240-414V without a transformer
I have been told verbally, some time ago, and just watched a vid in Utube a dual voltage motor being used to generate 415V 3 phase from 240V single phase without using a step-up transformer.
Is this really possible, if so does anyone have any info on how to wire it up.
What's happening here, Darren:
It requires a WYE-wired motor... hook it up for high-voltage operation- this puts the windings in series.
Connect the MIDDLE of each winding to your power source... this 'drives' the motor.
You'll find that high-voltage appears on the 'outer' windings.
What's happening here, is that you're putting power to one set of windings, and the second set acts as an 'autotransformer'.
Unfortunately, the electrical output will be half, or less, than the motor's rating... AND the mechanical output of the motor (if you use it) will be half, or less, than the motor's rating...
And if you try to pull power at the same time as mechanical load, you'll have a whole lot less of both.
But... it does work... Not as good as a transformer, though.
Thanks Dave, I was wondering how efficient it might be. You have cleared that one up nicely.
Think I'll stick with the transformer method then.
You will need more than just a transformer to do your Job.
The place to look is on Ebay at seller called drives_direct_nottingham
He has a number of single to three phase inverters listed.
To see which one you need , some questions.
1. Can your motor be wired for both star (415) and delta(240V) ?
If so the 240V single phase in to 240V three phase out are cheaper than the 415
2. What is the HP or Kva rating of the motor.
If it only states Kva or Watts then 1 HP = 746 Watts or 0.746Kva.
The advantage of using one of these inverters is that you can vary output frquency
and hence the motor speed over a big range and there is a feature called 'Soft Start'
and 'Soft Stop' which allows you to cause your motor to accelerate from rest to full
speed and decelerate to a stop. You can either drive the unit from its control panel
or control it via a cable with with a little control panel with 'forward' 'reverse' switches
and a potentiometer for speed control.
Rgds David C ( In Warrington)
Darren, et al,
What a great thing PM is! I was just musing (in the next thread) whether or not this very thing was possible. I ended up by doubting that it would work, and asking for input from experienced members. And now, a few days later, I learn that this is at least possible, although possibly of questionable practicality. I'm going to watch the video and perhaps overheat my small, weak brain trying to figure out how this approach arrives at the (to me) odd voltage of 415.
The Utube complicates the explanation.
The way the thing works (almost) is as follows.
The three coils on the motor are each designed to handle 240V.
On a 240/415 motor , both ends of each coil are brought out to a terminal block.
If the motor was to be connected to 240V three phase, the three coils would be daisy chained , like a triangle using the said terminal block, and the three phases are connected to the three points of the triangle. This is called Delta connected from the Greek letter Delta symbol.
If the motor was to be connected to 415V then at the terminal block one end of each of the three coils would be connected together forming what is called the star point. This is known as 'Y' connected, and I am sure you can see why !!
The three phases at 415V are applied to the other ends of the three coils.
When running on 415V, nothing is connected to the star point.
Now.... to get three phases at a nominal 415V from a three phase motor with only a
single phase supply. the theory is as follows.
A spare (and the bigger the better ) three phase motor is connected at the
terminal block in 'Y' format.
Three wires are connected from this motors terminals to the motor that you want to
drive, but which is not switched on yet.
In readiness but not switched on yet you connect your single phase live to any one of
the SPARE motors three phases, and the neutral to the spare motor's STAR POINT.
If the single phase was swiched on now, the spare motor would just vibrate which is
not what you want, so you need to have wrapped a short length of cord ,(WITH NO
PERMANENT ATTACHMENT) round the motor shaft..
On switching on the single phase, you yank on the rope like starting a lawnmower,
and the spare motor will spin and accelerate to full speed .
The single phase creates a field in the one coil, which magnetizes the spinning rotor
which then generates voltages in the other two coils of the Spare motor, which
appear on the motor's terminals, there ready to power the motor on your machine.
The direction of spinning of your machine's motor will depend on which way you pull-
started the spare motor.
The output voltages will each be each quite different and this would NOT be the way
to provide a reliable 3 phase supply.
Rotary converters work on this principal with a lot of extra components to make the
three phases more equal.
