 |
|
Improvement of Stepper Motor Efficiency
|
|
1.INTRODUCTION
|
- In this paper, we shall discuss ways
of improving the efficiency of phase-modulation
type stepper motors (hereafter referred to
as "SMs"). As low-priced, compact
and accurate motors for simultaneously controlling
speed and position, SMs are used in OA equipment
and many other types of machinery. Today,
there is a growing need for SMs of higher
output power, smaller size, faster speed
and lower power consumption, in keeping with
the development of downsized but higher-
performance electronic equipment. In response
to these needs, improving the torque performance
would not be sufficient. Improvement of electromechanical
conversion efficiency (hereafter, "efficiency"),
including the reduction of SM internal loss,
would also be necessary. Nevertheless, very
few studies have been reported on SM efficiency,
so we have decided to, first, grasp the actual
SM efficiency levels through measurement
and, second, discuss ways of improving efficiency.
|
|
2. MEASUREMENT OF SM EFFICIENCY
|
- Figure 1 shows a diagram of the SM efficiency
measurement system that we employed for the
present study. As the motor analyzer for
torque measurement, we used an EMA-1 (which
was developed by Sugawara Research Institute)
equipped with a hysteresis brake. While an
SM was driven under a specified condition,
a load was applied to it by the hysteresis
brake; measurement was made with regard to
voltage Eo in the drive circuit, average
input current Io, and effective currents
Ia and Ib, both of which flowed in the A-phase
and B-phase coils, respectively. The motor
analyzer, DC power supply, and ampere meter
were controlled by a computer which automatically
recorded torque T, rotation speed w as well
as Eo, Io, Ia and Ib. Then, the values of
input power Pi, output power Po, efficiency
n, and copper loss Pc were derived by the
following equations:
- In equation (3), Ra and Rb are the
resistance values of the A-phase
and B-phase
coils. Pl in equations (1), (4) and
(5) indicates
the total SM loss, including the
iron loss
and bearing friction loss.
|
 |
| Fig.1: Measuring System for Efficiency. |
| Stepper motors were driven by a stable voltage
of 2-phase bipolar excitation, and the supplied
voltage was regulated so that the SM's axial
output power stabilized at 3W. |
|
3. RESULTS OF EFFICIENCY MEASUREMENT
|
- As samples for measurement, we employed FDK's
SMT35-48 model stepper motors (35mm in outer
diameter, 12.4mm high, and with 48 steps).
Figure 2 compares the ratios of Po, Pc and
Pl to input power Pi at various pulse rates,
as were found from our measurement. The ratio
of Po to Pi indicates that the efficiency
level was approximately 40-45% at a 1,200
pps or higher pulse rate. It was also found
that 30-40% was lost as copper loss caused
by coil resistance and that the remaining
20-30% was iron loss and other losses.
|
 |
| Fig.2: Ratio of Loss to Input Power |
|
4. EFFICIENCY IMPROVING FACTORS
|
- Figure 3 shows the relation between input
electric power, loss and output mechanical
power. The electric power that is input into
the SM is consumed as copper loss by the
coil. Then, the remaining electric power,
after conversion into magnetic energy, is
consumed as iron loss by the yoke. (This
iron loss can be divided into an eddy current
loss and a hysteresis loss.) At the axial
bearing the remaining magnetic energy, after
conversion into mechanical energy, is consumed
as mechanical loss, such as friction and
air resistance. The remaining mechanical
energy provides the output mechanical power.
In order to improve electromechanical conversion
efficiency, we experimented with the following
matters so as to increase the output power
and reduce copper, iron and other losses:
|
|
(1) Amount of rotor flux (in terms of om)
(2) Coil constant (Gc = N2/h)
(3) Yoke materials
(4) Yoke annealing |
Input power  |
Conductive resistance loss |
Copper loss |
 |
| Magnetic energy |
Eddy current loss |
Iron loss |
| Hysteresis loss |
|
| Mechanical energy |
Bearing loss |
Mechanical loss |
|
| Mechanical output power |
|
| Fig.3. Various loss in motor. |
|
5. AMOUNT OF ROTOR FLUX
|
- We prepared stepper motors whose rotor flux
volumes are greater than that (om = 21.9
uWb) of SMT35-48 model stepper motors. We
found that an increase in rotor flux did
not improve efficiency. That is, although
an increase in rotor flux reduced copper
loss, it increased negative torque (torque
loss), so these factors offset each other.
