Electricity + Control September 2016

DRIVES, MOTORS + SWITCHGEAR

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.

Triangular

Trapezoid

Figure 4: Triangular and trapezoidal moves.

MoveTimes Refer to Figure 4 .

All articles referred to are available in the flipmagazines online as well as in E+C Spot On.

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.

• Forces, velocities, inertia and power all interact. • Using an S curve reduces jerking. • Peak power depends on acceleration.

take note

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 glyn@techlyn.co.za

Electricity+Control September ‘16

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