High Voltage Switches

[Darren]

I'm looking to make up an electrical control panel that will involve a couple of reversing switches.

Need to swap over two poles backwards and forwards.

I have one switch, but need another....but for 440-600V


Something like an intermediate lighting switch (that's a good tip for lower 240V use btw... )
Or a DPDT switch

Trouble is I'm having a job finding any rated at the higher voltages, any ideas ??

Current is not particularly high, it's just the voltage rating.

[Bluechip]

Darren,

Had to do a similar thing years ago for a 400V 2/3 hp dc motor. Could not get suitably rated switches or relays either.

Had to do it with contactors. These are usually 440 VAC, or so. Fortunately had two going begging, they are NOT cheap.
The contacts had no rating for DC, but motor only took about 1.5A, so, what the hell eh?.

Diag attached. I think. Done from memory. You need to be sure that there is an effective interlock to prevent any chance of both contactors being energised at same time .. otherwise ' Phuttt ....  who's turned the soddin' lights out ? etc. '





Check diag. if you use it. Think it's OK, .... have been wrong before tho'.

Dave.

Edit  All my drawings slope to right, to let spare electrons drain away 

[Darren]

Thanks Dave,

I can see how that works, might have to resort to such a thing if I can't find a suitable switch.

You wouldn't credit how hard they are to find. It seems these days everyone uses contactors and remote switching = complicated....

What's wrong with a simple switch with the proper ratings....

[Darren]

I found this, that might do it

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=2225936

[Weston Bye]

Darren,
Switches for directly controlling voltages that high are seldom used, save for mains power disconnecting.  The issue involves arcing as the contacts open.  Most high voltage switching makes use of snap-action contacts to very quickly open and close the contacts to momomize arcing.  This makes the switch operating mechanisms necessarily cumbersome and require high operating forces.  Also, for safety reasons, the structure of the switch needs to provide physical isolation for insulation to keep the voltage or arcs from reaching the operator's fingers.

For these reasons most control panels operator stations use pilot-duty devices for pushbuttons, pilot lights and switches.  That is, lower voltages are used at the operator station to control relays, motor starters and contactors to switch higher voltages elsewhere in the control panel.

One notable exception is the drum switch typically used at the motor of a Bridgeport or other machine tool.  If you absolutely need to directly switch the high voltage, I would recommend this last option.

I have dealt with these issues for the last 35 years, 18 of them as a self-employed controls system contractor.

[Weston Bye]

I found this, that might do it

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=2225936

Good choice, Darren.

[Darren]

Thanks guys,

I guessed arching was the main problem, then isolation from the operator. (in order of difficulty not safety.. )

But many older machines have such devices that have survived and are still in use today, so it can be done. Remote switching via low voltages is not beyond my scope, but I'd rather not go that way due to expense and bulk.

I'm in no hurry, and have little time today (Stews coming over..... ) but I'll post something at a later point.


In brief, I wish to make a control panel on an adjustable boom for my mill as the electrical controls are presently well out of reach from the working position.

I have one main motor reversing drum switch, and the high speed transverse is fitted with a drum switch as standard.

The slow transverse motor switch is out of reach, currently a simple on/off contactor with a thermal safety cutout.
Direction/off is via a clunky lever in front of the bed operating a set of gears. It works but is not precise making it easy to reverse the bed instead of stopping it. So I was thinking of controlling this motor electrically. It's 1/3Hp

It's just a whim, maybe I should try to become better acquainted with the controls first before making any decisions.



One question I don't know the answer to (reg wise) ......can you put 3ph and single in the same control panel....
Or should they be kept separate?
It would be a switch for lighting and poss a coolant pump.

[John Rudd]

One question I don't know the answer to (reg wise) ......can you put 3ph and single in the same control panel....
Or should they be kept separate?
It would be a switch for lighting and poss a coolant pump.

I see no reason for having 1ph and 3ph in the same enclosure..

Where I work we have motor control centres that have 415 3ph coming in and the control ccts are 110v ac off a suitable transformer......

Heck we even have 6.6kv vac breakers with 110v controls...

[Darren]

Thanks John,

I believe that if you take a single phase from one of the 3 phases and use a transformer to give the desired voltage then you are OK.

But I'm using a rotary converter to generate 3ph from a single phase.
What concerns me is that the original single and the manufactured 3ph may be no longer in sync. (it might be? I dunno)
But I suppose 3ph never is in sync between the legs anyway so it probably isn't an issue.

There is nothing I can find on the net to cover this, and I'm certain it was never covered when I did my apprenticeship many years ago.

I just wondered if someone here knew, but I guess it's a long shot.

My hunch is that it's ok cos the trips will/should take care of any problems. But the Regs can be funny sometimes.


Edit:
What concerns me is that the original single and the manufactured 3ph may be no longer in sync. (but it might be? I dunno)

Thinking about it they have to be in sync, held so by the original incoming phase...?

