Myford S7 VFD problem

[Andrew CrowParticipant @andrewcrow91475]

Hi Glyn, I have exactly the same set up as you and I bought VFD 2 years ago. I hadn’t noticed a problem before because I don’t use top speed very often.

I have just checked it after reading this thread and it does exactly the same as yours. It was only a quick test so I will run a few more tests over the next couple of days based on some of the suggestions above and get back to you.

I think a second reported problem to NT may prompt a better response from them.

Andy

[Neil LickfoldParticipant @neillickfold44316]

My setup is a 750 watt 240v 3phase with a VFD , the VFD is from China. I got mine as a working set up from a local supplier. They still sell the motors and inverters, but do not program or set them up for the hobbyist any more. The setup is for the S7 ML7 lathes, and works when in high gear at 100% 50 Hz. Tried it at 60Hz as well, and works just fine with the clutch to start or stop the spindle. Has a 120% / 60 Hz max setting and 10hz is the minimum setting. In high gear it works with the clutch at 10Hz, 50Hz, 60hz.

NT do have a problem with their inverter settings is My take on it.

Neil

[derek hall 1Participant @derekhall1]

I have the same NT set up on my Super 7.

I always use the clutch – force of habit I guess

It does happen very occasionally if when engaging the clutch the lever slips out of my hand and the clutch suddenly engages. This does trip the inverter.

First time it happened I was scrabbling to find the warranty! after 5 mins all was ok. It has only happened twice in total.

I can live with that, I always make sure I gradually engage the clutch anyway.

I know that you can leave the clutch engaged and just use the NT controls to start and stop the motor when required, however once the motor is running it stays running until I am done. Only adjustments I make is occasional pulley belt selection and speed control on the NT.

I am very pleased with NT. I have used them before many years ago, I would suggest that you contact NT and raise your concerns.

Regards

Derek

[EmgeeParticipant @emgee]

Most if not all VFD’s have the option to change the time and duration of motor overload so the unit doesn’t trip at every increase beyond motor full load current event, as suggested in another comment perhaps these parameters need tweaking to accommodate the use of the clutch as stated by some users.

I fail to understand why anyone buys a VFD with the ability to change parameters but is denied access to do so without invalidating the guarantee, would they buy a house air conditioning unit without being able to change the temperature ?

Emgee

[Dave HalfordParticipant @davehalford22513]

Having watched the video and ignoring the lumpy sound the featured lathe clutch makes.

Without the soft start circuit the sudden inertial load from the chuck when you engage the clutch might be overloading the VFD run settings. This will be worse with a larger chuck than standard and again with a heavy work piece.

Is it the same at the lowest speed on the high range belt?

The NT warranty makes interesting reading, the user controls are not covered, neither are the motor bearings in any way.

[Emgee]

On Emgee Said:

Most if not all VFD’s have the option to change the time and duration of motor overload so the unit doesn’t trip at every increase beyond motor full load current event, as suggested in another comment perhaps these parameters need tweaking to accommodate the use of the clutch as stated by some users.

I fail to understand why anyone buys a VFD with the ability to change parameters but is denied access to do so without invalidating the guarantee, would they buy a house air conditioning unit without being able to change the temperature ?

Emgee

Not really valid, all the average Joe wants to do is change speed and direction, which you can. Messing with the other parameters requires you to understand the manual, which in my case runs to well over 100 pages. Get it wrong and you might release the magic smoke. You can’t change overload settings on your aircon system.

[Emgee]

On Emgee Said:

……………….

I fail to understand why anyone buys a VFD with the ability to change parameters but is denied access to do so without invalidating the guarantee, would they buy a house air conditioning unit without being able to change the temperature ?

Emgee

One reason might be, if it’s sold as part of a package including the motor, with a 3 year warranty on the motor and a 10 year one on the inverter section, the supplier world have no control over someone mis-programming the VFD.

Bill

[duncan webster 1]

On duncan webster 1 Said:

On Emgee Said:

Most if not all VFD’s have the option to change the time and duration of motor overload so the unit doesn’t trip at every increase beyond motor full load current event, as suggested in another comment perhaps these parameters need tweaking to accommodate the use of the clutch as stated by some users.

