Adjustable Speed Drives -
(a.k.a. Variable Speed Drives)
What They Are, How They Work
Application Information
OTHER APPLICATION FACTORS Constant Torque Speed Range - On large motors, minimum speed
limitations may be necessary for self-ventilated motors, since their cooling is entirely dependent upon motor
speed and, there fore, diminishes as speed is reduced. Where rated torque operation is required continuously at
lower speeds, either a higher rated drive motor or supplemental motor ventilation, such as a motor mounted
cooling blower or external air duct, is required. Torque Limitations - Most adjustable speed drives
feature a torque limiter to protect the drive and the machine from torque overloads. The torque
limiter (current limit) is normally adjusted to 150% of rated torque to allow extra momentary torque for
breakaway, acceleration or cyclic overloads. Most drive systems are capable of sustaining the 150% torque
overload for one minute or less. Duty Cycle - Certain applications may require continuous
reversals, long acceleration times at high torque due to inertia loads, frequent high rate acceleration, or
cyclic overloads which may result in severe motor heating if not considered in the selection of the drive. Most
drives with 150% overload capability will operate successfully if there are compensating periods of operation
where motor temperatures can be normalized. MEASURING MACHINE TORQUE To measure the torque required to drive
a machine, fasten a pulley securely to the shaft which the motor is to drive. Fasten one end of a cord to
the outer surface of the pulley and wrap a few turns of the cord around the pulley. Tie the other end of
the cord to a spring scale. See Figure 22. 
Pull on scale until the
shaft turns. The force in pounds or ounces, indicated on the scale, multiplied by the radius of the
pulley (measured from the centerline of the machine shaft) in inches gives the torque value in Ib-inches
or oz-inches. On some machines, this torque may vary as the shaft rotates. The highest value of torque
must be used when selecting a motor. The running torque required by a machine will be approximately equal
to the starting torque if the load is composed almost entirely of friction. If the load is primarily inertia
or windage, the producing elements must be determined. The running torque of a machine can be accurately
determined by making a test run with an armature controlled DC drive (with a shunt wound or permanent magnet
DC motor) of known horsepower rating. The DC drive should have an ammeter in the armature circuit so
significant current readings can be observed and recorded throughout the speed range of the machine. Since
armature current and torque are directly proportional within very close limits, the current readings will
provide accurate information for selecting the drive rating required by the machine. Most machines require
a higher torque value to break it away, but once running, the torque requirement will decrease. Many drives
have 150% load capability for one minute, which may allow the required additional breakaway torque to be
obtained without increasing the drive horsepower rating. If the running torque is equal to or less than
the breakaway torque divided by 1.5, use the breakaway torque divided by 1.5 as the fullload torque required
to determine the motor horsepower. If the running torque is greater than the breakaway torque divided
by 1.5, but less than the breakaway torque, use the running torque as the
full load rated torque required to determine the motor horsepower.
Adjustable Speed Drive
Application Information provided by:
FINCOR
Automation
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1724 Tomich Court