Overview
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Universal motors rotate by either AD or DC. Since universal motors have
a simple structure which is robust, compact, and capable of high speeds,
they are used in home appliances and industrial electric tools.
Also, in universal motors, the rotation speed is determined by the load
when field coil and armature coil are connected in series.
This note presents the use of magnetic field analysis to obtain the characteristics
of the universal motor, including torque versus current (T-I), torque versus
speed (T-N), and magnetic flux density distribution. |
Magnetic Flux Density Distribution
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Figure 1 shows the magnetic flux density distribution at 4,000 rpm and
8,000 rpm. More current flows through the universal motor at high speed
than at low speed, so torque increases as the magnetic flux density becomes
higher. However, low speed tends to cause magnetic saturation, which may
result in deteriorating the characteristic of the motor. Therefore, it
is ideal to design magnetic circuit and motor that cause no magnetic saturation
even at low speed . |
Speed-Torque Curve / Torque-Current Curve
Figure 2 shows the speed-torque curve and Figure 3 shows the torque-current
curve.
The graphs show that, in the universal motor, the torque decreases when
the rotation speed increases and the torque increases when the current
increases. |
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