10hp RPC Amp readings

I just got done building my 10hp rpc and going through the balance procedure using fitchs document. I basically built this guy to run 2 different pieces of equipment. 1 is a makino rmc55 cnc mill. This mill has all power going through a transformer on the side of the machine and outputting 200v. The 2nd is a 36v forklift battery charger. The nameplate on the forklift charger is 240 at 32 amps

My main question is how do i know if am pulling too many amps through the phase converter since im not really driving motors direct? Do i only care about final amps being pulled through my 3 phase panel or do i need to watch the amps on the 3 wires coming from the pony motor?

Motor is a baldor 10hp. Tag says 240 and 28 amps. Is that 28 amps what i cant go over?

These are numbers idle and with the battery charger running

While the battery charger was running i took amp readings on the 3 wires coming from pony motor

1 was 14
1 was 14.5
1 was 31

I guess im just not sure what i need to watch out for

When i turn the cnc machine on i get almost no amp draw. Im thinking because its going through the transformer?

Thanks in advance. Im just confused.

Yeah the rms sum of the amps through each of the 3 motor coils is going to heat the motor.

You may be able to exceed the nameplate amps on one of the coils by 25% (it would be producing 50% more heat than it is designed for) if the rms sum of the three is less than nameplate.

Remember.... an RPC really only supplies ONE THIRD of the total power to the load motor.

The "output" of the RPC is the current/power on the generated leg. The other two legs going to the loads do not go through the RPC, they are a direct connection.

The input power to the RPC is only what the idler needs in order to supply the generated leg power plus losses.

With the output being 1/3, the input current is going to be around 60% to 70% of normal full load current. That means you can substantially exceed the rated current on the generated leg as long as you can stand the voltage drop that results.

The 25% overcurrent that Johansen suggests seems reasonable. That would bring the 28A up to over the 32 amps that you quoted for one of the loads.

The way to tell is to monitor the actual idler motor temperature. It will FEEL really hot even if it is basically loafing, so you really want numbers for the temperature if you measure it. if an accessible part of the winding is no higher than around 80 to 90 C, you should be OK for nearly any motor used as an idler. Most tempp classes are good for 110C hot spot, and hot spot is commonly reckoned to be 10C over surface temperatures.

If you measure winding temps, be careful.... I would assume the thermocouple or whatever might be hot with AC volts, that is a conservative assumption.

OK i think i understand what you guys are saying. I will keep an eye on the temp of the idle motor. Can i use an inferred temp gun on the outside of the case?

So as long as that 3rd generated leg doesnt get over the 28amp nameplate (or possible 25% over if i want to keep an eye on temp) I will be ok. So in my picture it shows with the battery charger running pulling about 20 amps on the 3rd generated leg so im actually under the nameplate and in the ok.

Because i am running 2 different pieces of equipment with very different loads what would you try to balance to? With the battery charger running it seems the 207v is a little low. But im worried about the 250 something going into the older 230 input of the mill transformer

The case temp is only "sorta representative".

You can use an IR gun, but see if you can get a reading on windings, and pick the highest one. The usual allowance for hot spot has been 10C higher than what the surface of the windings shows, with an IR gun, I;d allow a bit more, maybe 15C. The IR gun reading depends on the way the surface emits heat, and could easily read low.

alienturtle said:

I just got done building my 10hp rpc and going through the balance procedure using fitchs document. I basically built this guy to run 2 different pieces of equipment. 1 is a makino rmc55 cnc mill. This mill has all power going through a transformer on the side of the machine and outputting 200v. The 2nd is a 36v forklift battery charger. The nameplate on the forklift charger is 240 at 32 amps

My main question is how do i know if am pulling too many amps through the phase converter since im not really driving motors direct? Do i only care about final amps being pulled through my 3 phase panel or do i need to watch the amps on the 3 wires coming from the pony motor?

Motor is a baldor 10hp. Tag says 240 and 28 amps. Is that 28 amps what i cant go over?

These are numbers idle and with the battery charger running. When not just sitting on their virtual ass awaiting orders, anyway.

While the battery charger was running i took amp readings on the 3 wires coming from pony motor

1 was 14
1 was 14.5
1 was 31

I guess im just not sure what i need to watch out for

When i turn the cnc machine on i get almost no amp draw. Im thinking because its going through the transformer?

Thanks in advance. Im just confused.

Click to expand...

Don't make a problem out of a solution.

When you "turn on" a CNC critter, that usually means its control electronics are alive, reading position sensors and WAITING for code to tell it to run the spindle, positioning servos, any toolchanger or live tooling. The control electronics may not need any more power than a basic Pee Cee, so no - no real load until you have it making chips.

