Whats the optimum frequency for the ossilator

[dgolka]

I have put together a unipolar board using the L297. I basically looked at the spec sheet and went from there. I would like to know how you arrive at the optimum frequency for the input of the ossilator pin (16). I know there is a better way than the trial and error method I used with the bread board. I have soldered up 2 of these boards and am drivng PK264-01A Vextra motors @ about 1000 rpm with no problem.

[H500]

You should use a higher voltage if possible. I'm using about 35v and can get more than 5000 rpm (no load) with my 4.5v 1.4 amp motors, using a similar type of chopper. Of course the motor is usesless at that speed, since the torque is low, but it's interesting to see what is possible.

[dgolka]

I am going to put together a powersupply that will give me about 36V. I am currently using the power supply from an old QMS color script that I salvaged. It has an output of about 20V. I was wondering about the size of capacitor and resistor that connects to the ossilator conection on the L297 though.

[Gilii]

I have the same doubt about chopper freq.
I have installed one 22K resistor and a 3.3nF cap at pin 16 (osc) of L297, and this combination give me 20KHz chopper freq. (1/0.69*R*C)
I'm feeding motors with 40Vcc and speed is not problem - sound is!
There are a very high pitch sound, probably because 20KHz freq - inside human audible range.
My question is - what is the impact at the FETs (i'm using IRL630) if I trhow frequency more high, for let's say 30KHz or even a little more (outside audible range).

Thank you and best regards (sorry my poor English)
Gilii

[H500]

20khz is not within the audible frequency of most people. The squeal you hear is well below 20k.

I think it's caused by electrical noise causing the chopper to continuously change frequency. I don't think increasing the chop freq will make a difference, but you could try it anyways since it might change the tone and make it less offensive.

[Gilii]

H500,

Thanks for the answer.
I will try to change the freq, but i agree with you - that ugly sound probably is due to eletrical noise.
My doudt is If the FETs will accept a faster frequecy, and not cause lost steps, or maybe due to higher freq will work at higher temperatures.

Gilii

[H500]

Gilii, if your switching freq is too high for your fets, they will just get hot. I'm using irl530's in a similar chopping circuit and they have no trouble with 50 khz. I have them on a heat sink even though it's theoretically not needed.

[Gilii]

H500,

Thanks again - I'll try to change freq to something higher and see what will happens.

Gilii