Stall current

From Combat Robot

The current draw of a permanent magnet DC electric motor at a given voltage is inversely proportional to the speed of the motor. Minimum current draw is at maximum no-load speed, and maximum current draw is at zero RPM (stall) where the motor is unable to turn. Maximum current draw is referred to as 'stall current' or 'stall amps'.

Knowing the stall current of a motor is valuable when planning for worst-case design perameters, although a well-designed robot should never encounter a stall condition.

A safe, simple, and accurate method for estimating stall current of DC motors is called the D-cell method:

- Start with a new, fresh alkaline D-cell.
- Clamp the motor shaft to prevent turning, if required.
- Place the motor and D-cell in series with an ammeter capable of reading at least 15 amps.
- Close the circuit, take the amperage reading, and quickly disconnect the circuit.

The reading gives you the stall current for the motor at 1.5 volts. Stall current is proportional to voltage. To estimate the stall current at full operating voltage, multiply the reading obtained with the D-cell by (V / 1.5), where V is the operating voltage for yout robot.

Example: The stall current for your motor at 1.5 volts is 6.0 amps. Your robot has a 12 volt battery. Estimated stall current at 12 volts will be:

6.0 * (12 / 1.5) = 6.0 * ( 8 ) = 48 amps