1. Correspondence
ECONOMICAL STEPPER MOTORS FOR SPEED-CONTROL APPLICATIONS
Indexing terms: Stepping motors, Velocity control
Abstract: It is shown that automobile alternators can provide a low-cost alternative to a conventional stepper
motor in speed-control applications. Typical performance curves using a transistorised inverter are provided.
Introduction: Stepper motors are widely used in industry, Measured line voltages and line currents at different
and elsewhere, for two principal purposes;position control motor speeds are shown in Fig. 2. At low speeds the current
and speed control. To these ends a wide variety of motors waveform has the expected form while, at higher speeds,
differing in step size, speed, drop-out torque and cost have more distortion is present. As discussed by Tal3 this
been developed and marketed. In general the emphasis has constant-current excitation maximises the output power
been on higher resolution at the expense of speed and cost. for a given thermal dissipation in the motor. These wave-
Such motors are ideal for operating machine tools and forms, although distorted, are closer approximations to
other applications where an accurate position must be sinusoidal currents than square waves and give close to
maintained for some process time and then changed. optimal performance.
In many applications, however, these motors, while In practice, with a 25—40 V d.c. busbar, constant motor
suitable, are overdesigned for the purpose. For example, currents of 10 A can be achieved with d.c. busbar currents
in chemical plants a constant rate of flow through some of the order of 3—2 A when the motor is unloaded. At
positive-displacement pump requires a constant-speed higher loads the d.c. current and the maximum line current
motor. The step size is, in this case, largely irrelevant, and tend to become more equal. The exact ratio between these
significant savings in cost can readily be achieved using currents depends on the d.c. busbar voltage, the inverter
lower-resolution motors. Such motors are not usually switching speed and efficiency, the motor core and I2R
available from traditional stepper-motor sources, but losses, and the mechanical load.
automobile alternators have been found to be extremely Performance curves for a typical automobile alternator
useful and almost universally available. They are available used as a stepper motor driven with a constant current
with power ratings up to ~ 1 kW at much lower cost than transistorised inverter are shown in Fig. 3. In this case the
conventional stepper motors. maximum power output is 180 W at 1800 r/min. The motor
has a useful holding torque of 1 -3 Nm and a maximum
Description and results: Automobile alternators are usually speed of 2500 r/min. Higher d.c. busbar voltages are
star-connected 3-phase machines with, commonly, 8 or 12 effective in increasing the maximum speed and higher
poles giving 24 or 36 equivalent steps/revolution. To (constant) currents increase the holding torque.
modify an automobile alternator for use as a stepper motor
it is only necessary to disconnect the rectifier stack and d.c. busbar «•
connect a 3-phase inverter directly to the stator windings.
Rotor excitation is conveniently provided using the slip
rings on the machine. In explosion-critical environments
such external rotor excitation can be dispensed with by
replacing the rotor winding with toroidal permanent
magnets on a (new) nonmagnetic shaft.
A suitable inverter configuration is shown schematically ^® A
in Fig. 1. In essence this inverter is less complex than that
required for a 2- or 4-phase system, as it only requires three
'totem-pole' drivers. Following Nagasaka and Shinohara,1 current
constant current excitation has been used, but the select /control
implementation, in this case, is rather novel as it uses the
inductance of the motor in conjunction with the flywheel
diodes and transistors as a high-efficiency switching
regulator.
Transistors 1—6 are switched in the usual 'quasi-square-
wave' or '6-step' sequence described by Miller and
Lawrence.2 The three currents to the motor are monitored Fig. 1 Schematic diagram of transistorised inverter
using electronic ammeters / , , / 2 , and / 3 . The magnitude Transistors Tl — Tf are any suitable switching transistor (e.g. 2N305 5)
of the largest current is selected, and, if this magnitude is The diodes should be fast soft recovery types
above a predetermined level, the whole inverter is turned
off until the current drops to slightly below that level.
Switching the inverter off and on is accomplished by simply Conclusions: Automobile alternators are ' designed to
switching whichever of transistors 2, 4, or 6 are active in operate over a wide range of conditions in a very hostile en-
the 6-step sequence. The hysteresis in the current level at vironment. When they are used as stepping motors they
which switching occurs is chosen to ensure that the on/off still have these performance characteristics. Coupled with
switching rate is kept well into the ultrasonic area where it this ruggedness the lower cost of an automobile alternator
is not aurally objectionable. often means that a complete stepper-motor system (motor
ELECTRIC POWER APPLICATIONS, FEBRUARY 1979, Vol. 2, No. 1 27
0140-1327/79/010027 + 02 $01-50/0

2. plus inverter) can be made for less cost than the equivalent
stepping motor alone.
l200
500 1000 1500 2000 2500
speed, r/min
Fig. 3 Measured torque and power characteristics of a typical
automobile alternator type BOSCH 14 V, 35 A (G.M. part number
9929209)
Rotor excitation 2 A d.c., busbar voltage 40 V
Inverter constant current setting 7 A
J.T. BOYS 25th September 1978
Department of Electrical Engineering
University of Auckland
Auckland
New Zealand
G.R. DUNLOP
Department of Electrical Engineering
University of Canterbury
Christchurch
New Zealand
References
1 NAGASAKA, M., and SHINOHARA, K.: 'A study of large
capacity electric pulse motors', Electr. Eng. Jpn., 1974, 94, pp.
101-109
Fig. 2 Measured line voltages and line currents for a typical 2 MILLER, D.W., and LAWRENCE, R.G.: 'Variable-frequency
automobile alternator at different speeds inverter and its application to motor drives', Electron. & Power,
Upper 60 r/min. middle 300 r/min, lower 1200 r/min 1976, 22, pp. 675-678
The d.c. busbar is an unregulated supply of 40 V 3 TAL, J.: 'Control modes of step motors', IEEE Trans., 1977,
All current waveforms (upper traces) are 7 A peak AES-13, pp. 56-61
T284P
28 ELECTRIC POWER APPLICATIONS, FEBRUARY 1979, Vol. 2, No. I