2 pole or 4 pole for Myford ML7R

Thanks old mart, I had a hunt through the parameters but couldn't find anything relating to NO/ NC, so best work with it as is I guess. Magnetic base for the Estop sounds useful, I'll take a look

The earth connection on the inverter is already really crowded with two wires connecting and I'll struggle to get a third in there (it's very small) so I was thinking an oversize (40 x 30 x22cm) flame retardant ABS enclosure would be easier as it needs no earth. I know it won't transmit heat as a metal case would but we only use the lathe for short periods of time and it'd be an option, with the inverter well away from flying chips, to open the door if we needed to use it longer. Plus we could easily drill a few holes and cover with mesh to keep insects out. Any other reason not to use a plastic enclosure?

Edited By Chris152 on 28/08/2023 08:03:03

Don't overcrowd the earth terminal : one big wire with an eyelet is best. The problem is dealt with by an earth block like this example from Screwfix:

Ideally the block is bolted to the metalwork near the VFD, but it can be free-standing. The main requirement of earthing is that all connections be secure - you don't want earth wires vibrating off an overcrowded terminal screw. Insulated terminal blocks are available too:

ABS if you must, but metal cases are stronger, fireproof, and shield Radio Frequency emissions. An unfiltered, unshielded VFD is a potent source of electrical filth. Putting the VFD inside an earthed metal box helps contain its anti-social output.

Dave

Aha, I'd not seen an earthing block before. All clear.

Would the correct thing now be to remove all the connections from the inverter and recut them, leaving extended earth cables and the other cables normal length, allowing the earths to connect to the block/ enclosure?

It'd be easier if I could post pics when trying to explain things - do I need to find a photo hosting site and put them there, then link to them from here in my posts? I'm sure there's a guide somewhere on here, I'll take a look now.

Thanks.

Got it!

If you are going to earth the cable screens at the VFD you need to use the correct earth bonded gland or other suitable arrangement.

Emgee

I didn't even think of earthing the screen wiring. I've just searched earthing gland and see it can be an earth tag or gland - presumably this connects to the same earthing block that will be attached to the metal wall enclosure, and that the cable earths attach to before connecting to the inverter?

Thanks for pointing this out.

A metallic case also reduces interference. You can use an ABS box but you should fit the VFD and any filters onto a metallic plate that is then installed in the ABS cox. All earths, includng cable screens / armour, should be connected as directly as possible to the plate.
With plastic box the best way to cround the screen is with a metallic clamp. You need 360 degree contact. Even an inch or two of tail reduces the effectiveness of the screen.
If your VFD does not explictly have an EMC filter you should fit one. it also needs to be mounted on the plate.

Ther are two disadvantages to putting the E-Stop in series with the normal stop input:
1/ The machine will not stop as quickly.
2/ if ther is a fault in the normal stop circuit the E-Stop won't work either.

Robert.

Mine stops more quickly using the normal stop as that brakes the motor electrically. Using the Estop to cut all power means that it will stop no matter what the fault.

I'm now more or less decided we'll use a metal box.

Concerning EMC filter - the description of the inverter states:

"PWM CONTROL:
By optimizing PWM control technology and electromagnetic compatibility, it satisfies users' requirement of low noise, low electromagnetic interference."

Under the specs, it says "Filter - Fitted", but it doesn't specify what kind of filter.

Is that sufficient to control EMC or should I fit a filter?

Thanks.

Edited By Chris152 on 28/08/2023 15:18:34

Thanks Duncan - by normal stop, do you mean the Estop on the VFD controller/ inputting to the VFD, or do you mean fitting the Estop to the power cable? I think Robert said above cutting power in the cable takes longer, as the VFD Estop operates with a brake? Sorry if I'm confused.

Hi Duncan,

In a properly set-up VFD system using the E-Stop input will stop the machine faster than the normal stop button.
Cutting the power to a VFD with the E-Stop is not recommended as the VFD stores significant energy in the "bridge" capacitor(s). The electronics won't even recognise that power is cut until the voltage on this drops. Thus the stopping time is much longer than even a non-VFD motor with a power cut E-Stop. This is made worse by using an oversized VFD or when the load is light.
So a dedicated E-stop input is the best solution.

Chris,

As they say it has a filter built in you probably don't need an external one.
In theory a full set of electromagnetic emissions measurements should be carried out to ensure compliance with regulations. In practice this is seldom done for one-off designs even by professionals. The cost, typically a few thousand pounds, has a lot to do with it.

Robert.

Sorry to be slow Robert, I had an eight-hour round trip to Stansted to pick up my lad from the airport yesterday, after his flight back to Bristol on Monday was cancelled. How do people cope with travelling regularly on the M25?! Horrible.

Anyway, thanks for that, we'll skimp on the emissions measurements. Our steel wall box is ordered from RS Components and arriving tomorrow, I think I know what we're doing now!

Glands.

If I use a steel gland connected directly to the screen mesh (clear cover removed from just that part of the cable) and earth the gland, do I no longer need to earth the screen?

On the other hand, if I earth the screen and use a plastic gland, I only need to earth the screen, not the gland?

Is either scenario acceptable?

A steel gland that directy contacts the mesh (screen or armour) on the cable and is fastened through the metal box is the ideal solution. No other connection to the mesh / screen is required in this case. The matal case must of course be connected to mains earth. The inside of the box under the gland nutsmust be free of paint or similar finishes.
If you use a plastic gland on the box then leave the mesh and outer insulation intact up to within a few inches of the VFD. One good way to earth the screen in this case is to use a 15mm compression fitting for water pipe. Fold the mesh over the olive and clamp it with the nut. Iflike most, the olive has a sharp edge break it with a file so it doesn't cut the mesh. Mount the fitting on a angle bracket fastened ot the same metal plate / box wall as the VFD.
The plumbing fitting trick is not good for the outside of the box because it does not provide mechanical support for the cable.

Robert.

Thanks Robert - we'll go for the steel gland option I think. We really appreciate your advice.

Hopefully a quick one - will 0.75mm2 cable be adequate to connect the Ebrake to the vfd, or should we go for 1.5mm2?

If you are talking about the control switch, yes 0.75mm2 is fine.

Yes, thanks Robert.

Is this how you'd expect the screen to be connected to the earthing gland? It looks a bit scrappy to me, but the wires are held firmly to the gland. The gland was described as suiting the size of cable but was in fact larger, so I put an O-ring between the inner insulation and the screen wires to take up the slack.

I don't know the gland design , but from a EMC performance respect that is perfect. You just need to make sure it has adequate mechanocal support.