Gallery, Projects and General > How do I??

High Voltage Switches

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Darren:
Thanks John, you have inadvertently answered my question,

Silly me should have thought of this earlier.... :doh: 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..... :thumbup:

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! :coffee:

[Edited: I removed reference to auto transformer]

John Hill:
I found this:


--- Quote ---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
--- End quote ---

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

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

 :lol: :lol: :lol:

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....!!!

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