[John Hill]

Darren, I dont know the details of your rotary converter but if it involves an induction motor then that will involve slip  and slip means the phase is continually changing with respect to shaft position.  In such an environment I think there is a high chance that at any time any phase of the 3ph could be at 180 deg to the 1ph,  i.e. 440(?) + 240 between the two.  That is assuming of course that the 3 phase is not totally isolated from the 1ph.

However, if your converter is just a rotary transformer where two wires go in and three come out of what is otherwise a 3ph induction motor I dont think there would be any phase change at all.  But I am not sure how you could do that in the UK with only one phase to begin with as I feel you would end up with very low voltage 3 phase, different in the US where they have two phases to begin with.

P.S. How about sneaking an extension cord to the neighbour's garage?  They are probably on a different phase to you!

[Darren]

Thanks John, you have inadvertently answered my question,

Silly me should have thought of this earlier.... The rotary converter has single phase AND manufactured 3ph in the same box and that meets the regs as it's a modern commercial unit.
So all ok on that score then.....

I'm certainly no expert on motors, but I was under the impression that motor slip varied according to load? ie it doesn't constantly slip in ever increasing angles until it comes full circle again?

This is how some of the old record players regulated their precise speed. They used a non magnetic disc and a magnet to create a brake. Vary the position of the magnet and you vary the load hence the speed of the motor.

The rotary has a step up transformer, but without checking I'm not sure if it's a full pair of separate windings or a simple autoformer. I would guess the latter.

[John Hill]

Ah right, so the rotary converter is a commercial unit and not something built from the collected wisdom of the Internet!  Excellent!

As far as I know slip does indeed vary with load but in the presence of friction there is always some load, besides I suspect iron losses show as load too.  If I understand correctly slip is continuous though not constant.  There are types of motors  with less slip called synchronous motors but I think the majority of readily available motors in the, say, 1 to 3 HP sizes are simple induction motors.  It is the slip that induces the voltage in the armature that produces a magnetic field out of phase with the stator which produces the torque.

I have heard of converters where a transformer is used to step up 220V to 380V and that is applied to one winding of a 3ph motor and with capacitors for starting the motor you have 3ph at the required voltages.  I cannot imagine any phase slip in that configuration.

I have no experience of such things though I have all the bits in the garage!

[Edited: I removed reference to auto transformer]

[John Hill]

I found this:

Producing the three Phases in a Booster™
In European or IEC Boosters, a transformer steps up the single phase input voltage to 400V single phase.
North American or NEMA Boosters don’t have transformers.
A group of precharged power capacitors are, on demand, discharged into an internal high efficiency
three phase motor. This motor is used as a generator to produce three perfect sinusoidal
waveforms with a correct phase angle of 120 degrees.
Depending on load conditions, more or less capacitors are involved in the process of charging and
discharging.
Boosters are based on solid-state capacitor switching. Within a sinewave switching is performed at
zero voltage and zero current transitions.
This minimises stress to capacitors, components, motors and to the supply lines.
Electronic switching is free of sparks and stress. No EMI or RFI emission is found with a Booster.
No line disturbances will be generated. Lifetime of all components is extremely long.
For starting motors, Boosters are capable of producing up to 600% of their maximum output power for
a short period of time.
If motors start under load (e.g. submersible pumps, vehicle lifts, cranes, lathes) or have to change
their direction instantly (as in CNC machines), this is a unique and very useful feature.
Life Time

http://www.eurotech.co.nz/text/Understanding.pdf

[CrewCab]

Goddamit Lads ......... my head is throbbing from reading this thread  ................ can we have a version for those of us whose first language is English

 

CC

[Darren]

John,

I have built a rotary converter from scratch, I won't say how on here but it was basically the same design as my commercial unit.
They are not particularly hard to put together if you have the right bits and know how to set the voltages with the capacitors.

The real problem I had was getting a high enough voltage step-up transformer with enough current capacity. I used a transformer that is very common in the UK but it only gave me about 350V, a little shy of the 415V that was really needed.

It worked fine and would probably run my mill OK. But my lathe has a two speed motor and it just couldn't quite get the high speed going. If you spun the spindle by hand it would sometimes run up to speed. Sometimes not.

So I searched for a transformer winder and got some quotes. The price was frightening. Put it this way, if I added £100 to the price of the transformer I could buy a brand new commercial rotary converter, so that's what I did. It was worth it for the nice box it came in plus all the safety relays etc.


Re slip, it's not a constant, it varies with load as far as I know.

The two types of motor are synchronous and asynchronous, it's the latter used in the record players that use a disk/magnet to set speed.
I don't know which one our machine motors are, I forget so much over time.....



You can run a 3ph motor on single phase without a converter, but it will be reduced to 2/3 power. A friend runs his Harrison mill and lathe this way and reports no problems at all.
Anyone reading that please note you can't just put a 13amp plug on a 3ph motor, it just won't work and it's a little more complicated. But not much.




