18th Dec 2010
Penfold Motor Conversion

There has recently come to light another way of achieving a synchronous motor discovered by a British coiler, Clive Penfold. This utilises old washing machine motors and merely involves a small amount of work with a soldering iron. The full details can be found in these newsgroup postings.

TCML Post

Electrokinetica.org post

4HV.org post

Original SRSG Conversion by Milling Method.

Do This Modification At Your Own Risk

(These are 50Hz motors, but the process also works on 60Hz motors)

Above: 1440 rpm 4 Pole (Becomes salient pole 1500 rpm)

Above: 2880 rpm 2 Pole (Becomes salient pole 3000 rpm)

Above:  Another 2880 rpm I machined. This used a 43% width.

Above:  The Machined 'Flat' Is Ringed In Red. (motor is a two pole 2880 rpm. Flat = 40%)

Do These Modifications At Your Own Risk

An ordinary induction motor is an Asynchronous motor which means it will always rotate slightly slower (2% - 6%) than the rotating magnetic field created in the stator, this is called slippage.
To make a Synchronous Rotary Spark Gap though you will need either a synchronous motor, or an ordinary AC induction motor that has been converted to 'salient pole' operation. The word 'salient' in this context meaning dominant.

The rotor (or Armature as it may be called) of a synchronous or salient pole motor will always rotate at the same speed as the rotating magnetic field created by it's surrounding stator*. The speed of this rotating field being dictated by the frequency of the AC mains supply and the number of poles the motor has.
*In reality I think I read that they speed up slightly, then put themselves in check and slow down, repeating this cycle so quickly, that the speed, to all intent and purpose, is 3000rpm)


The Theory

Let us suppose we have a synchronous or salient pole motor running while you are monitoring the sine wave of its AC supply. Then when the supply voltage is at the peak of its sine wave, you were able to magically freeze both the motor's shaft and the AC sine wave.
If you then made a mark on the shaft in relation to the casing, you would find that every time the marks subsequently aligned, the AC wave will always be at its peak. This is often the best time to arrange for the gap to fire so you can be sure the capacitor is fully charged.
Do the same trick with an Asynchronous motor and you will find the marks will not align on the next revolution of the shaft.
Dependant on the motor's speed and number of poles it has, you usually have either 2 or 4 positions around the rotational path of the shaft where the AC voltage peaks.

As mentioned a normal induction motor's rotor revolves slightly slower than the magnetic field influencing it, but by machining some flats on the rotor you cause it to lock too, and rotate at, the same speed as the rotating field.
As this speed is directly linked to the frequency of the AC supply sine wave, which in turn is responsible for charging the capacitor, you can now arrange things so the electrodes align and the gap fires, at a time when the capacitor should be fully charged.


With Asynchronous motors the mains cycle could be at any point the sine wave curve when the electrodes align. The capacitor therefore may not be fully charged resulting in a missed firing. Plus if at the same time you also have a resonant condition in the charging network, this could result in very high voltages occurring that can damage the capacitor and transformer. (Resonant charging is discussed here)
Asynchronous motors can still be used as long as the breaks per second (bps) of the spark gap is higher than around 400. This is because the effect of an undercharged coil is not so great then.


An excellent in-depth analysis of rotary spark gaps can be found here at the very knowledgable site of Richie Burnett.


If the motor is a 1440 rpm (1800 on 60Hz) it has four field windings and will need four flats milled onto the rotor, each at 90 degrees to one another. If the speed is 2880 rpm (3600 on 60Hz), it only has two field windings and consequently only needs two flats at 180 degrees to each other.
After modification the speed will (should) have increased a little to either 1500 or 3000 rpm (1800rpm or 3600rpm on 60 Hz), and it may run a bit hotter with less power.

The amount removed when forming the flat is reasonably critical. Too little and the motor will not be synchronous and its speed will constantly surge or 'hunt'. Too much and it will loose too much power and overheat.

