+1 for that. Rather easy to do these days without breaking the bank. This typical example on ebay is £15. The monitor can also be used to hunt down whatever is eating all the electricity in your home. Quite an interesting thing to do, some appliances are terrible: I found a hifi unit consuming 30W on standby. Pretty bad, and I found switching it on without playing anything reduced the waste to only 10W. The standby function was worse than useless!

Irrespective of the cause of his £85 electricity bill Sean's idea about replacing an oversized single phase motor with a smaller 3-phase model is still good. It's not smart to over-power equipment because of the damage it can do when something goes wrong. And a 3-phase motor performs better on a lathe than single phase. It's not smart to seriously under-power it either!

A mains monitor can used to establish how big the replacement should be. Plug the lathe in via the monitor and take a deep fast cut out of a bit of steel pipe. The monitor will tell you how much power was consumed taking the cut. By testing the lathe off load (not cutting) you can run through the various gearbox, change-gear and power-traverse combinations and find out how many watts it takes just to turn them over.

Buy a motor about 20% bigger than the maximum cutting requirement and – depending on how bad the tick-over measurement is and how long you expect to continuously run the lathe – consider paying for a higher duty cycle as well. (For hobby use a cheaper 50% duty motor should be fine: but not if it never gets a chance to cool off!)

Doesn't need to be accurate and if you can't be bothered to measure it you can get a good idea by comparing the size of your lathe with similar sized machine. I'd guess anything between 600W and 1500W would be fine.

Dave

I feel trepidaceous about comment on condensing tumble driers and heading off piste again… but!

We had one for several years, it was a very simple one that drew in fresh air through an aluminium cassette with hollow fins the 'output' air went back through. The main advantage was that it didn't steam up the kitchen, but it also seemed to work faster and use less juice than a conventional drier. Cleaning and emptying water was just a matter of habit.

I suspect the 'expensive' ones are just trying to be too clever, that simple passive condenser added no energy usage and had little potential to go wrong.

Neil

Edited By Neil Wyatt on 09/12/2018 14:33:34

I bought a cheap power meter from the lamented Maplin's. Hard to read display but very useful, and it measures volts, amps, power and power factor. Probably not particularly accurate but useful from time to time.

I checked mine with an old school General Electric moving coil clamp meter. I have two 1/2hp motors one on the mill and one on the lathe (which should have a 3/4hp) both run at about the plated 4A current on idle or working, start up is 6A on the mill and 13A (big chuck to move and not a cap start either). The compressor is 2HP with 20A start and 11.5A run and plated at 15.5A. In the case of the lathe and mill off load means no belt on the motor pulley.

So for some reason the little ones do not use less off load.

Measuring the current of an AC motor is completely meaningless as a way to infer the power consumption. You need to measure the power, which is Voltage * current * Cos(power factor). the little plug in wat meters will do this. An Ammeter won't.

Pet peeve of mine, since I spent 7 1/2 years measuring the electrical output of power station generators during performance tests all around the world.

Correct do a search for models with the lowest consumption.

Going back a few years when electric was cheaper and from the big 7 an AEG worked out at 23.5p/hr whereas next best the best Bosch at 25p/hr. Even other A rated ones came in at 36p/hr. The general low budget were of the order 40p+ p/hr.

Mrs has our AEG on for hours daily, annoys me when the suns out. 4hrs a day 6 or 7 times per week adds about £23 p/m. Budget A/B to run of the order £10 p/week.

Going back 8 years i did run a test on consumption with a 3hp 3ph 400v motor, from memory:-
Transawave 5.5hp convertor on standby no load used 39p/hr, powering a 2.2kw motor used 52 p/hr.
Digital 10hp invertor standby no load 12p/hr, powering 2.2kw motor 35 p/hr.

Well I don't disagree with Robert and Mark but how important is power factor in a home workshop?

Most of us don't know the power factor of our motors or understand what effect PF might have on our electricity bills. I guess an old-fashioned electromagnetic disk meter would read higher than it should, but a new electronic meter might be smart enough to compensate.

However, my feeling is that a home workshop uses too little energy to make power factor worth worrying about. Am I wrong?

Second question, my understanding of VFDs is that they start by converting single phase AC into DC. Wouldn't that conversion act as a buffer such that the AC supply wouldn't 'see' that the VFD output was connected to an inductive motor load? On the input side what sort of load does the DC stage of a VFD offer to the mains – I guess capacitive?

This turns out to be a another subject I don't understand adequately!

Dave

Edited By SillyOldDuffer on 09/12/2018 22:32:38

The old electromechanical meters accumulated real power, and that's what you paid for, even if your power factor was awful. However, the utlility had to generate the full apparent power, while only being able to charge for a proportion of it. Electronic meters have no problem measuring volts, amps and power factor and thus can accumulate real power and apparent power. Or at least the IC chip sets I've used professionally can do so.

