Electric motors

By way of background, I recently purchased a brand new but old stock ( 2012 ) 1HP 3 phase motor to use with a VFD. Not installed as yet but everything looks good although it is a very heavy beast- finned cast iron shell, 19 mm shaft etc.

This led me to the question of how one would select the most appropriate motor for a given task. Assuming for the purposes of discussion we are sticking with the 1HP motor, available motors not only come in 3 phase, single phase and DC but in a whole range of physical sizes.

Which to choose and why seems to be the bit missing in all the references I have found. Obviously size and weight could be the limiting factor in some applications but where this is not an issue what is the guiding parameter?

E.g. is bigger best - smoother running, more reliable, longer life, or is it that experience with the performance of the newer lightweight motors has just not filtered through to general thinking.

I know I am of the old school which tends to think that bigger must be better but am I correct in this case?

Comments would be welcomed.

A factor in selecting a motor is its duty cycle. How long it has to operate at full power.

A couple of other factors:

- start/stop - if you are frequently starting and stopping the machine, 3-phase is generally better than single-phase.

- speed control - possible with 3-phase but not single-phase.

My vertical mill has a motor with electronic speed control of something like 200-10000 rpm, nominal 2HP, which I believe was designed for washing machine use. It has one useful thing for heavily-loaded slow speed motors - a separate cooling fan. Even with 3-phase motors with a VFD for low-speed control, a built-in fan might not be enough at those lower speeds.

Paul - Good point. I think most if not all of my motors are rated S1. However like most hobby machinists mine are more likely to be operating in intermittent mode ( S3 to S8 ). In some ways I can imagine this would be harder on a motor than continuous operation. However I note that if in doubt S1 is the default specification widely recommended.

Nealeb - Thanks for the comments. I'm not sure how much of an issue start/stop is in the home workshop. My ML7 and drill press do most of the frequent starting and stopping and have yet to have a problem. However they are stopping with no/minimal load which could make a difference..

Speed control of ( some ) single phase motors is possible but not much used on larger machine tools due to loss of torque. It is more often used on things like rotary tools. high speed grinders etc. If we continue with the 1HP theme I think it can probably be ruled out as a viable option

Your comment on cooling is pertinent. It would suggest that higher pole numbers would be preferable to achieve maximum cooling at low speeds - e.g. for a lathe with a typical max speed of 1000 rpm a six pole motor would be preferable to a 2 pole for slow speed operations such as threading.

It seems there are many considerations other than just HP rating that we should be considering when selecting a motor. More so than in the old days when motors were selected based on the availability of an old fridge to plunder.

Pero

Those wishing to change motors is quite limited. Generally failed motors are exchanged for a direct replacement - the original was carefully chosen by the manufacturer.

It was only the minority that changed motors, from 3 phase, to single in years gone by (before cheap VFDs became available) because used commercial machines wee cheap - for that very reason.

Nowadays, it tends to be the other way round because the 3 phase motors were always superior - given a suitable power supply was available.

Then, along came cheap brushed DC motors - but the cheapness often led to failures (often not so cheap for the user) - and now brushless, but also cheap, DC motors. An improvement but not idiot-proof, all the same.

Changing from DC or single phase induction is always a matter of what modifications need to be made to accommodate the new drive. Every situation may be different. The induction motor is the usual choice but universal motors can be found - snd require a different means of speed control.

Bearings are, I think, more likely to be ball bearings on 3 phase motors ( more long-lived). Changing from DC to single phase my include other bearing arrangements.

Foot or flange mounted motors are often not (easily) interchangeable. Foot mounts come in various sizes/congiurations.of corse.

There are plenty of cheap, light-weight, chinese motors around these days. Price usually denotes som advantage/shortcomings.. There are different ranges of motors, even from one brand. Cheapest, less cheap, lower starting torque torque or the increased starting torque range. Braked or unbraked is yet another category.

Your choice, of course, but there is rather more to motor choices than initially meets the eye. You may even think ahead and decide to buy dual voltage 3 phase motors - yet another choice to consider.- before jumping in on the girst available option. As is the options of powering the machine to which the motor is to be fitted,. VFD or historic static (or rotary) converter, for instance.

The other variable to consider on 3 phase motors is: how many poles?

What are the trade offs between torque and speed?

Rod

I tend to look for brushless if I'm choosing

It's like electronic ignition for cars in the 1980s, it's just better

Maybe not very helpful but my view on this is....

Yes, "things" (generally) aren't built as big/heavy as they used to be. But equally materials and manufacturing technology have moved moved on significantly. So smaller/lighter can't always be taken as an indicator or poorer quality or poorer performance.

