Do This Modification At Your Own Risk

Above:  1440 rpm Four Pole Motors (Becomes 1500 rpm)

Above:  2880 rpm Two Pole Motors (Becomes 3000 rpm)

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

The Rotor Modifications For a Synchronous Motor.

Do These Modifications At Your Own Risk

To make a Synchronous Rotary Spark Gap you will need either a synchronous motor, or an ordinary AC induction motor converted to 'salient pole' operation. The word 'salient' in this context meaning dominant.

The rotor 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 is dictated by the frequency of the AC mains supply and the number of poles the motor has.

Whilst an Asynchronous motor (an ordinary induction motor) will always rotate slightly slower (2% - 6%) than the rotating magnetic field. This is called slippage


The Theory

Let us suppose we have a synchronous or salient pole motor running while you are monitoring the AC voltage sine wave of its supply. Then when the supply voltage is at its peak, 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 the best time to arrange for the gap to fire, so you can be sure the capacitor will be as fully charged as it is ever going to be.
In reality, dependant on the motor's speed and number of poles, you may have either 2 or 4 positions (or more) around the path of the shaft where the AC voltage peaks.

A normal induction motor's rotor, as I mentioned, revolves slightly slower than the magnetic field that is 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 sine wave mentioned above, you have achieved your goal of making sure the capacitor is as ready as it will ever be when it is discharged.

A normal induction motor before modification, is an Asynchronous motor because it is not linked to the AC cycle. This is because the rotor is revolving slightly slower than the rotating magnetic field created by the stator. So if you do the same magic trick above and freeze the action and mark the shaft and casing, you will find that they will not align on the next revolution of the shaft.
This means the mains cycle could be at any point on its sine wave curve when the electrodes align. The capacitor therefore may not be fully charged resulting in a missed firing. If 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.
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 Richie Burnett's site.


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 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.




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.

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, meaning I only need four rotating electrodes.

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








I used John Freau's circuit for altering the phase angle whilst the SRSG is running. This is a very handy little circuit and full details can be found here. (sometimes this link seems to be down)
It works by making the inductance of the Variac and the phase capacitor into a filter that resonates with the mains frequency. The inductance of the motor when it is running also come into the equation, so the only way to get the right value is by trial and error.
I found I needed a 93uF capacitor with my 2Hp motor (3000rpm) and a 34uF with the 0.5 Hp motor (1500rpm).

I found the best way to ascertain the capacitance value is start with around 30 to 40uF and the Variac set to zero, so it has no inductance in the circuit. You then need to slowly increase the Variac and at the same time monitor the voltage across the motor, looking for a rise.
You only really need to have a 5 or 6 volt rise after which it should then drop again as you increase the Variac. You need to go slowly as it is easy to get 30 volt rises or more very easily, with the risk of damaging the motor. My 0.5Hp motor actually uses a 15v rise without suffering however.
If you find you get no rise at all you need more capacitance. Too much voltage rise means the capacitance value is too large. With both my motors I found 1 or 2 uF can make quiet a difference, whether all motors are like this I do not know though.

You often find that even with a capacitance value which is correct (i.e.giving a nice 5 or 6 volt rise) that as you approach maximum inductance the motor will loose sync'. As long as you have around 70 to 80 degrees of phase adjustment you will probably find that is sufficient to operate with.
How you determine how much phase movement you have can be a little fiddley, unless you have an oscilloscope. Probably the best way is to mark the rotor and then view it under a neon lamp which will flicker at the mains frequency.
If you do have an oscilloscope you can set the rotor to trigger a photoelectric cell which is connected to one channel of the 'scope, while the other channel has a low voltage AC mains source as a comparison.



Variac

It is possible to use your normal variac quiet easily by just choosing to use the output of the variac.
If you do this be sure to insulate the pins on the now unused inlet plug of the Variac, as they will become live.













(Javascript needs to be running)


'YouTube Video of Phase Control"


'An updated YouTube Video of Phase Control"


200bps 50Hz 1500 rpm

The spikes you see are trigger pulses from a photo electric cell that replaces one of the fixed electrodes. This is being triggered by the revolving electrodes as would happen normally when the gap fires. The reference AC sine wave is from a 10v AC supply on the second channel.
In the opening shots you can see a complete cycle of the mains with the 4 firings 90 degrees apart, a firing occurring every 5ms. At the 41 second point in the video, I expand the scale so the scopes horizontal divisions are now 1ms apart which is 18 degrees (90/5). You can now see the fine control this gives you.

The Variac is a 0 to 270v unit and 1ms movement on the scope's Y axis (18 degrees of phase) corresponds to 0 to 60v on the Variac's scale. So about 81 degrees total movement possible over the 0 to 270v scale. Ideally you would aim for 90 degrees.

The trace of the trigger pulse is a bit jittery on the scope due to the fact that the photo electric cell and motor are vibrating on a wooden floor.
The capacitor value, which is quiet critical, for the 0.5 HP motor I used, was 34uF. This gives a 15v resonant rise at one point (around 120v mark on the Variac scale). John Freau recommends less resonant rise than this, but my mains voltage is 235v and the motor is 250v, so no harm can occur.