Introduction to Single Phase Induction Motors
Induction
motors are the most popularly used motors among all the other electric motors
in industries. They are ac fed asynchronous motors as discussed in "induction motor basics”.
Since the AC supply is available in three phase or single phase, induction motors are
designed to operate either on three phase ac supply or single-phase ac supply.
In its three-phase form means three phase induction motors, these are used in
high power applications such as cranes, hoist, conveyor lines in manufacturing
plants, drill machines, lathe machines etc. While in its single-phase form
means single phase AC induction motor, these are most pronounced in low power
applications such as household fans, washing machines, AC, pumps, vacuum
cleaners, kitchen equipment etc.
What is a Single-Phase Induction Motor?
A single-phase induction motor is an induction motor designed to operate on single-phase AC supply. These motors are used in low power applications, as discussed above, and are available in fractional kilowatt ratings, hence also known as fractional kilowatt Motors.
Construction of Single-Phase Induction Motor
Like
any other electric motor, a single-phase induction motor consists of two main
parts i.e. stator and rotor. The stator is the stationary part and rotor is the
rotating part.
Stator of the Single-Phase Induction Motor
The
construction of the stator of the single-phase induction motor is similar to
that of the stator of the three-phase induction motor (explained in detail in
“induction motor basics”). The only key-point here is that the single-phase
induction motor’s stator is wound by a single-phase distributed winding, rather
than a three-phase winding.
Rotor of the Single-Phase Induction Motor
Unlike
three phase induction motors, where two types of rotor constructions are used,
that is squirrel cage rotor and slip ring rotor, single phase induction motor
is essentially used squirrel cage rotors.
Working of the Single-Phase Induction Motors
The
basic working principle for all the induction machines is the same, that is
based on Faraday's Law of Electromagnetic Induction, where the torque developed by the interaction of stator magnetic field (produced by the given AC
supply) and rotor magnetic field (produced by the current flows in the short-circuited
rotor's conductors).
In a single-phase induction motor, when the single phase AC supply is applied to stator winding, it produces a sinusoidally distributed rotating magnetic field in the air gap, whose amplitude is pulsating in nature.
This
rotating magnetic field rotates at synchronous speed in the air gap and cuts
the rotor’s conductors which induces voltage and current in the rotor’s
conductors.
This induced current in the rotor conductor produces its own magnetic field which interacts with the stator magnetic field, similarly to three phase induction motors. But single phase induction motors fail to start on their own due to the presence of the double revolving field.
Double Revolving Field Theory
According
to double revolving field theory, the pulsating magnetic field produced by the
single-phase winding contains two components of equal magnitude, rotating at
synchronous speed in opposite directions: one in forward direction and the
other in the backward direction. The forward field produces the forward torque
which tends the rotor to rotate in forward direction. Similarly, the backward
field produces the backward torque which tends the rotor to rotate in the
opposite direction. As both of these torques are equal and opposite, these
cancel out each other which results in the rotor standstill at its
position. Hence, we can say that the motor fails to start.
Mathematical
Explanation
As
discussed above, when a single-phase AC supply is applied to the stator
winding, it produces sinusoidally distributed MMF whose peak value pulsates
with time.
The equation represents the sinusoidally distributed MMF in the air gap is
F = Fpeak Cos θ
where, θ is the angle measured from the axis of winding.
Since
the peak value of the MMF pulsates with time, we can express it as:
Fpeak =
Fmax cos ωt
Substituting
this value in the MMF equation, we get,
F
= Fmax cos θ . cosωt
Using
Trigonometric identities, this expands to:
F
= ½ Fmax cos (θ - ωt) + ½ Fmax
cos(θ + ωt)
By
observing the above equation, we can conclude that the magnetic field produced
by the single-phase winding comprises two components of equal magnitude,
- One is forward magnetic field, rotates in forward direction at synchronous speed, Ff = ½ Fmax cos (θ - ωt)
- And the other is a backward field, rotates in backward direction at synchronous speed, Fb = ½ Fmax cos(θ + ωt)
Since there are two magnetic fields having the same magnitude forward magnetic field and backward magnetic field. The forward magnetic field produces forward torque which tends the rotor to rotate in forward direction similarly, backward field produces backward torque which tends the rotor to rotate in backward direction. As these torques is equal, they cancel out each other, preventing the motor from starting. That's why we say that single phase induction motors are not self-starting. Hence we need external staring methods of single phase induction motor.
However,
if any external force is applied in any direction, the motor will continue
to rotate in that direction. This characteristic allows a single-phase
induction motor to operate in both directions.
Single Phase Induction Motor Slip
As we
discussed above, the rotating magnetic field generated by single phase winding
consists of two components: the forward field and the backward field.
