Rotoray spark gap motor power

[Patric]

Hi,
here's a newbie question: all of these RSG synchronous motors and also those converted induction motors, they seem to be immensely oversized for the job they actually need to do. I mean all they do is turn a (considerably heavy) disk. Ok, accelerating it to ops speeds does take some torque, but once it's spinning, there should be hardly any torque needed right? So why are we seeing > 1kW monsters wheeling around here?  Is it in fact the necessary starting torque to sync up with mains frequency? Scratching my head on that one.
Cheers, Patric

[the_anomaly]

I built a rotary spark gap years ago with a small DC motor.  I liked being able to control the frequency with a variac.  I think it has to do with availability; 1-2hp motors are very common and cheap to buy used.

[Mads Barnkob]

With a rotating disc with a few studs there is not much load to drive, unless there is some spark gap forces I am not aware of that influences the rotation force.

But as the disc or arms get bigger, you introduce a great deal of air resistance and bigger motors are needed. F.ex. check out the size of the RSG from Bill Wysocks 13M system: https://teslauniverse.com/tesla-news/galleries/bill-wysocks-model-13m

[profdc9]

I used a $15 angle grinder for an asynchronous rotary spark gap and controlled its speed with a triac (like a lamp dimmer but higher current).  These spin at very high RPMs and don't have a lot of torque, because angle grinders cut with speed not force.  I was using a phenolic disc with screw heads which I think is probably around the same moment of inertia than a stone cutting disc.  Many synchronous motors are designed for heavier duty loads, with maybe the exception of timing motors, as brushed motors or shaded pole motors tend to be used for lighter loads.

Dan

Hi,
here's a newbie question: all of these RSG synchronous motors and also those converted induction motors, they seem to be immensely oversized for the job they actually need to do. I mean all they do is turn a (considerably heavy) disk. Ok, accelerating it to ops speeds does take some torque, but once it's spinning, there should be hardly any torque needed right? So why are we seeing > 1kW monsters wheeling around here?  Is it in fact the necessary starting torque to sync up with mains frequency? Scratching my head on that one.
Cheers, Patric

[MRMILSTAR]

My SRSG uses a modified 1/2 HP 3600 RPM induction motor. The rotor is a 10" diameter 1/2" thick piece of G10. It has 4 flying tungsten electrodes. It comes up to speed in a few seconds. At speed, it moves a LOT of air. It is just about the right power. If you use an under-powered motor, it will not stay synchronous.