VFD question

[Tai Fu]

I noticed some VFD only accepts 3 phase input... especially for larger HP ratings (in the case of Delta largest VFD they make that accepts a single phase input is 3HP).

Why should VFD care about the phase of electricity coming in when all they do is turn them into DC and back into 3 phase AC current? Can those 3 phase input VFD be modified so that they don't care (such as just filling all 3 leads with the same phase)?

[Rod Sheridan]

Hi, the issue is that the filter and rectifier components cannot handle the currents involved in converting more power into DC from a single phase source.

If you had an outboard rectifier/filter capacitor section you could run the larger VFD from a single phase source............Rod.

[Tai Fu]

So what do you do if you say have a 5HP 3 phase motor you need to run from a single phase source? RPC?

[Rod Sheridan]

Yes, you need a converter, doesn't have to be electro-mechanical, could be a suitably rated rectifier/inverter assembly............Rod.

[David Kumm]

Almost all vfds will run from a single phase source, even those listed as three phase input. Need to derate. In my simple world you double the size. IE 10 hp inverter to run a five hp motor. Just check the imput amperage on the vfd. Should be 1.5-2X what you need for output amps. Dave

[Richard Coers]

I noticed some VFD only accepts 3 phase input... especially for larger HP ratings (in the case of Delta largest VFD they make that accepts a single phase input is 3HP).

Why should VFD care about the phase of electricity coming in when all they do is turn them into DC and back into 3 phase AC current? Can those 3 phase input VFD be modified so that they don't care (such as just filling all 3 leads with the same phase)?

You're joking right? (such as just filling all 3 leads with the same phase)? You plan on just making up your own wiring diagram? If you don't understand electricity, or components, I don't recommend making it up. Lots of fires every year from folks doing the wrong thing with electricity. A Father and son just died in a house fire near here. They blame an overloaded "junction box" as the cause. Hire a professional, it's a lot better than burning your place down. Maybe with family members lives at risk!

[Ryan Mooney]

Almost all vfds

The key here is "almost all" and you usually want to use some line conditioning tools like a DC link choke (some folks get away without it, but you might fry the vfd otherwise). The other kicker is that the input AMPs are non trivial - a 10HP motor requires a 60a single phase circuit to feed the vfd (5hp is around 30-35 amps depending on the closest vfd size) and it may be worse than that if your motor has a higher FLA (full load amp) requirement (vfd size = max(horsepower required, motor fla)).

Note that I've found up to 5HP VFDs rated for single phase input (Yaskawa at least has some).

[Mike Henderson]

A VFD takes the input AC power, converts it to DC, then outputs it on the three phases, usually using pulse width modulation (PWM).

The problem with supplying a three phase output VFD from a single phase supply is that the single phase supply goes to zero voltage twice per Hz but the output voltage will be significantly greater than zero on at least two of the phases. To be able to provide power while the input is going to zero, the VFD must have storage devices which provide power during the zero transitions of the input power. This is usually large capacitors. But the current requirements for a 10 Hp motor, for example, are quite large and that would require big capacitors.

When the input is three phase, there is always voltage being fed to the DC portion of the VFD.

Mike

[Of course, the reason we have VFDs with three phase input and three phase output is so that the frequency of the output - and thus the speed of the attached motor - can be varied.]

[John McClanahan]

You're joking right? (such as just filling all 3 leads with the same phase)? You plan on just making up your own wiring diagram? If you don't understand electricity, or components, I don't recommend making it up. Lots of fires every year from folks doing the wrong thing with electricity. A Father and son just died in a house fire near here. They blame an overloaded "junction box" as the cause. Hire a professional, it's a lot better than burning your place down. Maybe with family members lives at risk!


That's why we ask before doing. Not the first time this type of VFD question has been asked.

John

[David Kumm]

It gets pretty expensive to run motors larger than 7.5 hp on single phase input. I use one for dust collection ( 7.5 hp ) but the rpc or even the Phase Perfect cost becomes reasonable pretty quickly. Dave

[Michael Mahan]

It gets pretty expensive to run motors larger than 7.5 hp on single phase input. I use one for dust collection ( 7.5 hp ) but the rpc or even the Phase Perfect cost becomes reasonable pretty quickly. Dave

David do you mean 'Un-reasonable ' ? ?

[David Kumm]

No Mike, when you run bigger stuff the cost of the power supply doesn't seem so bad. I'm mainly a used guy but have paid 800 for a 15 hp Kay, 1500-2000 for used 10 hp PP, 2500 for 20 hp and 3700 for a 30 hp PP. The PP are worth the upcharge ( I can make you a deal on either a 10 or a 15 hp Kay ). I found I couldn't run vfds off the Kay as the third leg voltage was too wild so any speed control machine needed single phase input. The PP third leg is 208 volts so as long as the vfd can handle that you can run it in conjunction with the PP. Dave

[Ryan Mooney]

I found I couldn't run vfds off the Kay as the third leg voltage was too wild so any speed control machine needed single phase input. The PP third leg is 208 volts so as long as the vfd can handle that you can run it in conjunction with the PP. Dave

Good to know that might save me a few turnover $$'s someday. So far I've been just trying to size everything so single phase in works, but we all know how this slope slides...

