Parker Hannifin Corporation
Load characteristics, performance requirements, and coupling
techniques need to be understood before the designer can
select the best motor/drive for the job. While not a difficult
process, several factors need to be considered for an optimum
solution. A good designer will adjust the characteristics of the
elements under his control including the motor/drive and the
mechanical transmission type (gears, lead screws, etc.) to
meet the performance requirements. Some important param-
eters are listed below.
Rotational force (ounce-inches or pound-inches) defined as a
linear force (ounces) multiplied by a radius (inches). When
selecting a motor/drive, the torque capacity of the motor must
exceed the load. The torque a motor can provide may vary with
its speed. Individual speed/torque curves should be consulted
by the designer for each application.
An object's inertia is a measure of its resistance to change in
velocity. The larger the inertial load, the longer it takes a motor
to accelerate or decelerate that load. However, the speed at
which a motor rotates is independent of inertia. For rotary
motion, inertia is proportional to the mass of the object being
moved times the square of its distance from the axis of rotation.
All mechanical systems exhibit some frictional force, and this
should be taken into account when sizing the motor, as the
motor must provide torque to overcome any system friction. A
small amount of friction is desirable since it can reduce settling
time and improve performance.
This number is defined as a motor's rated torque divided by its
rotor inertia. This ratio is a measure of how quickly a motor can
accelerate and decelerate its own mass. Motors with similar
ratings can have different torque-to-inertia ratios as a result of
Load Inertia-to-Rotor Inertia Ratio
For a high performance, relatively fast system, load inertia
reflected to the motor should generally not exceed the motor
inertia by more than 10 times. Load inertias in excess of 10
times the rotor inertia can cause unstable system behavior and
inefficent power usage.
Whenever possible, a motor/drive that can provide more motor
torque than the application requires should be specified. This
torque margin accommodates mechanical wear, lubricant
hardening, and other unexpected friction. Resonance effects,
while dramatically reduced with Compumotor's microstepping
systems, can cause a stepper motor's torque to be slightly
reduced at some speeds. Selecting a motor/drive that provides
at least 50% margin for steppers, and 20% for servos, above
the minimum needed torque is good practice.
Because available torque varies with velocity, motor/drives must
be selected with the required torque at the velocities needed by
the application. In some cases, a change in the type of me-
chanical transmission used is needed to achieve the required
The positioning resolution required by the application will have
an effect on the type of transmission used and the motor
resolution. For instance, a leadscrew with 4 revolutions per inch
and a 25,000-step-per-revolution motor/drive would give
100,000 steps per inch. Each step would then be 0.00001
Servo motors can produce peak torque for short time intervals
as long as the RMS or average torque is within the motor's
continuous duty rating. To take advantage of this feature, the
application torque requirements over various time intervals need
to be examined so RMS torque can be calculated.
Solving Duty Cycle Limitation Problems
Operating a motor beyond its recommended duty cycle
results in excessive heat in the motor and drive. This can
destroy the motor and drive package. The duty cycle may be
increased by providing active cooling to the drive and the motor.
A fan directed across the motor and another directed across the
drive's heatsink will result in increased duty cycle capability.
Note: Motors will run at case temperatures up to 100
F)--temperatures hot enough to burn individuals who
touch the motors.
To Improve Duty Cycle:
· Use a motor large enough for the application
· Mount the drive with heatsink fins running vertically
· Fan cool the motor
· Fan cool the drive
· Put the drive into REMOTE POWER SHUTDOWN when it
isn't moving, or reduce current (Steppers Only)
· Reduce the peak current to the motor