Speed - Torque Curves for Stepping
How they are created and what they mean
When selecting a stepping motor, you try to pick a motor that meets your
speed and torque requirements plus some safety margin. But how do you compare
motor performance between motor suppliers. Most suppliers provide speed – torque
characteristic curves to provide an idea of what performance can be expected
from a motor. Stepping motor speed - torque curves show how much torque is
available from a stepping motor at a given speed when combined with a particular
driver. This means that depending on different motor and driver combinations,
different performance can be expected from the stepping motor system. This
article will describe how a speed - torque curve for a stepping motor is
generated and what are the important points to look for on a curve.
A well defined speed – torque curve, such as the ones shown below, should
include the following information.
- Power input: This is the voltage that is supplied to the
driver. For DC input voltage drivers, this same voltage is usually applied
directly to the motor windings. For AC input voltage drivers, the AC voltage
is rectified to a DC voltage before being applied to the motor windings. For
example, for an 115VAC driver, the applied voltage to the motor windings is
- Driver type: This states what type of driver was used to
create the curve. Either a unipolar or bipolar driver should be shown. The
driver type will also states if the driver is of the constant current or
constant voltage type.
- Damper use: While not required, a damper can help to
create a more typical performance curve by representing an inertial load on
the motor. The curve should state if a damper was used and what its
- Step angle: This is the step angle the motor was driven
at when creating the curve. Curves will commonly show what the basic step
angle (1.8°, 0.9°, 0.72°, 0.36°) of the motor or what driver resolution (full,
half, microstep divisions) were used.
- Motor winding configuration: This describes how the motor
was connected to the driver and what current was applied to the windings.
Motor connections could be unipolar, bipolar series, bipolar half coil and
- Torque units: The vertical axis shows the amount of
torque and in what units (e.g. oz-in, N-m, etc).
- Speed: The horizontal axis shows the shaft speed of the
motor and in what units (e.g. rpm, pps, Hz, etc).
- Maximum No-load starting speed: The maximum no-load
starting speed is the maximum speed at which the motor can be started in
synchronism with no load attached and no acceleration used. It is usually
shown as a tick mark labeled “fs” on the horizontal axis.
- Holding Torque: This is the torque that the motor will
produce when the motor is at rest and rated current is applied to the
- Pull-out Torque curve: This curve represents the maximum
torque that the stepping motor can supply to a load at any given speed. Any
torque or speed required that exceeds (goes above) this curve will cause the
motor to lose synchronism.
- Pull-in Torque curve (no load): This curve represents the
maximum torque and speed combination that an unloaded stepping motor can start
or stop without any acceleration or deceleration. Since the pull-in torque
curve for a stepping motor varies depends on the inertial load attached to the
motor, the pull-in torque curves are not shown in the speed – torque curves
shown in catalogs. In order to operate above the pull-in torque curve, the
motor must be accelerated into or decelerated out of the slew range.
- Pull-in Torque curve (inertial load): This curve
represents the maximum torque and speed combination that a stepping motor with
an inertial load (i.e. damper) can supply to a load and start or stop without
any acceleration or deceleration. In order to operate above the pull-in torque
curve, the motor must be accelerated into or decelerated out of the slew
- Self start range (start/stop region): When in this area,
the stepping motor can start, stop or change directions in synchronism with
the input pulse without the need for acceleration or deceleration.
- Slew range: The slew range is where stepping motors are
usually operated. A stepping motor can not be started directly in the slew
range. After starting the motor somewhere in the self start range, the motor
can be accelerated into or load applied into the slew range. The motor must
then be decelerated or load reduced back into the self start range before the
motor can be stopped.
- Maximum response frequency: This is the maximum speed the
motor can be operated when no load is applied to the shaft.\
The speed – torque curves are created by spinning a step motor up to a known
speed and then gradually applying torque to the output shaft with a brake and
measured with a torque transducer. The load is slowly applied until the motor
loses synchronism (stops). At the moment that the motor loses synchronism, the
torque that was applied to the motor shaft at that same moment is recorded. This
process is repeated three times at each speed point. The average of the three
torque values is then used as the value that will be displayed on the speed –
torque curve. This process is repeated at several speed points. The torque
points are then plotted at the various speed points to create the complete
curve. See figure below.
As was mentioned earlier, the speed – torque characteristics are determined
by the stepping motor and driver combination. In general, the higher the applied
voltage to the motor windings, the faster the motor will rotate. For example, in
the curves below, the speed – torque curve for the CMK245APA/CMK245BPA indicates
that 24VDC is applied to the motor windings, while the curve for the UMK245BA
was created with 162VDC being applied to the windings. As you can see, the
torque at speed of the UMK245BA is held out to a much higher speed.
In summary, the speed- torque curve can be a useful tool for selecting the
right stepping motor for your application.
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