Overview
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Induction motors have been widely used in general industries since they
have simple structure, and are affordable, robust and highly efficient.
When an induction motor rotates at synchronous speed, no torque is produced.
However, when it has a proper slip, the maximum torque can be obtained.
In a cage induction motor, when current flows in the cage, the loss is
caused. So, the duration of rotation needs to be controlled depending on
the amount of heat generation. This note presents the use of magnetic field
analysis to obtain the torque characteristics of a cage induction motor. |
Torque Characteristics
Figure 1 shows the s-T curve at each current amplitude. Figure 2 shows
the I-T curve and Figure 3 shows the I-V curve.
As shown in Figure 1, the slip at which the maximum torque is obtained
varies with current amplitude, and it is determined by the electric resistance
of both coil and cage as well as leakage reactance. Therefore, when the
leakage reactance changes with increase of current, the slip at which the
maximum torque is obtained changes.
As shown in the slope of I-T curve in Figure 2, the torque constant of
this induction motor is about 0.8 N·m/A. Since the applicable
primary voltage is limited, the primary voltage at which the maximum torque
is obtained needs to be evaluated from the I-V curve. |
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Eddy Current Density Distribution / Joule Loss
| Figure 4 shows the eddy current loss density distribution at current amplitude
of 10 A. Figure 5 shows the Joule loss of the cage at each current value.
Both are results when the maximum torque is obtained. As shown in Figure
4, large current flows in bars so the density is seen to be more than 2
x 107 A/m2. The heat generation becomes so large that it is hard for the motor to
rotate continuously. In Figure 5, the Joule loss increases like a quadratic
curve as a function of current. By estimating the heat generation from
the amount of losses, the duration of rotation at the maximum torque can
be determined. Thus, to obtain the maximum torque of the cage induction
motor, the amount of current flowing in the cage and the amount of losses
need to be evaluated. |
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