Re: Speed controller for a 12V Brushed DC motor

On Mon, 19 Jan 2009 23:21:53 +0000, Peter Fairbrother
<zenadsl6186@xxxxxxxxx> wrote:


I think I should correct some misunderstandings, whether real or
imagined I'm not sure.

A PWM circuit just switches on and off very fast, and adjusts the length
of the ON and OFF pulses according to some input, often a static voltage
from a potentiometer.

This is useful as a sort of "partly-on" electronic switch, where using
eg a variable resistor would waste power (dissipated in the resistor),
as modern power FETs can switch on and off very quickly.

Normal simple voltage controlled PWM's will vary the ON-ness ratio
according to a static voltage, derived from the position of a
potentiometer - but this will *not* keep a motor at a specified speed
when under varied load, more circuitry is required to increase the ON
pulse width when the motor begins to slow down under load.


A speed controller needs more, it needs to have some method of measuring
the speed [1] - then compare that measured speed to the desired speed -
then create a difference input to the PWM.

If it does that fast enough, which if should be able to do, then the
speed of the motor shouldn't vary much whether it's unloaded or loaded
at full torque.

My point is, a speed controller will have this circuitry included - a
simple PWN won't. Others have said this, but I'm unsure who understands
what here.



[1] this is easy when a permanent magnet motor is used, as the
transitions between segments on the rotor, with changes in the flow of
the current, are easy to detect electronically


-- Peter Fairbrother

Just a bit more info. on how things work.

If you're using good permanent magnet motor fed from a constant
voltage source over a reasonable speed range (say 2 or 3 to one)
the the speed regulation is often good enough for the general run
of metal cutting activities (e.g. spindle speed and cutter feed).

In this case the speed regulation is controlled by the
motor armature resistance - ideally zero - the bigger the motor
the lower the resistance and the better the speed regulation.

To plug some typical numbers in - a 24v motor with 1 ohm
armature resistance will show a speed drop of about 10% when the
mechanical load causes it to draw 2.4A from the supply.

Any attempt to control speed by series resistance
directly adds to the undesirable armature resistance and results
in dreadful speed regulation.

Voltage control by a pulse width modulator (PWM) works
very differently. In this case power is applied to the motor by
high frequency semiconductor on/off switching and the apparent
voltage, as far as the motor is concerned, is the supply voltage
times the % on period (the duty cycle) of the switching waveform.

If the semiconductor switches are zero resistance
this is a perfect voltage changing system - it acts as a constant
voltage source and does not increase the effective series
resistance in the armature circuit. Modern MoSFETS and IGBTs Have
the "on" resistance measured in milliohms and come pretty close
to this ideal.

If the PWM is supplied from a near zero impedance DC
source (such as a large car battery) this an excellent variable
voltage system which will not degrade the basic speed regulation
inherently resulting from the armature resistance of the motor.

Things are a bit different with a rectified AC supply.
The DC output of raw full wave rectified AC is 0.9 x the RMS
value. A lighly loaded motor behaves a bit like a capacitor -
when the instantaneous value of the rectified supply drops, the
back EMF of the motor keeps the voltage nearer the peak value
(1.4 x RMS). When you connect a lightly loaded motor across a raw
rectified supply the voltage increases!!.

At full load the effect is much reduced and the voltage
drops back to near the 0.9 RMS value. This large undesirable
voltage change is of course in addition to any resistive voltage
drop in the transformer windings.

A partial cure is to connect a large capacitor across the
supply so that the load always appears capacitative. To swamp the
effect the time constant of the capacitor plus motor load should
be at least 30 milliseconds.

The apparent full load resistance of our sample motor is
24V/2.4A = 10 ohms. Time constant in mS is uF x ohms so at least
3,000 uF is needed.

An alternative cure is to add a large inductance in series
with the motor. Although this can give better results it needs a
physically large, specialised and expensive component

This is the basic PWM controlled system. Slightly more
elaborate systems can monitor the current drawn by the motor and
use this to increase the motor voltage sufficiently to at least
partially cancel the efects of motor armature resistance and DC
supply impedance. However various nonlinearities limit the
improvement that is possible - 2 or 3 to one is reasonable aim.
Beyond that a servo loop using the motor back EMF as the speed
monitor is the better approach.

Jim



.

Relevant Pages

  • Re: DIY speed control of ~1hp PM DC motor?
    ... Without significant resistance the SCR ... possible motor speed. ... peak valued full-wave rectified line voltage. ...
    (rec.crafts.metalworking)
  • Re: Easy question about Solar Cells / Electric Motor specs
    ... > need to power this motor ... resistance, so you probably know that the voltage across a resistor is ... If the solar cell is open circuit no current flows in the resistor so ...
    (sci.electronics.basics)
  • Re: Motor speed control via back-EMF detection
    ... More load on the motor shaft means more current drawn by the motor (the ... voltage has increased or the resistance of the armature winding has decreased. ... An ideal motor would have a winding resistance of zero. ...
    (sci.electronics.design)
  • OT, but kinda on topic - optical encoder elex
    ... I found a small DC motor at the dump with an optical encoder on it. ... It's a Rae Corp motor, but the sticker is stamped "Aug 27 1976", so I have no hope of finding specs for it. ... does it make sense that the detectors would have the same resistance in both directions? ... a resistor on each of Y & Blue in series to voltage, signal taken at Y & Blue? ...
    (rec.crafts.metalworking)
  • Re: Generator
    ... The Back EMF limits how much current the motor will ... Different motor types act differently when the voltage is ... to slow down and draw more current. ... by pulling more amps whenever voltage dropped. ...
    (sci.engr.joining.welding)