Soft Start For Electric Motor

[JOHN BRIDGE 1Participant @johnbridge1]

I am restoring a Colchester Student Roundnose it came with a 0.75 hp motor pretty useless really I have put a single phase 3 hp motor in now and the Lathe performs as it was intended. The problem is that the load this motor imposes on my domestic power supply when it starts causes the lights to dip and the tv in the house to shake about a bit ( the picture not the tv ) all is ok once the lathe has started, is there such a thing as a soft start for the 3 hp motor.

John

[JOHN BRIDGE 1Participant @johnbridge1]

[Nick_GParticipant @nick_g]

.

Yes.

But the chances are that it would be far, far cheaper to purchase a new 3 phase motor and combine it with a new inverter and configure a slow ramp-up time on the inverter than the single phase option.

Full of good news ain't I.?

Nick

[Ian PParticipant @ianp]

Nick is right. A single phase 3 HP motor is going to put a strain on any domestic supply.

Only two sensible options I can see, one, leave the motor running and have some sort of clutch, two, get 3 phase motor and VFD (someone will want your old motor so you should be able to sell that).

You will never regret getting a VFD!

Ian P

[Speedy Builder5Participant @speedybuilder5]

I used to be the DPmanager using some BIG mini computers. As they drew about 40 amps, we has in-line chokes fitted to the single phase supply. In effect these were big 1:1 transformers which smoothed out any blips in the supply. I guess these would work in reverse to stop your motor sending blips the other way

[JOHN BRIDGE 1Participant @johnbridge1]

Posted by Ian Phillips on 14/05/2014 20:36:21:

Nick is right. A single phase 3 HP motor is going to put a strain on any domestic supply.

Only two sensible options I can see, one, leave the motor running and have some sort of clutch, two, get 3 phase motor and VFD (someone will want your old motor so you should be able to sell that).

You will never regret getting a VFD!

Ian P

There are some cheap VFD's on Ebay are these too cheap.

**LINK**

[Ian PParticipant @ianp]

Not too cheap, but a fair price (ignore the manufacturers RRP!)

You will need a fairly high wattage VFD for a 3HP motor. Apart from the Transwave's and Newton Tesla's of this world there is a seller that frequently advertises on the Home Workshop site, all are worth talking to so you can get some more specific advice.

Ian P

[John Stevenson 1Participant @johnstevenson1]

Be aware that although they ship from a Portsmouth bonded warehouse any dealings is with China direct via email.

Not saying they are good or bad but just be aware that that flag doesn't mean British.

[Nick_GParticipant @nick_g]

.

My experience with larger single phase motors is that they are a pain in the a6$e and more trouble than they are worth.

Combine this with a domestic set up and I fear it will be a constant thorn in your side.

If it's causing the picture on your TV to drop out I would not be surprised in it was doing the same to those living local to you that are on the same phase. Not always, but very often every 3rd house is on the same phase. The supply companies try to do this in an 'attempt' balance the loads of their legs of supply.

Nick

[Tony RayParticipant @tonyray65007]

John, I agree that VFD is the way to go but do you really need 3HP ? You could save a fair bit on the inverter by going down to 2kw that's still 2 2/3rd HP.. I have used Newton-Tesla professionally and Gavin (homeworkshop) I for my own lathe and can recommend either, I would much rather use someone in the UK over an unknown in China for something as costly as a VFD.

[JoParticipant @jo]

John,

A silly question: Have you run a 45Amp spur to your Colchester from the main fuse board or have you plugged directly into your 30A ring mains?

If you return to a 3 phase motor, I suggest that you don't waste your time with a VFD and buy a static converter like I have on my Master: the Student has adequate gearing to be able to run at all the speeds you need without the compromise of lack of power at the lower speeds that a VFD imposes .

Jo

[Swarf, Mostly!Participant @swarfmostly]

Please forgive me if this is a silly question but:

What size motor did Colchester fit to the Student as original equipment?

Best regards,

Swarf, Mostly!

 

Edited By Swarf, Mostly! on 15/05/2014 07:49:36

[Ian PParticipant @ianp]

Posted by Jo on 15/05/2014 07:41:32:

John,

A silly question: Have you run a 45Amp spur to your Colchester from the main fuse board or have you plugged directly into your 30A ring mains?

If you return to a 3 phase motor, I suggest that you don't waste your time with a VFD and buy a static converter like I have on my Master: the Student has adequate gearing to be able to run at all the speeds you need without the compromise of lack of power at the lower speeds that a VFD imposes .