[jack forsberg]

much of what is said here is old. single phase input VFDs are here in the 10 HP range. 2 words . eBay china. most of them vector drive and very advanced. not that us woodworker need that.


http://www.ebay.ca/itm/VARIABLE-FREQUENCY-DRIVE-INVERTER-VFD-7-5KW-10HP-34A-/251194556907?pt=LH_DefaultDomain_0&hash=item3a7c5cc1eb&_uhb=1

[Tai Fu]

So what exactly is 3 phase? I mean in Taiwan that seems to mean 3 live wires that are 110v each, whereas "single phase" meant 2 of those phases connected together to make 220v. So in the US 3 phase means 3 live wires 220v each? I'm not sure how to get 3 phase but I tried asking around and it's complicated. They said it would cost many thousands of dollars and the power company wouldn't even run 3 phase to your place unless it was a factory. I know many talked about having lots of HP for their tools but being stuck to a 50amp 1 phase circuit means that the biggest motor I could run is a 5hp motor... anything bigger and it's 3 phase.

[Michael Mahan]

No Mike, when you run bigger stuff the cost of the power supply doesn't seem so bad. I'm mainly a used guy but have paid 800 for a 15 hp Kay, 1500-2000 for used 10 hp PP, 2500 for 20 hp and 3700 for a 30 hp PP. The PP are worth the upcharge ( I can make you a deal on either a 10 or a 15 hp Kay ). I found I couldn't run vfds off the Kay as the third leg voltage was too wild so any speed control machine needed single phase input. The PP third leg is 208 volts so as long as the vfd can handle that you can run it in conjunction with the PP. DaveI see what you mean now
good to know , I'm in a used buying mode

[Dan Friedrichs]

So what exactly is 3 phase? I mean in Taiwan that seems to mean 3 live wires that are 110v each, whereas "single phase" meant 2 of those phases connected together to make 220v. So in the US 3 phase means 3 live wires 220v each? I'm not sure how to get 3 phase but I tried asking around and it's complicated. They said it would cost many thousands of dollars and the power company wouldn't even run 3 phase to your place unless it was a factory. I know many talked about having lots of HP for their tools but being stuck to a 50amp 1 phase circuit means that the biggest motor I could run is a 5hp motor... anything bigger and it's 3 phase.

I believe the power distribution system in Taiwan is the same as in the US (120V/60Hz). I don't think you're understanding it correctly, though.

Typical residential power in the US is 2 "hot" legs and a neutral. The hot legs are 120V (RMS, relative to neutral). Because the voltage is alternating (as a sine wave), they are designed to be 180 degrees out-of-phase with each other, and thus, have 240V potential between them. Thus, you connect between a "hot" and neutral to get 120V, and between the two "hots" to get 240V. We call this "single phase".

For many reasons (including the reason that Mike mentioned earlier), there are benefits to using more than 2 "hot" wires. Typically, commercial and industrial power is "3 phase", meaning there are 3 "hot" legs, each being sine waves 120 degrees apart. Depending on how these are supplied, you can derive 120V, 240V, 208V, and 277V from these (as well as 480V, 600V, and others in some industrial settings).

3 phase is not typically available for residential service (in the US, at least), for non-trivial reasons. Often, 3-phase transformers are not available in residential areas, making it a much more complicated matter than just "adding a 3rd wire".

Thus, if you want to power 3 phase machinery, you need a VFD or RPC. I think what is atypical, here, is that your service is only 50A? Most (new construction) residential service in the US is 200A.

[Tai Fu]

My house is old and designed without air conditioning in mind. Modern buildings here are at least 200 amps and 3 wire. The infeed line is 14mm square so I think it can handle 150 amps but I would need to change the main breaker which I don't know how because I can't shut off the meter.

[Terry Therneau]

My house is old and designed without air conditioning in mind. Modern buildings here are at least 200 amps and 3 wire. The infeed line is 14mm square so I think it can handle 150 amps but I would need to change the main breaker which I don't know how because I can't shut off the meter.

Two things:
1. I grew up on a farm and we did most of our own wiring, I did all the wiring in my shop and most in my house. Still, I hired an electrician to change the main panel, without hesitation. If it involves pulling a meter, let the pros do it. You want it done right, including the required building permit and electrical inspection.

2. I am running 3 phase motors off single phase power, through a pair of ABB VFDs that I got off ebay. The 7.5 HP vfd runs a 3 HP motor and the 15 HP runs a 5 HP motor.
a. To run a 3 phase motor off a VFD with single phase (220 volt) input, the VFD needs oversize capacitors. There are two ways to get this: a VFD that is marketed as one that allows single phase input, or use a VFD that is 2x times bigger than needed.

b. The motor plate of the 3 HP motor indicates a draw of 8 FLA (full load amps), more of course when it is starting. The power delivery of 8 amps 3 phase = the same as 8* 1.73 = 14 amps from a 2 phase circuit. So I have a 20 amp circuit (12/2 with grd) feeding the VFD. The 5 HP motor's VFD has a larger circuit.

c. High quality VFDs do a lot of checks: motor speed, current, over and under voltage, .... If you are (un)lucky enough to get one that also checks the input power, then it will not be "hook up and go" with 2 phase input. The VFD will stop and put up an error code, all in the name of helping you out. I don't have wide experience in this area, but my impression is that this is not a common feature.

[Danny Barber]

I haven't really seen any micro drive accepting 1ph going higher than 3hp. I've used a Hitachi WJ200-022SF (http://www.clrwtr.com/Hitachi-WJ200.htm) in a VT application with a 5hp motor.

[Mike Heidrick]

Speedstar drives at driveswarehouse has all kinds of single phase 220V AC drives/VFDs above 3hp.

Hre is a 10hp for example: http://driveswarehouse.com/p-2125-pc1-100.aspx