VFD (Variable Frequency Driver) for 3-phase motors.

[BobK]

I was under the impression that 3 phase lines were run most everywhere and the transformers on the poles take 2 of the phases and convert them to 220V center tapped single phase for residential use.

Bob

 

[Robo_Pi]

That's my impression to. In fact, the main lines run right though my property. There are three high voltage lines. When I look up at my transformer I can see that it's only connected to two of them. I would imagine that they would just need to connect to the third line to get the 3rd phase. Of course they would no doubt need to swap out the transformer for a 3 phase transformer as well, and then run three lines into my home instead of just two.

But I'm pretty sure the 3-phase is right there ready to be hooked up to. It couldn't be any closer. It run directly over my vegetable garden. So I've been putting up with this electric right-of-way all my life. I may as well start using all three phases.

By the way, I just called my utility company. The engineers were out to lunch. So they'll be calling me back shortly and I'll find out what the scoop is. I never really even thought about looking into having the 3-phase hooked up directly. That would be the ideal solution for sure. Assuming the monthly bill isn't going to skyrocket! That I don't need. A one-time installation fee wouldn't be bad. Even a $300 install fee would be peanuts as I would easily invest that much in a good rotary phase converter. So if it's in that ballpark I might go for it if I can.

I'll let you know when they call back.

 

[Minder]

I was under the impression that 3 phase lines were run most everywhere and the transformers on the poles take 2 of the phases and convert them to 220V center tapped single phase for residential use.

Bob

That's correct, but I think you may run into some resistance in N.A. when trying to convince the service Co to run 3ph into a residential panel.
Some jurisdictions may vary.
It will also cost Way more than a VFD.
M.

 

[duke37]

3 phase motors have contactors which drop out if one phase is missing due to the excess cuurent in the remaining winding. A capacitor will add a small amount of power to the missing connection to get the motor started. There will not be rwo phases with any connection, it will either be three phases or single phase.
Single phase power has periods when no energy is availble and the rotor has to supply this energy to the third phase. I would think that the convertor motor will need to be much bigger with a large inertia than the driven motor to do this.

 

[Robo_Pi]

From what I've been reading the idler motor needs to be at least 1/3 larger than the motor it's driving (absolute theoretical minimum),.in practice it should actually be larger than that.

What I've seen as being a typical application for hobby machinists is a 7.5 hp idler motor to drive a 3hp 3-phase machine. Although these numbers are all maximums. In other words they assume the motors are doing the maximum amount of work they are capable of.

I just saw a video where a guy is running two 3hp machines with a 5 hp rotary phase converter. He says that it works fine as long as he doesn't try to start both machines at the same time. He can run them both at the same time. He just needs to get one up to speed before turning the other one on. He's also most likely not running the machines hard so they aren't drawing their max currents.

It can really all depend on what the motors are driving. If the motors are driving hard all the time, then they'll be pulling their max current all the time too. Like say an air compressor or refrigeration unit. Those are just designed to turn on and off at full power. But with things like machine shop equipment the amount of power needed can depend on what the machine is actually doing. If you're making deep cuts you're going to need more power than if you are just making light cuts. So if you are doing light machining work, say machining aluminum or plastic, you can get away with far less power than the actual rated hp of the motor.

 

[Y2KEDDIE]

It should be interesting what you find.
If you have 3 ph lines close by, it can help but that's only a start.
Your local utility will need to add 3 transformers to step down the primary line voltage to what you need. Depending what you plan to use in 3 ph power will determine if they will consider adding you a service. They have to offset the cost of the pole, transformers,conductors, and maintenance.
You will have to purchase conductors from pole to your building, meter set, main disconnect, and service panel. They probably won't provide two services (Single and 3 phase) so you may have to buy an additional single phase dry transformer to tap one leg for your existing 120/240 circuits.
I'm guessing you part: around $5K for a small 400A service , if they will go that small.
Then of course there's the monthly minimum use fee.
You can buy many VDF's, Static, and rotary converters for that kind of money.
You may want to rethink buying that new car.

 

[Robo_Pi]

I'm guessing you part: around $5K for a small 400A service , if they will go that small.
Then of course there's the monthly minimum use fee.
You can buy many VDF's, Static, and rotary converters for that kind of money.

Absolutely. If that's how much it cost I wouldn't even consider it. At this point I'm just going to ask them for a price. If its in the thousands of dollars for a hook-up it ain't gonna happen. But getting a quote is free. So no harm in asking.

You may want to rethink buying that new car.

I already bought it. It's 'new' to me, but certainly not a new car. It's a 2003 Caravan



It's got some rusty spots on it down low by the rocker panels. But nothing serious. If I take care of that this summer I can stop it in its tracks from spreading. The hood was replaced and hasn't yet been painted. Other than this it's in pretty good condition. It has really good tires on it. It's nice and quite when driving it. No rattling. And the air conditioner still works. The people who owned this car had it serviced on a reguar basis. So mechanically it's pretty sound. It should last me at least 10 years.

I paid $300 for it. That might give you an idea of my financial thinking. I don't think I've ever paid more than $500 for a car in my life. For me, when numbers start having more than 3 digits in them I'm walking away. That's just where I live in the financial universe.

So yeah, if it's $5000 for 3-phase service you won't be seeing it at my house.

