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Soft Start For Electric Motor

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JOHN BRIDGE 114/05/2014 16:47:56
104 forum posts
11 photos

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

Nick_G14/05/2014 16:53:49
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1808 forum posts
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.

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.? indecision

Nick

Ian P14/05/2014 20:36:21
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2437 forum posts
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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 Builder514/05/2014 22:17:38
2143 forum posts
150 photos

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 114/05/2014 22:22:37
104 forum posts
11 photos
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 P14/05/2014 22:31:23
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2437 forum posts
101 photos

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 Stevenson14/05/2014 22:37:50
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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_G15/05/2014 00:18:52
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1808 forum posts
744 photos

.

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 Ray15/05/2014 00:36:36
138 forum posts
25 photos

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.

Jo15/05/2014 07:41:32
198 forum posts

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 crook.

Jo

Swarf, Mostly!15/05/2014 07:49:11
547 forum posts
47 photos

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 P15/05/2014 08:31:56
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2437 forum posts
101 photos
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 crook.

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 115/05/2014 08:32:48
104 forum posts
11 photos

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

WALLACE15/05/2014 08:40:10
304 forum posts
17 photos
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 115/05/2014 08:41:35
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1016 forum posts
127 photos

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.

Jo15/05/2014 09:02:17
198 forum posts
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 115/05/2014 09:02:37
104 forum posts
11 photos

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.

Jo15/05/2014 09:46:00
198 forum posts

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 Stevenson15/05/2014 09:47:53
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Moderator
5068 forum posts
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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.

Jo15/05/2014 09:53:54
198 forum posts
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

 

disgust 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

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