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PWM for dc Motor

Started by HardySpicer April 25, 2012
How much more efficient is PWM over ordinary dc? eg in terms of
battery useage, can I double the life between charges?


Hardy
On 4/25/2012 11:55 AM, HardySpicer wrote:
> How much more efficient is PWM over ordinary dc? eg in terms of > battery useage, can I double the life between charges?
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 <something> 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...
HardySpicer wrote:
> How much more efficient is PWM over ordinary dc? eg in terms of > battery useage, can I double the life between charges?
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...
On Wed, 25 Apr 2012 12:14:14 -0700, Joel Koltner wrote:

> HardySpicer wrote: >> How much more efficient is PWM over ordinary dc? eg in terms of battery >> useage, can I double the life between charges? > > 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. >
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
Tim Wescott wrote:
> On Wed, 25 Apr 2012 12:14:14 -0700, Joel Koltner wrote: > >> HardySpicer wrote: >>> How much more efficient is PWM over ordinary dc? eg in terms of battery >>> useage, can I double the life between charges? >> >> 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. >> > > PWM as opposed to a linear amplifier would save some power, if the PWM is > fast enough to take advantage of the motor inductance.
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...
On Apr 26, 8:40=A0am, Joel Koltner <zapwire-use...@yahoo.com> wrote:
> Tim Wescott wrote: > > On Wed, 25 Apr 2012 12:14:14 -0700, Joel Koltner wrote: > > >> HardySpicer wrote: > >>> How much more efficient is PWM over ordinary dc? eg in terms of batte=
ry
> >>> useage, can I double the life between charges? > > >> PWM is just a technique for controlling how much power gets to the > >> motor. =A0Other than some (usually small) losses in the switches, it > >> doesn't directly influence efficiency. > > > PWM as opposed to a linear amplifier would save some power, if the PWM =
is
> > fast enough to take advantage of the motor inductance. > > 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...
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
HardySpicer wrote:
> On Apr 26, 8:40 am, Joel Koltner <zapwire-use...@yahoo.com> wrote: >> Tim Wescott wrote: >>> On Wed, 25 Apr 2012 12:14:14 -0700, Joel Koltner wrote: >>>> HardySpicer wrote: >>>>> How much more efficient is PWM over ordinary dc? eg in terms of battery >>>>> useage, can I double the life between charges? >>>> 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. >>> PWM as opposed to a linear amplifier would save some power, if the PWM is >>> fast enough to take advantage of the motor inductance. >> 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... > > 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. >
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/
On Wed, 25 Apr 2012 14:31:21 -0700, Joerg <invalid@invalid.invalid>
wrote:

>HardySpicer wrote: >> On Apr 26, 8:40 am, Joel Koltner <zapwire-use...@yahoo.com> wrote: >>> Tim Wescott wrote: >>>> On Wed, 25 Apr 2012 12:14:14 -0700, Joel Koltner wrote: >>>>> HardySpicer wrote: >>>>>> How much more efficient is PWM over ordinary dc? eg in terms of battery >>>>>> useage, can I double the life between charges? >>>>> 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. >>>> PWM as opposed to a linear amplifier would save some power, if the PWM is >>>> fast enough to take advantage of the motor inductance. >>> 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... >> >> 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. >> > >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.
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 | | 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.
HardySpicer wrote:

> How much more efficient is PWM over ordinary dc? eg in terms of > battery useage, can I double the life between charges? > > > Hardy
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
Jamie wrote:
> HardySpicer wrote: > >> How much more efficient is PWM over ordinary dc? eg in terms of >> battery useage, can I double the life between charges? >> >> >> Hardy > > The idea with PWM is that the control circuit has very little > resistance when on and virtually infinite R when off.
But careful, don't forget that diode :-)
> ... 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. >
Heat? Heat! Woohoo ... this could lead straight into another global warming debate ... :-) <duck and run> -- Regards, Joerg http://www.analogconsultants.com/