# How to calculate for single to three phase



## swood1 (Oct 24, 2018)

Hi all,

I have a BCA machine currently with a single phase motor on it but I would like more speed control and I have a spare three phase motor which originally came with the machine.   I was wondering can I buy an off the shelf device that changes single phase to three phase and gives me speed control with this motor?  If so how do I calculate what I need?  Also, it looks like the motor is a three phase two speed motor from the details on the side?  Can this be utilised?

The details on the motor are:-

Rating: Cont 0.5HP
Volts: 400/440
Amps: 0.8/1.5
RPM: 710/1420
Phase: 3

Regards

Steven


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## abby (Oct 24, 2018)

You can buy a single to three phase converter which will give speed control , however you do not say what voltage your mains supply is.
In the UK mains voltage is around 220 volts so you could run a dual voltage 3 phase motor but not the one that you have (as far as I an aware).
I have fitted my BCA with a DC motor from a motorised treadmill along with the speed control electronics , this gives me speeds from zero to 3000 rpm or even faster if I risk damaging the bearings.
I have 2 pultra lathes fitted with 220/440 volt 3 phase motors run from an inverter but I have found the inverters I have bought via ebay to be unreliable , hence the DC conversion for the BCA.
Dan.


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## Wizard69 (Oct 24, 2018)

What you want is known as a VFD or Variable Frequency Drive.   Purchase the proper one and it will convert single phase to 3 phase.   

Unfortunately I don’t know enough about UK electrical system to go into details.  It would be a good idea to get a decent electrician involved at your end.   

In any event in order to have usable torque over a wide enough range of RPM to be usable it is normal to up size the motor.  That may mean 3-4 times the horse power that originally powered the machine.  This will give you a reasonably wide range of RPMs.  However you may still want a significant reduction ratio for smooth operation at very low speeds.  

There are two very common types of drives, the regular VFD and what is known as a vector drive.   Vector drives give you somewhat better performance across a wider operating range.   You can easily get involved in very complicated drives but plain VFDs and standard 3 phase motors are good enough for a home shop and very economical.


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## swood1 (Oct 25, 2018)

Hi all,

Thanks for replies.  I am UK based, so mains voltage is single phase, approx. 240v.   I shall do some research on VFD's, I don't know whether it's more economical to convert to DV or use the 3 phase motor I have and buy a VFD...  need to do some research on the options.

Ta

Steven


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## Wizard69 (Oct 25, 2018)

swood1 said:


> Hi all,
> 
> Thanks for replies.  I am UK based, so mains voltage is single phase, approx. 240v.   I shall do some research on VFD's, I don't know whether it's more economical to convert to DV or use the 3 phase motor I have and buy a VFD...  need to do some research on the options.
> 
> ...



The question of going DC. Verses AC 3 phase isn’t always easy to decide.    If buying new DF motors are more expensive for a given horse power.  However the controllers can be cheaper.  In the home shop it is often the cheapest solution that wins.   

I notice that people often focus on used treadmill motors for a DC solution.  These can work but sometimes are not the best solution due to damage.   In any event it isn’t impossible to find used AC 3 phase motors.  Unless you are real lucky I would suggest new electronics In either case.   As for all things electronic there have been huge improvements over the last 5 decades.  

As for damaged DC treadmill motors i only mention that due to helping a guy debug a spindle speed problem he was having with a used treadmill motor.   In this case a motor bought online.  In the end his lack of torque came from a partially missing field magnet.   If you go treadmill I suggest salvage from a local machine that is going to the dump.  That is something you suspect is working.


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## xpylonracer (Oct 25, 2018)

Hi Steven

You won't be able to use your existing motor unless the motor has 6 winding ends in the terminal box, that would allow you to connect the windings in Delta configuration to use at 240v 3 phase supplied from a 240v single phase input 240v 3 phase output VFD.

If there are only 3 terminals in the connection box the motor will be wired in Star for the stated 400v 3 phase.
Your closest motor repair shop may be able to bring the connections required out to the terminal box, that would save you buying another motor.

xpylonracer


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## Brian Lawson (Oct 25, 2018)

I can't comment on the VFD required in the UK, but I'm pretty darn sure that in order to make proper use of that expensive two-speed motor and using that option, you will need to have two contactors (relays) that are both electrically, and also preferably mechanically,  interconnected.  The windings of a normal two-speed motor are usually separate windings.   Two-speed AC motors are not usually Wye-Delta connected either, which is so common in single speed squirrel-cage AC motors to control starting current and power consumption costs.  

