More?? - How PWM Works?

Interested in Robotics? Here's the place to be.
Post Reply
dyarker
Posts: 1752
Joined: Fri Aug 22, 2003 1:01 am
Location: Izmir, Turkiye; from Rochester, NY
Contact:

More?? - How PWM Works?

Post by dyarker » Sat Jan 17, 2004 3:27 am

Is there a "rule-of-thumb" for picking the PWM frequency relative to the motor's time constant?<p>It seems to me, that if PWM frequency was much too high, only high percentage duty cycle pulses would result in useful power. Low percentage duty cycle pulses would be gone before any significant current flow. (ie- 1% to 50% = 0 oz-in., 55% to 100% = 1 oz-in. to 50 oz-in..) The range where there is control over torque or RPM is reduced.<p>If the PWM frequency was much too low, motion would be jerky, and probably hard on the gears.<p>I also suspect that the PWM period should not be hamonically related to the mechanical time constant of the motor. Else resonent vibration shakes the motor apart, or at least an unpleasant noise.<p>Gut feeling is that PWM frequency should be set so that a 5% to 10% duty cycle pulse is about equal to the time constant.<p>Anybody have any facts to share?<p>Thanx
Dale Y

hlreed
Posts: 349
Joined: Wed Jan 09, 2002 1:01 am
Location: Richmond, TX
Contact:

Re: More?? - How PWM Works?

Post by hlreed » Sat Jan 17, 2004 8:20 am

Think of it this way. For a DC motor to run at full speed you feed it DC.
If you want it to run half speed, feed it half DC and half off. And so on.
The frequency comes from the repetition.
Motor
Read Input
Write pulse related to input
GOTO Motor
Harold L. Reed
Microbes got brains

dyarker
Posts: 1752
Joined: Fri Aug 22, 2003 1:01 am
Location: Izmir, Turkiye; from Rochester, NY
Contact:

Re: More?? - How PWM Works?

Post by dyarker » Sat Jan 17, 2004 7:42 pm

No Harold, it's not that simple. Motors are inductive. They also have a mechanical time constant.
Dale Y

Chris Foley
Posts: 146
Joined: Mon Jan 13, 2003 1:01 am
Location: Chicago IL
Contact:

Re: More?? - How PWM Works?

Post by Chris Foley » Sat Jan 17, 2004 11:53 pm

Hi, Dale. While there are a number of other factors that you could take into consideration, generally you just say that a frequency which is too low will result in mechanical resonances/excess noise, and a frequency which is too high will result in too much lost power from the switching transistors/MOSFETs being in the linear region. A good rule of thumb for smaller DC motors is 1 KHz to 5 KHz. If possible, sweep the PWM switching frequency, and observe the results for the motor you've chosen (listen for excess hum/buzz, check your switching transistors for excess heat/scope 'em out).<p>Good luck
Chris

cato
Posts: 366
Joined: Sun Sep 07, 2003 1:01 am
Contact:

Re: More?? - How PWM Works?

Post by cato » Sun Jan 18, 2004 5:48 am

I suggest something in the range of 40Khz to 1 MHz. If you pulse at 5 Khz, youre gonna hear it. It can get annoying fast.

josmith
Posts: 340
Joined: Tue Dec 04, 2001 1:01 am
Contact:

Re: More?? - How PWM Works?

Post by josmith » Sun Jan 18, 2004 2:54 pm

The GE servos I have seen run at 400hz. It would seem that the inertia of the system would be a big factor. YOu would want to go as low as possible without causing vibration.

toejam
Posts: 253
Joined: Wed Dec 05, 2001 1:01 am
Location: n.c.
Contact:

Re: More?? - How PWM Works?

Post by toejam » Thu Jan 22, 2004 5:07 am

What i have been finding in experimenting with dc motors in the 100-200 watt range switching speeds of ~ 10-25 khz seem to be the most efficient but I believe that is dependant on the amount of commutations the motor makes. As the frequency increases, so does the gate current needed to charge the gate cap. If you have too little gate current, the mosfet spends too much time in its linear mode causing it to overheat. I have also found that the reverse diode's switching speed seems to make a differance.good luck
tj

Post Reply

Who is online

Users browsing this forum: No registered users and 3 guests