Phase Converter Sizing - Is there any reason a 20hp won't work for this application?

I'm building a new shop. I will have a 10hp planer on the wood side and am currently shopping for a dust collector. One that I have found has a 7 1/2 hp motor. Assuming the motors start separately, is there any reason a 20hp converter wouldn't be large enough to run both the planer and dust collector. If it matters, I'm looking at either the American Rotary AD20 or AI20 for the shop.

I have the AD20 to run a 10 hp Okuma LS lathe. You want an RPC that is rated at about twice your load.

ezduzit said:

I have the AD20 to run a 10 hp Okuma LS lathe. You want an RPC that is rated at about twice your load.

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I understand this for a single motor load. This is how I ended up with sizing a 20hp converter. Where I get confused is with the second motor. One seller is recommending a 20hp and another seller recommends a 25hp converter.

The answer depends on how much load you are actually going to draw from the RPC via the first 10hp load; when the second load starts up.

In the case of a dust collector, does it actually draw 7.5 hp?
And your 10hp planer is it ever going to see 10hp load long enough to cause both motors to overheat and burn up due to low voltage on the third leg?

AndyF said:

...Where I get confused is with the second motor...

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To avoid becoming confused, call tech support at American Rotary. 888-743-6832
I will be extremely surprised if they tell you it is OK to run 17.5 hp on an AD20.

RPCs are rated in different ways. I have an ARCO, and they at least used to rate theirs with two ratings. They had a rated single motor size, that no connected motor should be larger than.

They also had a total connected load rating, which for "those" RPCs from "that" company, was 3x the single motor rating. So you could run 3 motors of the maximum single motor load, or any combination that added up the same.

Other makers, especially any that do not supply the idler, generally do not do that, and I have no idea what the breakdown is, what their largest single load is, vs their largest total. Call them and ask.

It is also true that connected motors, once running, can act as added idler capacity. This is only if they are not loaded, once they have a load on them, their capacity to assist drops rapidly.

The dust collector would be loaded most of the time*, but the machine may not be. So the dust collector would not be likely to help start the machine, but the reverse could be the case.

As for running the loads, if the 20 HP has any meaning, it should mean that the converter could run up to a total of 20 HP. But again, I do not know how they rate their stuff.

And, if they do not supply the idler, then THEY do not know what it will do either, because so much of the performance depends on the idler. After the converter is started, the idler is doing all the work, so IT controls what happens. If they do not supply it, relying on you to connect some random design of idler, then nobody really knows what the system will support.

So they will likely be very conservative, as they should be.

* The dust collector is loaded whenever it is moving air. If all the gates are shut, then it is nearly unloaded, because it is just whirling the same air, not moving it through the pipes..

JST said:

...As for running the loads, if the 20 HP has any meaning, it should mean that the converter could run up to a total of 20 HP...

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It does not.

ezduzit said:

It does not.

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I have spoke to the company made my static converter box I used to make RPC with. It is rated at 15 hp max single motor and I think 19hp combined motors. They sized it to match the 15hp idler motor I used. They are adament that it will run a 15hp motor, since that is what it is rated for. I called and asked this exact question recently when I got a "new" machine with a 15 hp drive motor. He did tell me that it probably needed to be running the idler motor to it, as a static only it might have trouble.

Rpc is rated to start what the nameplate says. Are you thinking of a vfd that is using single phase input to run 3 phase motor? Pretty sure that would need to be de-rated by half.

Rob F. said:

I have spoke to the company made my static converter box I used to make RPC with. It is rated at 15 hp max single motor and I think 19hp combined motors. They sized it to match the 15hp idler motor I used. They are adament that it will run a 15hp motor, since that is what it is rated for. I called and asked this exact question recently when I got a "new" machine with a 15 hp drive motor. He did tell me that it probably needed to be running the idler motor to it, as a static only it might have trouble.

Rpc is rated to start what the nameplate says. Are you thinking of a vfd that is using single phase input to run 3 phase motor? Pretty sure that would need to be de-rated by half.

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I was referring to the OP's AD20 by American Rotary, an exact copy of which I own.

AndyF said:

I understand this for a single motor load. This is how I ended up with sizing a 20hp converter. Where I get confused is with the second motor. One seller is recommending a 20hp and another seller recommends a 25hp converter.

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Your actual loads are all over the map, lathe most of all.

IF.. the 20 HP is OK, I'd just buy a 7.5 HP or 10 HP as a supplementary idler.

No more stating or control hardware than the 20 HP already has.

- supplementary idler, one contactor, two caps for it.

I don't EVEN use a 20. Ten starts more easily.

- 10 HP Pilot & Phase Craft starter/control

- 3 HP, 7.5 HP, 7.5 HP

The two 7.5 HP have different run cap balancing.

