What passes as Pulse Width Modulation in DC Motor Control?

"John Fields"

** It's worth repeating:

John knows what he posted is wrong and is now just playing games with words.

He has never admitted error in his whole life and it not about to break the habit.

Being the psychotic, lying asshole that he is.

...... Phil

--- Your rational technical stuff is generally a good read and might be worth repeating, but your grotesque attempts at dog-paddling by using insults in order to keep from going under, when you're clearly sinking, aren't.

I've already, graciously, given you the opportunity to prove me wrong by soliciting your technical rebuff of my position but, from the lack of anything technical coming this way, you seem to not to want to go there.

Moreover, since I generally respond to constructive criticism in a positive way, I can only conclude that your refusal to delve into the details means that you're afraid you'll lose even more face in the arena when the lions descend upon you.

Why worry about it?

You're pretty much down to bone now, as it is, and you weren't all that good-looking when you started.

----

--- Ah, words...

"A Rose, by any other name", even called a turd, "would smell as sweet".. .

Thanks, Bill, for the original :-)

---

OK, you get another chance: Using your best words, _Why_ is my description of PWM wrong?

---

--- "it not?"

Well, I thought I was wrong once, but then I found out that I'd made a mistake in thinking that.

---

--- That's two.

Which reminds me of a joke.

Wanna hear it?

JF

"John Fields"

** You are playing silly games with words.

You have ALREADY been proved wrong.

So FUCK OFF !!!

Asshole.

.... Phil

--
By whom, and how?

But it would be PWM control if the "width" of a dc pulse was a function of a sinusoid waveform.

Of course, if the frequency of the sinsoidal wave was really low, the motor's speed would change at the frequency of the sinusoidal wave.

this is a long thread, here's just my opinion, i'll try to keep it simple:

the speed of the motor is directly proportional to the supply power. turning the supply source on and off very fast is a way of varying the supply power. In practice, the supply voltage is usually constant, the best way to switch on and off the supply is to put a FET or some kind of high speed switch on the supply rail, to connect or disconnect it. when you switch on and off that switch really fast, square wave will from, constant supply voltage for ON and 0v for OFF. in the switching control circuit, there's a clock. in the PWM the clock frequency is usually fixed. and the Duty Cycle which is the % of ON time over 1 period (period(T)=3D1/switching frequency(Fsw) ) controls the average power being supplied to the motor.

and example of duty cycle control is, if you using a clock of 300kHz, then the period is 1/300k=3D3.33uS. if you ask for 10% duty cycle, the switch will be turned on with the rising clock edge, and turned off after 0.1*3.33uS=3D0.33uS. and stay off in the rest of the period until being turned on again with the next clock rising edge. the turn on timing is control by the clock, but the control of when to turn off the switch is not by timing. normally is by measure the voltage or current and feed into a comparator. they are called Voltage Mode and Current Mode control.

in many cases we don't define things and implement them, but rather find a way to implement then put it into context. in this case we don't define it has to be a square wave, but rather the square wave is the easiest we can make and control.

now, if supply voltage is 1V, the impudence of the motor is 1ohm, then DC supply will allow 1A flow, give the motor 1W of power. but if you turning the rail on and off with 10% duty cycle, the average current is now 0.1A and the average power becomes 0.1W. the motor will slow down.

re

sure you can do that, it's a waveform on top of a DC signal. what matters is the average power goes into the motor. by integration, it's the area below the waveform that matters.

you can, BUT sinusoidal same as the sawtooth, are signals with fixed

50% duty cycle (think about it). and the idea of PWM is to be able to control the duty cycle to change the power. so those signals won't fit here. and if you asking if it has to be square wave, the answer is no. as i said above, it's just happen to be the easiest we can get. and with some capacitance on the rail, if you look closely, there's always round edges and corners in the wave form. and with certain capacitance, 50% duty cycle square wave will looks like sinusoidal same as the sawtooth waves!

hope it helps.

If you use a sine wave, you lose efficiency, and raise the losses as waste heat.

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There are two kinds of people on this earth: The crazy, and the insane. The first sign of insanity is denying that you're crazy.

There is no lss if you chop a waveform into pulses with a switch. Running the motor steady, with dc motors, all pulses are at some set voltage and a set constant duration. Speed up the motor by extending the pulse width, not the dc voltage.

If you chop the sinewave to a universal motor with a switch (however which way) still no loss as waste heat when speed controlling the motor. Slightly less *motor efficiency* probably because pulses do not retain the most efficient voltage level. I mean, for a 12v dc motor I imagine that for*motor efficiency* the most efficient level for a pulse is dc 12v. Or perhaps we can afford to have a greater pulse level, like say dc 20v.

Yea, if I have a 12v dc motor, is it better to have say 20v pulses? Why stick to 12v pulses with PWM.

Industrial DC motors are often controlled via PWM acting on the AC mains (in a rectifying configuration). Switching losses are not a problem.

Best regards,

Bob Masta DAQARTA v4.51 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, Sound Level Meter FREE Signal Generator Science with your sound card!

What I've seen rectifies the AC with a full wave bridge, does minimal filtering and varies the pulse frequency so they can go both above & below the rated speed. AKA a 'VFD'

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There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you\'re crazy.

a

The original post was in reference to DC motors, which are not controlled by frequency but by average voltage. The controllers are essentially just fancy rectifying lamp dimmers. They vary the duty cycle to control the power to the motor. The one I own uses feedback to adjust the duty cycle to maintain a given motor performance setpoint.

Best regards,

Bob Masta DAQARTA v4.51 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, Sound Level Meter FREE Signal Generator Science with your sound card!