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Re: [TCML] **External Email** Re: DC motor fuse/rectifier issue

Much snippage.. but here's some thoughts:
1) The motor is pretty tough for transients in the hundreds of volt range - Most consumer equipment sees a "hi-pot" test in the several kV range. So even if there's a transient on the power wires, it won't short to ground. And if it does, it won't kill the motor, it just punches a tiny hole in the insulation, but since the motor is running at 100V or so, air makes a fine insulator.

2) The diode bridge, on the other hand, will be instantly killed by 1-2kV transients.

What you might have is a common mode voltage problem - that is, both wires to the rectifier and motor are raised above "ground". As long as there's nothing "ground" near the rectifier, the rectifier doesn't see kilovots across it. Ground the rectifier (i.e. bolt it to a grounded heat sink) and all of a sudden, it cooks.

The motor can tolerate multi-kV common mode voltage spikes - either it punches through the winding insulation or it doesn't.

Any sort of varistor or ZnO transient suppressor will eventually die in this sort of application - each time they "clip" they die a little, and their leakage current increases - either it winds up leaking enough and blowing a fuse, or it melts and catches fire.

A semiconductor transient suppressor (back to back zeners) will clip and recover, but the power dissipation might still cook it - you don't have a lot of current, but you do have the clip voltage (400V) across it, so even 1 mA is 0.4 W. If the transients are short in duration, you're ok.

EMI filters might work (often do work) - you need to decide if it's a common mode (line/neutral to ground) or a differential mode (line to neutral). Since your bridge doesn't blow when it's hanging free, I'm going to guess common mode. And anyway, if the wires from Variac, to bridge, to motor are close together, there's not much "differential model" coupling possible. Plenty of scope for common mode coupling though. What you look for is the "loop area" that's catching the transients. And getting a "big loop" for common mode is easy in a TC. If your "ground wire" goes via a different path than the "power wires", there's your loop.

On 7/29/20 8:05 AM, William Noble wrote:
The electrolytic has too much impedance at higher frequencies, it does nothing.  You need to shield the cables, including the ground, and ground the shield at both ends.  At the driving end, you need an RLC filter that works from about 1/2 your coil freq to at least 10X the coil freq.  I'd use ferrite beads, twist the lead

----- Original Message -----
From: "Terry Oxandale" <toxandale@xxxxxxxxxxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Tuesday, July 28, 2020 9:31:59 AM
Subject: Re: [TCML] **External Email** Re:  DC motor fuse/rectifier

Thanks Steve. Do you have any suggestions on the filter? I was kinda headed in that direction anyway, but not sure what to look for.
     On Tuesday, July 28, 2020, 09:15:12 AM CDT, Steve White <steve.white1@xxxxxxxxx> wrote:

I use a modified induction motor for synchronous operation using AC so its not exactly like your setup. I have done several things to protect the motor.

1. Independent ground for motor case and shield around power cable. Motor case and shield are tied together.
2. I added a line filter at the motor mounting point under the Tesla coil base.
3. I added another line filter on the motor power cable back at the control cabinet.
4. I added a varistor to the motor power cable at the control cabinet end.

I've been operating this coil at 6.5 KVA for about 4 years now with no problem.

Steve White
Cedar Rapids, Iowa

----- Original Message -----


My ARSG control assembly is set up pretty much the way you’ve described yours, and I had that very same issue with my coil when I grounded (mains, not RF) the outer metal casing of my FWB rectifier (also rated @ 1kv/50 amps). My FWB rectifier worked perfectly fine in this arrangement when only driving the motor, as long as the coil wasn’t ‘sparking’. However, once I turned the voltage feed variac to my transformer up enough to break down the spark gap and initiate sparks from the top load, the FWB rectifier shorted out almost immediately! I believe I let the magic smoke out of two or three of them in this manor! Once I replaced the blown out FWB rectifier with a fresh one AND removed the ground from its outer casing and just allowed it to ‘float’, it worked flawlessly throughout coil’s active sparking range of power throughput (and the voltage input range for the motor) and I had no more problems.


Sent from my iPhone

On Jul 27, 2020, at 6:54 PM, shaun <snoggle@xxxxxxxxxxxxxx> wrote:

Does the motor case and variac share the same ground.

I'm using a permanent magnet DC motor for my ARSG. Power is through a
variac, to a diode bridge, and then out to the motor under the coil.
The issue is that I can't keep the fuses protecting this circuit, nor
rectifier, intact. I use a 10 amp fuse at the control cabinet for
protection (just off the 120vac hot leg of a 240vac source), and the
DC supply cable for the motor is surrounded by grounded braided
copper. I have an ammeter to monitor the motor, which indicates
proper draw during runs (2-3 amps depending on speed), yet the cheap
(50 amp/1000 v) gets warm fast, and also does not endure an extended
run on the coil. A while back I created a filter of sort to smooth
the full-wave pulses (cap/reactor), but that made things worse, so
that was removed.  I don't have a sufficient understanding of the
induced currents associated with these coils, and hoping someone here
can help. I have yet to determine if a failed bridge is the cause of
the fuse blowing, so I may insert some fuses at different points of
the circuit to find out. My assumption is the rectifier (very cheap
item) could be at fault, but there is a lot of knowledge I lack about
what impacts of these coils on surrounding/supporting circuits. Any
thoughts on what can do to resolve this?
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