I would like to know what formula(resonant frequency) to use for a
cmos hex inverter(CD4009) based oscillator/PWM circuit I am building
to power a 36 volt PMDC motor??
I have read somewhere that a frequency of 1500 Hertz would be sufficient for
the output powerfet's.
Thanks.

 

Instead of the CD4009, use a CD4093, which is a quad 2-input NAND gate with Schmitt triggers on each input. The CD4009 does not have Schmitt triggers on the inputs, and slow-transitioning signals on such IC's are not good, as that can lead to high current consumption along with resultant heating of the IC.

If you go to National Semiconductor's website:
http://www.national.com
and download the datasheet for the CD4093BC, you will find a schematic and formula for determining the frequency of a Gated Oscillator in the "Typical Applications" section, starting on page 4. To modify the circuit so that it always oscillates, tie the two inputs together, or tie the CONTROL input to Vcc (logic "1").

As always with CMOS, ensure to tie any unused inputs to either ground or Vcc to prevent uncontrolled oscillations.