VFD inside or outside the control box?

[Kitwn]

One of the main ways in which noise will get coupled between parts of the system is by running cables next to each other. I wanted to make everything neat by having all the cables for the spindle, motors and limit switches running through one set of drag chains but this is obviously the worst case for this. At one point I had the spindle feed looped up with the dust extraction hose to keep it separate but wasn't happy with it. This is why you want to screen all the cables and ground them to a single point at the control box/VFD end. Capacitors in parallel with the limit switch inputs to the breakout board are a great help as well. My understanding is that it's the high frequency, high amplitude noise from the stepper and spindle drives couling back into the limit switch inputs and possibly then into the controlling computer that are the main risk.

Kit

[Muzzer]

Yes, screening (and grounding the screens) is just good practice. Most drag cables have sets of screened cores within a screened bundle, so you can run signals and steppers etc in the same cable. Good idea to run the VFD>>motor separately though. Decent quality CNC controllers are fitted with noise filtering on the IO, so additional measures shouldn't be necessary if you have a good construction and follow good practice.

[Paul53]

Well, It looks like I have started an interesting discussion.

I have watched countless Youtube videos on wiring/connections etc, and most of the builds have the VFD inside the cabinet, now whether that is the right or wrong way, I wouldn't have a clue, but what I will say that a majority have an EMI filter on the mains feed from the VFD, I was also going have the VFD/spindle on it's own power supply, as I said, this is all new to me and it is a very steep learning curve, so expect loads of questions and advice while I put together all the components, none of you live in Hampshire by any chance? Would make my life so much easier just pop round for some one to one advice

I have just picked up a metal box/enclosure, so I will very shortly be starting the electrical build, wish me luck!

Thank you for all the input and advice.

[Kitwn]

none of you live in Hampshire by any chance?

Sorry no, I live in Exmouth. The one in Western Australia. I'm not sure my advice is worth a cup of tea anyway.

Earthing everything to a single point is supposed to prevent earth loops, the cause of much mains hum in audio equipment. This is probably the thinking behind only connecting screens at one end. As you'd be earthing the body of your machine back to the same point with fairly short wiring anyway it probably doesn't matter. Belt and braces is to make the cable so you CAN connect at both ends but be able to disconnect if you have problems.

On a safety note: The output of your VFD is at potentially lethal voltages and is isolated from the mains input so you get no protection from an RCD or other 'safety switch' on your mains supply. Make sure the spindle body is well earthed and use insulating sleeving over each connection inside the plug (this will help prevent you detonating your VFD as well). Recent spindles from Huan Yang have the body earthed via a fourth pin on the socket, but earlier versions had only 3 pins or the fourth pin was not connected.

Kit

[Muzzer]

On a safety note: The output of your VFD is at potentially lethal voltages and is isolated from the mains input so you get no protection from an RCD or other 'safety switch' on your mains supply. Make sure the spindle body is well earthed and use insulating sleeving over each connection inside the plug (this will help prevent you detonating your VFD as well). Recent spindles from Huan Yang have the body earthed via a fourth pin on the socket, but earlier versions had only 3 pins or the fourth pin was not connected.

Kit

I'm afraid that's incorrect. VFDs do NOT isolate the motor (output) side from the mains. So the RCD would provide some degree of protection. So, you should consider the motor phase connections to be at mains potential (which they are) and use appropriate insulation / creepage and clearance distances. A good quality VFD should tolerate a phase to ground short circuit without "detonating". I speak as an engineer who has developed VFDs throughout my career.

There are some high end industrial installations where the entire VFD high voltage circuit (AC and output) is isolated from ground by means of a dedicated power transformer ("normal" mains is grounded on the neutral side). This is to allow a single fault (phase to ground) to exist without the whole system crashing immediately. These are used in factories such as paper and steel mills where loss of one VFD in a series of interconnected VFDs could be catastrophic. I doubt any of us will ever see such a system.

[Kitwn]

Muzzer,

You obviously have more experience of this than I do. I'll have to do some rectifier drawings and follow the currents.

Kit

[Muzzer]

Generally speaking you will find a bridge rectifier, then DC bus caps, then a 3-phase / 6-switch (IGBT) bridge. Nothing much else needed on the power circuit.

With single phase 240V input, only 4 of the 6 rectifier diodes get used but as I said, most VFDs don't check for a missing phase. The manual may define a slight derating when you use single phase instead of three phase input but the rectifiers are pretty robust and unlikely to be an issue for intermittent / light hobby use. The DC bus is going to be around 340Vdc and obviously you can't generate an AC voltage that exceeds 240Vac from that, hence you normally need to connect your 3-phase motor in delta if it's a 240/415 star/delta machine.

On higher power VFDs (>6kW or so), the bridge rectifier will become an active PFC ie another 6 IGBTs instead of rectifiers. And they may then be bidirectional ie can regenerate back into the mains during overrun.

Some VFDs come with built-in EMC filters but that's the exception rather than the rule. It's a good idea to fit one if you can.

[Doddy]

This is why you want to screen all the cables and ground them to a single point at the control box/VFD end.

For emissive cables (steppers, spindle) you should bond the screen at both ends to the respective enclosures.

<Opens popcorn>

[Muzzer]

Haha, nice try. Some (most) VFD manufacturers say one end, others say both ends. Probably doesn't really matter for the sort of work we do but this is an old chestnut. Let's just say there is no right or wrong here...