Specifications

CHOOSING YOUR MOTOR
ADVANCED MICRO CONTROLS INC.
A
46
Determining Your Motor Current Setting (continued)
Torque curves show the performance of the motor at the
stated current, which is the maximum setting for the
motor or the drive, whichever is less. If you decide to use
a lower current setting than the value listed in the curve,
be aware that a reduction in current proportionally
reduces the holding torque. However, a reduction in cur-
rent may not lead to a proportionally reduction in torque
at high speeds, especially if the motor is series connected.
At high speeds, motor torque is limited by the voltage bus
of the drive and the inductance of the motor. (The sim-
plest explanation is that the drive does not have enough
time to establish the full current through the motor before
it must switch the current to the winding.)
Because of this, its difficult to calculate the exact amount
of high speed torque a motor will give you when you
reduce its current setting. Its often easier to determine
your optimum current setting by testing your machine at
various current settings and then deciding which setting
gives you the best performance.
A Note on Microstepping
Many microstepping drives control the peak current
through the motor. At low speeds, this type of current
control drops the available torque of a microstepped motor
to approximately 70.7% of that available when the motor
is full stepped.
However, by default, the SD17063 controls the RMS cur-
rent through the motor. Therefore, the current supplied by
the SD17063 when microstepping is always the power
equivalent of the full step current. This means that the
motors’ full torque is always available. RMS current is
maintained down to 0.005 revolutions per second. Below
this speed, the SD17063 automatically switches to peak
current control to prevent motor damage. If you decide to
use microstepping, AMCI suggests using RMS control.
If you are full stepping the motor, or you are using the
SD17063 to replace a drive that used peak current control,
then you can configure the SD17063 to use peak current
control by changing a DIP switch on the top of the drive.
Unipolar Ratings
By convention, most motor specifications, including maximum motor current, are based on a unipolar motor
connection. The first stepper drives were called Unipolar Drives because of the way they controlled the rota-
tional direction of the motor. A typical stepper motor has four windings. A Unipolar Drive uses two of these
windings to drive the motor clockwise, and the other two windings to drive the motor counter-clockwise.
Therefore, two of the windings in the motor are always off, which means the available torque is less than if
you could use all four windings together. A Bipolar Drive, such as the SD17063, has the additional electron-
ics that allow it to switch the direction of current flow through its output drivers. Therefore a Bipolar Drive
can use all four windings at the same time, thereby increasing the available torque from a motor.
SM23 - 240D
0
50
100
150
200
250
0 1020304050
Speed (RPS)
–– Torque (oz-in) ––
- - - Power (W) - - -
Series
2.0A
Parallel
4.0A
Figure A.3 SM23-240 Torque Curves
0
50
100
150
200
250
300
0 1020304050
Speed (RPS)
––Torque (oz-in) ––
- - - Power (W) - - -
Series
2.2A
Parallel
4.3A
SM34 - 250D
Figure A.4 SM34-250 Torque Curves
SM34 - 425D
0
50
100
150
200
250
300
350
400
450
0 1020304050
Speed (RPS)
–– Torque (oz-in) ––
- - - Power (W) - - -
Series
3.2A
010
Parallel
6.3A
Figure A.5 SM34-425 Torque Curves