I think your question needs to clarify what type and size of work to expect to do. Big subject indeed.

Edited By Brian Oldford on 17/12/2016 21:03:23

"Came across this today, might help you with the differences between brushed and brushless DC motors"

Seems to me that it shows that a 500W motor is more powerful than a 250W motor, or did I miss something?

Many smaller (and generally cheaper) machines use DC motors, mainly I think because they can easily be speed controlled so saving on mechanical speed reduction. (Mechanics more expensive than electronics these days.) Some DC speed controllers are a bit fragile, especially it seems those fitted to small mills and lathes (e.g. the X1 mill, see various tales of woe here). Brushed motors are traditional but the brushes can wear out. Brushless require electronics to drive them – they have only appeared on amateur machines in the past couple of years, so reliability perhaps an unknown quantity. They can also be more powerful in a given size I think. I have a small CNC mill (Denford Novamill) with a 1/2 hp brushed DC motor run from a "proper" KB Electronics controller and more than once I have stalled the drive (doing something silly) – once the fuse blew but the drive survived, so they can be robust.

Single phase induction motors are cheap to make, very reliable, quite efficient, but need AC to make them work – but AC comes out of the mains so isn't a problem. They can't be efficiently speed controlled so you need belts/gears. Because of the way they work their torque varies cyclically slightly out of phase with their rotation, hence vibrate a bit. But most small machine tools have done very well with them for decades. Also (in reasonable power versions) they need special means to make them start, usually consisting of an extra winding and a capacitor, and generally a centrifugal switch. The capacitor and/or switch sometimes give trouble. By changing the way the extra winding is connected they can be reversed. Maximum power is generally up to 3/4 hp or so. They generally come in 2-pole, ~2900 rpm; or 4-pole, ~1425 rpm (on 50 Hz). Poles and volts are unrelated.

3-phase induction motors are also cheap to make, very very reliable, efficient, but need a 3-phase supply. They start on their own and can be reversed by swapping over two connections. Again, 2-pole and 4-pole are common. Speeds (on 50 Hz) the same as single phase. Industrially they sometimes run on 415V phase-to-neutral, in a star winding configuration. Such a motor will need rewiring in a "delta" to run on a standard 3-phase "domestic" supply which is only 230V phase-to-neutral. This may be easy or hard depending on construction! Rotation is very smooth and vibration free (as long as the motor is properly balanced). The magnetic circuit of a 3-phase machine is more efficient so they can give more power in a given frame size.

These days a 3-phase motor can be fed from an "inverter" or Variable Frequency Drive (VFD). These allow you to vary the speed from virtually stopped up to rated speed up to (in principle) as high as you like. I run my mill up to 75 Hz, 50% over plate RPM. The VFD can be fed from single phase (usually), or the higher power ones 3-phase.

There is nowadays no point whatever in having a 3-phase supply connected to your house unless you need a lot more power than you can get from a standard single phase supply, as you can run a 3-phase motor from a VFD. Indeed it's better using a VFD because you get variable speed as well.

In summary,

• If you are looking at a hobby size machine, if it's a type that has a DC motor then ask around on the ME forums to see what sort of reliability people are getting.
• There are some machines that are fitted with a 3-ph induction motor and VFD – I think these are likely to be better and more reliable. Industrial grade VFDs are readily available and quite cheap if you needed to replace the original one.
• If you buy a used machine, you would probably be very happy with a standard single-phase induction motor but could improve it by fitting a 3-phase motor and VFD. (Or if you get a used industrial machine with 3-phase, add a VFD and use it on your single phase supply.)
• Don't worry about getting a 3-phase mains supply unless you want to run a big muffle furnace or electric kiln!

But as you say Andrew, you have a lot of big machines!

Terry,

You might find this recent thread of interest: **LINK**

http://www.model-engineer.co.uk/forums/postings.asp?th=122544

MichaelG.

Lots of reference to VFD as if it's something that only changes single phase to three phase but Variable frequency drives are predominantly used in industry to change the frequency of three phase supplies in order to run motors at variable speeds and/or speeds not available with the basic supply.

If I had three phase to my workshop I'd still use VFDs because variable speed is an absolute godsend in a variety of circumstances, the sad part is that until you've run a machine with variable speed and made propper use of the facility you won't understand how useful it can be

– Nick

Some 3-phase machines are a bit too complex to simply accept a VFD, eg where there is a 3-phase transformer on the input with lots of different components powered from it eg a mixture of single and 3-phase.

You can't simply run a 3-phase transformer from the output of a VFD without some interesting (expensive) filtering between them, so instead you have to either power the whole thing from a phase converter or dive in there, remove the 3-phase transformer and provide alternative sources for each of the subsystems. It's usually only the spindle motor and the hydraulic and coolant pumps that require 3-phase (one or two VFDs) while the rest of the gear (the controller, switchgear, lighting, display etc) is usually powered from one phase, so can be provided with a single phase supply, either directly or via a (single phase) transformer.

Phase converters may be the simplest solution but given their cost it may often be worth taking a different approach. As you may have deduced, I'm in the middle of such a conversion myself. I see a cost-effective solution but it will require some reconfiguration of the innards of my machine to make it accept single phase.

Murray

Hi Martin – I wasn't replying to your post as such – not problem with it at all with it. Just thought this thread was a relevant place to make a couple of observations in the light of my current activities.

Murray