RPC question for large loads

What kind/capacity of electrical service does he have?

200 amp 1 phase 240 volt. standard residential. Just like mine.

karl

Karl, your plan should work very well, it is easy to do also if you have the electrical gear necessary, such as contactors.

i

You have more confidence than I. My RPC is essentially the same and the lathe can't be started in the top four gears without blowing a fuse. The idea of bringing in more caps during extreme load has me intrigued.

karl

That's going to be a huge load to place on a residential service. I know you aren't going to be running 50 hp at full load but even 20 times let's say

800 watts at 240 is over 66 amps? I'd be checking with an amprobe and making sure all the connections are perfect and don't get hot. Not impossible but a little unusual so proceed with caution, especially if there are subpanels and long runs involved. I've seen a fair amount of large stab-on breakers heat up and burn the stab on the bus.

You can't switch additional capacitors into an existing cap bank without massive explosions. The power company guys have some kind of special relays that bring in the additional caps without directly connecting them together, so there are no huge currents between them. If you use standard contactors, there will be huge sparks when the contacts close, as the capacitors will be charged to different voltages at that instant, and so currents of thousands of amps (I'm not kidding, here) will flow for some microseconds. It may be possible to control this with small resistances of a few Ohms in series with the cap bank. The resistors will hinder the functioning of the capacitors, though.

Running a 20 Hp idler motor PLUS a 20 Hp spindle motor off a 200 A residential service sounds like big trouble. The power draw assuming perfect power factor isn't that huge, but idler motors tend to have TERRIBLE power factor. A PF correction capacitor bank will help a lot. But, the current demand when the motors start is going to be horrendous. The full load current on 3-phase 240 V for a 20 Hp motor is about 66 A. I don't know what the line current of an RPC plus a 20 Hp motor is on 240 single-phase, but it is going to be a lot higher, possibly over 100 A. I would think it makes a lot more sense to get a VFD, then you get an adjustable amount of slow-start, and can keep from dimming all the lights in the neighborhood. With all these idler motors, cap banks, relays and control circuits, you could likely have paid for a good used VFD by now.

Jon

What about zero crossing solid state relays?

Just to give you guys a data point. I have a RPC with two idlers, 10 and 7.5 HP.

When I start the 10 HP one, it takes somewhat under a second, and the current draw is 120 amps. It does not dim lights in my house. Starting the 7.5 HP idler is near instantaneous, "pop" and it is running.

They have soft starts on ebay pretty cheaply, this may be the ticket for Karl if he can get spindle-at-speed signal work correctly.

If Karl's spindle motor is a regular motor, maybe he can get a smaller one.

Mazak should tell Karl what wiring etc is needed to hook this up, so we can see if the statring current is manageable. I would call Mazak.

i

I agree.

Gunner, running a Clausing 1500, with 7.5hp motor just hunky on a 5hp rotary converter...as long as he starts the motor FIRST, before engaging the spindle.

I am the Sword of my Family and the Shield of my Nation. If sent, I will crush everything you have built, burn everything you love, and kill every one of you. (Hebrew quote)

??? I'm doing it now. My existing converter starts a 15 with run caps, drops the start caps, then engages a 25 and a second bank of run caps. hard to believe its impossible to switch in say 500 MFD of additional capacitance on a running converter. But, I'm not the expert here.

The system I'm planning is for an entire shop of three phase equipment. I'm wanting to build the best possible system for many years of use. A vfd might bve the best solution for the large lathe but they are very expensive and the lathe works now, it just could be better. Surely some incremental improvements are possible over the one that's running today.

Karl

I happen to have an 80 hp..yes...80 horsepower VFD at my disposal..if anyone needs one.

It will be cheap too.

Gunner

I am the Sword of my Family and the Shield of my Nation. If sent, I will crush everything you have built, burn everything you love, and kill every one of you. (Hebrew quote)

I spent some time looking. Can't seem to find 20hp. and 220 volt. They are all for 440 in the higher horses. I did find a few rated 20hp at

440 and 10hp at 220. So, I looked at 40 hp. units. only a couple but no mention of multivoltage and they are up in VFD price range.

I'll watch a while, do you know if all soft starts work on both 440 and 220?

I'm not sure soft starts or VFDs are the answer anyway. You drive this lathe like an old truck. The manual says start in LO range (1-8) then shift to HI(9-16). This takes to much load so we shift four gears at a time till gear twelve then two at a time.

Anyway, the main subject here is how to build the best possible RPC for the kid's new shop. The lathe is just the worst example.

karl

Karl

if its 220 volt, i need it. But me thinks no way is 80 hp 220 volt.

karl

Im fairly certain it is indeed 220 volts. Ill check later in the day when I wake up.

Ive been out tuning and tweaking the Clausing lathe I got running yesterday. 14x52"

Im rather surprised that the 7.5hp motor runs nicely on the 5hp rotary converter Ive got powering the shop. I just cant start the motor AND the spindle at the same time. I have to fire up the drive motor, then use the clutch to fire up the spindle. Which works for me. Carraige lock bolt is busted down in the apron..got it out of the lock assembly..now I just have to fish the stub out of the hole. Cant find any of the damned .25 magnets I used to be swimming in. Shrug...Ill be up at noon-2pm and will fish it out, and use the lathe to turn a new bolt, and the new mill(s) to machine the head.

Crom its nice having decent machines finally.

Gunner

I am the Sword of my Family and the Shield of my Nation. If sent, I will crush everything you have built, burn everything you love, and kill every one of you. (Hebrew quote)

That might be the trick! Of course, if it ever fails to turn on right at the zero crossing, the relay will be history! But, that ought to work better than a big contactor, which would have a really short life.

Jon

If these caps are wired to the generated leg of each idler, that may be different. Generally, though, connecting banks of capacitors together, if the voltage in them is not the same, leads to HUGE currents, in the thousands of amps, and will severely damage a beefy contactor even the first first time it is switched on. Putting sufficient resistance in the circuit will reduce that peak current, so maybe that is why it works for you.

Jon