I'd suggest that (for the same rated HP) key consideration between bigger/smaller, new/old is comparison of the specification of the two units.

Andy

In the beginning there was darkness and then Faraday and Edison gave the world Light. DC was their vision and then Tesla got in on the act and the world became AC mainly because it enabled easier power distribution.

However DC motors are in many ways better than AC and the latest permanent magnet brushless motors are very compact and powerful. An electronic controller for a brushless DC is very similar, electronically, to a VFD for an AC motor so if you want variable speed the cost difference between AC and DC is minimal.

AC motor are best used at a fixed speed with variable load. DC motors can handle variable speed and high load as can be witnessed on a fully loaded Lisbon tram starting on a steep incline.

I'm for BLDC motors every time.

CS

I always use my brushless DC battery drills when hand drilling steel to 12mm

My poor 240v drills would start to smell if they were made to do any proper drill work

Many old AC motors had internal fan driven air ventilation but modern motors do not hence the fins. This is because if there is a severe overload situation or winding failure and the stator winding catches fire this remains contained within the case improving safety. The downside is that although the stator can be conduction cooled the rotor temperature becomes a limiting factor and frequent starting cycles is a frequent cause . The issue here is that heat is conducted from the rotor into the bearings through the shaft. However since the shaft is hot and the outer bearing race is being cooled by the case fins issues with bearing load arise. Unlike plain bushes used on early motors ball races can be lubricated for life so give better reliability especially for motors located where regular maintenance is difficult. Rather like long case clocks nobody thinks they need looking at until they stop. So it is with motors and lathe motors with their frequent stop/start use profile do suffer.

The soft start of a VFD in itself is a worthwhile feature for a lathe without a clutch with the variable speed as a bonus.

BLDC motors with a permanent magnet rotor don't suffer from rotor heating to the same degree that an AC motor does and they don't need as much iron in the stator so are more compact and lighter.

CS

Yes and no.

Yes: you can turn down the wick of a more powerful motor, but the smaller motor's output is fixed.

No: it has to fit physically in the available space.

There isn't a lot to go wrong in a 3-phase motor.

I tend to buy the best that I can afford - it generally works out cheaper in the long run.

Bigger brings greater mass and inertia. This can be a benefit or detrement depending on application. Good for constant running with variable not good for frequent stopping and starting.
BLDC has a lot of advantages but it's not perfect. Not an issue for us but they don't like high temperatures. For machine tool use they have to be well sealed or ferrous particles will foul the magnets.

For those who prefer fixed speed induction motors I see that there are quite a few being offered on eBay that come from industrial sewing machines at circa £20. The motors have mechanical clutches, a useful feature for a lathe, look to be easily mounted and are rated around 500W . Some being offered are 3 phase so can be used with an inverter.

CS

Well In usually look for cheaper second hand motors from respected manufacturers. 3 phase or single phase. I avoid cheap new motors from China. Found them noisy and not well balanced. Blow the nitty gritty technical stuff. Never had a failure with this philosophy and there are a lot more £s left in my pock.

Andrew.

The king of motors in industry has been the 3 phase totally enclosed fan cooled squirrel cage induction motor, 2 pole and 4pole by far the most common. If you want variable speed supply it from a VFD and if you want position control then fit an encoder to the motor. Robots these days favour brushless DC. The DC motor has some great characteristics that lend it to various speed control systems, other big applications were traction motors often found on trains and forklift trucks. I can’t think of much new equipment being fitted with DC motors since the early 1980s.in the home workshop I would tend to choose a 3 phase TEFC motor and run it from a VFD, particularly for lathes and mills. At the moment I think brushless DC is not an easy technology to pick up spare controllers and motors should you need them whereas the 3ph TEFC motors are readily available online and at your local factors, a VFD is easy to source and even if you need a replacement for an obsolete model they are almost a drop in replacement for any failed unit regardless of manufacturer, you may need some understanding of setting the parameters though.

Mike

Not exactly a "drop in" for my Myford 254 both in terms of the space available and the connexions, but I do know what you mean.

As Mike says, 3-phase squirrel cage motors are widespread for a good reason.

I didn’t realise that a myford 254 (or any other myford) was fitted with a VFD.🙂

Myford did a version of the 254 with 3 phase motor and vfd. It had a poly vee belt drive to the spindle. I think it was 2 speed (2 lots of pulleys, but I might be wrong.

Surely what matters is what the motor is intended to do, in service? A motor for a toy drone is not at all the same thing as a motor for a town-hall clock, or one for a dentists drill, or one for use in a coal mine?

Horses and courses, all that sort of stuff

Tim

Edited By Tim Stevens on 21/08/2023 15:23:01