DC 895 motor direct connect to Duet wifi

[sarvan]

Hey guys I would like to use my duet wifi as small CNC. For that I am going to connect the 12-24 V 12000rpm 895 DC motor to bed or heater. How can I control the speed? Do I need any external circuits? Below is the motor details
https://www.amazon.com/BestTong-DC12-24V-Generator-12000rpm-Upgraded/dp/B07Q8TJXB9/ref=mp_s_a_1_4?dchild=1&keywords=895+motor&qid=1629906119&sr=8-4

[alankilian]

@sarvan You can use an external controller to convert a PWM signal to a DC signal for your motor.

Here's a PWM-input speed controller that might work depending on your motor .
https://www.amazon.com/Motor-Speed-Controller-Spindle-Control/dp/B00OSWQC9Y

You would hook the input of the controller to a PWM-capable output pin.

I really don't know what would happen if you connected it to a bed heater output and tried setting the PWM on that pin.

[sarvan]

@alankilian you mean I can connect through any of fan PWM? And give the power to the external PWM converter from the same PSU?

[o_lampe]

@sarvan
Unfortunately we don't know the motor current for sure. There are formulas to calculate current from torque & RPM & input voltage, but I have to digg deep in my memory.
If you don't try to mill steel with insane speed, you might get away with the bed heater output. A current gauge between motor and Duet would be a good idea.
You could also add a potmeter to an external analog input (thermistor?) to dial in the max PWM duty cycle, you want to allow.

//edit according to the coarse numbers we have, I've calculated max current of ~50-70A (12-24V) at full torque.
Duet2 is rated 18A max. bed current, so you'd have to be very careful with direct control.

[alankilian]

@sarvan said in DC 895 motor direct connect to Duet wifi:

@alankilian you mean I can connect through any of fan PWM? And give the power to the external PWM converter from the same PSU?

Yes, exactly.

The spindle speed will vary with load, but you should be able to mill things that aren't difficult to mill like plastic, wood and maybe, maybe light passes on aluminum.

[dc42]

@sarvan the data for that motor says the starting current is over 10A @ 24V, the no-load current is about 2A and the maximum current is 16A. So one option is to connect a Schottky flyback diode with a rated current of at least 5A in parallel the with motor, observing correct polarity (e.g. diode cathode to the positive motor terminal), then connect that combination to the bed heater output.

If it turns it that the starting current is actually a lot higher than 16A and it blows the 15A bed heater circuit fuse, then you will need to use an external controller.

[sarvan]

Thanks guys. I think it's better to use external PWM converter than that of frying something on assumption

[sarvan]

Hello folks,
I got my controller, https://www.amazon.com/Motor-Speed-Controller-Spindle-Control/dp/B00OSWQC9Y
but, I have doubt again here,

so, I can just directly connect the FAN2 polarities to the PWM inputs right? or do I need to add any resistors?

[dc42]

@sarvan if the PWM input is max 12V and you are using 24V VIN, then you need to connect a series resistor between the van output and the PWM input.

[sarvan]

@dc42 thanks. If use the jumper with 5V. Then no need to add resistor?
If I use 24V VIN so to reduce the voltage to 12V - if I use 10 Ohm, how much watt needed? I am not very good at electronics

[alankilian]

@dc42 How about connecting the PWM+ to a 5 Volt supply on the Duet and the PWM- to FAN2-