A three phase motor running on mains true sine wave three phase supply are very smooth and quiet running as the effect of the three phase in the motor is to produce a uniform rotating field which provides uniform rotating torque unlike the pulsing torque produced in both a single phase or DC motor.
It follows that a three phase motor supplied with unequal voltages on the three phases wil not be as smooth and silent running.
The efficiency of the system can be improved and more power got out by adding two suitable size capacitors to transfer power from the live 240 volt input to the two unpowered arms of the Y. The capacitors go in series, one from 240 live (on the first arm of the Y) to the second arm of the Y and the other between the second and thrid arms. The thing becomes self starting if you temporarily connect a booster capacitor between the first and second arms.
These connections are an alternative way of building a static converter around three phase motor. The main advantage is that it can use a hard wired Y connected, 440 volt only, motor (as many of the older ones are). The usual UK / Euro converter requires delta connection meaning a dual voltage motor with 6 terminals, one for each end of every winding. The disadvantages are that its virtually impossible to get a decent 120° phase balance and that the winding currents are always imbalanced, often considerably. Significant power will be lost and large internal currents will occur causing the motor to run hot and eventually destroy itself.
Oh its pretty much pants as a transformer whatever you do.
The conventional delta connected 220 volt static converter with capacitor across the wild leg is quite legitimate engineering and, contrary to the usual American bodge experience, capable of producing a motor performance approaching that under its native 3 phase drive.
Thanks for the detailed explanation. I think our motors here in the states differ from yours, though, in that the TYPICAL three phase dual voltage motor is a nine lead motor and your approach demands a twelve lead motor. Here our nine lead motors are wye connected with one set of three windings permanently connected at the center to form the initial wye. I suppose you could dig around in there and find the connection, unsolder it and bring the leads out, but this is NOT a simple reconnection, where nicely numbered leads are mated with wire nuts.
I'm still not sure where the odd voltage of 414 comes from. I'm familiar with 110, 120, 208, 220, 240, 440 and 480. Looking at this now, I suppose I might have just answered my own question. 480 is 2x 240. 440 is 2x 220. I guess that makes 415 an approximation of 208x2. I'm not sure what I'd do with 415 volts, though, since the motors want 480 to run properly and 414 isn't even close to 10% under.
I watched the video and was unimpressed that the guy is using a VFD in conjunction with the Frankenmotor. Now, if I could use this basic setup with a few capacitors and minor additions to run a three phase 480 motor just like I'd run 240 or 208 motors with a plain RPC (bodge-style as Clive might say) then I'd be impressed enough to give it a try. But if I've got to mess around with a spaghetti-wired motor AND a VFD, sorry, I'll just get a single phase 240/480 transformer of appropriate KVA and use that to single-phase a derated VFD and be done with it. As has been pointed out on PM before, 480V VFD's are comparatively inexpensive and plentiful. Transformers, too, can be found at reasonable prices, making for a dependable and simple converter.
Looks like I've got some more curiosity satisfying to do. I am not ready to accept that what Clive refers to as a UK/Euro rotary phase converter, based on a delta-connected motor, is any more capable of producing clean and balanced three phase power than our wye-connected bodger. I'm not arguing that this is NOT a better converter, I am merely saying I need convincing.
One thing's for sure, the bodgers are not as bad as Clive makes out, and I'm sure many PM membrs can attest to this, having used home-brew converters successfully for many years without allowing any smoke to escape. If it can be shown conclusively that the delta converter is superior to the bodger (I like that word) I will consider hacking into a hapless victim motor to bring out the remaining three leads to make a super converter.
To some extent, though, this series of posts has satisfied me in one regard. Now if I describe the Frankenmotor setup to someone and they say I am an idiot and that such a scheme could never work, I can prove them wrong, on a demonstration level if not a practical one.
Thanks all for the interesting and stimulating conversation.
It is actually 415 volts and while I am not up on how it comes to 415V but the voltage is directly related to our 240V single phase.
Originally Posted by sa100
Like your 208volt three phase comes from your 110V single phase...
I have seen a diagram explaining it all somewhere..
240V to 415V.
For those interested where 415 comes from.
Square root of 3 x 240 = 415.692
In a three phase system with phase to neutral = 240V, then
Phase to phase is 415v (ish)