Turing a static phase converter into a rotary

[dan.taylor.1]

!!! I've got a chunk of that rubber, never even considered it as vibration damping. Brilliant! thanks for that idea

I went begging to my mums to find a spare car floor mat but they were all too thin, then it dawned on me that we go through at least one rubber a season while the cows are in and they are usually just lying about the yard, about 150mm wide, 20mm thick and probably 2m long. Ideal stuff as you can stick a screw through it and still have enough meat to hold where thinner stuff would pull out.

 

[dan.taylor.1]

I think (I'm no expert)

the idea behind the idler motor is it helps balance the phases out, if one phase is a bit low or a bit high they even out as the idle motor spins at a steady speed putting a bit more load on the high phase and less on the low phases (or even generating when you start a load up)

but... even if it's undetectable to the eye/ear the motor isn't really spinning that steadaly, if you hook it up to the right gear you can literally see power spikes/vibration etc. as the magnets passes windings - what looks to us as one steady revolution is really a bunch of pulses as the motor shaft is pushed around

adding a bit of mass to it helps it run more smoothly :-)

Yeah I know it won't be running right as its been run by two phases 90° apart and one phantom phase, where they should be 120° apart. I'll maybe try a load on it and report back, I've got some big cast taper lock pulleys that may fit (it's probably a 30mm shaft on the idler!)

 

[m_c]

@knighty you're partially right, but the main point of an idler motor is to act as a generator.

When you're running a static converter, you're essentially using a big capacitor to create a phase lag to start the motor spinning (the same principle as how a capacitor is used to start a single phase motor). Once running, the run capacitors are doing the same thing, but to a lesser degree, relying on the inductance/inertia of the motor to help generate the required phase lag to power the false phase.

By adding an idler motor, it's main aim is to help generate the false leg in a more sinusoidal way using capacitors to help boost/balance the voltage.

Ideally you want as big an idler motor as possible, as the higher inertia goes further to help smoothing the continuous acceleration/deceleration as the motor goes between being powered on two phase, to having to generate a phase. Off course, the more motors you have running, the smoother the output becomes, as every motor helps generate/smooth the false phase.
This is why you often see rotary converters built with huge idler motors, which need a pony motor to start them spinning. There is however a point of reducing gains.

I have been considering rebuilding my rotary, but I'm toying with the idea of trying a 240V in/400 V out VFD through a filter (to stop any voltage spikes being fed back into the VFD) into my idler motor, rather than the existing transformer/relay setup.

 

[dan.taylor.1]

Here's a question, mine makes about 420v, 416vand 383v, do I remove or add capacitors to bring the voltage on the 3rd phase up to nearer the other two?

The motor I'm using is a big, old parkinson 7.5hp motor which I can just about lift, must be 60kg or something. So I'm pretty sure the rotary inertia is fairly high anyway, its one reason I got an older one not a new motor.

 

[Seadog]

Here's a question, mine makes about 420v, 416vand 383v, do I remove or add capacitors to bring the voltage on the 3rd phase up to nearer the other two?

Suck it and see. I think it may be an increase, it's a while since I played with my "project."

 

[dan.taylor.1]

Easy enough to fiddle with, only problem is the converter weights more than the motor and it took me and my brother to lift it onto its shelf!

 

[m_c]

Generally more capacitance means more voltage.