Hi Terry,

>From the experimenting with PWM that I have done recently, I think your
idea is excellent. The only problem is complexity.

Here is how I understand PWM (someone please correct me if I am wrong):

When you feed PWM to a DC motor, you might think that you are saving power
as you turn down the motor speed. It is true that less mechanical work is
being done,but more power is being wasted in the winding resistance. This
is because the same average current needs to flow (if the frictional torque
remains the same). If we have a motor that generates 10v of back emf at
full speed and has 1 ohm of winding resistance, then with 12v PWM at 100%
duty cycle, we have a continuous 2 amps flowing, we are using 24 watts of
electrical power,and doing 20 watts of mechanical work (approximately,
assuming that the transfer of power due to back EMF is purely mechanical).
The efficiency is 20/24 = 83%

If we now drop to 50% duty cycle, we still need an average current of 2
amps to oppose the friction,so we have 4 amps flowing during the on time.
That's 48 watts of power for 50% of the time,or,again,24 watts of average
electrical power. However, the back emf is now only 8v ( 12-8=4, 4/1ohm = 4
amps),so we are only doing 8*4=32 watts for half the time, or 16 watts
mechanical work. The efficiency is now only 67%,and it reaches 0% when the
duty cycle gets so low that the resistance doesn't allow enough current to
flow to oppose the friction (in this case, we couldn't operate the motor
below 16% duty cycle). As we decrease the duty cycle, the speed vs. duty
cycle curve is nonlinear and drops expecially fast as we near 16%.

With a switcher,however,we could forget PWM and just supply a continuous
variable voltage to the motor. The current would stay the same and the RPM
would linearly follow the voltage. The efficiency would not change much
over the whole RPM range.

Sean


At 03:39 AM 7/26/99 +0800, you wrote:
>Hi, i was just thinking, instaed of using a semiconductor switch to control
>the motors, why not use a switch mode power supply to directly drive the
>motors?
>
>If your circuit doesn't require reverse motor drive, wouldn't it be ideal
>to cut out the switches and just use the SMPS's drivers?
>
>Battery powered controllers would benefit even more from the SMPS's ability
>to step up the battery voltage and squeeze out every last drop from the
>batteries, (don't over squeeze to the point of battery breakdown tho..)
>
>Current sense in the SMPS could double as overload protection too. If
>response time is somewhat more critical, use a smaller filter cap for the
>outputs.
>
>What say you folks? Viable?
>
>Terry
>
>
>
>
>At 10:24 PM 7/24/99 -0700, you wrote:
>>At 01:55 AM 7/24/99 -0400, you wrote:
>>>I am a bit confused here: why would the FETs require more precautions? The
>>>only thing I can think of would be that the power dissipation for a FET
>>>goes as the second power of current,and for a darlington/BJT, as the first
>>>power of current. Granted,this makes a big difference as current goes to
>>>infinity, but in certain cases, you might burn up the BJTs way before the
>>>FETs if the RDSon of the FETs is really low.
>>
>>It's actually even worse than that. The R of the MOSFET increases with
>>temperature and current, so unless you go overkill or have protection you
>>have a big problem. With comparable devices, the bipolar will usually take
>>a lot more abuse(assuming it has enough drive current). Generally in a
>>small application it's easy enough to use a larger MOSFET, but I've
>>designed 1200 amp motor controllers and it makes sense to put in good
>>protection so you can use fewer MOSFETs.
>>
>>Cheers,
>>Bob
>>
>
|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
\--------------=----------------
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