As the induction motor is located from
no load to full load, its speed decreases hence slip increases. Due to
the increased. load, motor has to produce more torque to satisfy load
demand. The torque ultimately depends on slip as explained earlier. The
behaviour of motor can be easily judged by sketching a curve obtained by
plotting torque produced against slip of induction motor. The curve
obtained by plotting torque against slip from s = 1 (at start) to s = 0
(at synchronous speed) is called torque-slip characteristics of the
induction motor. It is very interesting to study the nature of
torque-slip characteristics let us divide the slip range (s = 0 to s =
1) into two parts and analyse them independently.
i) Low slip region :
torque is directly proportional to slip. So as load increases, speed
decreases, increasing the slip. This increases the torque which
satisfies the load demand.
Hence the graph is straight line in nature.
ii) High slip region :
Now when load increases, load demand increases but speed decreases. As
speed decreases, slip increases. In high slip region as T ?1/s, torque decreases as slip increases.
But torque must increases to satisfy the load demand. As torque
decreases, due to extra loading effect, speed further decreases and slip
further increases. Again torque decreases as T ?1/s
hence same load acts as an extra load due to reduction in torque
produced. Hence speed further drops. Eventually motor comes to
standstill condition. The motor can not continue to rotate at any point
in this high slip region. Hence this region is called unstable region of
So torque – slip characteristics has two parts,
1. Straight line called stable region of operation
2. Rectangular hyperbola called unstable region of operation.
In low slip region, as load increases, slip increases and torque also
increases linearly. Every motor has its own limit to produce a torque.
The maximum torque, the motor can produces as load increases is Tm which occurs at s = sm. So linear behaviour continues till s = sm.
If load is increased beyond this limit, motor slip acts dominantly
pushing motor into high slip region. Due to unstable conditions, motor
comes to standstill condition at such a load. Hence i.e. maximum torque
which motor can produce is also called breakdown torque or pull out
torque. So range s = 0 to s = sm is called low slip region,
known as stable region of operation. Motor always operates at a point in
this region. And range s = sm to s = 1 is called high slip
region which is rectangular hyperbola, called unstable region of
operation. Motor can not continue to rotate at any point in this region.
At s = 1, N = 0 i.e. start, motor produces a torque called starting torque denoted as Tst.
The entire torque – slip characteristics is shown in the Fig. 1.
|Fig. 1 Torque speed characteristics|
When the load on the motor increases, the torque produced increases as
speed decreases and slip increases. The increases torque demand is
satisfied by drawing motor current from the supply.
The load which motor can drive safely while operating continuously and
due to such load, the current drawn is also within safe limits is called
full load condition of motor. When current increases, due to heat
produced the temperature rise. The safe limit of current is that which
when drawn for continuous operation of motor, produces a temperature
rise well within the limits. Such a full load point is shown on the
torque-slip characteristics torque as TF.L.
The interesting thing is that the load on the motor can be increased
beyond point C till maximum torque condition. But due to high current
and hence high temperature rise there is possibility of damage of
winding insulation, if motor is operated for longer time duration in
this region i.e. from point C to B. But motor can be used to drive
loads more than full load, producing torque upto maximum torque for
short duration of time. Generally full load torque is less than the
OC upto full load condition allow motor operation continuously and
safely from the temperature point pf view. While region CB is possible
to achieve in practice but only for short duration of time and not for
continuous operation of motor. This is the difference between full load
torque and the maximum or breakdown torque. The breakdown torque is also
called stalling torque.
When the slip lies in the region 0 and 1 i.e. when 0 ?s ?1, the machine
runs as a motor which is the normal operation. The rotation of rotor is
in the direction of rotating field which is developed by stator
currents. In this region it takes electrical power from supply lines and
supplies mechanical power output. The rotor speed and corresponding
torque are in same direction.
When the slip is greater than 1, the machine works in the braking mode.
The motor is rotated in opposite direction to that of rotating field.
In practice two of the stator terminals are interchanged which changes
the phase sequence which in turn reverses the direction of rotation of
magnetic field. The motor comes to quick stop under the influence of
counter torque which produces braking action. This method by which the
motor comes to rest is known as plugging. Only care is taken that the
stator must be disconnected from the supply to avoid the rotor to rotate
in other direction
run the induction machine as a generator, its slip must be less than
zero i.e. negative. The negative slip indicates that the rotor is
running at a speed above the synchronous speed. When running as a
generator it takes mechanical energy and supplies electrical energy from
negative slip, generation action takes place and nature of torque – slip
characteristics reverses in this generating region.
|Fig. 2 Regions of torque – slip characteristics|