CC,
Sorry, it's a bit difficult to simplify the language. The terms are (as far as I remember) correct, if we used anything else John and I (or anyone else) wouldn't have a clue what the other was on about.............sounds like a normal course of events....!!!

[John Hill]

Darren, yes, slip is continuous in that it is always there in an asychronous induction motor in.  A typical two pole 50Hz motor would would assume would run at 1500RPM? But the name plate will show something like 1480RPM, the difference is slip.  Leastwise thats how I understand it.

There is prubably a transformer I  could have found for you but freight would have been a challenge!   Of course if current was your only issue you could have used multiple trannies in parallel (?).  Being careful to observe phase of course!!!!   If you need 440V  I think you could use the secondary winding of an old isolation transformer in boost, if I recall correctly all transformers are insulation rated for 1Kv but you would still want to think about it before making the connection.

I cant even get my lathe (very much the same as Mr Bogs has over at Bandit's place) to start in the top ranges without tripping circuit breakers but it does spin up OK if I take the chuck off!  I could up the circuit capacity but I am tending towards some sort of clutch to get around this little problem,  I dont see 3 phase as attractive for that job.


I have tried a 3 phase motor on single phase using the fairly well advertised methods but with only modest success and not having a real need for it I never perservered.
 

[Darren]

Had a quick look at Wiki....http://en.wikipedia.org/wiki/Electric_motor

Our Machine motors don't have slip as they are AC Synchronous, it's asynchronous that has slip.


AC Synchronous are in sync with the mains frequency, that's what controls their speed. (unlike DC motors where voltage controls speed)

With an AC Synchronous you can vary the voltage but the speed will not alter, though the power will. That's where these new fangled VFD's come into play as they vary the frequency to the motor to change the speed.

So, no slip in machine motors.

[Darren]

John, what size breaker do you have on the lathe circuit and are you using a radial or ring circuit?

What is the power of the lathe motor and speed?

What is your domestic line voltage and what is stated on the lathe motor?

Questions, questions, questions.....

[Darren]

Try a bigger start cap, that might do it.

It should give a boost for starting and if it's the type of motor I think it is the cap is cut out of circuit when the motor spins up.


But the previous quests are still relevant....

[Darren]

Ah, I think I know what you mean about slip now, (I was thinking of it being used in a different way) yes there has to be some for the motor to work.... otherwise the motor wouldn't chase itself round and round....... though some of the 1,500 - 1,480 speed loss is magnetic losses.
But this only aplies to single phase motors. The slip is generated by the run capacitor.

Older Iron cased motors have less losses than the new alloy cased motors and is partly why the older motors are more desirable then the new Chinese ones.

Three phase motors don't need slip as the phases do the chasing.

[John Hill]

The breaker that trips is only 10 amps (and is at the outlet) so hardly suprising, the main breaker for that circuit is 27 amps.  We do not have ring wiring in our house (in fact I dont know if it is even used in NZ).

Motor is 2 Chinese horsepower, capacitor start, capacitor run, dont know the RPM, maybe 1480.

I think the motor is capable of starting the lathe and any boosting by way of more start capacitance is not likely to gain anything as that breaker will still trip.  I could just short the 10 amp breaker (it is a discretionary installation) and it would likely be OK but I dont like to hear that Chinese motor struggling to get started under load as I have already had one fail and would rather nurse this one to old age if possible hence the clutch concept which will likely be no more complicated than a swinging mount for the motor and a lever control to slacken the belt when starting the higher gears.

I have to think twice regarding 3ph not requiring slip as the three phases set up a rotating magnetic field that is true  but current must still flow in the bars of the armature for the armature to have a magnetic field to interact with the stator field and I do not think there would be any voltage induced if the bars were not moving in relation to the rotating field.  This is what someone else says on the matter..

In a three phase induction motor, the windings on the rotor are not connected to a power supply, which are basically short circuited. The squirrel cage winding, which is the most common type of rotor windings,

it looks like a running wheel used in cages for pet gerbils. When the motor is turned on the rotor is stationary, the rotor conductors experience a changing magnetic field sweeping by at the synchronous speed.  This result in current around the rotor windings, the level of this current depends on the impedance of the rotor windings.  This causes the motor action to be fulfilled, and the rotor experiences a torque and starts to turn.  The rotor can never rotate at the same speed because there would be no relative motion between the magnetic field and the rotor windings and no current could be induced.  This is one reason why induction motor has a high starting torque. 

  More at http://www.electricmotorsale.com/threephasemotors.html

[Bluechip]

Hi Troops ..

Don't know where we are with this .. but there are breakers designed for use on inductive loads ie motory things ( and high inrush linear PSU's for that matter ) etc.

Different Curve

Here's one I grabbed at random, there are more..


http://www.aelgroup.co.uk/hb/hb040.htm

May be relevant, may not

Enjoy ...

Dave BC