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VFD's and me


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Some of you may be aware that I've just bought a 3 phase drill and I need to power it, the other part of the problem is I am significantly useless at electrickery 

Going by the motor plate below it looks like the hp  is .9.9.9.9 I'm led to believe that this is the power at different speeds ?????

I'm looking at buying a 2.2kw VFD to power the drill. I would appreciate any advice on if this will run the drill and if anyone is successfully doing this and would be kind enough to put me in the right direction in words that a 5 year  old can understand thanks in advance regards Stephen 

 

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I don't know a huge amount but I would say your right. It's 0.9hp at what ever speed.  1 hp = 746 watts so your some where around 675watts 3 phase. 

I'm guessing your using the vfd to go single phase to 3 phase? 

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9 minutes ago, MECCANO said:

 

I'm guessing your using the vfd to go single phase to 3 phase? 

That's the plan, I did look at some references to vfd's on the forum but just ended up confused as to what some members had to do to get it to work is it really as simple as " vfd will sort.that for you" regards Stephen 

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10 hours ago, MECCANO said:

Excuse my stupidity, but Isn't it 1.732x 420x 2.8 = 1552w given it's 3 phase?

Could well be, didn't stick around the electrics course long enough to get much into 3-phase :P

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I use a VFD to power a lathe and milling machine in my workshop.

Not going to claim to be any expert but this is what I found out and has worked for me.

There are basically two ways to connect one up:

Wired direct to the motor, this means you will be using the VFD to start and stop the motor not the controls on the machine, the advantages of this is the drive only needs to be the same capacity as the motor (or a bit bigger for safety), it can be used to start the motor gently to prevent a start up surge, the controls on the drive can be used to change the speed of the motor. Downside is to shut it down you need to use the VFD controls, you can get control remotes to mount on a machine, the machine shut downs / emergency stops etc will be bypassed. I am sure someone with enough knowledge could work the wiring to over come some if not all the problems.

Using the machine controls, not all set up will allow this, the drive will need to be at least 3 times the power rating of the motor, this is to handle the start up draw surge, as drives get bigger they get a lot more expensive but all the original controls can still be used.

In my set up I do both, the lathe is powered direct, it is a clutch driven lathe so the motor run continually and the spindle is engaged via a clutch. The milling machine is connected as is and I use the machine controls to stop / start and change rotation direction. The lathe is three times the power of the mill so this works for me but I have to physically unplug the machines to change from one to the other.

 

I got my set up from "drivesdirect.com" and found them helpfully and advised me on the set up options I could use, I use a 240v single input to 415v 3 phase output drive rated at 10hp, it wasn't cheap!.

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I think the thing you need to be careful of, is that typically a vfd is used to control the same voltage and phase input / output. For example 420v 3ph in 420v 3ph out. But you want to go 230 1ph in to 420 3ph out, which is a inverter vfd ( I believe from my googling). So you need to make sure you are buying one that can do this. There may be some trickery to convert 3ph in 3ph out to a 1ph in 3ph out, but its just easier to get the right one in the first place.

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3 minutes ago, MECCANO said:

I think the thing you need to be careful of, is that typically a vfd is used to control the same voltage and phase input / output. For example 420v 3ph in 420v 3ph out. But you want to go 230 1ph in to 420 3ph out, which is a inverter vfd ( I believe from my googling). So you need to make sure you are buying one that can do this. There may be some trickery to convert 3ph in 3ph out to a 1ph in 3ph out, but its just easier to get the right one in the first place.

Yep should have added that, I have a inverter type and 1ph in 3ph out which also makes it more expensive.

I believe some 3ph motors can be run in 230 3ph which takes one step out but not something I have any knowledge of beyond what I can see on google.

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I have a Colchester Student, which has a 3hp motor.

When I was researching phase converters I came to conclude that the digital inverters wouldn't allow me to use the machines own controls without a whole lot of rewiring.

I was keen to use the machine's own switchgear as it included the emergency-stop foot pedal. Basically I wanted to just plug in and go, to which end I purchased a 3kw rotary converter from Transwave.

Also I'm not bothered about the variable phase function. The machine has sufficient speed options. I just need phase conversation.

It's a complicated world out there for novices, when you have Transwave and Direct Drives offering conflicting advice. I got the feeling there was some animosity between the two companies.

The rotary converter is an easy option, but it's big, heavy, noisy and consumes power when the machine tool isn't running.

I'd love to use a digital inverter if I ever learn more, but it would also need to be able to run my Bridgeport. I was told from one source that the Bridgeport's motor wasn't capable of being used this way. Something to do with star and delta wiring IIRC, and not being able to be set to the right configuration. I forget which is which, but one is 3 ph 220v and the other 3 ph 415v.

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Single phase or three phase in to 3-phase out isn't a problem - all VFD's rectify the incoming mains (i.e. turn it into DC) before chopping it back up again to give the required output. However, most VFD's are only capable of giving an output voltage the same as the input voltage i.e. if you want to run it off a normal 230V single phase input then you can only get 230V three phase out of it. 

This is OK so long as the motor you want to use is rated 400V star and 230V delta but if (as appears to be the case with the OPs motor) you have only got access to one end of the windings (the other ends all being connected together inside the motor) then you can't power the motor off a single phase input unless your VFD also increases the output voltage. (I have to say I have never seen one which does, but from the above conversation they apparently exist, although I'm sure there is a substantial price premium.)

