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Registered Member #3824
Joined: Sun Apr 10 2011, 08:29PM
Location: Edmonton, Alberta, Canada
Posts: 54
Hey everyone, I am about to run the first test of my rotary spark gap and I just have a question regarding starting it. It is one of those 1/4hp marathon motors. Coupling the disk to the shaft was a pain in the ass but I think its on there (unknown how much force it can take). Anyway, I have a variac that I'm planning on using to start it, but I am having some thoughts on what kind of inrush I'll be expecting.
If I start my motor form a variac and slowly ramp the voltage up, will I be seeing rather large currents when the voltage is that low? I know the speed of the motor depends solely on frequency, but I want to limit the inrush somehow (obviously not using any variation of a VSD).
Is it better to be starting the motor directly from the mains line through a switch, or through my variac? The load on the rotor is essentially zero (just my polycarbonate disk with brass electrodes).
Registered Member #3902
Joined: Fri May 20 2011, 08:55PM
Location: Burlington,Wisconsin and Springhill, Florida
Posts: 2
Im not familiar with that motor, if its ac induction you can ramp it up with a variac to your line voltage. Polycarbonate is not the best route for disk material. G-10 phenolic would be better. I would also use some kind of saftey shield. AC/DC universal motors from Dayton work good for Async spark gaps. 120volts to the Variac then run a full bridge rectifier and you can vary your breakrate with the Dayton.
Registered Member #3824
Joined: Sun Apr 10 2011, 08:29PM
Location: Edmonton, Alberta, Canada
Posts: 54
well I have already fashioned my disk of acrylic / polycarbonate (not exactly sure the chemical compound). I am hoping this will not blow to pieces when I turn it on! Oh well. I think if this coil fails to work very good I will just turn to SSTCing.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Based on some google searches and a very informative pdf, your motor will probably pull around 37.5-50 amps. According to Marathon electric's website, your motor pulls 5 amps at full load and is a split phase motor, and this pdf says that typical starting current is 750-1000% normal current, and starting torque is 250-350% normal torque.
A bit off topic, but it is interesting that shaded pole motors like in microwaves are typically below 20% efficient! Might as well use a tiny internal combustion engine (not really).
Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
isn't this the sort of thing that ac motor start capacitors are for? such as those in washing machines and ceiling fans. i think you could also use a big ballast inductor to limit the initial current inrush.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Forty wrote ...
isn't this the sort of thing that ac motor start capacitors are for? such as those in washing machines and ceiling fans. i think you could also use a big ballast inductor to limit the initial current inrush.
Well a split phase motor does use a start capacitor. The capacitor gives the start coil a pulse whose magnetic field is a different phase angle than the main winding, which makes the rotor rotate. An AC induction motor with no start winding can be energized, but without the initial kick the start winding provides, the rotor may not rotate and just sit there and vibrate. Especially if there is a load.
You could use something like to bypass a limiting inductor or resistor. You could replace the opto isolator with a relay (be sure to use a freewheeling diode across the coil of the relay).
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arsg motor starting.