As to "which" to balance for? Your battery charger is very likely the most forgiving of the lot. It shouldn't need "perfect" balance. CNC goods are generally far pickier - once up off their virtual ass and hard at work, anyway.

2CW

"While the battery charger was running i took amp readings on the 3 wires coming from pony motor"

What instrument did you use to take those readings?

An amp clamp is uniquely unsuitable for this purpose as it cannot differentiate between in phase and
out of phase currents.

Unloaded rotary converters measured by amp-clamp meters are notorious for generating posts like this.
I suggest you also measure the three voltages A-B, B-C and C-A and let us know how well this balances.

jim rozen said:

"While the battery charger was running i took amp readings on the 3 wires coming from pony motor"

What instrument did you use to take those readings?

An amp clamp is uniquely unsuitable for this purpose as it cannot differentiate between in phase and
out of phase currents.

Unloaded rotary converters measured by amp-clamp meters are notorious for generating posts like this.
I suggest you also measure the three voltages A-B, B-C and C-A and let us know how well this balances.

Click to expand...

Heating in motors is primarily dependent on amperage and rotor slip. So while it is possible to have entirely reactive power causing winding losses and no rotor losses, this isn't the case with an RPC since current is induced from the windings into the rotor, and back from the rotor into another winding. So current in this case, regardless of whether it is real or not, is a decent indicator of how hard you are being on the motor. Not perfect, but decent.

I second the notion of reporting the phase voltages.

jim rozen said:

"While the battery charger was running i took amp readings on the 3 wires coming from pony motor"

What instrument did you use to take those readings?

Click to expand...

His panel meters look identical to some I have here. My ones use the classical resistive shunts, external mount, not in the meter.

But "pony motor" - if indeed he even HAS one is wrong. Idler, we hope?

Excuse my terminology 10hp idler

the voltages you see in the picture are between legs L1-L2, L1-L3, and L2-L3. I also checked them with a DVM.

The amps shows on the meters are the round resistive shunts. I also use an amp clamp to take other readings as well.

alienturtle said:

Excuse my terminology 10hp idler

the voltages you see in the picture are between legs L1-L2, L1-L3, and L2-L3. I also checked them with a DVM.

The amps shows on the meters are the round resistive shunts. I also use an amp clamp to take other readings as well.

Click to expand...

Those are close enough for Daily drivers to give you warning, then. The DVM for verification was wise.

The only thing makes any sense to me is that the CNC was, as previously said, "standing by" awaiting orders to motate spindle and servos, not in the cut, nor even being rapid'ed about.

Not sure what the conditions were for the pics. One seems to be no load, it's the other I am not sure of.

The high draw on the one output for the charger may be because that output had the highest voltage. Typical for a rectified load into a capacitor filter....which the charger, if it is a newer one, may have. The highest voltage charges the filter up higher and reduces the currents for the other voltages.

Really old style chargers have no filter, but the highest voltage may still produce the highest charging current. But the difference is usually less between the highest and lowest. Any control circuitry in a somewhat more recent unit will modify that current, and can distort the currents to where the currents are not as expected.

JST said:

Not sure what the conditions were for the pics. One seems to be no load, it's the other I am not sure of.

The high draw on the one output for the charger may be because that output had the highest voltage. [...]

Click to expand...

I thought the whole matter was relatively clear but I assumed his statement: "These are numbers idle and with the battery charger running" referred to the two photos uploaded.

so I inferred that the rpc was supplying 17 amps to the battery charger from the generated leg, but the actual amps through the motor was 31.

OP, how much capacitance do you have connected?

Johansen that is exactly correct. These numbers were taken when the battery charger was on and the battery was almost dead according to the forklift. If it matters over the course of the 7 hr charge the amps slowly began to go down and volts back up. Right before the charge completed i was L1, L2 6 amps. L2, L3 4 amps. L1, L3 6 amps

L1 to L3 150mfd
L2 to L3 90mfd
L1 to L2 35mfd (power factor)
432 to 518mfd start caps

All in all the battery charger works and did charge the giant forklift battery. I just want to make sure im not doing any long term damage.

The mill was basically idle so once i get that back together and making chips i can see what it does

I used my temp gun and got about 115 degrees F on the outer case. This was near the end of the forklift charge

OK, I was not understanding where the 31 amps was measured, and was not seeing it on the meters If it was measured at the INPUT to the motor, that is different from what I thought I read (I thought he had one OUTPUT to the charger at 31 A, which is actually possible.