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Re: [TCML] vfd spikes and high current draw

I have a 3 phase 208v synchronous motor that starts up in the same
position every time. To sync it to the ac line as well, I had to build
my own vfd since nobody makes one with an external sync input. I used a
microcontroller connected to a 3 phase power driver board. I built a
daughter board that sits on the mcu to handle input and output. On it I
built a high precision zero crossing detector that makes a 30 usec
pulse. The mcu software homes in on and locks onto the ac line which can
be anywhere from 59.5 to 60.5 hz. When it has locked on I can push a
button and 6 pwm signals go out through optocouplers to the driver board
and the motor spins.

I built an led strobe that fires on the peak of the ac line and it shows
me that my disk is firmly locked in sync. The best part is a little
switch that allows me to shift the phase 1 degree of rotation at a time,
either clockwise or counterclockwise while still maintaining lock. The 3
phase power signals exit the driver board and go through a shielded
cable into my resonator box. All of this works great. But when I put on
a high voltage transformer, even a small 7.5kv, the 3-phase driver board
shuts off and the motor dies. When I kill the transformer, the motor
comes back up again. The status lights and switches on the mcu continue
as usual so I know that it's unaffected.

As an alternative to running the motor and spinning the disk I can
rotate it into electrode alignment and run it as a static spark gap with
a small fan for quenching. In fact I have some videos from last night!
See here:


I get 4 feet of sparks while running in this crazy motor stationary mode
but the whole rig pulls 30 amps. So I want to add some tvs diodes to
protect the driver board. Thank you for your time and attention!

On 15-07-24 07:56 AM, Phil wrote:
> Hello,
> You mention that the motor is synchronous (SRSG) but it's fed by a
> three-phase line. As the unit is a VFD are you varying the frequency and
> trying to match the motor rpm speed to the mains frequency to get
> synchronous operation that way? If so how are you varying the firing point
> as the John Fraeu phase controller is based on single phase working?
> On the issue itself, I would look at adding Ferrite cores on the leads, and
> the VFD in a shielded box, or better still getting rid of the VFD and just
> have a normal single phase motor modified to synchronous.
> I blew a switched-off VFD on a lathe once when the unit was inside a shed
> around 8 feet away from a 4 inch coil that had a good local RF earth running
> outside. That was solely because I left the plug in (UK plugs have an earth
> connection from the house earth) so I think that VFD's are always a
> liability when around coils.
> Depending on the bps used the current draw should be lower, as 120 or 240
> bps operation should give a better PF than a static can.
> Regards
> Phil Tuck
> www.hvtesla.com
> -----Original Message-----
> From: Tesla [mailto:tesla-bounces@xxxxxxxxxx] On Behalf Of teslafirma
> Sent: 23 July 2015 12:57
> To: tesla@xxxxxxxxxx
> Subject: [TCML] vfd spikes and high current draw
> I have finally got my tesla coil working and now I can start to
> optimize it. My biggest problem is that the 3 phase power line that 
> feeds
> my srsg motor picks up transients and shuts down the 3 phase driver
> board. I used a shielded cable but it's still too noisy. When I measure
> between the 3 legs my meter shows an uncanny 120.0 volts each. So my
> plan is to multiply 120 by root 2 which is about 170, buy 6 tvs diodes
> at slightly more, say 172 volts, and wire them up with a load - a small
> light bulb. This way, when spikes threaten my driver board, the diodes
> will vent them through the light bulb and I can see it flashing. I plan 
> to do this on both sides of the wire, next to the coil and inside my 
> control box. Does
> that sound reasonable?
> I can still run my coil without the spark gap motor by turning
> the disk to orient the electrodes into firing position and running it as
> a static spark gap. I put a small fan for quenching on one side and I
> get 4 foot sparks from my pair of 15kv/60ma nst's. The problem there is
> that I'm pulling about 31 amps and that's not sustainable. My
> transformers have 200 uf of power factor correcting caps and an rf 
> filter. If I
> clean up the motor line enough to spin the disk I'm hoping that I'll
> pull a lot less current. Any advice will be greatly appreciated!
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