These two fields rotate in opposite directions at synchronous speed Ns.
Now, let us say that the rotor rotates in forward field direction at a speed N. In this case the slip of the rotor with respect to the forward rotating field can be expressed as follows.
Sf
(Forward Slip) = (Ns - N) / Ns = s
(similar to the slip in three phase phase induction motor)
And the slip of the rotor with respect to the backward rotating field can be expressed as
Similarly,
if the rotor rotates in the direction of the backward field, the slip of the
rotor with respect to the backward field is s, while the slip of the rotor with
respect to the forward field is (2 - s).
Therefore,
the rotor slip in the case of a single phase induction motor with respect to
the two rotating magnetic fields are different and are given by the equation
(1) and (2) above.
Torque Slip Characteristics of Single-Phase Induction Motor
As we know, the torque equation of the induction motor is (refer Torque in Three Phase Induction Motor)
Now as we
at the starting of the single-phase induction motor, the speed of the rotor is
zero (N = 0). So, the forward slip and the backward slip is equal to 1
(calculated from the equations (1) & (2)).
Since the forward field and backward field are the same i.e. equals to 1, it means that the two rotating magnetic fields induce equal currents in the squirrel cage rotor. Consequently, these two fields produce equal torque in opposite directions, which cancel out each other, resulting in the net starting torque being zero. Thus, we can say that the single-phase induction motors are non self starting.
As, sb = sf
= 1, then Tf = Tb
However, if
the rotor is made to rotate at a speed N in any direction by any external
means. Then these two slips become different (i.e. s and 2 - s). In this
condition the backward slip (2 - s) is significantly greater than the forward
slip s.
Note :- In running condition N is nearly equal to Ns
and s is nearly equal to zero.
As a
consequence, the backward field induces a much larger rotor current compared to
standstill and has a lower power factor. Due to this large induced current, the
rotor produces a stronger magnetic field which opposes the backward field
significantly, resulting in the substantial reduction of the backward field.
On the
other hand, the low slip forward rotating field induces smaller current of
higher power factor in the rotor than at stand still. This leads to the greater
enhancement in the forward flux wave.
This
reduction in the backward field and strengthening of the forward field is slip
dependent. As the speed of the rotor increases, the forward slip decreases and
the backward slip increases, which leads to the greater reduction of the
backward field and the strengthening of the forward field. In fact, at a
speed close to the synchronous speed, the forward field may be several times
stronger than the backward field. As a result, there is a net running toque.
As we have discussed in the
above section, there are two magnetic fields: forward magnetic field and the
backward magnetic field and each of these magnetic fields generate its own
torques. So, the complete torque-slip characteristics is the sum of the torque
speed characteristics due to these two fields. And resultant of these two
characteristics represents the final torque slip characteristics of the single-phase
induction motor
Below the given figure illustrate the complete torque slip characteristics of a single-phase induction motor.
The weakening of one field and
simultaneous strengthening of the other field leads to the torque speed
characteristics of a single-phase induction motor like that of a 3-phase
induction motor in the speed region close to synchronous speed. The fact
of zero starting torque or non self-starting nature of single phase induction
motor can be clearly seen in the resultant torque slip characteristics of
single phase induction motor.
Starting Methods of Single-Phase Induction Motor
As
discussed in the above section, a single-phase induction motor is non
self-starting, so we need the external circuitry to start the single phase
induction motor. There are four various types of starting method of single-phase
induction motors. Based on these starting methods a single-phase induction
motor can be classified as follows,
- Split Phase or Resistance Split Phase Induction Motor
- Capacitor Start and Run Induction Motor
- Permanent Capacitor or Capacitor Start and Capacitor Run Induction Motor
- Shaded Pole Induction Motor
We
discussed these “starting methods of single-phase induction motors” in depth in
a separate article.
Difference Between Three-Phase Induction Motors & Single-Phase Induction Motor
Three Phase Induction Motors | Single Phase Induction Motors |
---|---|
Three-phase induction motors are self-starting, so they do not require additional starting methods. However, to limit the high starting current, certain starting techniques are used. | Single phase induction motors are non self starting, so they require additional starting methods, which require auxiliary winding, centrifugal switch etc. |
For the same rating design cost is less. | For the same rating design cost is more. |
For the same size they produce more power and torque. | For the same size they produce less power and torque. |
These are more efficient. | These are less efficient |
These motors operate on better power factor compare to single phase induction motor. | Comparatively Low power factor |
The absence of the backward rotating field in these motors causes less noisy operation. | The presence of the backward rotating field in these motors causes higher vibrations and noise during operation. |
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