2 pole or 4 pole motor for VFD

I understand that the rpm is half / double, but are there any other issue plus or minus in choosing a 2 pole or a 4 pole motor for VFD control - I can adjust the spindle speed by the pulley sizes.

You may be able to run the 2 pole (3000rpm) motor down to the same speed as the 4 pole (1500rpm) but it will only have half the torque there if it's the same power rating. So if your machine doesn't have a good reduction ratio, you may not have the low speed torque required for tapping and heavy machining of large diameter work.

Similarly, you can get a higher spindle speed out of the 2 pole motor but is your machine happy taking the full power at those speeds? You can run the 4 pole motor (at reduced power) above base speed with a VFD anyway, so unless you plan to do some serious high speed metal removal, the 4 pole may the safest bet. It's what I fitted to my Bantam.

Depends if you expect to challenge your machine at high speed or low speed. If both, get a bigger motor when you are at it!

Murray

In principle no although allegedly a 4 pole motor can be bit more efficient than a two pole but thats probably lost in the noise for the sizes we use. In practical its probably best to use which ever has a nominal speed closest to the "usual" speed for your spindle. 2800 motors tend to be noisy due to the higher speed cooling fan.

Where possible I try to stick close to the always safe ± 1/3 rd nominal speed range when using a VFD. There is little change in torque over that range and you can be certain there will be no cooling issues at low speed or bearing issues at high.

One advantage of the 2,800 motor if you need a high absolute speed range is that it lets you exploit the fact that torque fall off with overspeed is slower than with speed reduction. In general you also need less torque at high speeds which helps. So if you use a higher reduction ratio so that usual spindle speed is reached somewhat above the nominal motor speed you have more useful VFD speed reduction range before torque fall off becomes serious. This may help avoid a belt ratio change but you are still loosing out on absolute power compared to a mechanical shift unless the motor is somewhat oversized compared to a simple mechnical speed change set up.

Large speed ratio changes can be a PIA with belt drives unless you go to poly-vee and even then pulley sizes soon get inconvenient. Personally I try to avoid anything more than 3 to 1 or so variation with v belts and 5 to 1 (ish) with poly-vee for a single pair of pulleys. The concetric double reduction set-up, belt drive equivalent to a back gear is compact and relatively easy to arrange if you want an large speed change.

Clive.

Edited By Clive Foster on 13/05/2015 18:49:06

Clive

What is the 'concentric double reduction' set-up you are referring to, it sounds intriguing

Ian P

The higher speed motor will run cooler when run slower, if you use pulleys to get the same spindle speed.

Neil

Ian

This is what was taught to me as the "concentric double reduction pulley drive" set-up. No idea if that is the correct name or even whether it has an official one. Really needs a picture :-

Consider the motor shaft to be set concetric to the spindle it drives with sufficient space between the ends to permit both shafts and pulleys to turn without interference. Let there be another, intermediate, shaft spinning freely in suitable bearings arranged parallel to the motor shaft and spindle but offset by a convenient distance. Take two large pulleys of the same diameter and two small pulleys, also the same diameter. Fix one small pulley to the motor shaft and one large to the spindle. Fix the other pair to the intermediate shaft arranged so that the small motor pulley drives the large one on the intermediate shaft whilst its small compatriot drives the large pulley on the spindle. Clearly the overall reduction ratio is the square of the individual ratios i.e. if the pulley sizes give 3 to 1 reduction the total from motor to spindle is 9 to 1.

Clearly this arrangement is geometrically the same as a conventional lathe back gear. Indeed I have seen it described as a back belt system. Theoretically you can use the same length belt for each stage of the drive and tension both simultaneously by adjusting the offset shaft. In practice it will probably be needful to adjust the motor position away from absolute concentricity to get both belt tensions the same.

Like all useful engineering techniques this sort of set-up is amenable to a multitude of variations depending on the exact application. There is no need for the pulley pairs to be identical or even for the motor shaft and spindle to be concentric. The concentric arrangement is merely the most compact layout and, probably the easiest version to remember.

Clive.

Edited By Clive Foster on 13/05/2015 23:13:16

Why would it run cooler? The cooling fan will be running at less than it's design speed.

The motor on my mill definitely runs hotter at reduced speeds.

Russell.

I think he means the higher speed motor will run cooler than the lower speed motor. Sort of assumes that they have the same fan rotor.

Very few of us run our motors continuously anywhere near their max ratings. Windings are often Class H (180C) insulation rating these days, so you could fry eggs on them. Pain threshold for most people is in the 50-60C range.

Murray

Myford used a 2 pole/4 pole motor on the Tri Lever version of the? ML7 to give a two speed drive. At high speed, small diameter work, not a great amount of torque required, Where as with large dia lump of steel in the chuck the 4 pole/1450rpm, driving through the back gear gives plenty of torque, making the machine very flexable in it's capacity.

Ian S C