The world of 3 phase and VFD's

[HexusOdy]

In an effort to open up a new world of better (and hopefully cheaper) machines I've been looking at 3 phase and VFDs.

I have to admit the electrics are by far my weakest area. Even the most basic wiring has always baffled me. Having looked online there is a lot of very technical info and broad strokes bug not much of the basics.

Would I be right in thinking I would need a single phase 220v to 3 phase 220v VFD rated to the HP of the machine. For example 2.2kw for a TS?

Am I right in thinking the VFD would need a new mains spur wired in from one side and on the output you take the plug off the 3 phase connection and wire that into the VFD?

Anybody's advice, real life experiences and product recommendations would be appreciated.

 

[porker]

The most common VFD's convert 230V single phase to 230V three phase (but there are different types that step up voltage, allow plug and play etc.). They also normally require direct connection of the output of the VFD to the motor. This means that you may have to alter the switch gear on the machine. Depending what this is depends how simple this will be. You would also normally need to reconfigure a star configured motor to delta to allow it to be run from the lower voltage. This is normally fairly simple and just requires some links being reconfigured in the terminal box (assuming the motor is of a type that is easily configured). Yes, the HP of the machine would be similar to the rating of the VFD but this depends also on start up current as some machines require a larger amount of power to start requiring a higher rated VFD.

I installed a couple to drive my Bridgeport milling machine. I used two Teco drives from Drives direct, pulled out all the existing wiring and switchgear and refitted a simpler control box that gave me extra features like variable speed and reverse.

 

[DennisCA]

Just what would it take to get 3-phase anyway in the UK?

I mean here power is sent to homes as 230v 3-phase, we just split up the phases for single phase power and design it so the load is balanced as much as possible between the phases. Running actual 3-phase power from this is as 'simple' as running the wiring from your houses central and installing outlets. So the cost of some minor electricians work, probably no more than a VFD would cost.

And 230V 3-phase allows us to hook up motors in a Y-config and get 400V.

 

[porker]

It's a similar setup in the UK but generally the costs to get 3 phase installed that I have seen run to several thousand pounds although this does vary. I guess a call to the provider could get a figure but for small scale use I think it will be an expensive option, although I agree it should in most cases not be such a large charge.

 

[DennisCA]

I don't see why it should cost a lot yes if that is true. It's standard here to pull in 3-phase for the stove and other larger appliances and to have one 3-phase CEE outlet in the central. In many garages and workshops outlets are also included.

 

[porker]

It's not often that all three phases are brought into the domestic house but they will be on the pole or in the road. Maybe this is the difference.

 

[deema]

It's usually as cheap to replace the motor for a single phase motor and change the starter relay / thermal overload unit than it is to add a VFD. The only real advantages with a VFD are

1. If you need variable speed.
2. If you want to be able to stop the motor in compliance with ColRegs, the. A VFD can be fitted with a resistor to create the brake effect.
3. Soft start, reducing power consumption and the size of cabling required.

A single phase machine will be more valuable when you come to sell it than a 3ph machine with a VFD.

 

[DennisCA]

It's not often that all three phases are brought into the domestic house but they will be on the pole or in the road. Maybe this is the difference.

Ooh, that must mean you have very little electricity or some very big fuses and thick wires.

 

[HexusOdy]

It's usually as cheap to replace the motor for a single phase motor and change the starter relay / thermal overload unit than it is to add a VFD. The only real advantages with a VFD are

1. If you need variable speed.
2. If you want to be able to stop the motor in compliance with ColRegs, the. A VFD can be fitted with a resistor to create the brake effect.
3. Soft start, reducing power consumption and the size of cabling required.

A single phase machine will be more valuable when you come to sell it than a 3ph machine with a VFD.

But it is also more work right? Ive read thats its very difficult to get a new motor that will fit the switchgear and pulleys out of the box. Stories of grinding, getting new belts etc which sounds like a lot more work than some wiring. Also those advantages you list are quite nice advantages to have.

On saying that I hadn't considered the resale value of a saw once its single phase, but then it's supposed to be a saw for life hopefully.

 

[HexusOdy]

So how do these things physically wire up then?

Was I right in that you run a mains spur into the VFD for input and lop the plug off the machine and wire that into the VFD output?

 

[porker]

With a VFD running less than a standard plug (fused at 13A) there is no need to run any new mains spur to the VFD. That can still be supplied by the standard plug. VFDs normally have inputs of power, start, stop, reverse etc and an output to the motor that has to be directly connected to the motor (i.e. no switches). This is what can require adjustment of the switchgear.

There are 'plug & play' solutions available which basically just let you plug into the standard wall socket and the output takes the three phase wiring of the machine but they tend to be expensive. You can go down the route of rotary or static converters which also mean you can leave the machine you are running basically untouched.

Or as suggested, you can change the motor to a single phase but as you mention there may be mechanical issues to overcome in mounting and pulleys etc.

If you have a particular machine in mind there is probably someone on here who will be familiar with how they converted that. For me I was driving basically two three phase motors so I ripped out all the old 3 phase switchgear, and put in two VFDs. I configured both motors from star to delta so they run off 230V. The outputs from the VFDs were connected to the motors (main spindle and table traverse) and I bought an optional pendant with a start/stop reverse, speed pot and emergency stop on an umbilical, made a bracket and fixed this to the front of the machine. This is connected to the inputs of the VFD. The whole machine runs off a standard 13A plug. It has a 2HP spindle motor and a 0.5HP table traverse motor. For some machines where the controls are basically turning a motor on and off, it should be straightforward. Other configurations may need more thought.

Take a look on the Drives Direct site as there is useful info on there. (I have no connection with this company apart from being a satisfied customer and the guys there are very helpful).

 

[HexusOdy]

Sadly it sounds like it may be beyond my electrical prowess, which is very little. I have trouble fitting a new ceiling light.