A few other points worth noting (some already alluded to by others, I admit). Firstly, if you fit a VFD it should be connected directly to the motor, with no switchgear (not even a fuse or overload breaker) between the terminals on the VFD and the windings of the motor. This is because if you disconnect the output of the VFD from the motor while it is running the resulting voltage spike has a good chance of cooking the output electronics in the VFD. Any switchgear intended to turn the machine on and off must be between the VFD and the incoming power, not between the VFD and the motor. 

Most VFD's have a built in mechanism for starting and stopping the output which is usually operated with a control switch. However, unless this is specifically designated at a safety rated input (what is known in the business as a 'Safe Torque Off' input) then you should never rely on the VFD to stop the machine from moving while you are (for example) changing the tools. VFD's without STO MUST be isolated from the input power to be safe, 

The other thing to bear in mind when using the VFD to control the motor speed is that most AC induction motors have an integral fan attached to the rotor, and if you slow the motor right down the airflow drops exponentially, but the heating effect of the power in the motor does not. Thus, it's very easy to overheat the motor if you run it for extended periods at low speeds, and you'll need to fit an external fan if you want to do this. 

 

 

 

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1 hour ago, nickwilliams said:

Single phase or three phase in to 3-phase out isn't a problem - all VFD's rectify the incoming mains (i.e. turn it into DC) before chopping it back up again to give the required output. However, most VFD's are only capable of giving an output voltage the same as the input voltage i.e. if you want to run it off a normal 230V single phase input then you can only get 230V three phase out of it. 

This is OK so long as the motor you want to use is rated 400V star and 230V delta but if (as appears to be the case with the OPs motor) you have only got access to one end of the windings (the other ends all being connected together inside the motor) then you can't power the motor off a single phase input unless your VFD also increases the output voltage. (I have to say I have never seen one which does, but from the above conversation they apparently exist, although I'm sure there is a substantial price premium.)

A few other points worth noting (some already alluded to by others, I admit). Firstly, if you fit a VFD it should be connected directly to the motor, with no switchgear (not even a fuse or overload breaker) between the terminals on the VFD and the windings of the motor. This is because if you disconnect the output of the VFD from the motor while it is running the resulting voltage spike has a good chance of cooking the output electronics in the VFD. Any switchgear intended to turn the machine on and off must be between the VFD and the incoming power, not between the VFD and the motor. 

Most VFD's have a built in mechanism for starting and stopping the output which is usually operated with a control switch. However, unless this is specifically designated at a safety rated input (what is known in the business as a 'Safe Torque Off' input) then you should never rely on the VFD to stop the machine from moving while you are (for example) changing the tools. VFD's without STO MUST be isolated from the input power to be safe, 

The other thing to bear in mind when using the VFD to control the motor speed is that most AC induction motors have an integral fan attached to the rotor, and if you slow the motor right down the airflow drops exponentially, but the heating effect of the power in the motor does not. Thus, it's very easy to overheat the motor if you run it for extended periods at low speeds, and you'll need to fit an external fan if you want to do this. 

 

 

 

I have seen that before about only connecting direct to the motor, I don't know how but the ones from Drives direct do allow switching on the machine, they claim you can effectively use them to create a 415v 3ph ring main (provided you are careful with power requirements), the info from the front of mine is below. I use it to run a 3hp mill in plug and play mode and a 7 .5hp lathe in direct to the motor mode. I know in the direct mode the motor runs up to speed gently to avoid a start up surge and the converter will run a motor up to its stated power. In plug and play mode it needs to be able to accommodate  the start up surge so will only handle a motor up to about a third of its rated load, I think this depends on the sort of load the motor has on it when it starts up.

I have the converter going to a wall socket and each machine wired to a plug so there is no chance of disconnecting them whilst running or connecting them both at the same time, I switch the converter off at its input switch before changing plugs over, I think if I tried plugging the 7.5hp lathe in with the converter powered up it wouldn't end well.

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I never quite understood how all this works, just that it did and the info on the site went over my head so would be quite interested in an idiots explanation. I do know that what you said about there being a price premium is correct though!, this one has now done over 10 years with no issues and the cooling fan is the only real noise it makes. Some not very scientific testing with the household power monitoring meter indicates it doesn't draw much power when on but with no load on it, the workshop lights draw significantly more.

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Just looking at the drill specs, how does it change speed, is there a gearbox, or is it switched somehow ? It looks like WKT 41/25 is the (catchy!) name for the drill not the motor. Looking at pics of them it looks like it has a gearbox, therefore the motor is probably just a single speed.

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It's perfectly possible to design a VFD which snubs voltage spikes on the output causes by disconnecting the motor but it involves adding some reasonably beefy components to the output stage, which costs money of course. Most VFDs are used in applications where they are permanently wired direct to the motor so adding the snubber components is needless expense. 

There's no such thing as a three phase 'ring main' - it's not electronically impossible, it's just nobody wires three phase up that way. However, I can see what the maker is trying to say and it's perfectly valid if the drive is designed correctly (which your one certainly appears to be). However, you can't do soft-start in that configuration (because soft start works by ramping up the output frequency and every motor on the circuit has to be supplied at the same frequency so any machine already running when you plug in the next one would slow down) so the loads you are plugging in directly need to be only fairly small in comparison to the rating of the drive. I think your assessment that Bad Things might happen if you plug your 7.5 hp lathe straight in is pretty much spot on!

50A max input current is pretty beefy. I hope it's wired back to your fuseboard with some fat string!

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