Torque and Speed of Rotation of DC Motor

The torque generated by the motor, and its speed of rotation, are
dependent on each other.
This is a basic characteristic of the motor ; it is a linear relationship and
is used to calculate the no-load speed and the start-up torque of the motor.

The curve for the output power of the motor is deduced from the graph
of torque versus speed.

The torque vs. speed and output power curves depend on the supply
voltage

The torque vs. speed and output power curves depend on the supply
voltage to the motor.
The supply voltage to the motor assumes continuous running of the
motor at an ambient temperature of 20ËšC in nominal operational
conditions.

It is possible to supply the motor with a different voltage (normally
between -50% and + 100% of the recommended supply voltage).
If a lower voltage is used compared to the recommended supply the
motor will be less powerful. If a higher voltage is used, the motor will have a higher output power but
will run hotter (intermittent operation is recommended).

For variations in supply voltage between approximately – 25% to + 50%,
the new torque vs. speed graph will remain parallel to the previous one.
Its start-up torque and no-load speed will vary by the same percentage
(n%) as the variation in supply voltage. The maximum output power is
multiplied by (1 + n%)2.

Example : For a 20% increase in supply voltage

Start-up torque increases by 20% ( x 1.2)

No-load speed increases by 20% ( x 1.2)

Output power increases by 44% ( x 1.44)