While the VFD may well be used to provide power, I think you will need to contact the VFD provider as to whether or not you may "switch" it from one speed to the other while under power.  Some are not rated for that duty.

Good luck.

Brian Lawson, Bothwell, Ontario.

ps...I put a 2HP  90 volt DC treadmill motor on my Deckel, with the spindle speed controlled manually through a built-in rheostat knob on a 110 VAC "router speed controller" into a 15 amp bridge rectifier to the motor.  Works a treat !!


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## ddmckee54 (Oct 25, 2018)

A VFD is a Variable Frequency Drive, which changes the speed of the motor by varying the frequency of the power it supplies to the motor.

Changing the speed of the motor while the motor is running is exactly what the drive was designed to do.

It is possible to get VFD's that will use single phase power as their input and then, they will in turn output three phase power.  The typical rule of thumb for sizing this type of drive is to double the motor HP and that will be the minimum HP for the VFD that can be used.

I don't know if you would be able to use the SEW motor without doing a lot of electrical work that it doesn't sound like you would be comfortable doing.  That is a 400/440 volt motor, +/- 10%.  You say you have 240 volt single phase mains power.  I don't know of any VFD's that will step up the output voltage so you'd need to step up the input voltage for the VFD to 440 volts, get a 440 volt single phase to three phase VFD, etc., etc., etc... You may not want to go down that rabbit hole.

It IS possible to get single-phase to single-phase VFD, but these also have their drawbacks.  You need to de-rate the motor so you'd need a motor with more HP, and the motor will also run hotter.

The bottom line is - how much do you REALLY WANT to run at a different speed, and how much are you willing to spend to be able to run at that speed?

Don


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## DJP (Oct 25, 2018)

I vote for simple is better. I would swap the 440V 3 phase motor for a single phase that runs on 240VAC then adjust spindle speed with belt pulley selection.  It never fails, you don't need a bigger hp motor  and you don't have 440 volts in the shop which gets into the dangerous range.

Simple is better, in my opinion.


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## abby (Oct 25, 2018)

Simple is better but unfortunately the drive arrangement for the BCA is not very helpful because the clearance between the spindle and it's housing limits the diameter of the largest pulley to around 3 inches.
This is fine for endmilling but is too fast for fly cutting.


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## lemelman (Oct 26, 2018)

I'm in UK.  Modern 3-phase motors here are usually easy to convert from the '*star*' format used for *440v* operation, to the '*delta*' format required for *220v* operation. The older types are not so easy because the ends of the three windings are joined together to form a '*star-point*' which is buried inside the windings. The windings themselves are unchanged, its just the way they are connected together that determines if its *star *or *delta *format.
When I purchased my Centec mill it had 440v 3-phase motors, one for the power-feed and one for the spindle. I removed both motors, burst the star-point of each, and extended the free ends to the connection box where I reconnected them in delta format. I then purchased a couple of VFDs from the 'net' and I now have full variable speed for both.
I'm not certain how to reconfigure a *2-speed* 3-phase motor to delta, but a motor rewind shop will be able to advise. Don't listen too closely to scare stories about VFDs - I've helped 4 people install them with no problems at all.


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## skyline1 (Oct 26, 2018)

I doubt you will be able to use your existing motor

Looking at the rating plate it seems to be a twin wound machine with an 8 pole low speed winding (750 RPM less slip so around 710) and a 4 pole high speed winding (1500 RPM less slip so around 1420) both Rated at 400 to 440 Volts.  U.K.  3 Phase power is around 415 Volts Phase to Phase right in the middle of this range.

It Will most likely have 6 terminals but these are one set of 3 for each winding 6 in total, not one at each end of the 3 phase windings allowing for dual voltage operation.

It has been mentioned that they are normally used with two mechanical/electrical interlocked contactors so as not to energise both windings at once.

VFD drives of the single to 3 phase type work by electronically generating a variable frequency 3 phase output from the single phase input this varies the speed of the motor which is directly related to the supply frequency.

The problem with this is that as the speed drops so does the torque of the motor and at the same time so does the efficiency so the heat generated rises. and the 3 phase supply is a a lower 240 Volts 

Practice may have changed now but when I was using them it was the norm to over rate the motor to by 2 to 3 times and never run them at less than half their normal speed.

D.C. Machines however used a phase angle controlled drive not dissimilar to a domestic light dimmer (although more complex and usually a closed loop system)

D.C. motors can theoretically at least develop maximum torque at standstill if they are separately cooled and so have traditionally tended to be used in these sorts of applications however the downside is that they need  more maintenance having Carbon Brushes to wear.