Jo

A static inverter is a retrograde step compared with a VFD and the end results between the two are worlds apart. (Google it)

Jo, your speed and torque requirements are limited by what the lathe manufacturer chose, with a VFD you still retain all of those, but, in every belt selected speed you have the variability and CONTROL! of the spindle. Soft start, soft stop, jogging, and a myriad of other features. In practice you would rarely change belt positions but do all your day to day turning just using the speed pot.

The lack of torque at low rpm is an imagined problem only (just use a lower belt speed speed) but I find it incredibly useful for tapping purposes and as a general safety feature,

Ian P

[JOHN BRIDGE 1Participant @johnbridge1]

Tony,not sure I understand what you mean are you saying I need a 2 kw motor?

Jo, The power supply is not the problem it is the load imposed on it by the starting current of the motor, what is a static converter, it sounds interesting?

Swarf, They where supplied with a 3 hp 3 phase motor.

Thanks to you all for your replies.

John

[WALLACEParticipant @wallace]

I’ve a 2hp on my Harrison l5a which is sort of student size and that’s just fine.

The original was a twin speed 1.5 / 3 hp monster that I replaced with a new one and a home built vfd. Occasionally, on a nasty parting off dig in, the vfd overload will trip, otherwise it’s fine even when taking prolonged heavy roughing cuts – the temperature controlled fan on the vfd heatsink has never come on

So basically I think 2hp would be just right !

W..

[Bob Brown 1Participant @bobbrown1]

Swarf, Mostly!

3hp 3 phase if lathes.co.uk is correct

John,

If the motor is connected via the ring main I would suspect voltage drop in the cable, it may be worth checking the actual drop on start up.

If it is connected via the ring main then the first move would be to get it changed to a dedicated supply direct from the consumer unit, if fitting a dedicated 45amp supply ensure the cable size is at least 10mm.

Note: In the UK this is not a DIY job.

Workshop power supplies ideally need to be on a dedicated supply from the consumer unit but some are just a spur off a ring main (not legal in the UK), I have even seen workshops run off an extension lead. I have made allowance in the rewire to allow a 45amp supply to my new workshop once it get built.

In my previous workshop the largest motor I ran was 2.5hp single phase driving a belt driven compressor without any problems but this was on a dedicated 32amp supply.

Once you have eliminated the supply only then I would then look at changing the motor to 3 phase and a VFD but its not going to be cheap.

[JoParticipant @jo]

Posted by Ian Phillips on 15/05/2014 08:31:56:

A static inverter is a retrograde step compared with a VFD and the end results between the two are worlds apart. (Google it)

Jo, your speed and torque requirements are limited by what the lathe manufacturer chose, with a VFD you still retain all of those, but, in every belt selected speed you have the variability and CONTROL! of the spindle. Soft start, soft stop, jogging, and a myriad of other features. In practice you would rarely change belt positions but do all your day to day turning just using the speed pot.

The lack of torque at low rpm is an imagined problem only (just use a lower belt speed speed) but I find it incredibly useful for tapping purposes and as a general safety feature,

Ian P

Ian: The Student and the Master are geared head lathes, we don't have to mess around with belts.

The lack of torque from a VFD at low speed is a fact on my Sixis mill that I have one fitted to. And I particulairly do not like the way it lets you overdrive the motor (the motor gets excessively hot which is not good!) and the trip function is a pest when you are working the machine hard.

I have three static converters they all work very well driving the seven 3 phase machines I own. I have one VFD and have been very dissapointed with it. If you have only one machine then think of the VFD as integral to the machine, then you buy another VFD and another for each additional 3 phase machine.

Jo

[JOHN BRIDGE 1Participant @johnbridge1]

Tony from lathes.co.uk recons this lath needs 1.5 hp just to run and recommended I fit a 3 hp motor which I have done, I would like to use a 3 hp motor but not as it is at the moment, I am leaning towards the VFD.

[JoParticipant @jo]

Then I recommend you think of the VFD as purely a glorified transformer and continue to use the gear selector switches for speed control.

You will have to think about where you are going to mount the VFD box and how you are going to turn the power on and off: The switches on the front are small and fiddly, which normally forces people to add a pendent, have you thought where is that going to go? Remember it has to be secure so that you can hit it to turn the power off in an emergency.

And before you start ripping out the original Colchester electrical wiring to use the existing switches to control the VFD, remember that that will impact the machine's long term value.

Motor type: Yes three phase has less of a start up spike (I think my 3Hp peaks at 15A) but you should still think of running a separate mains ring/spur (on their own fuse ) when it comes to these more powerful machines and the fact that it is seperate will stop your machines glitching the rest of the household's mains power.