With four pole 1440 rpm motors I measured the overall diameter of the armature and removed one quarter of this distance as a flat.
For a later two pole 2880 rpm motor that I also modified, the width of the flat was 40% of the rotor's diameter.
I did try 30% initially and this worked with just the bare 10 inch disk, but when I added all the Tungsten electrodes and their holders the added weight caused it to 'unlock' and it become asynchronous again. By removing another 10% to make it 40% overall, the motor then worked perfectly in synchronous mode.

Both of my modifications resulted in no discernible loss of torque and very little, if any, increase in the heat generated. The 1500rpm motor was a 0.5 Hp so any loss of power would have been noticeable, whereas the 3000 rpm is a 2 Hp, so it had plenty of leeway.


An alternative method:

The method of determining the amount to remove mentioned above seems to be based on another method that came to light in a 2001 posting from 'Scott D' on the TCML newsgroup.
This uses a formula that uses the area under an imaginary arc drawn from the rotor's centre. This can be best seen below.


First you find out if you have a 'dead' pole winding or not. ('Dead' pole refers to an area where you have a pole with no winding around it)
If you do have a 'dead' pole you use a 40 degree arc, otherwise if not you use a 38 degree arc. The formula used for the 'dead' pole 40 degree option is 2*(sqroot(radius^2 - (radius*0.9397)^2)) [0.9397 is the co-sine of 20 degrees (40 halved)]
While that for a non-dead pole is 2*(sqroot(radius^2 - (radius*0.9455)^2)).

These give figures of around 34.2% for a 40 degree 'dead' pole motor, and 32.5% for the 38 degree option. With this method there is no distinction between 1448 or 2880 rpm motors however. I have used these figures in the past and found that while they worked for the 1448 rpm motors they did not always work on the faster 2880rpm motors, as more needed removing.
Basically it is still very much a try it and see situation, but overall I prefer to use the 25% or 40% method.




A Good Example of A Poorly Designed SRSG

The 1500 rpm motor had an 8 inch rotor with the electrodes on a 7 inch PCD. The 8 electrodes giving a break rate of 200bps at 1500 rpm. As the tungsten electrodes are 0.25 inch diameter the mechanical dwell time of the rotor was very poor. I attempted to overcome this by having staggered (in the vertical plane) electrodes.
This is achieved by having the right hand one adjustable up or down.

As can be seen despite the staggered electrodes I still suffered badly from power arching, caused I think by the closeness of the electrodes on the disc, and possibly the fact that I used a conducting ring on the rear of the rotor. Also the motor was only 1500 rpm, so for that reason I built the one shown below with a bigger rotor.

Above is my second attempt using a 10 inch diameter disc. This is still 200bps but without the conducting ring on the rear of the disc. This meant I needed two pairs of stationary electrodes, with the front set positioned at the rotor's mid height (say 9 o'clock) and the other rear set (hidden) at the 4:30 position on a clock face. When this is combined with four rotating electrodes on a 1500rpm motor it gives 200bps.



Unfortunately the 0.25 inch diameter electrodes combined with the 1500 rpm, still gave a poor dwell time, with power arcing at much over 3KW. For that reason I then put the 10 inch disc onto a 2Hp 3000 rpm motor, which meant I only needed four revolving electrodes and one stationary pair.

2HP 3000rpm Synchronous Operation 10 Inch Rotor

Above is shown the finished working SRSG (my third attempt) that used my 10 inch rotor from the second build but this time mounted on a 3000rpm 2 HP motor. It has two stationary electrodes and four revolving. All electrodes are 0.25 inch Tu.
The safety gap will be the two square pillars on the left, this consists of a 'horn' gap using two 0.25 inch brass rods. The horn shape means it acts like a mini Jacob's ladder and the arc extinguishes easily.
At the time my MMC handled 38Kv and I was using a PDT with a BIL rating of probably in excess of 60Kv, so I set the safety gap at 10mm giving a breakdown of ~30kv.

I used copper bar as much as possible rather than wire, with the large copper bar at the rear going up to feed the primary.