivalent circuits are connected in series.
• The current, power and torque can be calculated
from the combined equivalent circuit using the Ohm Law
• The calculations are demonstrated on a numerical example
Starting torque
• The single-phase motor starting torque is zero because of the pulsating single-phase magnetic flux.
• The starting of the motor requires the generation of a rotating magnetic flux similar to the rotating flux in a three-phase motor.
• Two perpendicular coils that have currents 90° outof- phase can generate the necessary rotating magnetic fields which start the motor.
• Therefore, single-phase motors are built with two perpendicular windings.
• The phase shift is achieved by connecting
– a resistance,– an inductance, or
– a capacitance in series with the starting winding.
• Most frequently used is a capacitor to generate the starting torque.
• Figure shows the connection diagram of a motor using a capacitor to generate the starting torque.
• When the motor reaches the operating speed, a centrifugal switch turns off the starting winding.
• A less effective but more economical method using shaded pole motors
• The motor has two salient poles excited by ac current.
• Each pole includes a small portion that has a short circuited winding. This part of the pole is called the shaded pole.
• The main winding produces a pulsating flux that links with the squirrel cage rotor.
• This flux induces a voltage in the shorted winding.
• The induced voltage produces a current in the shorted winding.
• This current generates a flux that opposes the main flux in the shaded pole (the part of the pole that carries the shorted winding).
• The result is that the flux in the unshaded and shaded parts of the pole will be unequal.
• Both the amplitude and the phase angle will be different.
• These two fluxes generate an unbalanced rotating field. The field amplitude changes as it rotates.
• Nevertheless this rotating field produces a torque, which starts the motor in the direction of the shaded pole.• The starting torque is small but sufficient for fans and other household equipment requiring small starting torque.
• The motor efficiency is poor but it is cheap
• The motor has two salient poles excited by ac current.
• Each pole includes a small portion that has a short-circuited winding.
• This part of the pole is called the shaded pole
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