In Part 1, we covered motor efficiency due to electrical and magnetic power losses in the motor itself. For a complete motion-control system, however, motor-drive loss must also be taken into account.

The motor model, valid for PMS and brush motors, is shown below.

The power "overhead" of the drive can be modeled simply as a fixed power loss, P_z, that does not vary with motor speed or torque. Starting once again with the basic torque-speed equation,

Mechanical output power is:

Electrical input power is the motor drive power (first term) plus the drive power loss:

Efficiency is then

This reduces to

For no drive loss, P_z = 0, and h  = w /w ₀.

The second term in the denominator of h is the ratio of the fixed drive loss to the motor electrical power loss, V_s²/R, which is also T_{0× w 0}. (Recall that T_{0× w 0}/4 is the maximum motor output power for a given V_s.) This ratio, z, is a measure of the fraction of drive loss to that of a characteristic motor power value. Efficiency is plotted against speed below, with z as a parameter, for no drive loss (z = 0) up to drive loss equal to motor electrical loss.

The speed at which the maximum efficiency occurs can be found by differentiating h with respect to w , setting , and solving for w  = w _p.

For w  < w ₀,

Its plot as a function of z is shown below.

From the plot of h (w , z) above, note that the peak decreases in frequency as z increases. This is consistent with the plot of w _p/w ₀ = w _p0, as plotted above. At infinite z, motor-drive loss completely dominates efficiency and the peak efficiency occurs at the degenerate value of zero speed. As the motor-drive power loss approaches zero, the motor speed at which peak efficiency occurs approaches the no-load speed, w ₀. This is the same direction of shift in peak efficiency as when motor losses approach the ideal.

In the final part of this motor efficiency article, we will look at fan-load efficiency.