Belt Grinder Speed Control

[RogueGeek]

Good day,

I'm in the process of building a variable speed 2x72 grinder.  As I have calculated it so far, my grinder should have a top speed of around 6800 SFPM.

I'm currently building the electronic motor controller (my other hobby and day job is electronics) and as it is currently designed, I will be able to adjust belt speeds in increments of 50 SFPM.

Does this sound like a reasonable step size or should I build it for a smaller step size?

 

Just in case anyone is curious, I will be using a 2HP permanent magnet DC motor from a treadmill. It will have a speed sensor to get accurate speeds and the controller software will be written to maintain the set speed as loading on the belt varies during use.

 

thanks

 

[Kozzy]

50 is over-kill for operation.  Probably 200 is plenty..but...

Since you want to ramp up speeds from start-up (soft start) or a smooth transition when changing speeds, that is the reason to go with a higher resolution.  WIth 8 bits on the DAC you'll transition by about 26 FPM...

Guessing that some people will chime in that the whole prospect is complicating things more than they need to be but sometimes it's about doing a fun project, not the realities of actual use.

Arduino based?

[RogueGeek]

It is definitely overkill but like you said, it is fun building it.

50 was chosen arbitrarily as the feet per minute step size. I primarily chose it since it allowed a reasonably fine control and still didn't take to long to go from MIN to MAX.

I'm using an MSP430 series microcontroller.  Similar to a Microchip PIC.

[New axe maker]

Have you thought about wiring in a dimmer switch to control the speeds by adjusting it with the dimmer?

[ThomasPowers]

Is  that dimmer a rheostat or a chip based version that clips the wave?

[New axe maker]

I'm not aure. I was thinking for my project to use a dimmer switch from a light switch 

[ThomasPowers]

Which could be either type but modern ones use the wave clipping chip why you can't run most motors from them.

[New axe maker]

My motor is from a treadmill so it has seen control built in. How would I beable to control it. I think 6800rpm is a bit fast for a belt grinder

[Frosty]

That's 6,800 FPM (Feet Per Minute) not RPM. I'm thinking if you posted the specs of your motor and controller our Roguegeek and Mr. Kozzy could fill you in on what you have and need.

Another option would be to walk into the showroom of Hayden Electric on the Old Seward Hwy. and ask. They've always been helpful guys and know what they're about.

Frosty The Lucky.

[RogueGeek]

Sorry it took so long to get back to this thread, we are in the middle of moving from NJ to FL.  I never realized how much crap I have.

NewAxe,  99% of treadmill motors are DC and require a somewhat special speed controller. I'm choosing to build mine since I like to do things the hard way. I also wanted to over-complicate it by using a microprocessor and having it automatically compensate for belt loading to maintain a set speed.

For your motor,  if you have the control panel and other bits from the old treadmill, you most likely have the necessary speed controller.  There are hundreds of YouTube videos and how-to articles that talk about using treadmill motors as the drive motors for drill presses, lathes, mills and more.

As Thomas mentioned, the newer light dimmers don't simply turn down the voltage, they alter the AC waveform.   See this site for more info, look at the Forward Phase Dimming. 
Using a rheostat (variable resistance) type dimmer can work for some motors but it is very inefficient and doesn't offer a wide range of control. Typical light dimmer rheostats are likely not made to handle the current draw of a large 2+HP motor.

Most brushed DC motors can be controlled using a PWM (Pulse Width Modulation) speed controller.  This delivers full voltage/current to the motor in short pulses.  The shorter the pulse the slower the motor goes,  the wider the pulse the faster it goes.  See the diagram below for what PWM looks like.  

Regarding the belt speed to RPM calculations.  Do a google search for 'belt sfpm'  It will provide you with hours of entertaining reading.

Since I don't know your level of electrical knowledge, I'm going to offer a few words of CAUTION!  Treadmill motors operate at high voltage DC, typically 90VDC or higher and the controller circuits can deliver 10+ amps to the motors at full voltage.  These voltage and current levels CAN KILL YOU!!!  Please exercise extreme caution when you are working with your Treadmill parts.  Always remember to unplug everything BEFORE you reach in to make an adjustment.  If you don't understand electronics, please educate yourself or ask for help before trying anything.  I would rather help you here than hear about your unfortunate accident on the news. And remember, just because you plugged it into a GFCI outlet, doesn't mean it is 100% safe.  GFCI isn't a get out of the morgue free card.

 

 

 

 

 

 

[New axe maker]

Thank you for the info. I'm know enough about electrical to get myself killed so I think I have A LOT of reading and research before I continue with the belt grinder. 

[RogueGeek]

Please don't let my warnings discourage you.  Electricity isn't rocket science or magic.  You just need to learn to do's and don'ts to keep you and your family safe.  You might want to check with a local technical school and see if they offer any electrical classes for beginners.  You might be also to find something at a reasonable price.

The NAVY has also released their NEETS modules (Navy Electricity and Electronics Training Series) to the public.  They will provide you with hours of fun and entertaining reading.   They can be found all over the internet, but this guy http://jacquesricher.com/NEETS/  has them all in once place.  Oh, and if you ever suffer from insomnia, any NAVY book or manual is a miracle cure and will put you to sleep in minutes.

Search around online, there are several good belt grinder how to articles.  More than one belt grinder has been built without a speed control.
Keep in mind you can also use pulleys to get a few fixed speed options.
I would recommend starting simple and get a basic fixed speed belt grinder working.  The designs are tried and true and reasonable easy to build.  Once you have that working, use it for a while, if it does what you need, then your done. No need to change it.

If on the other hand it is lacking something, you can make modifications to your existing machine to fix the short comings or you can start work on a totally new machine and incorporate what you have learned into the new design.

[New axe maker]

I'm thinking a simple on off toggle switch ill line with the power cord should turn It on and off than using a removable pulley would change the spoon so a bigger pulley would in theory slow the belt down just like gearing in a vehicle will change the engine speed.