3 phase vs single phase

There is a project in Jan 2019 issue of “Everyday Practical Electronics” to build a speed controller for upto 2HP single and 3 phase induction motors ,with how it works explained.

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.

When it is off, it is off - not waiting to speed up - so safer. Better to have an on-off switch than using the ES. The ES certainly needs testing occasionally. If it is switched off, it will not be using any power, so I don’t quite understand your reasoning.

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.

Most inverters should have a stop mode that does not require powering it down, turning the pot down will leave the drive in a run mode and any disturbance could set it in motion. If you are going to get your hands involved with the job then it should be in an off condition that does not mean you have to power down.

Mike

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

He is not running his motor on no load, he is stopping it by decreasing the VFD output frequency to zero, which will effectively provide no power to the motor, so your calcs are not really applicable? It is the safety factor - when disconnected from the supply, the motor is definitely dead. Bump that potentiometer and away it could go!

I hate electric chainsaws which are plugged in and switched on - they may be stationary, but could start if picked up improperly. At least with a petrol driven one, when the engine is stopped you know it cannot ’bite’ you!

It is the lack of inductive load at initial start that consumes a large amount of current - the motor windings, when stationary, are almost a resistive circuit.  Change of kinetic energy in the drive is decreased to zero, whethervthe motor remains running or not; that kinetic energy has to be returned to the drive, either by the motor starting or through a clutch if the motor remains spinning.  Not a lot of difference, really.

Most ventilated motors run a fan on the shaft, which consumes the same power at no load as it does a full load - that makes a difference to efficiency as well.

Edited By not done it yet on 10/12/2018 12:11:27

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

I'd never leave a motor activated, even if not rotating, while making measurements. Off should be off, or clutch disengaged, otherwise one is courting disaster. Neither would I use the emegency stop, except in an emergency. Depending upon the setup the ES may well stress the components far more than an orderly stop using the normal button

The large currents when starting an induction motor direct-on-line aren't really to do with inductance. When running at rated speed there is a significant backemf induced in the windings from the magnetic field associated with the rotor. The current in the windings is dictated by the small difference between the applied voltage and the backemf. When starting the backemf is not present, so the full voltage across the windings means the current is correspondingly large, until the motor starts rotating and the backemf appears. With a VFD the VFD simply ramps the voltage up slowly to keep the current at a sensible level.

In theory an induction motor can be run at 0Hz. The systems I worked on for electric vehicles did just that, full torque at zero speed. It's the equivalent of holding a vehicle on a slope by slipping the clutch. Whether the facility is needed on a machine tool is another matter. One also needs to consider the VFD. A simple V/F unit won't be able to control at 0Hz, you need vector control. Originally you also needed an independent measure of rotor position, but there are now sensorless vector control units.

Again in theory, an induction motor can be run above it's base speed, usually at 50Hz in the UK. What happens will depend upon the motor design. If you supply a nominal 50Hz motor with 100Hz it will try to run at double the speed. Whether it does so, or flies apart will depend upon the mechanical design of the motor.

To summarise there is no problem, in theory, running an induction motor from 0Hz to 1000Hz, but as always the devil is in the detail. I think most general VFDs only go up to 400Hz, which is nominally 24000rpm for a 2-pole motor. That's what my high speed spindle on the CNC mill goes up to.

Andrew

Hello all, thnks for the replies. When I turn the pot down, the motor RPM reduces with the lowering of the frequency on the display. Theres no danger of the spindle rotating because I disengage the clutch. I rarely touch the Pot, just leaving the VFD at 50hz.

Hi Mark , the 2hp motor was total overkill . The lathe is 12x24 Fortis (clausing derived). I've now fitted the 1/2hp motor now , I do notice a difference and in future will have to be more patient in my turning ...ie... smaller cuts 🤔. I'll probably look for a 3/4-1hp motor in the future .