|
|
6. COIL CONSTANT
|
- We compared three different levels of coil
constant Gc--the same as, above, and below
the coil constant of SMT35- 48 model stepper
motors. Figures 4 and 5 report the measurements
of efficiency and copper loss in relation
to coil constant. As is apparent from Figure
4, the greater the coil constant, the lower
the copper loss. However, when the coil constant
was 6,400S and the pulse rate was 1,500 pps
or higher, efficiency stopped increasing.
This was because a higher rotation speed
increased negative torque as well. Thus,
an increase in coil constant above 6,400S
does not noticeably improve efficiency. These
results indicated that copper loss is the
dominant factor at low rotation speeds. At
high rotation speeds, we failed to improve
efficiency, because negative torque (such
as iron loss) becomes the dominant factor
at these speed levels. Accordingly, the next
step should be to reduce negative torque.
|
 |
| Fig.4: Efficiency vs. Coil Constant |
 |
| Fig.5: Copper Loss vs. Coil Constant |
|
7. YOKE MATERIALS
|
- We prepared stepper motors using four different
yoke materials in order to study the possible
reduction of iron loss--specifically, eddy
current loss. Then we measured the amount
of negative torque or torque loss inside
each SM and the value of efficiency. The
four yoke materials were: 1] a conventional
electromagnetic soft iron plate ("SUY"),
2] a conventional zinc-plated steel plate
("SEC"), 3] a new Fe-Si alloy with
a 1% Si ("Fe-Si"), and 4] a new
Fe-Cr alloy with a 12% Cr ("Fe- Cr").
All four materials were formed into yokes
by pressing, while magnetic annealing treatment
was omitted. To determine negative torque,
we used a torque sensor which measured the
torque that was generated when the SM's rotor
was rotated by an external DC motor. Figure
6 shows the negative torque characteristics
of SMs in relation to different yoke materials.
The negative loss inclinations in the graph
were more moderate in the Fe-Cr and Fe-Si
stepper motors than in the SUY and SEC stepper
motors. This indicates that the Fe-Cr and
Fe-Si stepper motors generate less eddy current
than the SUY and SEC stepper motors. Between
Fe-Cr and Fe-Si stepper motors, the former
generates less eddy current. The four materials
were equivalent in Y-piece negative torque,
although Fe-Si stepper motors had a slightly
lower negative torque than the others. These
results indicated that the four materials
had similar total values of mechanical loss
and hysteresis loss.
|
 |
| Fig.6: Effects of Yoke Materials on Negative
Torque |
| Figure 7 summarizes the SM characteristics
in relation to the yoke materials. Fe-Cr
and Fe-Si stepper motors were more efficient
than conventional SUY and SEC stepper motors
by as much as 10%. Between Fe-Cr and Fe-Si,
the latter was more efficient at 1,600pps
or at lower pulse rates, but the former was
more efficient at pulse rates of 1,600pps
or over. The higher efficiency of Fe-Cr and
Fe-Si stepping motors over conventional SMs
was attributed to a sharp reduction in eddy
current loss. |
 |
| Fig.7: Effects of Yoke Materials on Efficiency |
8. YOKE ANNEALING
- The effect of annealing was examined with
a view to reducing hysteresis loss. The above
Fe-Cr alloy was employed as yoke material
because of its large efficiency-improving
effect. Annealing was carried out at 850oC
for one hour; then, negative torque and efficiency
were measured.
|
Table 1 compares the negative torque values
of annealed and non-annealed Y-pieces
at
the zero pulse. The negative torque
was nearly
halved as a result of annealing, thus
substantially
cutting hysteresis loss.
|
| Table 1: Negative Torques at 0pps in Relation
to Annealing |
