Another DC motor question

Switching when the rectified voltage is high will increase the current losses in the motor, probably without affecting your speed variation all that much. If you never stop the motor then pulsing it only simplifies your circuit, it doesn't give you any advantages in the controllability of the motor.

Building a real switching amplifier that presents a low-impedance load to the motor may help a lot, but to really make the motor go a constant speed you need to take the suggestion already given and wrap the motor with a closed-loop controller.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

A fairly close approximation of the right source impedance to effect speed regulation is to set it at the additive inverse of the coil resistance. Regardless of whether the stationary field is setup by permanent magnets or a winding, this drive arrangement can be thought of as a flux forcing function, holding the derivative w.r.t. time proportional to the control voltage. This occurs only at a particular speed, at least on a timescale where the inductance can be ignored. The output impedance plus the coil resistance should stay in the left half-plane in order to avoid instability.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

Pulsing does have some effect when a motor is stalled or nearly stalled. For instance, model railway speed controllers often supply pulsed power, to better handle voltage drops in the wheel to track contact, and to "buzz" the motor into breaking friction. However, this is a special case where you have a low mass motor and a lot of "stiction", so the "buzz" factor really works. Additionally, many model railway pulse controllers sense the DC generated by the motor between pulses, use this as a measure of motor speed and control the pulse width in a feedback circuit.

However, stall torque always drops with reduced voltage, regardless of pulse waveshape, and I don't think you will get the amount of benefit you need. DC motors always change speed somewhat under load. If your motor has field and armature in series, it is very subject to changing speed under load anyway. A shunt motor is much better in this regard.

Feedback from a tachometer makes an enormous difference, no matter what kind of motor you have.

Roger

Is this a PM or universal motor?

On PM motors, the "back EMF" will remain when the switch is off. You can use this fact to get a crude speed regulation out of it. Linear technology makes a "isolated flyback regulator" chip. If your motor is PM I suggest you look at their data sheet and App-notes. You can treat the back EMF like the flyback voltage.

There is a trick you can use on some universal motors. You can put a large rectifier across the field winding. (Be very careful of polarity; you want it off when the transistor is on) What this does is allow the current in the field coil to continue briefly when the transistor shuts off.

If you are lucky, you will see the drain of the MOSFET do this:

.......*................. .......**................ .......*.*..************* .......*.*.*............. .......*.**.............. .......*.................

*******................. ^ ! This bit is new and indicates the speed (sort of)

You would really be better off not feeding high frequencies to the iron in the motor. If you power it through an inductor, delaying the turn on may not be such a bad idea.

--
--
kensmith@rahul.net   forging knowledge

In article , Tim Wescott wrote: [...]

You can make the output impedance negitive and do even better. Run the motor unloaded at some speed and measure the voltage and current. Attach the load and adjust the voltage for the same speed and measure voltage and current. You now know that you need to increase the voltage by about X volts when it draws Y current. You don't want to increase it by any more than X so you may want to fall a little short of the voltage you measured.

--
--
kensmith@rahul.net   forging knowledge

Simple, interesting, can you elucidate your thinking?

--
 Thanks,
    - Win

[snip]

Do you have LTSpice from Linear Technology? It can be used to 'experiment' with ideas like that, and see if they are worth trying for real. A rough model of a PWM'd dc motor is shown below. It is my rough guess, which is still under construction.

Vsupply+---+---------------+ | __________|___________ | | | | | | \ MOTOR | | | R / | | | \ | | | | | | | )|| | | | L )|| | | | )|| |____ | | | | | | +----+--+--+----+

PWM'ing a motor is not as straightforward as PWM'ing (say) a solenoid. That generated Vbemf influences everything.

So I was trying to model something that automatically produced a resultant Vbemf, which might then provide a more accurate representation of the PWM'd currents.

A Capacitor in parallel with a Constant Current sink seems appropriate. The sink represents the static torque load, and the energy stored on the capacitor is the equivalent of the kinetic energy stored in the rotating parts. The voltage across the capacitor should represent the rpm.

There is a neat resemblance between the energies. 0.5.C.V-squared for the capacitor and 0.5.I.V-squared for the rotating parts, (I being the moment of inertia and V being the rotational velocity). Since both V's represent rpm, then the value of the capacitor should equate to the Moment of Inertia of the rotating body.

However, without an actual mfr's figure for MoI the value of the capacitor can only be stabbed at in the first instance. The no-load current is a measure of the torque required for the losses. If the motor is then switched off the rundown time is a measure of the MoI.

As a first stab I have assumed that the capacitive equivalent is given by a simple C.V = I.T.

But the model is still under construction, and if anyone wants to correct/improve please feel free to contribute.

--
Tony Williams.

I've added values from the data sheet of the Maxon dc motor, 148867. This was the subject of another thread recently. 137mA is the no-load current at 24Vdc and the value of C1 was calculated to hold 4 Joules at 24V, which I think is the stored energy in the rotor at no-load speed, (134 gm.cm-squared at 7580rpm).

A question......

If an actual motor is clocked at 7.8KHz and 50:50 duty cycle, what speed should it reach?

--
Tony Williams.