PTS Depression Controller

[Tony]

Bruce,
The current in any inductive circuit tries to keep flowing when the circuit is broken.
That is exactly how the initial arc is struck with an an arc welder.
A capacitor connected across an opening contact can reduce this arcing, as is commonly done with the ignition points in a distributor.

With higher voltages, it is also usual to fit a low value resistor in series with this capacitor to help reduce sparking on contact closure.

Choosing a suitable snubber is not an exact science, but for what we are doing, something fairly large, say 1uF with the lowest available value series resistor, that might be maybe 22 ohms to 47 ohms. This will probably be the largest size snubber available. This should be wired directly across the relay contact.

That should make a noticeable improvement.

Another thing you might like to try is connecting a Metal Oxide Varistor (mov) directly across the wires coming out of EACH motor. These movs stay open circuit at normal mains voltages, but will absorb any high voltages produced by the motor when the relay contacts open.

For the 110 volt American system, a mov rated at say 140 volts might be appropriate. The exact voltage rating is not that important. For 230 volt systems, a mov rated at 270 volts might be about right.
Again, get the largest physical size available, which will probably be about an inch in diameter. These will quite likely get quite hot each time the relay opens, so it needs to be large enough not to burn up.

If you have problems with your movs overheating, connect more than one in parallel.

The RC snubber goes directly across the relay contacts, the mov directly across each motor. Both can be used together. This is not an exact science, a bit of experimentation is in order here.

Connecting multiple snubber capacitors across the same relay contact will reduce arcing even further. But it will then probably cause a big fat "splat" as the contact closes. The optimum snubber is a compromise between the violence of contact opening and the violence of contact closing.

Pablo,
The incoming heavy mains wiring into the bench, just goes through the SCR power block to all the motors connected in parallel. No relays required.
An individual fuse at each motor, rated very close to the measured full load current of each motor might be a good idea. That way, a motor that commits suicide will blow it's own fuse without hopefully destroying your SCR power block. These SCRs are really tough animals, and can handle massive short term overloads. But carefully rated individual motor fuses are still something to think about.

The incoming mains into the bench, also goes to the plug pack for the PID controller, via your master motor on/off switch.

The reason for switching the incoming power to the PID controller on and off, is so that the PID controller internal "soft startup" feature can gradually accelerates all the motors up to speed each time the ower to the controller is turned on.

So basically you leave the PID test pressure setting where it is, and just switch off the power to the PID controller. All the motors will stop. Turn the power back on, and the motors will gently accelerate back up to the exact set test pressure. This slow acceleration startup feature avoids any massive power surge and inrush current. That enables the full available mains current to be used without fuses blowing or breakers tripping at power turn on, with all the motors connected together.

If your bench draws maybe 80 Amps flat out, you could fit one large 100 amp SCR power block to run all the motors combined.
Another way to do it would be to use two 50 amp SCR power blocks, with each running half the motors.
The dc control voltage from the PID controller then goes to the inputs of both these SCR power blocks.
All the motors will then go up and down in speed together.

One advantage of using more than one SCR power block is that they can be run off separate phases where three phase power is available.
In Australia and Europe, three thirty amp SCR power blocks running from three phases would be far more desirable than trying to draw 90 amps from only one phase.
That will be of absolutely no interest to Americans, the whole US power distribution system is quite unique, very different to ours.
But in other countries around the world, spreading the load across two or three phases becomes an absolute necessity for a really powerful bench.

[pablo]

Tony,
Thanks for the great info. on the PID wiring.
I think I'll try the RC snubber first on my relay contacts. Just need to check out some P/N's and pricing.