The need of a relay for a quite spindle?

Hi all
I own a x-carve from the year 2015. It’equipped with a powersupply 24v, an arduino and the, to this time, attached motor driver board. Now i must replace the original spindel due to bearing worn out.
I want to change to the 300 watt quite spindle. Inventables write, in addition to the powersuply, i also need a relay. This i can not understand. The original spindle was speedcontrolled by pwm from the arduino pin D11. The voltage regulator contains a pwm input. Why i can not connect this pwm input directly to the pwm output of arduino?
Please help.
Regards, Paul

Power requirements from spindle may be different.
Please post more info in regards to the 300 watt spindle.

I put a quiet spindle on my machine when my original one, one of the now legendary first batch that died almost immediately a couple of years ago.
When I ordered it, I believed it was a 400 watt unit but may in fact be a 300 watt.
I dont care either way,It works.
It is on a separate 48 watt power supply. I have a pot switch to manually control the speed, but rarely adjust it, and also have a optical relay switch from Inventables to control on and off from the software.

You may need to look at the documentation for youre spindle to confirm the voltage requirements.
Although it was an extra cost, I like having my spindle separate form the stepper control hardware. If something fries in the spindle it’s less likely to take the arquino/gshield with it.

So far, thanks.
I know, i need an other powersupply. The spindel needs 48 volt. And since the inventables powersupply seems not to contain a voltage regulator, i also need this part. All clear so far. What i can not understand is, why the pwm input in this voltage regulator can not be used to controll the speed instead of using the potentiometer and an additionsl relay? Mmmhhh. If the voltage regulators pwm input is ttl level, and also the arduinos pwm output is ttl level, this should not be a question. Am i completely wrong?
Thanks for further help. May be from inventables themselve?