Biomedical Engineering Reference
In-Depth Information
FIGURE 3-51
Torque and current
versus slip for an
induction motor.
[Adapted from
(Kuphaldt 2008)]
The frequency of the current induced into the rotor conductors is only as high as the line
frequency at motor start and decreases as the rotor approaches synchronous speed. This
rotor frequency,
f
r
(Hz), is
f
r
=
sf
(3.57)
Slip at 100% torque is typically 5% or less in induction motors. Thus, for
f
=
50 Hz line
frequency, the frequency of the induced current in the rotor
f
r
=
5 Hz.
Figure 3-51 shows the torque, speed, and current relationships of a typical induction
motor. It can be seen that the starting torque, known as locked rotor torque (LRT), is higher
than 100% of the full-load torque (FLT), the safe continuous torque rating. The locked
rotor torque is about 175% of FLT for the example shown in Figure 3-51.
Starting current, known as locked rotor current (LRC), is 500% of full-load current
(FLC), the safe running current. The current is high because this is analogous to a shorted
secondary on a transformer. As the rotor starts to rotate, the torque generally decreases
slightly to a value known as the pull-up torque. This is the lowest value of torque ever
encountered by the starting motor. As the rotor gains 80% of synchronous speed, torque
increases from 175% up to 300% of the full-load torque. This breakdown torque is due to
the larger than normal 20% slip. The current has decreased only slightly at this point but
decreases rapidly beyond it. As the rotor accelerates to within a few percent of synchronous
speed, both torque and current decrease sharply.
Slip will be only a few percent during normal operation. For a running motor, any
portion of the torque curve below 100% rated torque is normal. The motor load determines
the operating point on the torque curve. While the motor torque and current may exceed
100% for a few seconds during starting, continuous operation above 100% can damage
the motor. Any motor torque load above the breakdown torque will stall the motor. The
torque, slip, and current will approach zero for a
no mechanical torque
load condition
(Kuphaldt, 2008).
0
.
05
×
50
=
2
.
Efficiency:
Large three-phase motors are more efficient than smaller three-phase motors
and almost all single-phase motors. Large induction motor efficiency can be as high as
