Author Topic: Mill Power feed  (Read 57416 times)

Offline John Rudd

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Re: Mill Power feed
« Reply #125 on: July 02, 2009, 06:09:22 AM »
I have a question John....

If say you wanted half speed, you would set the timer to pulse the power delivered for 50% cycle time...

Wouldn't 50% on 50% off at full power be the same as 50% power continuous ?

eg, take 1HP for simplicity sake,     1/(100/50)= 1/2 power/torque

Also what about this pulsing power, I realise the freq would be high, but it would be pulsing all the same wouldn't it?

I'll answer these as best I can...I dont hold and engineering degree or Phd in electronics, I'm merely a hobbyist..

Answer 1..yes 50% would equal 1/2 speed assuming say the motor ran at 3000rpm at full volts and disregarding losses in the motor.

Answer 2...Seems logical to me captain..but because you are applying the full 12volts then the motor runs at its max speed for a short duration that the power is on to it...The following probably describes thnigs a bit better

To control the speed of a d.c. motor, it needs a d.c. power source.If you take a 12v motor and switch on the power to it, the motor will start to speed up: motors do not respond immediately so it will take a small time to reach full speed. If the power is switched off sometime before the motor reaches full speed, then the motor will start to slow down. If the power is switched on and off quickly enough, the motor will run at some speed part way between zero and full speed. This is exactly what a p.w.m. controller does: it switches the motor on in a series of pulses. To control the motor speed it varies (modulates) the width of the pulses - hence Pulse Width Modulation.
See the waveforms in the pictures I've attached... If a motor is connected with one end to the battery positive and the other end to battery negative via a switch (mosfet, power transistor or similar) then if the mosfet is on for a short period and off for a long one, as in A above, the motor will only rotate slowly. At B the switch is on 50% and off 50%. At C the motor is on for most of the time and only off a short while, so the speed is near maximum. In a practice the mosfet is switched at something like 20kHz. This is too fast for the motor to even realise it is being switched on and off: it thinks it is being fed from a pure d.c. voltage. It is also a frequency above the audible range so any noise emitted by the motor will be inaudible.

I hope this answers your questions without insulting your intelligence...Some of this you may already know..
« Last Edit: July 02, 2009, 06:15:21 AM by John Rudd »
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Offline Darren

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Re: Mill Power feed
« Reply #126 on: July 02, 2009, 06:36:15 AM »
Hi John,

Yep that does it nicely... :thumbup:

As you know I already have a power feed set-up on my mill and use it quite a bit now that it's there.

If there is a better way of delivering the power then I would certainly consider a change. At the mo I'm yet to be convinced.
My thinking is with a windscreen motor the worm drive gives quite a large gear reduction ratio, so the motor is not turning particularly slow at the lower speeds.
My other finding is that the lower speeds are not really of much benefit in real life and I tend to use it flat out 99% of the time.
Half speed can sometimes give a slightly better finish, but not for everything or every type of tooling/material I have used.

I think too much effort has been put into designing a precisely variable speed (voltage) output on my part. But this of course in in hindsight.

Two speeds, one at 5V and another at 18V would be more than adequate in my opinion. With this in mind I'm thinking an old computer power supply with 12V & 5V outputs would be more than sufficient for those electronically challenged. It's prob the way I would do it now if I had to start over.

A 9/18V transformer and rectifier would also be a very simple, cheap and reliable way to do this too. Don't worry too much about the exact voltages, it really is not that important  :thumbup:
Oh, and these motors seem to be quite happy to run at 25V without any signs of distress too....

I hope that helps.. :dremel:

Any chance of the schematic, I've never used a 555 but I know what it is... :ddb:
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