I hope these comments help clear things up a bit

Best Regards Mark


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## davidyat (Oct 26, 2018)

When I purchased some old machines, Lathe & Mill, they were 220 Volt 3 Phase and I only had 220 Volt One Phase. I was very lucky. There was a company you will see advertised a lot, Phase-o-Matic in Palmdale, California, USA. And I only lived 15 miles from them. YAHOO!! There are 2 kinds of phase converters, static and rotary. If you have a 5 hp motor, 3 phase, you only need a 5 hp static converter which is rather expensive but compact. A rotary uses an electric motor with Phase-o-Matics' system and is cost effective. If you have a 5 hp, 3 phase motor, you'll need a 7 1/2 hp electric motor (50% more on the hp for the rotary phase converter motor). A friend had a spare 7 1/2 hp electric motor he gave me, I wired it up with the help of Phase-o-Matic and in 15 years, only had to replace an on/off switch. You will end up with 2 switches. One to charge some capacitors, then switch on the electric motor, switch off the capacitors and turn on your 3 phase machines. Mine's been working great.


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## Nick Hulme (Oct 26, 2018)

skyline1 said:


> The problem with this is that as the speed drops so does the torque of the motor and at the same time so does the efficiency so the heat generated rises. and the 3 phase supply is a a lower 240 Volts



This is far less of an issue in practice than is suggested by the way this is trotted out whenever use of a 3 phase with a VFD is mentioned. 
I use a 750W motor with a Siemens VFD on my Super 7 lathe and with the belts set one below top speed I don't have to resort to the back-gear for tapping M12 or 1/2" 20 UNF, I just control the speed with the VFD. 
A job I do turning Titanium has the lathe running for extended periods at 350rpm without motor overheating issues. 

 - Nick


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## BobNZ (Oct 26, 2018)

The 220VAC single phase to 220VAC three phase VFDs I have had good success with, can control the speed but with a caution to not exceed double the rated motor RPMs for mechanical reasons, nor significantly lowering the rpm below the motor rated rpm, if the motors are under heavy load. 



For light load I can drop the  rpm to about half the rated motor rpm OK, although with discretion the rpm can be a lot slower than that which is  ideal when jogging during setup.  The reason for keeping a heavily loaded motor at the rated speed, is that if speed is reduced then the current will rise and may cause over heating.  Using discretion and watching current draw, you can vary the speed widely.



There are some things VFDs don't like and the big one is that you must NOT include a switch the  3 phase lines between the VFD and the motor.  Power can be switched to the VFD but control of power to your motor must be through the VFD.    Your motor ON and OFF is controlled through the VFD.



The three phase lines from the VFD MUST see a motor load at all times the power is on.   That is simple enough to manage.



You can vary the motor speed as well as speed of ramp up and ramp down or switch to reverse.   An emergency stop can be also  programmed into the VFD as well as setting up a braking resistor use to slow the motor quickly.



If you wish to buy a VFD with a braking circuit them tell the seller that so the  braking circuit is included in the  VFD, and you may like to get a braking resistor from the supplier as well.



Programming the VFD is manageable using the many helping hands on the  web.  It is a good idea to buy a VFD rated  higher than the motor you wish to drive.  Some say double the rating but I have found adding say a 50% capability to work out fine.  You can monitor the current on the  VFD display if a heavy load to the motor is expected and so get an better idea of what strain you may be putting on VFD and motor.


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## Nick Hulme (Oct 26, 2018)

BobNZ said:


> The 220VAC single phase to 220VAC three phase VFDs I have had good success with, can control the speed but with a caution to not exceed double the rated motor RPMs for mechanical reasons.



Some cheap motors will have an issue below 2x rated RPM, some good motors have no issues well above. 
I run a 1hp motor at 7000rpm on a daily basis, having had no issues over 5 years. 

If you buy a good quality VFD the rating is the rating, you definitely do NOT need a higher rating for Siemens, Jaguar, ABB and any other quality brand, YMMV if you are cheap when shopping for a VFD


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## BobNZ (Oct 26, 2018)

Agreed Nick.

A motor may not take the increased centrifugal forces of higher RPM.   Balance and shaft stiffness along with bearing design are relevant factors.  A sloppy guide may be if the  motor is 4 pole and a similar motor of the same brand and style is 2 pole, then you are probably OK for 2 X rpm.

