Electricity + Control September 2016

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DRIVES, MOTORS + SWITCHGEAR
MoveTimes
Refer to Figure 4.
The Triangular move accelerates the load for half the time and decel-
erates for the remaining time. This profile is used for shorter moves.
Note that the deceleration time can be shorter than the accelera-
tion time if there is significant friction as this will aid the brak-
ing torque provided by the motor. For a given motor
torque, this profile gives the minimum move time.
The times are dependent on load inertia and
available motor torque. It is standard practice to
use only 60% of the available motor torque to
allow for unforeseen extra demands on the mo-
tor. From the longevity point of view it is much
better to run the motor well within its performance
capabilities.
The Trapezoidal move is used for long moves as
the motor will have a finite practical maximum available
top speed. Maximum speed with a stepper motor is constrained
by the falling torque produced at higher speeds. In general a sensible
upper limit for stepper motors is 600 RPM (10 rev/sec).
Conclusion
With brush motors, peak speed and torque are constrained by com-
mutator and brush considerations. Brushless motors provide win-win
performance, as the rotor is a permanent magnet with no propensity
for self-heating. In addition the stator (on the outside of themotor) can
be convection and conduction cooled. Brushless motor construction
and operation was covered in an earlier article.
Note that standard gearboxes will often tolerate only modest
input speeds, due to friction heating of the oil seals and lubrication
problems. Gearboxes intended for use with induction motors would
normally not see much more than 1 500 RPM.
As shown in Figure 4, it is common for each of the three sections
to occupy a third of the move time.
In this case it can be shown that the maximum speed during the
plateau section has to be 50% higher than the average speed of the
complete move.
take note
Bibliography
[1] Craig G. A bench top dynamometer for drives testing. Electricity
+ Control, May 2015.
[2] Craig G. Brushless Servo operating principles. Electricity + Con-
trol, July 2015.
[3] Craig G. Computer control of a small lathe. Electricity + Control,
March 2016.
[4] Craig G. Matching load and Servomotor sizes. Electricity + Con-
trol, July 2016.
All articles referred to are available in the flipmagazines online as well
as in E+C Spot On.
• Forces, velocities, inertia and power all interact.
• Using an S curve reduces jerking.
• Peak power depends on acceleration.
Glyn Craig is a director of Techlyn. He has been involved in
the mechatronics field for many years. Enquiries:
Tel. +27 (0) 11 835 1174 or email
Figure 4: Triangular and trapezoidal moves.
Triangular Trapezoid
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