PWM for dc Motor

Huh? I'm unclear about your question...

Are you using the PWM for speed/torque control? How does your "ordinary DC" approach achieve that control (i.e., are you just dissipating power in from a fixed supply voltage)?

Putting a "household lamp dimmer" on a lamp that you always operate at full brightness doesn't save you anything. OTOH, if the alternative means of operating it at N% brightness (where N is an arbitrary number) requires you to dump power in a rheostat...

PWM is just a technique for controlling how much power gets to the motor. Other than some (usually small) losses in the switches, it doesn't directly influence efficiency.

There is definitely some PWM setpoint at which your batteries will last twice as long (measured in minutes) -- just as there's a setpoint at which the batteries will last 4x or 10x as long! Whether or not the motor is putting out enough power at any of those points to do "whatever it is you want it to do" satisfactorily you'll have to judge for yourself...

PWM as opposed to a linear amplifier would save some power, if the PWM is fast enough to take advantage of the motor inductance.

OP: Could you elaborate?

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

OK, I took "DC" as in, "connected directly to a DC source" [such as the batteries mentioned] ... but I suppose it could be a DC output from a linear amplifier that was powered by the batteries...

ry

is

It would be dc through a dc amp to a motor to simply switch the motor and open loop control the speed. I always thought that PWM was more efficient than ordinary dc.

100 years ago when I learned my control theory, dc servos used pure dc. Nowadays they all appear to be PWM with H bridges.

Hardy

PWM is more efficient. If a DC amp regulates a 12V motor to 6V and the motor draws 10 amps at 6V then the DC amp burns off north of 60 watts. A good PWM stage can reduce that to just a few watts.

--
Regards, Joerg

http://www.analogconsultants.com/

But is 6V, derived by PWM from 12V, any more efficient than 6VDC ?:-)

(Vague OP.) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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I love to cook with wine.     Sometimes I even put it in the food.

The idea with PWM is that the control circuit has very little resistance when on and virtually infinite R when off. In short, this means very little heat will be generated from the electronics and all that is needed is to meter the on time power source to the motor.

Basically, resistance under load generates heat, heat is considered a lost of power. So, using electronics that turns on the switch with very little R in the circuit will not waste power and all power needed will only be in the load, the motor.

Analog type drives must set themselves to create a resistance to meter the flow of current. This will generate heat in the electronics which translates to power that you never get to use for the motor, hence, more power from your batteries will be used for heat generation along with the motor, too.

Jamie

But careful, don't forget that diode :-)

Heat? Heat! Woohoo ... this could lead straight into another global warming debate ... :-)

--
Regards, Joerg

http://www.analogconsultants.com/

You haven't heard? ...

British Climate Scientist Recants His Alarmism

...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

=A0 =A0 =A0 =A0 =A0 =A0 ... In short, this

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Jim-out-of-touch-reality-Thompson doesn't know is that James Lovelock is primarily famous for the Gaia hypothesis

It's an interesting idea but rather less than mainstream.

As British climate scientists go, James Lovelock rates as a borderline nutcase. If he's decided that global warming isn't as serious as he first thought, it may get a lot of attention in the denialist press - who don't exactly have a liberal supply of persuasive evidence to propagate - but nobody else is going to take it seriously.

-- Bill Sloman, Nijmegen

Seems as if cosmic rays *do* matter.

--

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro   acquisition and simulation

=A0 =A0 =A0 =A0 =A0 =A0 ... In short, this

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More denialist propaganda from The Register. If you were paying attention, you'd have noticed that Professor Henrik Svensmark has devised all sorts of alternative explanations for global warming, and can be relied on to trot them out whenever it suites the denialist propaganda machine.

None of his explanations stand up to close inspection, but sadly you don't do close inspection or rational scepticism.

-- Bill Sloman, Nijmegen

What's radical about trying to find explanations for past ice ages, tropocal growths covering the planet, mass extinctions, 5000 PPM CO2 levels, and incredible bursts of speciation? Are you denying that those things happened?

How does your "close inspection" explain all that?

--

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro   acquisition and simulation

PWM is more efficient _if_ it is fast enough to use the motor inductance. Make it too slow and you'll save power in the amplifier, but still burn it up in the motor.

If you can hold the motor in your hands and don't need tweezers to install it, a PWM frequency between 5 and 15kHz is probably good. If you two men and a boy to install it, you can probably go down to 2kHz. If you need a crane and a safety guy with a yellow vest -- consult the data sheet, I'm out of my experience range.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

No, of course not. But, the 6 V supply offers no speed or torque control. A PWM stage can offer speed and/or torque control, and the losses can be quite minimal. You can get efficiencies above 90 %, and speed control from practically zero to 100% quite simply.

Jon

I know that. You dropped my comment "(Vague OP.)" ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

Motors aren't great inductors. You might expect some eddy-current losses.

--

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro   acquisition and simulation

=A0 =A0 =A0 =A0 =A0 =A0 =A0 ... In short, this

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All these are very interesting subjects, and people have been looking a them for quite some time. Some of those people have even comes up with explanations that make sense.

Henrik Svensmark isn't one of them, as you would be aware if you knew anything much about the science involved.

I've recommended Tony Hallam's "Catastrophes and Lesser Calamities" - ISBN 0-19-280669-8 - here before. It was first published in 2004, but it would probably tell you quite a bit that you don't already know. Henrik Svensmark isn't one of the authors that Tony Hallam cites.

-- Bill Sloman, Nijmegen