I fail to understand why anyone buys a VFD with the ability to change parameters but is denied access to do so without invalidating the guarantee, would they buy a house air conditioning unit without being able to change the temperature ?

Emgee

Not really valid, all the average Joe wants to do is change speed and direction, which you can. Messing with the other parameters requires you to understand the manual, which in my case runs to well over 100 pages. Get it wrong and you might release the magic smoke. You can’t change overload settings on your aircon system.

Duncan

I clearly give the average Joe more ability than you especially with a well written manual and telephone support.

You can’t change the o/load on the air condition unit however you can vary the temperature.

 

Emgee

[Andrew CrowParticipant @andrewcrow91475]

I have run a few more tests on my machine and found that it doesn’t trip on the slower two speeds but does on the higher two if you engage the clutch quickly. (remember when you had kangaroo petrol in your car)

If you engage the clutch more slowly it doesn’t cause the problem. Having said that using the VFD to start and stop at the higher speeds seems to be a much smoother operation, that’s maybe why I haven’t noticed a problem before.

Andy

[John HaineParticipant @johnhaine32865]

I think once one has a VFD one realises that a clutch is not really necessary, but Myford users have got accustomed to it.  Few other lathes have a clutch AFAIK and nobody complains about those.  The Myford clutches seem reasonably reliable but it is just another thing to wear and need repairing.

[derek hall 1Participant @derekhall1]

I use the clutch on my myford as I can engage it slowly keeping an eye out on the chuck and workpiece as it gets up to speed.

If I leave the clutch engaged, I have to look down to make sure I press the green button, crucially taking my eyes off that chuck and workpiece etc.

I will continue gently engaging the clutch, it feels to me “safer”.

On large Colchester lathes I used to work on, there was a large stop start brake lever that you could put your hand on whilst keeping an eagle eye out checking all that rotating mass of spinning metal.

[NealebParticipant @nealeb]

I think that the clutch is a useful addition if you are using the standard single-phase motor – it avoids the starting loads involving the centrifugal switch, starting capacitor and so on. But once we have the three-phase motor that is essential for use with the VFD all those switching and mechanical loads go away, especially with the soft-start capability that comes more-or-less for free with a VFD. I used to have a S7 with its chunky clutch and used it all the time but I now have a lathe with three-phase motor and VFD and start-up is even more gentle via the speed ramp-up.

Coming back to the original question – one of my complaints about the “sealed for life” VFD enclosure would be the inability to see any error codes on the VFD when it does trip out. There have been a few suggestions effectively looking at various overload conditions but there is another possibility that I suffer from in my own VFD. Most VFDs will have a three-phase bridge rectifier at the input, used to charge a DC bus capacitor which then feeds the inverter section that produces the output. This input rectifier configuration is also happy to accept single-phase input and again this will charge the capacitor. However, the three-phase input will charge with a series of 300Hz pulses where single-phase only gives 100Hz charging pulses. Apologies to people with more insight into what is actually happening here but at a hand-waving level it’s about right! When running, the inverter circuitry is drawing charge from the capacitor and the voltage will pop up during each charging pulse but “sag” between pulses. If the charging pulses are further apart then this “sag” will be more pronounced. Generally, if you are running within the VFD’s ratings this should not matter too much but under heavy load, the sag can exceed allowable VFD limits. In fact, in the case of my own lathe and VFD, at high speed just the drag of the machine is enough that this voltage sag trips the VFD protective circuitry and the reason that I know this is the error code that comes up tells me so.

I wonder if this is also the problem reported here?

In my own case, I know why it happens – it is actually a 415V input/output VFD, fed via a voltage doubler from 240V single-phase which means that the charging pulses are only at 50Hz. This allows the inter-pulse voltage sag to be even greater. Fortunately, this only happens above about 2K RPM and I tend not to run at those speeds very much – it’s an elderly lathe and in any case an 8″ three-jaw chuck just looks intimidating at that speed…

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