I do HAVE some 3/4 HP, 1 1/2 HP, 1 3/4 HP loads that the 3 HP will run, but otherwise it is a "balancer" to "trim" larger arrays as to generated leg stiffness, etc.

Four idlers, run as any combination or all of the above, 3 HP to 28 HP.

Just two is good.

My one is more to support experiments & R&D than for Daily Driver, so

"YMMV"

ezduzit said:

To avoid becoming confused, call tech support at American Rotary. 888-743-6832
I will be extremely surprised if they tell you it is OK to run 17.5 hp on an AD20.

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I seriously doubt a dust collector and planner will draw full load amps. I bet the dust collector won't even pull 50%.

Dualkit said:

I seriously doubt a dust collector and planner will draw full load amps. I bet the dust collector won't even pull 50%.

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Actually it is the dust collector which is constantly loaded, with the planer's load being variable.

Nevertheless, an easy phone call to the manufacturer will quickly clear up the OP's confusion.

A dust collector should be the hardest working motor in the shop. I run my 7.5 hp DC on a vfd so I can limit the RPC or Phase Perfect to 10-12 hp starting. Slow start on DC is a nice feature and the ability to adjust speed to maximize the cfm and stay within the FLA of the motor is also a benefit. Based on the way AR sizes their rpc I would guess that a 20 hp is generally OK but marginal. I'd verify with them for sure. Kay sizes their units similar to ARCO. A 10 hp Kay is about the same as a 20 hp AR. Dave

ezduzit said:

To avoid becoming confused, call tech support at American Rotary. 888-743-6832
I will be extremely surprised if they tell you it is OK to run 17.5 hp on an AD20.

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Consensus seemed to be that I should call American Rotary Tech Support. I did that this afternoon. Their recommendation is an ADX25 rather than the AD20 because the dust collector is considered a hard starting motor. This will be more than I originally planned on spending, but I'd rather spend a little more and get something that will work reliably than cobble something together. I've spent the last 40 years cobbling, and am at the point now where I want to just have things work rather than fart around trying to make them work.

Yes, they were conservative, but that's OK.

The planer should help start the dust collector, and would not be loaded without the collector operating, but that's OK.

Yes, the collector is a hard start, it has a lot of spinning mass so it is an inertial load and takes a longer time to come up to speed than most loads. So a heavier draw for longer.

It also helps if you start the collector with the gates closed and open them only when needed. I have about a 5 amp difference between open and closed. Dave

beckerkumm said:

A dust collector should be the hardest working motor in the shop. I run my 7.5 hp DC on a vfd so I can limit the RPC or Phase Perfect to 10-12 hp starting. Slow start on DC is a nice feature and the ability to adjust speed to maximize the cfm and stay within the FLA of the motor is also a benefit. Based on the way AR sizes their rpc I would guess that a 20 hp is generally OK but marginal. I'd verify with them for sure. Kay sizes their units similar to ARCO. A 10 hp Kay is about the same as a 20 hp AR. Dave

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Yup. As a fixed load, the motor/fan should be sized that it sits at roughly FLA when handling design airflow. There's little reason to oversize the motor except too-frequent starting; you're wasting money, chewing up service capacity (because of the extra reactive current), and it's running below peak efficiency.

On the other hand, if you are essentially using it as an extra idler with a really big flywheel attached, it probably works fine as long as you can start it.

If you're using a VFD, one option is to add a pressure sensor and maintain a constant vacuum on the dust extractor inlet, regardless of the flow/how many gates are open, and how dirty the filter is.

SomeoneSomewhere said:

Yup. As a fixed load, the motor/fan should be sized that it sits at roughly FLA when handling design airflow. There's little reason to oversize the motor except too-frequent starting; you're wasting money, chewing up service capacity (because of the extra reactive current), and it's running below peak efficiency.

On the other hand, if you are essentially using it as an extra idler with a really big flywheel attached, it probably works fine as long as you can start it.

If you're using a VFD, one option is to add a pressure sensor and maintain a constant vacuum on the dust extractor inlet, regardless of the flow/how many gates are open, and how dirty the filter is.

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Where and when a DC is that "central" a player, it should not even BE sharing the same RPC as machine tools.

If not a VFD (Dave's case, your self-adjusting additions..) THEN a dedicated RPC of its own. 7.5 HP is within bounds for a 1-P motor, too.

"If it isn't part of the solution, then it must be part of the problem." applies at-present. But it doesn't need to REMAIN that way.

Give it an RPC of its own.
Give the lathe "etc" THEIR one.
Starting and sizing problems become trivial.

Annnd with a bit more plotting and scheming? Each RPC becomes close to same capacity as a rapidly "re-assignable" unit. Uncluding being paralleled-up.

So EACH can BACK UP the other.. and you gain lessened risk of delay, "emergency" chaos... or of missing a deadline off the back of unplanned downtime.

More to it than just "how MUCH" capacity.

The "HOW" can be as important as the "much".