While high quality VFDs are a good path to tread, taking advantage of cheaper lines such as HuanYang  VFDs,  may make a budget project manageable.

Many cheaper synchronous motors also have elevated KW ratings with low duty cycles at rated power and substandard mechanical design.  

Some old motors with well known brand names can also be a risk regarding aging insulation.    A bit of homework before purchase is common sense.

I also run a single to three phase converter I constructed some years ago, and while that has done good service it runs at 50 Hz.  The facilities provided by VFDs are very convenient.


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## Niels Abildgaard (Oct 27, 2018)

abby said:


> You can buy a single to three phase converter which will give speed control , however you do not say what voltage your mains supply is.
> In the UK mains voltage is around 220 volts so you could run a dual voltage 3 phase motor but not the one that you have (as far as I an aware).
> I have fitted my BCA with a DC motor from a motorised treadmill along with the speed control electronics , this gives me speeds from zero to 3000 rpm or even faster if I risk damaging the bearings.
> I have 2 pultra lathes fitted with 220/440 volt 3 phase motors run from an inverter but I have found the inverters I have bought via ebay to be unreliable , hence the DC conversion for the BCA.
> Dan.



What solution DC or VFD is most pleasant noise-wise?


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## Nick Hulme (Oct 27, 2018)

That depends on several factors including the fan, I use air filter foam on the air intake of some of the 3 phase motor fans to reduce the noise when I run them faster.


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## lemelman (Oct 27, 2018)

skyline1 said:


> I doubt you will be able to use your existing motor
> The problem with this is that as the speed drops so does the torque of the motor and at the same time so does the efficiency so the heat generated rises. and the 3 phase supply is a a lower 240 Volts



This is not actually true.
As a general rule of thumb when using a VFD, the motor will run at constant torque at speeds below the normal speed, and at constant power at speeds above.
Since power is RPM x torque, power reduces at slow speeds and is constant at high speeds.


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## BIGTREV (Oct 27, 2018)

What is a BCA machine?
Thanks


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## biqut2 (Oct 27, 2018)

It's been my experience that torque drops with lower rpms on a vfd. At least this is true with the 2.2kw 24000rpm motor on my router and the 3600rpm motor on my mill. Both are 3 phase, the former is 400hz and the later is 50hz.


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## lemelman (Oct 27, 2018)

biqut2 said:


> It's been my experience that torque drops with lower rpms on a vfd. At least this is true with the 2.2kw 24000rpm motor on my router and the 3600rpm motor on my mill. Both are 3 phase, the former is 400hz and the later is 50hz.



Many people use a VFD to get a slower spindle speed without changing the gearing. Using a *lower gear *actually *magnifies *the available torque, but if the VFD is used in the expectation that it would behave in the same way as gearing, then it *appears *that the slower motor has reduced torque because it isn't magnified by the lower gear..


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## BobNZ (Oct 27, 2018)

Niels Abildgaard said:


> What solution DC or VFD is most pleasant noise-wise?



It depends on what noise you wish to reduce.  Constant fan hum can be a bit distracting and is unnecessary when the VFD is not driving a motor or is under very light load.  The fan is for cooling so ambient air temperature must be taken into account when looking at modifying fan operation.
There are several options such as having a series of speed steps for the fan related to the need for cooling or the simplest one is to have an on / off arrangement with the fan being turned on as the heat sink becomes warm.  
This utube video is one persons approach.



Electrical interference generated by VFD is another issue.  Harmonics and radio frequency emissions ( RFI) can be a problem if other devices sensitive to these emissions are effected.   There are solutions for most of these "problem" side effects of VFDs.

So it depends on what "noise" you need to reduce as to what measures may be needed.


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## Ken I (Oct 28, 2018)

I have found most single phase motors can be rewound to 3 phase and the number of active poles can be changed as well - just find a competent rewinder.

Further you can use a VFD to run the beans out of the motor - typically generating many multiples of the horsepower.

I do this all the time in my line of work (robotics) where I need much more power to weight ratios than are typically available from standard 1500 / 3000 rpm configuration squirrel cage motors.
(See attached article)
Most motors can be spun up to 3000 - 12000 rpm for commensurate winds and appropriate consideration to bearings. balance, cooling and other issues.

Most persons are going to chime in and tell you why not - but I am talking from real world practical application here.