Yes the Student needs a 3Hp motor, it is a powerful professional machine capable of much more than lesser lathes.

Jo

[John Stevenson 1Participant @johnstevenson1]

You don't have to use the variable speed function if you have the gear head and are happy with it. We used to fit literally hundreds of VFD's and in the early days before they got as refined as they are today just so they could run thee phase machines of single phase.

I agree a phase converter is a retrograde step and a far more expensive setup for a one machine application.

3Kw convertor is about £780 and they only generate 2 phase, the third phase is ghosted

One the side of the converter though, if you have two speed 3 phase motors it virtually the only way to go and many fit these to obtain a three phase ring main.

[JoParticipant @jo]

Posted by John Stevenson on 15/05/2014 09:47:53:

3Kw convertor is about £780 and they only generate 2 phase, the third phase is ghosted

 

When did they get that expensive, I was quoted £500 for another of my all singing all dancing 3HP Transwaves last autumn.

Edit:  Just checked 3HP converters: Rotary Converter £726, Hi-Torque Static £462.

Jo

Edited By Jo on 15/05/2014 10:24:28

[Tony RayParticipant @tonyray65007]

John,

Inverters are sized in kW eg 1.5, 2.0 ,2.5 each time you jump up a size the cost increases. Your lathe was made for industrial use 5 days a week most home users find that they can downsize a bit save cost.

A VFD used correctly is a superior solution to a rotary or static converter – if it wasn't then Mitsubishi, Hitachi, Huyundai, Scheider, ABB etc, would be making static and rotary converters – they are not.

I have had VFD on my M250 for a year, how many times have I use the VFD to change spindle speed ? – not once.

Why ? Its like riding a bicycle with gears. The gears allow you to go faster whilst maintaining the same pedal rpm, you could go faster or slower by changing your pedal rpm but there comes a point where changing gear is more efficient. So I use the lathes gearbox to change spindle rpm because that is the most efficient way of working. You can use the VFD instead but there is a point at which changing gear is more efficient and as Jo ? said you will lose torque at low rpm (and risk overheating the motor too) Probably not the most techically correct explanation but it works for me.

Ignore claims that a VFD solution is less efficient – the losses in the VFD are not worth considering

[BazyleParticipant @bazyle]

It's easy to get confused by the names. 'Phase Converter' is a different animal from a 'VFD (Variable Frequency Drive)' or from an 'inverter' depending on who is using the term ans what they think it means. Check carefully before you buy what the thing actually does.

I'd go with the 'check wiring first' advice. Try relocating your workshop to the kitchen and using the oven feed. You can alwyas live on takeaways.

[jason udallParticipant @jasonudall57142]

Static and phase converters…as I understand produce pseudo two phase..and as such deliver less power than expected from your nominal 3phase motor.
Also you still have that big clitch when you engage the motor…the power has to come from somewhere. .also this does not provide the softstart the op asked for.
All the above “change the motor for a 3phase”..with whatever converter preferred. ..Is there no single phase solution..maybe like the start/run capacitor solution. .

Given all that I would myself prefer the vfd solutions. ..and low torque at low rpm?..that might depend on your inverter…

Edited By jason udall on 15/05/2014 13:29:05

[JasonBModerator @jasonb]

I would not want to be without my variable speed now. Its ideal when cutting threads using a die on the lathe, I can turn down a bit of hex to the required size at say 1000rpm then just back out the tool, bring up the tailstock, turn the speed down to 50rpm at the same time then run the thread down the part before running in reverse to back the die off at a faster speed then back upto 1000rpm to tidy the end with a file. Far easier than stopping the lathe, fitting a spindle crank, winding that to cut thread and then back it out.

Likewise when drilling a hole in stages, start with a small drill at full speed then just keep changing bits as you work up through the sizes to a big blacksmiths drill no need to keep stopping and moving gear levers or worse change belts.

Same with the mill its nice to slow it down for countersinking, running an out of balance boring head or using the dial type centering gauge without the needle being a blur. I would have thought a VFD on your Sixis with its 1000rpm slowest speed belt was ideal if you want to use it with anything else than small milling cutters.

Used sensibly with the lathe belts/gears set for the main work in hand and the VFD to drop it down on occasion makes things very nice to use.

I think most stories of low torque are from the imported machines that use DC motors on variable speed rather then VFDs on AC motors

 

J

Edited By JasonB on 15/05/2014 13:35:59

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