Specifications

hen sizing a motor, torque is the most critical element to
know. Once you know the motor requirement, sizing the
drive is easy. The torque required from the motor is linearly pro-
portional to the torque required to move the load.
There are two simple methods for calculating torque:
Specify the running torque for applications where the motor
operates continuously in one direction, or for infrequent rever-
sals, where reverse time is not critical. This includes conveyors
and similar loads. Use the calculation below for single-quad-
rant drives such as XL and MM Series.
Torque (T) = F x r
F= Force necessary to move load at desired speed
r= radius or distance of load from center of motor shaft
NOTE: WHENEVER “FAST” ACCELERATION TIMES ARE REQUIRED, USE RG DRIVES AND
NOTE ACCEL/DECEL TORQUE CALCULATIONS (SEE RIGHT).
Specify the Accel/Decel torque in any application where
the time to accelerate and/or brake the load is critical.
This includes indexing tables, winders, wrappers, strap-
pers, as well as heavy inertial loads such as centrifuges
and presses using flywheels. Use the calculation below
for second and four-quadrant drives like the RG, NRG
and BOSS.
Torque (T) = J x a = J x x
J = Inertia of load (in-lb-sec
2
)
a = Acceleration rate (rad/sec
2
)
Dv = The change of velocity (Rpm)
Dt = The required time limit for the
velocity change (Seconds)
If the torque required by the load exceeds the rating of the
motor and drive, then they could run very hot. RG drives
are the most susceptible since peak currents may exceed
6 times the current limit setting to assist in ultra-fast accel-
erations and braking. Continuously operating at these
peak currents (instead of using a larger horsepower motor
and drive) will result in blown fuses or circuit breakers as
well as thermal degradation of the motor and drive. For
maximum performance, size a DC motor and RG drive as
if sizing a servo system.
DC drives and motors are specified by horsepower. Thus,
once torque is known, calculate the horsepower required
for the application using the equation below:
Horsepower (Hp) = T x N/63025
T=Torque (in-lb)
N=Speed (rpm)
Torque
1. Running Torque
2. Acceleration/Deceleration Torque
W
DV
Dt
1
60
0 81
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K
REFERENCE