1.5V to 30V fixed frquency PWM generator
I had suggested a 555 timer circuit, but Marcus Müller reminded me it needs to start at below 1.5V.
 A transistor-based astable multivibrator should be able to work at such a low voltage (possibly down to 1V). Here is one:  







simulate this circuit &ndash; Schematic created using CircuitLab  

A standard basic design is not recommended, because one edge would be changing slowly instead of suddenly. This would waste energy as heat in your output transistors and could burn them.
 The improved design includes an additional diode and a resistor on each side (D and Rc2 in the schematic).
 C3 is definitely needed because the circuit might not start oscillating without it. Its value could be as low as 0.1µF or 100nF. If it's in parallel with the DC motor, it would have to be at least 100µF.
 Q3 is needed to buffer the signal and cause less loading on the circuit. If you could show me the schematic of what exactly you intend to do, I could tell you better values of the resistors and whether to add another buffer transistor.
 The T1 (time period of one cycle) is about:
T1 = 0.7(R1+Px)C1  --> Px in this case depends on the slider position.
 If it is in the middle, then it's about 23kΩ, so we get:
T1 = 0.7(1500+23000)100E-9 = 1.7ms
The total time period is T1+T2, and with the slider in the middle it is 2 times the single time period, in this case 3.4ms. The frequency is the reciprocal of the total time period, in this case about 300Hz.
T=T1+T2
f=1/T

Most importantly, you can adjust the duty cycle of your output, the ratio is probably around 17:1 (6-94% adjustment range, if I'm correct).
Please note that I might be slightly wrong in some of the recommended resistor values, but those can be easily changed.  

UPDATE: Since you require a fixed 60% duty cycle, you don't need the potentiometer. Now you can use a wider range of resistor values.
I would start with the Rc and Rc2 resistors; change all of them to 3k3 (the 1k value at 20V would dissipate around 0.4W).
The R1 and R2 would need to be 33k and 51k to give you around 40-60 duty cycle ratios.
 Since the frequency is mostly dependent on the R1C1 and R2C2 values, I would suggest placing some sort of a voltage regulation either for the whole oscillator or just for the base resistors R1 and R2.  You could simply place 2 diodes like 4148 or 4001 in series to get around 1V to 1.2V and a dropping resistor from the power source to this circuit. You would have a much more constant voltage that way.  







simulate this circuit

Rdrop would have to be at least 1/2W resistor at 20V; at 30V it needs to be at least 1W.