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Low stepping rate
High
stepping
rate
Figure 9.13
Basic constant-voltage drive circuit and typical current waveforms
Stepping Motors
329
to appreciate the severely limited performance of the simple constant-
voltage drive.
Current-forced drive
The initial rate of rise of current in a series
R
–
L
circuit is directly
proportional to the applied voltage, so in order to establish the current
more quickly at switch-on, a higher supply voltage (
V
f
) is needed. But if
we simply increased the voltage, the steady-state current (
V
f
/
R
) would
exceed the rated current and the winding would overheat.
To prevent the current from exceeding the rated value, an additional
‘forcing’ resistor has to be added in series with the winding. The value of
this resistance (
R
f
) must be chosen so that
V
f
=
(
R
þ
R
f
)
¼
I
, where
I
is
the rated current. This is shown in the upper part of Figure 9.14,
together with the current waveforms at low and high stepping rates.
Because the rates of rise and fall of current are higher, the current
waveforms approximate more closely to the ideal rectangular shape,
especially at low stepping rates, though at higher rates they are still far
from ideal, as shown in Figure 9.14. The low-frequency pull-out torque
is therefore maintained to a higher stepping rate, as shown in Figure
9.16(b). Values of
R
f
from 2 to 10 times the motor resistance (
R
) are
common. Broadly speaking, if
R
f
¼
10
R
, for example, a given pull-out
torque will be available at 10 times the stepping rate, compared with an
unforced constant-voltage drive.
V
f
L
R
R
f
Low stepping rate
High
stepping
rate
Current
Time
Figure 9.14
Current-forced (L/R) drive and typical current waveforms
330
Electric Motors and Drives
Some manufacturers call this type of drive an ‘
R
/
L
’ drive, while others
call it an ‘
L
/
R
’ drive, or even simply a ‘resistor drive’. Often, sets of pull-
out torque–speed curves in catalogues are labelled with values
R
/
L
(or
L
/
R
)
¼
5, 10, etc. This means that the curves apply to drives where the
forcing resistor is
W
ve (or ten) times the winding resistance, the implica-
tion being that the drive voltage has also been adjusted to keep the static
current at its rated value. Obviously, it follows that the higher
R
f
is
made, the higher the power rating of the supply; and it is the higher
power rating which is the principal reason for the improved torque–
speed performance.
The major disadvantage of this drive is its ine
Y
ciency, and the con-
sequent need for a high power-supply rating. Large amounts of heat are
dissipated in the forcing resistors, especially when the motor is at rest
and the phase current is continuous, and disposing of the heat can lead
to awkward problems in the siting of the forcing resistors.
It was mentioned earlier that the in
X
uence of the motional e.m.f.
in the winding would be ignored. In practice, however, the motional
e.m.f. always has a pronounced in
X
uence on the current, especially at
high stepping rates, so it must be borne in mind that the waveforms
shown in Figures 9.13 and 9.14 are only approximate. Not surprisingly,
it turns out that the motional e.m.f. tends to make the current wave-
forms worse (and the torque less) than the discussion above suggests.
Ideally therefore, we need a drive, which will keep the current
constant throughout the on period, regardless of the motional e.m.f.
The closed-loop chopper-type drive (below) provides the closest ap-
proximation to this, and also avoids the waste of power, which is a
feature of
R
/
L
drives.
Chopper drive
The basic circuit for one phase of a VR motor is shown in the upper part
of Figure 9.15 together with the current waveforms. A high-voltage
power supply is used in order to obtain very rapid changes in current
when the phase is switched-on or o
V
.
The lower transistor is turned on for the whole period during which
current is required. The upper-transistor turns on whenever the
actual current falls below the lower threshold (shown dotted in Figure
9.15) and it turns o
V
when the current exceeds the upper threshold. The
chopping action leads to a current waveform that is a good approxima-
tion to the ideal (see Figure 9.10). At the end of the on period both
transistors switch o
V
and the current freewheels through both diodes
and back to the supply. During this period the stored energy in the
Stepping Motors
331
Plate 9.4
Bipolar constant-current chopper drive with size 17 hybrid motor. This
versatile drive draws its power from a d.c. supply (between 24 V and 80 V) and the
output phase current can be set (using programming resistors) to any value in the range
0.3 A to 7 A. A resonance adjustment is provided. (Photo courtesy of Astrosyn)
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