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Re: [TCML] vfd spikes and high current draw

You are suffering, as you know, from EMI/EMC problems - the means to control
the interference are well known, but very tedious.  Put a shield around the
motor and another overbraid over the whole cable - make sure that there are
no entry points for the RFI - you will probably need a continuous conductive
gasket, you may need a grounding brush on the motor shaft, you will need the
smallest possible clearance between the motor shaft and the enclosure, a
non-conductive connection into the spark gap box, and probably another
separately grounded shielded enclosure around the spark gap.  You may find
that driving the motor drive optically helps, then you only need to worry
about conduction through the power cable and the motor shaft, and the power
cable can be filtered with commercial or home brew EMI filters (which you
must enclose in a separate shielded enclosure soldered otherwise sealed to

There are whole books on EMI and EMC prevention and shielding, there are MIL
standards for the levels - it will just be a lot of tedious work to find and
suppress or eliminate all the ingress points.  Try the "cookbook" methods
first, and you may be done.

-----Original Message-----
From: Tesla [mailto:tesla-bounces@xxxxxxxxxx] On Behalf Of David Boyle
Sent: Saturday, July 25, 2015 7:03 AM
To: tesla@xxxxxxxxxx
Subject: 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

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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