How to build a low-loss stepper motor

May 1, 2010 12:00 PM,

Nick Johantgen North American Technical Product Manager Oriental Motor USA Corp. Torrance, Calif.

The quest for energy efficiency has become important even for motors used in positioning systems.

For a stepping motor in action, losses break down into driver and motor losses. The biggest contributors during high motor speeds are iron losses.
Select figure to enlarge.

Stepper motors are well known for their ability to perform accurate positioning in an open-loop mode. But they have a reputation for generating a lot of heat when rotating at high speeds. Generation of heat, of course, implies that the motor is operating at a low efficiency. Manufacturers have increasingly realized that efficiency can be an issue even for positioning motors. So they have adopted construction techniques aimed at boosting the efficiency of steppers, especially when rotating at high speeds.

To understand how to construct a step motor that is energy efficient, it is useful to review the losses that arise when a step motor rotates. Within the motor itself, most losses are copper and iron losses. Copper losses arise from current flowing in the stator windings. Iron losses are generated by the flux change in the rotor and stator core. The flux in the core changes as the rotor turns and as current levels change within the stator windings. Thus iron losses can be classified as those from the electromagnetic field and those from the stator winding.

Iron losses can be further subdivided into eddy current losses and hysteresis losses from magnetic field reversals. Other losses include mechanical loss and stray load loss. But these are small enough to be ignored compared with iron and copper losses.

To understand field iron losses, consider that a common hybrid-type stepping motor has a rotor containing a permanent magnet and teeth on its outer diameter that function as inductors. There are also teeth on the inner diameter of the stator core. Iron losses arise when the rotor rotates and the teeth periodically face each other, changing the flux in the stator core.

Iron loss is given by the expression

W₀ = (2π/60)• N • T
1

where W₀ = field iron loss, W; N = rotational speed, rpm; T = torque, Nm. Then eddy current loss W_e and hysteresis loss W_h per unit mass are respectively

W_e = c_e • B_m² • t² • k² • N²
2

W_h = c_h • B_m^1.6 • k • N
3

In a low-loss stepping motor, field iron loss exceeds the iron loss at speeds above 2,400 rpm because of field weakening. A comparison of conventional and low-loss steppers shows that copper losses rise slightly because current is boosted to make torque more uniform, but stator iron loss drops by 81%.
Select figure to enlarge.

where c_e and c_h = iron loss constants determined by the material used; t = the thickness of the lamination sheets, mm; k = constant determined by the number of pole pairs; B_m = flux density, T; and N = rotating speed, rpm.

From these expressions it becomes clear that eddy current loss is proportional to the square of the rotational speed, and the hysteresis loss is proportional to the rotating speed. The iron loss is the sum of the eddy current loss and the hysteresis loss and it is proportional to somewhat less than the second power of rotational speed.

It should also be noted that there is a loss associated with the motor driver electronics. Driver electronics are designed so that a constant current is provided to the motor regardless of motor load. Thus loss is greatest when the motor has no load, because the power to the motor does no work. Nevertheless, motor losses are relatively large compared with driver losses.

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