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Motor control circuit

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David Siddle 128/03/2019 09:46:50
12 forum posts

OK. Now I have my VFD, remote pod and a nice, large enclosure. What about contactor/circuit breaker? Are these strictly necessary in a home workshop? This project is starting to get expensive and I don't want to spend money I don't really need to. If so, what exactly do i need on my shopping list? The motor is .37kw 3ph (wired Delta now) to be run off a 1ph domestic supply.

I looked up the contactor Mitsubishi, (the VFD manufacturers), recommended and it was pretty pricey. Is there a cheaper alternative?

Many thanks people.

Andrew Johnston28/03/2019 10:02:07
5401 forum posts
625 photos

When I added a high speed spindle and VFD to my CNC mill I didn't bother with a contactor. Just a double pole mains switch and a COTS filter to reduce current harmonics being fed back into the mains.


John Rudd28/03/2019 10:05:06
1368 forum posts
62 photos

What vfd do you buy?

The makers manual should have a typical connection drawing showing the incoming 1ph supply with their recommended protection devices and whatever is shown on the outgoing side to the motor. Following their installation recommendations should suffice in terms of meeting any protection requirements.

Typically, fused protection or rated mcb on the input and direct connection to the motor on the outgoing side with maybe a filter.

There is certainly no need for anything too complicated.

For your own protection there should be an nvr switch on the primary side.

Emgee28/03/2019 10:08:19
1445 forum posts
212 photos

Hi David

The VFD will run OK with a supply via a correctly wired 13A socket, best to use a DP switched type to give isolation to live and neutral.
Any contactor or such must be fitted in the supply side of the VFD , not between the VFD and motor.
No other devices should be needed unless you want an Emergency Stop but that can be wired from the VFD control circuit.


John, no volt protection is built in to the VFD ?

Edited By Emgee on 28/03/2019 10:10:07

Clive Foster28/03/2019 10:16:58
2146 forum posts
73 photos

Presumably this contactor is to go in the supply line to the VFD as normally a VFD is connected direct to the machine and its controls superset the machines own.

In the home workshop the primary reasons for a contactor are to provide a "No Volt Release" so that the circuit is broken if power is lost so the machine cannot self restart when power is restored and to provide overload protection if the motor draws to much current, e.g. when stalled. A properly set up VFD automatically provides both functions so no need for a contactor in the supply to the VFD.

Some VFD boxes can be configured for automatic restart when power is restored but, usually, the option is safely buried inside the standard incomprehensible detail part of the manual.

However I would fit a nice clear lever operated switch in the power line to the VFD so you can clearly see whether or not things are turned on and ready to go. I like the Lewden MW single phase **LINK** and ISO three phase **LINK** ones as they are fairly small and can be see all the way across the shop. Can be a bit hard to find.


John Haine28/03/2019 10:25:34
3007 forum posts
160 photos

Or use a double-pole switch with indicator light as used on immersion heaters. Cheap, readily available, you can see at a glance that it's on 'cos the light goes on.

David Jupp28/03/2019 10:33:09
723 forum posts
17 photos

The VFD can provide many of the functions of a contactor (overload and no-volt release) if configured appropriately - note it is possible to configure in ways that would not provide these expected protections.

Does your VFD include STO functionality? If it does (and if the STO circuit is wired correctly) that may well satisfy even requirements for safe working on the machine.

Provision of a manual isolator switch, or removable plug for the supply to VFD would be sensible.

John Rudd28/03/2019 11:33:52
1368 forum posts
62 photos

I knew it was a mistake to reply to this thread.....

SillyOldDuffer28/03/2019 11:34:04
5612 forum posts
1154 photos
Posted by David Siddle 1 on 28/03/2019 09:46:50:

... What about contactor/circuit breaker? Are these strictly necessary in a home workshop?


I looked up the contactor Mitsubishi, (the VFD manufacturers), recommended and it was pretty pricey. Is there a cheaper alternative?

The contactor is a safety feature and no-one today can tell you it's OK not to fit one. It's down to you to assess the risks, and these have to be taken much more seriously if someone else is involved. Do you ever have visitors in the workshop? Is there an insurance problem if you burn the house down due to a home-made electrical installation?

That said, many older machine tools have unacceptably poor safety features by modern standards and people use them privately without ending up in hospital, being persecuted by the HSE, or getting sued by the neighbours. Exposed gears and belts whizzing round on machines with dodgy 1925 electrics and no interlocks are manageable provided the owner takes reasonable care. Although the hobby has it's risks, I think they're relatively small. Model Engineering is considerably safer than Amateur Radio where chaps get killed and maimed falling off masts and roof-tops, or having heavy aerials land on them, plus the odd RF burn, electrocution, and lightning strike. Let's not mention microlights, horse-riding, or motor-bikes!

I would definitely build Mitsubishi's recommended circuit if your machine already has safety interlocks because deliberately rendering interlocks ineffective to save money isn't smart. Conversely, if an older machine doesn't have safety interlocks, adding a contactor is likely to be a waste of time.

For H&S and commercial reasons Mitsubishi are unlikely to recommend anything but the very best contactor. There are plenty of cheaper alternatives available: prices start just below £14 on Amazon, other suppliers available.


martin perman28/03/2019 11:44:00
1808 forum posts
78 photos
Posted by John Rudd on 28/03/2019 11:33:52:

I knew it was a mistake to reply to this thread.....


Like everything in life there is the minimalist view and OTT view of everything, its up to the user to decide whats safe and whats not, he is the one thats going to use the machine smiley

Martin P

David Siddle 104/04/2019 07:47:31
12 forum posts

Many thanks to all those who have replied with the benefit of their experience. The VFD is model

FR-D720S-042SC-EC. It's to power a Fobco drill. I'm not concerned about NVR and it's only me using the equipment.

Russell Eberhardt04/04/2019 09:20:02
2573 forum posts
85 photos

IMHO that unit will be fine when connected via a fused 13 A plug. I would unplug it when not in use or when working on it. O.K. you might forget to unplug it but equally you might forget to switch any isolator switch off. Check that the NVR function is enabled by running the machine, unplugging and plugging in again.


Edited By Russell Eberhardt on 04/04/2019 09:21:06

Peter Spink04/04/2019 10:24:25
87 forum posts
37 photos

Two buttons (one make and one break) and a 10k pot are all you need for basic control of this VFD.

If the volts disappear from the device it will need a start command to run again so NVR is sorted anyway!

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