Motor Speed Control

Résumé du document

The aim of this report is to examine the main properties of a DC motor, its main applications and the characteristics of certain circuits that are used to control it. The report also seeks to give a general background on the equipment, including a slightly more in depth explanation of the DC motor itself. An explanation of certain key terms is included in the appendix. The goal of the motor speed control experiment is to explore the use of feedback in DC motor speed control applications and how this can be used to improve the speed regulations properties. Depending on what kind of feedback is used, the DC motor can be used in several different applications.

Extraits

[...] INTRODUCTION The goal of the motor speed control experiment is to explore the use of feedback in dc motor speed control applications and how this can be used to improve the speed regulations properties. Depending on what kind of feedback is used the dc motor can be used in several different applications. dc motors, although rather old, have several important applications in many modern devices these days. These applications vary from complicated robotics (automated tightening of bolts) to simple everyday household appliances mixer). For these devices to operate correctly it is necessary to be able to control the motors accurately. This is done through the use of several different control circuits. [...]

[...] This can be seen in θ' versus ia graph plotted below (fig. 5). Discussion Using current-derived feedback control can be very useful in certain applications when there is a risk of the motor jamming or blocking as it can prevent having to replace the whole motor. Another useful application of current-derived feedback is when screws must be mechanically tightened by robots. If a voltage-derived feedback control circuit were implemented the motor would burnout every time the screw was completely screwed on. [...]

[...] With the brake unconnected θ' against va and ia was plotted for speeds up to 1200 rpm (fig. 2). Experimental Results Discussion Plotting va or ia versus θ' we can see that the motor does not start moving until a certain threshold voltage va min (which corresponds to va = 1v or ia = 60mA on the graph) is achieved. The area between zero and va min, when the current and voltage are increasing but the motor is not moving, is known as the dead band. [...]

[...] Experimental Procedure A circuit as in page 43 figure 8b of the 2nd year Electronics Laboratory booklet was implemented. The aim is to create a current feedback system that has a speed of 500 rpm when unloaded and to have the motor stall at ia = 1.5 A (ia max). At 500 rpm the armature current ia can be determined and by extrapolation θ'max can be calculated. R2, vref and ia are given therefore R3 can be calculated. R4 is determined by setting ia = 1.5 A when the motor is stationary. [...]

[...] In this way a time-varying signal can be added on to the input. With the input for the time-varying signal set to zero, vref and the brake load are adjusted to give ia = 0.5 A at 500 rpm. The brake load is then fixed. A squarewave input is then applied at a 0.5 Hz frequency and of big enough amplitude to give a tachogenerator response of 1Vp-p. The amplitude of the squarewave at the amplifier output was measured and used to determine the value of C with the equation Δθ' = CΔva. [...]