Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!ncar!boulder!sunybcs!kitty!larry From: larry@kitty.UUCP (Larry Lippman) Newsgroups: sci.electronics Subject: Re: Variable speed motor control ... need help. Summary: DC motor control is not so simple... Message-ID: <2903@kitty.UUCP> Date: 5 Jan 89 19:17:43 GMT References: <288@heurikon.UUCP> Organization: Recognition Research Corp., Clarence, NY Lines: 48 In article <288@heurikon.UUCP>, lampman@heurikon.UUCP (Ray Lampman) writes: > I need a circuit that will provide variable speed control of a DC motor. The > operator control should be via some device with a knob attached. Would this > be a variable resister? The control should be roughly linear, let me explain. > > | / When the control knob is set half way between > control | / minimum and maximum, the motor should run at a > setting | o rate half way between the rate at the minimum > | / knob setting and the maximum knob setting. In general, with either a series or shunt wound DC motor, using just a variable resistor you don't have a prayer of a chance of achieving a linear transfer function between the control setting and effective RPM - even at no load. Loading the motor makes the matter significantly worse. Of course, you could always have a custom-wound non-linear variable resistor :-) - but this is not what you are seeking. Now, if the DC motor is permanent magnet or has a field winding that is separately excited at constant current, the transfer function between a linear variable resistor setting and effective RPM gets "straighter" - but by no means is it perfectly linear. Should there be any change in load upon the motor, this transfer function will go to hell in a handbasket real quick-like. :-) Unfortunately, it is really not possible to achieve the _linear_ relationship which you seek without having an electronic motor control circuit that utilizes feedback through back-EMF sensing and/or a tachometer measurement. Such motor control circuits are not that complex, but they are going to require transistor or SCR control. Under these circumstances, the variable resistor would be a 3-terminal voltage divider providing only a low-current control voltage to the motor control circuit, say, 0 to 10 volts for 0 to full RPM. Even under these circumstances the transfer function will not be perfectly linear - but you can get it pretty straight with clever circuit design. Now as to a source for an appropriate circuit, I would suggest perusing application manuals from semiconductor manufacturers such as National, RCA, Signetics, etc. > I have an appropiate DC power supply for the system. Should I be able to > locate parts for this circuit at a Radio Shack store? Yes, believe it or not. :-) <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|ames|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {att|hplabs|mtune|utzoo|uunet}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"