Regards, Ken


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## deverett (Oct 28, 2018)

BIGTREV said:


> What is a BCA machine?
> Thanks


Here you go Trev:
http://www.lathes.co.uk/bca/index.html

Dave
The Emerald Isle


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## Niels Abildgaard (Oct 28, 2018)

Ken I said:


> I have found most single phase motors can be rewound to 3 phase and the number of active poles can be changed as well - just find a competent rewinder.
> 
> Further you can use a VFD to run the beans out of the motor - typically generating many multiples of the horsepower.
> 
> ...



Hello Ken

I have released a lot of inbuilt smoke from two old motors ,fooling around with a cheap inverter.
Your very well written paper has explained why I think.
It was a pleasure to read and thank You.

Kind regards

Niels


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## SpringHollow (Oct 28, 2018)

If you do not mind spending the money, I would buy a used 3 phase 1 HP motor and slap one of these on it:  http://www.electricmotorwholesale.com/9536-KBDA-24D/

They are very nice because they have analog (dials) controls that allow fast manual changing of the speed.  I have one on my mill and works nice.

Ken


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## Wizard69 (Oct 29, 2018)

A couple of things here. 

1. Most motor manufactures these days publish max speed recommendations for current or recent motor models.   So for newer motors you don’t have to guess about permissible speed ranges.  

2.   Further don’t assume every motor in a family has the same speed range ratting.  Some variants will have substantially lower max permissible rotor speeds.  

3. On rather older models motors I would be reluctant to go to a 120 HZ. 

4. Don’t confuse power with torque.  Torque is somewhat flat when the motor is driven by a VFD but that doesn’t mean you have power at low speeds.    A belt or gear speed reduction is still important at lower speeds.  

5.  The other reason for a gear ratio between the motor and spindle is to keep motor RPM up so that you get a cooling effect from the fan.  

6. All of the above implies simple low cost VFDs.   Use a vector drive, a drive with RPM feedback or other advanced features (AC servos) and you end up with different performance characteristics.


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## BIGTREV (Oct 29, 2018)

deverett said:


> Here you go Trev:
> http://www.lathes.co.uk/bca/index.html
> 
> Dave
> The Emerald Isle


Thanks Dave


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## stackerjack (Oct 29, 2018)

If you are in the Uk, there is a website where a guy called Gavin is always selling invertors/convertors/Variable Frequency Drives etc.
https://www.homeworkshop.org.uk/
Jack


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## ddmckee54 (Nov 1, 2018)

At lower motor speeds, it's all about the heat.  Most lower cost motors will recommend not to run the motor below 20-25% of rated motor speed when using a VFD.  This is not due to the fact that the motor won't run at those low speeds, it's because the cooling fan will not be turning fast enough to keep the motor cool.

If you force cool the motor you can run at virtually any speed.  As I sit here, I'm within a few hundred feet of  about a dozen extrusion systems that run on VFD's.  These are 300-600HP three phase motors with VFD's that will provide 100% rated torque at 0 RPM, and do that 24/7 as long as you want.  The only thing that's special about these motors is that they have a forced air cooling jacket.  Of course the forced air cooling blowers on these motors ALL have 5HP motors or above - and those cooling fans are running full speed all the time or the main motors DON'T run.

If you want to run the motor at low speed without forced cooling, then pay attention to the motor temperature.  When the motor temperature reaches the point where you can't comfortably KEEP your hand on it for a few seconds, then it's time to take a break.  Go have a cuppa, or a cool one or two, anything to let the motor cool off.  Remember, every degree over 140°C that those windings experience will knock about 10% off the motors rated life.

Don


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## BobNZ (Nov 1, 2018)

Air flow though some motors is better than other and some motors are encased as protection in volatile environments, so rely on external fins for cooling.  The field coils and the armature both heat while  the case temperature is affected more by the field coils heating in the short time frame.

Some manufacturers raise the power rating of their motors by increasing the field coils capability which usually means the field windings  warm more slowly, but the armature just works harder when the  extra load is applied.  It is more difficult to check the armature heating, and applying your hand to the outer case only gives an indication of total residual heating from both the field coils and the  armature after air cooling has taken some heat away.

A wide safety margin is a wise precaution particularly if running lower than rated speed with a reduction of AC frequency down from the  motors rating.  Many VFDs do provide a meter option so current draw can be monitored.

Heavy work loads at low spindle speed require gearing and some common sense as to how a VFD is used to give speed flexibility .


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## abby (Nov 1, 2018)

I have bought 2 inverters from Gavin, both have failed after a few months of light use so I would not recommend him.
Dan.


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## abby (Nov 1, 2018)

For bigTrev





A BCA is a small jig borer with a rotary table , it's the machine on the left of the picture.
Dan.


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