SPEED-TORQUE CHARACTERISTICS OF INDUCTION MOTORS
SPEED-TORQUE CHARACTERISTICS OF
INDUCTION MOTORS
The X axis shows
speed and
Slip. The Y-axis shows
the torque and current. The
Characteristics are
drawn with rated voltage and
Frequency supplied to
the stator.
During start-up, the
motor typically draws up to seven
times the rated
current. This high current is a result of
Stator and rotor
flux, the losses in the stator and rotor
Windings and losses
in the bearings due to friction.
This high starting
current overcomes these
Components and produces
the momentum to rotate the rotor.
At start-up, the
motor delivers 1.5 times the rated
Torque of the motor.
This starting torque is also
Locked rotor torque
(L RT). As the speed increases,
The Current drawn by
the motor reduces slightly (see
The current drops significantly,
when the motor speed
Approaches ~80% of
the rated speed. At base speed,
The motor draws the
rated current and delivers the
Rated torque.
At base speed, if the
load on the motor shaft is
Increased beyond its
rated torque, the speed starts
Dropping and slip
increases. When the motor is
Running at
approximately 80% of the synchronous
Speed, the load can
increase up to 2.5 times the rated torque.
This torque is called
breakdown torque. If the load on
The motor is
increased further; it will not be able to
Take any further load
and the motor will stall.
In addition, when the
load is increased beyond the
Rated load, the load
current increases following the
Current characteristic
path. Due to this higher current
Flow in the windings,
inherent losses in the windings
Increase as well.
This leads to a higher temperature in
The motor windings.
Motor windings can withstand
Different temperatures,
based on the class of
Insulation used in
the windings and cooling system
Used in the motor.
Some motor manufacturers provide
The data on overload
capacity and load over duty cycle.
If the motor is
overloaded for longer than
Recommended, then the
motor may burn out.
As seen in the
speed-torque characteristics, torque is
Highly nonlinear as
the speed varies. In many applications,
The speed needs to be
varied, which makes the
Torque vary. We will
discuss a simple open loop
Method of speed
control called, Variable Voltage
Variable Frequency
(VF or V/f) in this application note.
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| Induction Motors: Torque/Speed Curve |
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