 

[Minder]

Your local utility will need to add 3 transformers to step down the primary line voltage to what you need..

Here they would just need to add one extra for the three phase, or none if It was 440v.
That should all that should be required in most N.A. service supplies.
Either way, it runs to the high 4 figures.
M.

 

[Robo_Pi]

Either way, it runs to the high 4 figures.

If that's the case then there's no contest. A decent homemade rotary phase converter isn't going to cost more than $300 if built wisely. I was just looking at a 7.5hp 3-phase motor for $99. Surely it's not going to cost more than $200 for the capacitors, etc.

Why pay thousands when $300 will solve the problem?

In fact, the most economical solution would be to just convert the machines over to 110v. Based on what I've seen machines that have been converted to 110v motors bring a higher price on the market anyway. People don't want to be bothered with 3-phase so they are glad to buy a machine that already has a 110v motor on it. If you're planning on reselling the equipment at some point replacing everything with 110v motors is probably the best way to go anyway. That was my original plan. I just might go back to that. Sometimes the first idea is the best.

 

[Petkan]

Ok, I found the following schematic. It seems fairly straight-forward.

What I'm not clear on is how to hook up the 230 VAC at the input of the bridge. This schematic appears to assume a single-line 230 volt 1 phase. But what I would like to do is use a standard residence 220 VAC. That comes in as two 110 VAC lines that are of opposite phase with respect to a common neutral. Is that right?

I don't know how to hook that up at the input of the bridge to get the 220 VDC at the output. Would I hook up a 110v line to the switch and the other 110v line to where it says 230 volts on the schematic? And if so then what about the neural line?

I guess what I'm really asking is how to build a 220 VDC power supply using a standard residence electric service. Once I get a suitable 220 VDC power supply the rest of the schematic is pretty simple. Most of the magic will be in the PWM programmed into the MCU. That part I know how to do.

Petkan:
If you are in US, Canada, Japan etc, you may have 120Vac line, but according to your details you do have 220Vac.
The schematic above shows correctly what to do with normal 220Vac line. You rectify it and drive your IGBT bus from what is often called DC link cap. All motor drives ultimately work from HV dc bus.

 

[Minder]

. All motor drives ultimately work from HV dc bus.

With the exception of SCR/Bridge controllers that simply switch the AC line to produce a pulsed DC output.
KB drives and Treadmills for e.g.
M.

 

[Y2KEDDIE]

The schematic previously shown operates from a DC bus. The comercial vfd’s (similar to the schematic representation above) get their power from 3 phase mains, and have a 3 phase rectifier at the input. When operated from a single phase source only part of the 3 phase stack issued. This decreases the duty cycle of the input power. When the power input is limited in this fashion it is typical to add a large capacitor to the DC bus to beef it up.

 

[Tha fios agaibh]

Why pay thousands when $300 will solve the problem?

In fact, the most economical solution would be to just convert the machines over to 110v.

I agree but price aside, It's likely you can't get 3 phase service anyway.
More than one service fed to a residential house isn't allowed, and a 3 ph delta(4 wire) feed is usually denied by either the utility company or your local authority.

Better to change motor to single phase 240v (rather than 120v) where smaller conductors can be fed to the motor or just opt for an inverter which is also about $300

 

[(*steve*)]

and a 3 ph delta(4 wire) feed is usually denied by either the utility company or your local authority.

Really? I just asked and paid the money. But that was when the house was being built. As I recall, the major expense was several hundred dollars for the meter, and under $200 for the additional cable cost.

They may have asked me why, but it was just curiousity.

 

[Minder]

I checked with a friend of mine who has a position of authority in the provincial Hydro and he said definitely impossible to get 3ph in a residential building here. (Canada).
M.

 

[Tha fios agaibh]

Really? I just asked and paid the money.

Yes. It does exist here in residential applications but it is rare.

It works perfectly for places with 240v because you can perfectly ballance all 3 phases but if you have 120v household it gets a little unconventional having to mid-tap the transformer to create split phase 120v and 240v, 3 phase on a system.
This causes one high leg phase of 208v (to ground) which is not a common voltage and can't be reliably used by a heavy load because of fluctuations.

I presume most jurisdictions think it better to just say no, rather than deal with confusion by incompetent people that could pose a threat to themselves or others comming across an uncommon system in a residential home.

 

[(*steve*)]

but if you have 120v household it gets a little unconventional

Aaaah, that makes more sense. Because that 120V isn't just one of the phases, it's some funny "half a phase" thing isn't it.

Most new properties built here these days have underground power. This is delivered to a green "mushroom" that is placed close to a corner of the property near the crossover. My understanding is that these all have 3 phase power, so the difference between single and three phase power is the metering and cabling between the mushroom and the house.

In fact, even when there is overhead power, three phase is run individually to a mushroom. I was walking through part of my neighbourhood that is quite old and is having blocks redeveloped. The cables running to these mushrooms are clearly overrated for a single residence -- especially when only a single phase is in use.

 

[Minder]

The N.A. residential supply is 240v secondary of a 1ph transformer with a grounded neutral centre tap for 120v-0-120v.
In the likes of the UK, every so many residences on a street are fed from 230v 1 phase of a 3phase transformer with star point grounded neutral.
The instance of 3ph supply that I was involved in was because several heating storage units were installed in a residence and the 1ph just would not be sufficient to support the load.
M.