is needed? What is the peak speed
required?
These critical questions cannot be
addressed simply by choosing a motor
with a given horsepower.
The power output of a motor is the
combination of torque and speed which
can be calculated by a multiplication of
speed, torque, and a constant.
Due to the nature of this calcula-
tion, however, there are many different
combinations of torque and speed that
will yield a specific power output. Thus,
different motors with similar power
ratings can operate differently due to
the combination of speed and torque
t he y of fer.
Engineers must know how fast a
certain size load needs to move before
confidently choosing a motor that will
work best. The job being performed
must also fall under the motor’s torque/
speed curve. This curve shows how a
motor’s torque varies during operation.
Using “worst-case” assumptions (in oth-
er words, determining the maximum/
minimum amount of torque and speed
the job will require), engineers can be
confident a chosen motor has a suffi-
cient torque/speed curve.
The inertia of the load is another fac-
tor that should be addressed before div-
ing into the decision-making process
of choosing a motor. The inertia ratio
must be calculated, which is the com-
parison between the load’s inertia and
the motor’s inertia. One rule of thumb
says that if the load’s inertia exceeds 10
times that of the rotor, then tuning the
motor may be more difficult and per-
formance can suffer. But this rule varies
not only from technology to technol-
ogy, but from supplier to supplier and
even product to product. How criti-
cal an application is will also affect this
decision. Some products handle up to
30-to-1 ratios, while direct drives run
at up to 200-to-1. Many people don’t
like sizing a motor that exceeds a 10-to-
1 ratio.
Finally, are there physical limita-
tions that restrict one certain motor
over another. Motors come in different
shapes and sizes. In some instances,
motors are large and bulky, and there
are certain operations that cannot
house a certain size motor. Before an
informed decision can be made on the
best type of motor, these physical speci-
fications should be recognized and
understood.
Once engineers answer all these ques-
tions—speed, torque, horsepower, load
inertia, and physical limitations—they
can zero in on the most efficient size
motor. However, the decision-making
process does not stop there. Engineers
must also figure out what type of motor
best fits the application. For years, the
choice on type boiled down to one of
two options for most applications: a
servo motor or an open-loop stepper
motor.SERVOS AND STEPPERS
The operating principles for servo
and open-loop stepper motors are
similar. However, there are key differ-
ences between the two that engineers
must understand before deciding which
motor is ideal for a given application.
In traditional servo systems, a con-
troller sends commands to the motor’s
drive via pulse and direction or an ana-
log command related to position, speed,
or torque. Some controls may use a
bus-based method, which in the newest
controls is typically an Ethernet-based
communication method. The drive then
sends appropriate current to each phase
of the motor. Motor feedback circles
back to the motor’s drive and, if needed,
the controller. The drive relies on this
information to properly commutate the
motor and to send good information
about the motor shaft‘s dynamic posi-
tion. So, servo motors are consideredThese closed-loop stepper motors from Parker Hannifin, with their drives shown on the left,
are easy to use and offer good torque low speeds without the traditional stepper concerns
of losing position.GO TO MACHINEDESIGN.COM 47