Calculating amperage draw?

[Surgo]

Until I build a shop, I'm limited to a good old residential 110v, 15-amp-limit circuit. Yuck! Assuming I want to run a 3-phase motor with VFD, I'm trying to figure out the strongest motor I can run is. The question is proving...a bit complicated.

Take this fairly basic motor for example:

 (This might be mislabeled, because the pl;ate on it says it's 2HP). The rated amperage is 2.69 at max frequency. Now if I go by P=IV then at 110v in, I'd be looking at 10.76 amps. But...there's no way it's that simple. The VFD is doing a more than just stepping the voltage up or down, so it could be drawing something larger and losing it through heat. And then there's inrush current to the motor consider; will a VFD cut that off or will that still go substantially higher than 15amp and trip the breaker?

I guess what I'm really asking -- is there a page somewhere that breaks down how to calculate what the best you can handle on such a line is? I feel like there's no way a simple proportional P=IV calculation would be correct here because I know I shouldn't be able to run a 2hp motor on this line!

[Glenn]

What size wire are you using? How long is the run of that size wire between the breaker box and the motor?

[Surgo]

Breaker box to outlet length is unknown right now (I might have the house's electrical diagram laying around somewhere...hopefully). I'm guessing that it's 14 gauge based on the shape of the outlet via this lovely diagram: 

https://fthmb.tqn.com/tMILNUH02SPqlR5U2C5EWWqWTlo=/960x0/filters:no_upscale()/1-outlets-56a4a2855f9b58b7d0d7eec5.jpg

So I'd probably use wire from a 14 gauge extension cord to complete the wiring, which would add no more than 3 feet to the total length of the run.

Edited November 27, 2017 by Surgo
got my numbers mixed up

[Surgo]

According to the very helpful VFD buying guide at https://www.vfds.com/blog/vfd-buying-guide, a rule of thumb is VFD size on single-phase power of 2x the full load amperage draw of the motor. Which, I suppose, answers my question exactly. So that's good information for anyone else facing the same dilemma I am. Though I'm not entirely clear on *how* that rule came around. EG, wouldn't we still have to multiply the amperage draw by 4 to get what would be taken in the 110v line? I'm not an electrician, I only have a basic understanding of electrical components.

[Charles R. Stevens]

I think the point Glenn was trying to make was if the run is short enugh and the wire is heavy enugh it might be safe to put in a larger breaker. So pulling the outlet and verifying the wire size and figuring out how long the wire run is can certainly save you problems in the long run.

3 phase power is expensive to have run to the house for sure, buying big 220 motors and having 220 ran to your shop is probably a lot less expensive. 110 converted to 3 phase is not going to be as effecent to operate either. That said, I don't know your situation or reasoning.  

[Steve Sells]

section 400 table 430.138 in the NEC has the charts laying out the amp draw,  at 115 volts its full  load current 20a for 1.5 Hp and 24a for 2Hp but you also need to calculate the inefficiency of 80% or less for the vfd itself

[rockstar.esq]

Steve,  I'm working off table 430.250 full-load Current, Three phase Alternate-current motors because that's what he's trying to power.  I suspect you were using the single phase motor table 430.248 as the values you quoted correspond perfectly.

Choosing the 208V column of the 430.250 table, a 1.5 HP motor's going to draw 6.6 Amps.  and 7.5 Amps for a 2 horse.

There are VFD's available like this one which can take 120V single phase input and generate 3 phase variable voltage output.  It's got significant efficiency losses.

According to the data sheets on that drive, it looks like it could power up to a 1/2 horse 208 V 3 Phase motor while drawing 9.5 Amps at 115 Volts. Stepping up to a one horse 3 phase motor jumps the input current to 16.0 A which is exactly 80% of a 20A circuit.  

So near as I can tell, the answer to Surgo's question assuming the drive I've linked, working on a 15A 120V single phase circuit,  is a 1/2 Horse 3 Phase motor running at 208 Volts.

A more efficient drive might up the ante for you but it's worth pointing out that you're supposed to stay at roughly 80% of the branch circuit rating.  At 15 Amps circuit capacity, you've got 12 Amps to work with which is a net 1,440VA to work with .

At a perfectly efficient scenario, that limits your output current at  208V 3 phase to about 4 amps which correlates to a 3/4 Horse motor.  If you used 100% of your 15A circuit, you'd bring your 208V 3 phase output current to 5 Amps which will get you up to 1 Horse.  It bears mentioning that nothing is this efficient.

The real advantage of a  120V input VFD isn't maximum horsepower, or torque, it's speed control.  Another  advantage of VFD's is that you can reverse the motor's rotation using the drive.

Simplifying all of the above.  Without upgrading your 15A 120V circuit you could power up to a 3/4 Horse single phase motor (assuming no load starting), or you could power up to a 1/2 horse 3 phase motor at 208V using a VFD.  If speed control is important to you, go for the VFD and 3 phase motor, if durability and torque is important, go with the single phase.

I hope that helps.

 

[Surgo]

Thanks, that does help. Table 430.250 says half of what I expected, which is that P=IV is the ratio here. The piece I was missing was what the drive itself would end up consuming, which I now see via your answer that I could simply read off the rated input current from the VFD. Well, in retrospect that was stupidly obvious :headbang:. And that's some serious efficiency loss on that drive! Not that I'd really expect anything different given the monstrous manipulations you have to do to single-phase input.

I'll probably stick with the angle grinder and hand files and just wait to finish building my shop rather than temporarily purchase and use a 1/2 HP motor.

[Steve Sells]

14 hours ago, rockstar.esq said:

Steve,  I'm working off table 430.250 full-load Current, Three phase Alternate-current motors because that's what he's trying to power.  I suspect you were using the single phase motor table 430.248 as the values you quoted correspond perfectly.

that is what I get for  quoting from memory rather than looking it up

[Charles R. Stevens]

Glad to see your still human Steve, lol

[rockstar.esq]

17 hours ago, Steve Sells said:

that is what I get for  quoting from memory rather than looking it up

Steve,

I very rarely have to figure this stuff out because commercial work tends to have a big engineering fudge-factor built in.  Also, we rarely have a motor on it's own to hook up.  If the motor has a fan blade on it, the fan housing will have a nameplate, and that's what we're basing everything on.  It's good practice to go back to the basics.

Surgo,

Glad it helped.  Straightforward DC calculations don't work quite the same in AC.  With three phase power you need to factor the square root of 3 to get the correct VA.  Wherever digital/logic stuff crosses over into work/horsepower, it's good to be wary about what you're being told.  A good example of this is car audio amplifiers.  A lot of them are marketed by their wattage.  If you multiply the voltage (12V nominal) by the amplifiers fuse rating, you'll usually end up with a wattage number that's way lower than what they're advertising.  The tricky bit is that digital power devices are marketed by their effective  or peak output  for a limited duty cycle.

A fully charged capacitor can discharge very quickly which gives a "DC Offset" to the amplifier's output.  If you think of it as a battery assisting the main supply for a short period of time, the effective output for that moment is significantly improved.  For an instantaneous load like a drum beat, that might be fine.  For a constant load, it's not so good.  The difference between actual input power and claimed output power tends to be greatest on price-point items for the consumer market.  

 

 

 

[ThomasPowers]

I worked for a company that had a product with a stated draw of 630 amps at -48 Volts DC; well we got a query about that as providing battery backup was around $50K back then.  Since the hardware had had a lot of changes over a bunch of years we went out and measured the draw from a fully loaded system and updated the requirement to 75 amps....biggest drop was changing from the old Winchester removable media disk drives to more modern disk drives...

moral of the story is that sometimes even the documentation is out of date....