[thingstodo]

Another search of my junk pile results in a larger power supply ... a 24V charger, actually, that is rated for 7 amps at 25.2 VDC output. A quick check at the output connector identifies which pins on the connector are + and -, that there is 20.5VDC coming out, that the on/off switch works ... so far so good.

Use an alligator cord to connect the + on the charger to one side of the grinder's 120V plug and - on the power supply to the other side of the 120V plug. I know that the grinder turns at 10000 rpm from the label on the side. That's faster than the 120 V frequency of 3600 rpm, so I know that the grinder takes the 120 VAC and converts it to DC. And that the motor is actually a DC motor. So it's not a big deal which connection has + and which has -.

Turn on the charger and the grinder spins up. So far so good. The current meter on the charger slowly falls to about 1 amp when the grinder is at a steady state. After turning off the grinder, I add some electrical tape and a small reflector to the grinder disc to make a target. That way, I can use my optical (light) tachometer to check what the speed ends up being. It turns out, the no-load speed is about 3300 rpm.

My clampon meter, set for DC amps, and set to record the peak, shows a peak of 5.5 amps immediately when the charger is turns on. The current stabilizes at between 1.2 and 1.3 amps when the motor is up to speed.

This is very good news. I have the full range of the encoder available, since the encoder card can only keep track of .. was it 2929 rpm? When driving the encoder, I'll be using most of the available DC output of the PLC. The inertia of the grinder motor, gearbox, and rotating disc is likely high enough to average out the DC pulses from the PLC output and give a fairly stable motor speed. That's the theory, I just need to test that.

Next step, connect the charger through the PLC output and see how the averaging of the pulses works out with the grinder speed. The power fed to the PLC output card looks good. The output voltage makes my digital meter bounce a bit, but 4.5 - 4.8 VDC covers most of the values I see. The low is 3.7 and the high was above 5.

At this point, another alligator clip connects the common and the switched PLC output to the grinder. It appears to run OK. After a few seconds, the charger starts to sound bad, like there is a momentary short circuit across the transformer ... the noise that you hear when a welder strikes an arc, shorting out the secondary of the welding transformer. This would be BAD.

Disconnect everything and check out the power supply by itself. It does the same noise, repetitively (not constantly) with no load connected. The sound occurs quickly after power-up and for longer the first time after it has cooled off for a while. The output voltage drops to 0VDC (when I add wires to connect up the meter) and then bounces back up to around 6 VDC. It REALLY sounds BAD. The sound of the transformer 'shorting' appears to happen every 5 - 10 seconds. I think that there may be a self-resetting fuse (like a circuit breaker that resets when it cools off, after a huge amount of current likely goes through it) ... I guess it's time to take apart the charger. The thing about my junk pile ... garage sale stuff, auctions both online and in person ... some of it works for a while, some of it was sold for a reason ... and some of it is pristine and works great.

So no answer on whether the grinder rpm is stable with the PLC output pulses ... until I find another power supply.