[Jim Meyer]

The efficiency of a motor is calculated by taking the power the output shaft is doing divided by the electrical power coming into the motor. One way to calculate output shaft power is to multiply torque times angular velocity (in radians / second). For the motor to draw 20 A you must stall it. It draws much less current when the motor is not stalled. When the motor is stalled the angular velocity of the motor is 0 and therefore the efficiency of the motor at this state is 0. I should also note that the motor output power at free speed is 0 also. The motor can produce not torque at this speed.

The reported power number for the motor is measured at the point where the motor produces the most power. This is usually when the output is spinning at around half of the free speed. This is also the point where most motors are most efficient. I don't know what the motor draws for current at this speed so I really can't calculate the actual efficiency.

The white papers on this site have an excellent documnt that graphs the efficiency, torque, and speed curves for many of the motors we use. I encourage you to check it out.

I hope this helps.