# Torque Varies at Fixed Input Current for a Servomotor

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#### Darren Ng

Dear all,

I am currently working on a numerical model using the data collected from a servo motor. Based on some prior knowledge about the DC motor, the torque is supposed to be proportional to the current. However, to my surprise, the experimental results did not seem to be in line with my hypothesis.

In one of the experiment, the input current is kept constant and the servomotor is used to carry a weight at an equilibrium position. By slowly removing part of the weight, the position does not change at all even though the current remains constant. Does this actually tell that the output torque of the servomotor is not proportional to the input current?

#### oneye14

It's unclear what you are asking the servomotor to do in your experiments. Are you applying constant current to a winding(s) directly, or is the motor designed to have a constant current supply?

A servomotor is not a DC motor - the output torque of the servomotor may be proportional to the commands received and the load requirements imposed upon it.

So yes - you have demonstrated the output torque of the servomotor is not proportional to the input current - nor should it be.

#### William Hinton

oneye14

Many servo motors cah supply full torque at zero speed. This is how they hold position when powered up. many times it takes as much power (AMPS and VOLTS) to hold the shaft in place as it takes to rotate it.