Specifications

Speed Range
This is generally published as the fastest a motor can run trou-
ble-free divided by the slowest it will run before it begins to
“cog” (or “step”). Cogging occurs due to static friction in the
motor, inefficiencies in a pre-mounted gearbox and/or spacing
between the commutator slots of a DC brush motor. The fewer
the number of slots, the sooner (or higher speed) the motor will
“step”.
Fan-cooled motors rely on the fan to stay below the maximum
temperature of the motors. Often, a minimum fan speed is
necessary, thus narrowing the motor speed range. Many motors
can run at 1 1/2 to 2 times their rated speed. Their potential
speed range may be artificially high.
This is generally published as the maximum output voltage of
the drive divided by the minimum output voltage of the drive.
However, it should be published as the maximum output from
the drive (where it can properly regulate motor speed), divided
by the minimum output (when it can properly regulate motor
speed).
The drive regulates motor speed by changing its output voltage
in response to the motor load. More voltage will be applied
when a motor begins to slow due to increased loading. Once
the drive exceeds the maximum output voltage limit, it cannot
output any additional voltage. At this point, the drive surpass-
es the speed range and fails to regulate properly.
The same concept holds true at minimum speed. The drive
may be capable of running a motor, with “cogging,” at 1
RPM. However, if the motor stalls when loaded, the drive
speed range cannot be used at the lower end since the
drive allowed the motor to stall.
Those who specify a system, or end-users, are
usually only concerned with the most important specifica-
tion, the system speed range. However, the system speed
range is a difficult specification to obtain from a catalog.
Normal listings show motor speed range and drive speed
range only; rarely will you find system speed range listed.
We determine the system speed range by dividing the
motor’s speed at the maximum drive output voltage (with
proper regulations) by the motor’s minimum speed (before
“cogging”). We combine only the motor and drive para-
meters that limit the system speed range. The maximum
drive output is used because it is well below the maximum
speed of the motor. The minimum speed of the motor is
used because it is well above the minimum output of the
drive (usually 2 to 3 volts are required to overcome the
“dead zone” point of a motor).
Many things may affect the actual system speed range.
Difficulties can arise when defining the minimum and
maximum system speeds. For example, perhaps the bench
tests and burn in were unidirectional. The way the brush-
es seat on the commutator could affect speed range in
one direction. Ambient moisture might affect air gap flux-
es of the commutator. Temperature changes in ambient air
and/or the motor will affect magnetic field strength, thus
affecting system speed range as well.
The most often overlooked culprit in narrowing the system
speed range is the form factor of the drive output. The
form factor from a drive worsens as motor speed reduces
(lower output voltage) unless Minarik PWM drives are
used. This is a major concern when specifying motors, and
deserves its own explanation (see form factor discussion
on pg. 107)
Consult Minarik’s factory engineers for assistance in
selecting a motor and drive that will meet your system’s
speed range requirements. Typically, Minarik specifies SCR
drives for a 60 to 1 speed range. DC brush PWM drives
are 80 to 100 to 1 speed range, and brushless DC drives
for 80 to 1 speed range. Speed ranges beyond those list-
ed involve Servo applications. If you have questions, con-
tact the Minarik factory or sales engineers as they are all
factory trained on Servo systems.
1. Motor Speed Range
2. Drive Speed Range
3. System Speed Range
peed range is usually defined as the ratio of maximum
system speed to the minimum system speed. For exam-
ple, if the maximum speed is 1,750 RPM, and the speed range
is 100:1, the minimum speed will be 17.5 RPM. Using DC
motors and drives as an example, let’s analyze the three speed
ranges listed below.
1) Motor speed range
2) Drive speed range
3) System speed range
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REFERENCE
Call us toll free 1•800•MINARIK or download manuals at www.minarikdrives.com
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