Correct that a VFD converts the incoming AC to a DC link voltage and then uses that to produce the PWM signals that drive the motor. In the simpler VFDs the AC-DC conversion is simply a rectifier. Cheap, but it generates awful current harmonics. In theory you're not allowed to directly connect them to the supply within the EU. Instead you need an input filter, or DC reactor, to reduce the current harmonics. Higher quality, or larger, VFDs have a power factor corrector at the front end after the rectifier and before the DC capacitor. These are essentially a boost converter, but the control loop works to force the input current to follow the input voltage, ie, sinusiodal.

Obviously a PFC looks like a resistive load. For the case of a rectifier it's more complicated as the current waveform is highly distorted, even though the voltage and current waveforms are in phase. I suppose it looks like a time varying resistor.

With the advent of the electronic meters, and especially 'smart' meters, you'd better worry about power factor as you're going to be charged for it. Not at the moment, but believe me it will be arriving, along with variable kWh pricing. Just so you don't know what you're paying for electricity at any particular point in time.

Andrew

Edited By Andrew Johnston on 09/12/2018 23:33:47

This is all very interesting, so I have a question !. Knowing very little about electrics, on my lathe with its 3phase 3 hp motor powererd by a VFD, is it better to turn the Potentiometer down rather than keep switching off the motor ?. I often turn off the motor via the emergency stop button if Im not turning for a few minutes, not good for electrical consumption I take it ?.

Many thanks.

Not done it yet, what Im trying to say, is quite often, I will stop the the motor whilst doing a measurement or just a general pause, believing perhaps wrongly I am saving electricity, when it would probably be better to idle the motor via the Pot'. Thanks.

It's a good question! When you start an electric motor it draws an unusually high amount of power as it accelerates up to speed – it does work shifting the heavy rotor from rest against the resistance of the bearings etc. But once the motor is running the power consumption drops back to normal. If you can measure the power consumption during start-up, and the power consumption whilst idling, you can calculate how long the motor can be left idling before exceeding the start up cost. As most small motors accelerate quickly – seconds, rather than minutes, turning the motor off will soon save more energy than the cost of restarting.

Example with wild guess numbers:

• Motor consumes 10amps for 30 seconds when starting, then
• Motor consumes 1 amp whilst idling.
• Therefore cost of starting is 300 amp-seconds.
• Therefore motor can be left idling for 300 seconds at 1 amp before it's cheaper to switch it off

Real world motors start more efficiently than my example so switching off quickly (say 30 seconds) to save power makes sense. But there's another good reason to ramp up and down on the pot and to leave the motor idling rather than switching on and off repeatedly. Switching on and off saves power at the cost of stressing the motor and control circuitry electrically. Using the pot to slow and accelerate the motor avoids most of these stresses.

Another issue that might be worth considering is the temperature of your workshop and it's effect on your equipment. If you happen to run heavy machines fitted with plain bearings in an unheated out-house these stiffen up considerably when they get cold. Although it wastes power it may be necessary to idle such machines until they warm up, perhaps half an hour, and then to avoid letting them cool off again. Machines fitted with roller bearings are almost immune to this problem.

Saving power may not be worth the bother in a home workshop. I was surprised when I measured my 1100W mill and 1500W lathe just how little power they actually consume. It's mostly low because for a large proportion of an average session they're waiting for me to get my act together rather than cutting. What does consume a lot of power in my workshop is the lights – six fluorescent tubes. As there's insufficient natural light they are switched on all the time and it soon adds up.

Finally, not worth leaving a machine running to save a few pence if the moving parts are in the slightest bit likely to catch you out. 

Dave

Edited By SillyOldDuffer on 10/12/2018 11:42:48

Yes, if that's what he's doing. I assumed that Roger's set-up is the same as mine, ie turning the pot down does not stop the motor, rather it reduces it to a preset minimum speed. (150rpm at the chuck in high gear on my lathe.) I assumed (again!) that this is because AC motors stall if the frequency is reduced too much and the windings would gently cook between stall frequency and 0Hz.

Only Roger can tell us what his pot does?

But I might be wrong about AC motors stalling at some low frequency above 0Hz. I don't believe an AC motor will run above a certain frequency either.  How do 50 Hz 3-phase motors behave when fed between 0 and 1000Hz? Can anyone put me right? 

Apologies if anyone thinks this is all too theoretical, but I think the answer will help anyone setting up a VFD and motor from scratch. Most VFDs I've looked at can deliver power to the motor at frequencies down to 0Hz but is doing so wise?

Dave

Edited By SillyOldDuffer on 10/12/2018 13:00:48