Messages

251

General Mach Discussion / Re: Building a stepper motor dyno

« on: October 25, 2012, 11:20:22 AM »

As the saying goes, "there are many ways to skin a cat." Using a DCPM motor allows for a conceptually simple project that is easy for most folks to understand the theory of operation. Since I wanted to share the project with others this was a good place to start. As you say using a motor with field windings rather than magnets would allow for easier control of the load presented. You can also just switch in resistance, and monitor the current. Measuring the current (and voltage) eliminates the need for precision resistors.

I built a similar dyno for the university here for a student that was studying how to diagnose problems with three phase motors. Since they do a lot of tours through the lab it was going to be in I used 100W light bulbs as a load and measured the current/voltage on the DC side (well on AC side as well for diagnostic purposes). Anyhow it is kind of neat to watch it go through a test cycle and see the lights turn on/off. It makes a good visual for tours as well

252

Promote and discuss your product / Re: Special Gecko Stepper driver sale

« on: October 24, 2012, 12:42:25 PM »

I have not gotten into the servo drives yet. I started out with small desktop machines and products and am starting to branch out into larger machines/products. Servos and servo drives will be part of the line up eventually.

253

General Mach Discussion / Re: Building a stepper motor dyno

« on: October 22, 2012, 12:27:42 PM »

I posted the second installment a few hours ago. It covers modifying the motor to drive the encoder.

@EJParrot - The choice of drive and drive voltage also plays a big role. That is another thing I'm hoping to shed some light on: How does the same stepper motor, at the same drive voltage perform with different drives.

254

General Mach Discussion / Re: grounding your cnc

« on: October 21, 2012, 09:48:05 PM »

The situation, and hazards, with high-current welding equipment are quite different from what folks are dealing with here, and if someone is having problems with, for example, spurious E-stop or limit triggers, tying the machine ground to a local ground rod is exceedingly unlikely to make any difference at all with those problems.

If you'll read my first few responses I also said that unless there is some need, i.e. plasma table etc, then a local ground rod is not required.

It is quite common to have significant potential difference between the "local" earth ground, and the power panel earth ground,

Also please note I said the two ground rods should be tied together. The article Terry posted the link to says the same thing.

255

General Mach Discussion / Building a stepper motor dyno

« on: October 21, 2012, 09:41:38 PM »

I've been trying to stress the importance of choosing the right stepper motor for the job for years now. This runs contrary to the 'bigger-is-better' idea that is so popular among hobbyist. I often point folks toward stepper motor torque curves but as these are all done by different manufacturers, with different stepper drives, etc it makes it hard to try and compare two different motors. (And sometimes I'm not sure how accurate some of these torque curves are.)

In an effort to have more reliable data to work with I decided to build my own stepper motor dyno for testing. I just posted the first blog entry today that covers the build: http://www.soigeneris.com/Blog/Default.aspx Follow along with the process and let me know what you think.

256

General Mach Discussion / Re: grounding your cnc

« on: October 21, 2012, 09:35:46 PM »

Terry - That is a good article! Thanks for sharing. The one use for the term 'ground' they kind of skipped of was 'DC Common'. A lot of guys try to use the machine frame as a current return path for sensor, probes, etc. Since the frame is generally connected to the earth ground you wind up with small control currents running through earth ground which causes all sort of problems. 

Rich - Yeah there are fancy meters for testing how good a ground is. When installing plasma cutters, welding robots, etc we would do a quick 'red neck' test on the local ground rod for the machine. Hook a 100W light bulb between the neutral of the power source and the local ground rod. If the bulb was anything other than very dim the local ground rod was not good. After testing the local ground and building ground were tied together.

257

General Mach Discussion / Re: grounding your cnc

« on: October 21, 2012, 11:43:38 AM »

There is a difference between grounds from an electrical  and noise point of view.

Earth Ground is Earth Ground is Earth Ground, Earth Ground does not change into something else when being used for noise abatement. While the primary purpose of the ground wire being added to electrical system was safety having a low impedance path to earth (a path which had no current flow from any circuit) was also a perfect place to bleed off electrical noise.

A copper ground rod is for safety - it will do nothing at all for noise.

That is incorrect. Crack open the power supply on your PC, where do all the filter parts get bonded to? The Earth Ground! Why? It is a low impedance path to ground. In the case of a machine like a Plasma Cutting table, Automated TIG welding machine etc, the local ground rod provides a low impedance path to the earth rather than rely on the small gauge ground wire that runs all the way back through the plant to the electrical service entrance. Any local ground rod should be bonded to the buildings electrical ground to prevent a potential shock hazard between the two.

I'm not meaning to sound like a 'know-it-all' here, I just want to dispel myths about grounding. I used to design, install and service automated welding and cutting systems and have had first hand experience with about every type of noise issue there is. I just want to share that experience with others.

258

General Mach Discussion / Re: grounding your cnc

« on: October 20, 2012, 10:35:28 PM »

Yes, unless you are using a plasma table etc. On your machine all the items that need an earth ground should be tied to a single point (bonding bar) and you would connect your earth ground there.

259

General Mach Discussion / Re: grounding your cnc

« on: October 20, 2012, 10:04:02 PM »

A hole drilled beside the machine is typically a useless ground, think about it you are driving a rod into dry earth, that has been dry for years as there is a building over it and will thus remain dry and a lousy ground. Some machines, such as a plasma cutter, can make use of a local ground rod usually driven outside the building as close to the machine as possible. The local ground rod must be bonded to the electrical ground or will will create more problems for yourself. Two rods driven in the ground just 100' apart can have enough potential difference (voltage) across them to create a deadly shock hazard.

260

General Mach Discussion / Re: Plasma cutting with a MIG welder

« on: October 16, 2012, 03:03:42 PM »

I thought your question was "can I take a MIG welder I have now and use it for a plasma cutter", which can't be done. There are welding power supplies out there that can run in both CC and CV mode (constant current for TIG/Stick and Constant Voltage for MIG). I've not seen one of the 3-in-1 power supplies in person so I cannot comment on how they work.