Overview
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A brush motor rotates when the brush and commutator alternate the direction
of the current passing through the armature coils.
The torque needs to be evaluated based on the rotation speed of a motor
because the torque varies with the rotation speed if the supply voltage
is constant.
Evaluating the relationship between the torque and current during the design
stage is also advantageous because the torque is proportional to the current.
This example presents the use of a magnetic field analysis to obtain the
speed versus torque curve and torque versus current curve for a slot motor
that has 2 brushes, 6 poles, and 19 slots. |
Speed vs. Torque Curve/Torque vs. Current Curve
The speed versus torque curve is indicated in Fig. 1, and the torque versus
current curve is indicated in Fig. 2.
The torque of the DC brush motor decreases as the rotation speed increases,
as indicated in Fig. 1. This is because the current flowing in the coil
is reduced by the increase of back electromagnetic voltage produced as
the rotation speed increases. The relationship between the current and
torque is mostly linear, and the torque coefficient of this brush motor
is 0.049 N•m/A, as indicated in Fig. 2. |
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Magentic Flux Density Distribution
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The magnetic flux density distribution at a rotation speed of 2000 rpm
and 4000 rpm is indicated in Fig. 3.
The magnetic flux density increases at a lower rotation speed, as indicated
in Fig. 3. This is because the current flowing in the circuit is larger
as previously noted. A motor and control design that does not allow magnetic
saturation to occur at low rotation speeds is desirable because the performance
of the motor worsens with magnetic saturation. |
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