Wednesday, January 2, 2013

69, Rotary Convertor, part one.

I purchased my old thicknesser some 20 odd years ago, it was the first of my machines to be converted from 3 to single phase. Now that it had died, my timber machining couldn't proceed.
I unbolted the sickly 3hp motor and took it to the local "rewinds" for diagnosis. The news wasn't good, it had a short circuit in the run winding and the bill to repair it would exceed the replacement cost. An estimated $350 to $400 for a new motor was beyond me at the time and, as I had already been looking around for some time for similar sized second hand motors to no avail, some further options needed to be investigated.
Advice and a bunch of emailed links from a friend enlightened on the subject of rotary converters. These are a relatively simple way of converting single phase power to 3 phase.
The principle uses an "idle" 3 phase motor, of larger size than the machine motor that it is required to run. This is started and run by capacitors and generates a relatively smooth 3 phase supply.
Having accumulated numerous 3 phase motors over the years, mainly removed from machines being converted, my workbench was soon covered with them, along with the associated wiring. Some initial testing indicated that the system was feasible and a batch of various sized capacitors, as well as some volt and amp meters, were purchased from Hong Kong, via eBay.

Once the parcels had arrived  I delved into my stores of electrical bits and put together a switch panel to control the converter. For switching in and out the starting capacitors, I incorporated a timing relay.
Having a safer way of switching things, some more serious testing could be carried out.

A 7.5hp (5.6kw), motor, previously removed from the sanding machine was chosen as the most suitable for the Idler. Initially, all looked well with any number of subsequent unladen motors starting flawlessly, once the idler was in motion. Problems, however, were just starting....

The first issue to arise, (and the most enduring), was that, once the drive belts on the thicknesser were reattached and the motor had to start under load, the starting torque was unable to get the machine up to speed. The prolonged excessive current draw was then causing the main supply circuit breaker to trip. Reconfiguring the motors in either star or delta seemed to make no difference.
Dave, my mentor and researcher in this field, hit the Internet, looking for answers. The first thing that became apparent was that the design was gleaned from American sources, their power supply varies quite considerably from ours in Australia, theirs being more akin to a 2 phase supply. The Poms, whose mains supply is similar to ours, have found that a step up transformer is required to provide a 415volt input into the idle motor.
EBay came to the party again and a suitable 3 phase welder was purchased for $40.

We stripped the welder down and managed to tap onto an intermediate position on the main coil winding. Connecting the 240volt active wire at that point and the neutral wire to the end of the larger portion provided 440 volts across the whole winding, "near enough" for our application.

To reduce some of the bulk and weight I then set about cutting the transformer apart. The secondary coil was discarded and the laminated core was reduced down to a minimum height and rewelded. All being well, I'm hoping there is still plenty of metal to avoid any overheating issues.

With the voltage sorted, the next problem could be addressed, the starting capacitors. For this purpose I had purchased 5 x 100uF caps, trying different numbers of them until a suitable quantity was achieved. Using only 2, giving 200uF proved to be the most satisfactory, however, for reasons I have yet to determine, one by one, they failed. As entertaining as all the sparking and flashing was, the lot of them all ended up in the scrap bin. We figured, at first, that the higher voltage was too great for them, even though they were rated at 450v. I then ordered a further 2, at 400uF. These were arranged in series, to give a safer rating of 900v at 200uF, but the second time I used them to start the idler, they suffered the same fate as the originals. More head scratching was needed.
My simple solution came in the form of a two stage system. A smaller 1hp motor was added to the setup. I found that it was much easier to start, requiring only 5uF, and an equal amount to run. Once this motor was running, the larger motor, with it's separate capacitors, could then be started off it. A flywheel was, however, required to prevent the smaller motor stalling.
This setup also has the advantage that, if only a small, (less than 1hp), machine motor is required to be operated, the larger Idle motor can be left turned off. Current draw can then be minimised.

With all the preliminary testing taken care of, all the components could then be assembled. I calculated that most of the bits could be accommodated within the case of the trashed welder. For this I fabricated a frame, on to which the bits could be attached. This was then bolted to the base, along with the larger idle motor.
Now that everything was in place, the unit could then be properly wired. I have, at this stage completed all this with the exception of the wiring to the capacitors, as I am still waiting to purchase the 5uF ones, it's a bad time of the year for that!. For testing I have been using 4 larger caps, wired in series to achieve the correct values, but they can't all fit in the enclosure.
I have used the heaviest wire possible, right back to the distribution panel to minimise voltage drop, however there are still issues in starting the thicknesser motor, a 4hp, (3kw), 2900rpm, 3 phase job, under load, particularly when the bearings are cold. Some more research has revealed that this is apparently a common problem with rotary converters. I toyed with the idea using a clutch system, but are, at the moment, looking for a "soft starter" as a hopeful remedy. Initial testing shows that the machine torque is fine, once up to speed. Wood can be planed with no effort, although I have just had the blades sharpened.

Quite obviously it would have been much quicker and easier to simply replace the thicknesser's single phase motor. The theory is, however, that with this converter I will then be able to run any other 3 phase equipment as well. I already have a spindle moulder, a roller feeder for the same and a small drill press that, now, won't need conversion.

A few more helpful links on the subject;

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