Switching between lathe and mill (3 phase)

[John CoatesParticipant @johncoates48577]

Having wired up the lathe to an inverter and put a 3 phase motor on it, I am very impressed with the variable speed control so want the same for the milling machine. But I want to keep with the one inverter so need a way of joining the screened 3 phase supply cable from the inverter to some kind of switch box that would then connect between the lathe or the mill

 

Basically the 3 phase SY cable would come out of the inverter to the switch box, the switch box would be either connecting the lathe or the mill (it could have an off position), and the screening wire for the in cable and the two out cables would be attached to some kind of separate earth point

 

Anyone got any idea of what sort of switch box I should be looking for? Early Google attempts have turned up nothing like I am looking for.

 

Fall back position would be both machines having 3 phase supply cables with industrial connectors on and I would unplug and connect every time I wanted to use a different machine

 

Thanks for any help folks

 

John

[John CoatesParticipant @johncoates48577]

Want a switch box

[LADmachiningParticipant @ladmachining]

Most inverter manuals advise against having any sort of contactor or switch connected between the output of the inverter and the motor it is controlling.  I guess this is to avoid an open circuit fault condition which could damage the IGBTs on the output of the inverter.

 

The idea of using an industrial plug to switch between the two machines is probably the most feasible idea, but then careful choice of the type of plug should be given to ensure the integrity of the screeing is maintained.

[KWILParticipant @kwil]

Are both your motors exactly the same? If not, how are you going to setup the internals of the inverter to suit both?

[WALLACEParticipant @wallace]

Hi John

 

I’ve an inverter feeding 3 machines (not all at once !). There’s a ‘ring main’ of 3 phase from the inverter together with a ‘control box’ on the 3 machines.  Adjacent to the 3 pahse ring, there’s a control one made up of 10 way multicable. This carries a 12 volt supply line and the various analogue signals back to the inverter for speed, direction etc.

 

On each machine is a ‘control box’ with a couple of signal relays to put in/out of circuit the switches and potentiometer to give speed and run signals back to the inverter. There’s also a big realy in each machine in paralell with the signal relays to switch the 3 phase..

 

The control boxes have a small phono socket on the front and the workshop has JUST ONE shorted plug – so you can only ‘liven’ up one control box and therefore one machine at a time. 

 

It all works a treat – and at one time was going to do a write up for MEW  BUT decided against it after chatting to a very helpful guy on one of the inverter trade stands at an engineering show. He said the problem was that you’d probably do the inverter in if ‘the plug was pulled’ when it was running !

 

No doubt someone with time and more electronic knowledge than me could figure out a dead clever interlockled system than only allowed one machine to work at once – and wouldn’t allow the main 3 phase relay to go open while the inverter was running.

 

I should also add that the inverter is an electronic one – but very basic – it was built from a design published in Elektor magazine a few years ago and doesn’t have any fancy settings to adjust for different characteristics – just speed, direction and ramp up and down times. And the cable feeding each machine is armored (i.e. screened) to try and keep any RF escaping.

 

w.

 

 

[_Paul_Participant @_paul_]

A switch would be nice.

FWIW I asked the people who supplied my drive originally and they suggested the following, it’s OK but a nuisance to keep unplugging and plugging back in.

——————————————————————————-

From: paul floyd
To: Drives Direct
Sent: Saturday, May 08, 2010 5:59 PM

Subject: Inverter “Machine Selector Switch” could you help please.

Hello again,

Can you help, I would like to run 3 seperate machines from one VFD (E2-202 2HP) by using some kind of a selector switch to swap between the machines but only running one machine at a time.

What I need is some type of “Machine Selector Switch” do these things exist and if so do you sell them.
Preferably Break before make switch type & an interlock would be nice to prevent the inverter starting  before a particular machine is selected.

Regards

Paul Floyd

—————————————————————————
And this was their response:
—————————————————————————

Re: Inverter “Machine Selector Switch” could you help please.
Monday, May 10, 2010 2:02 PM
From:
“Drives Direct 2010” <administrator@drivesdirect.co.uk>

To:

“paul floyd” paul_s_floyd@*******

Hi,
 
well the best way to do this would be to take the inverter output to a socket(£15) and then have a lead connected direct to the motor on each machine and put a plug on the end of each lead(£5) to connect to the inverters output,
You can then use a remote POD(£35) on a fly lead and move it to the machine you have plugged in to give ON/OFF switching and SPEED CONTROL,

Give us a call if you need any more information,

Many Thanks
 
Dave.

Drives Direct(Inverters) LTD
 
Phone Office   :- 01773 811038
Mobile            :- 07976 766538

Fax                :- 08717 334875

 

————————————–

 

Regards

 

Paul

#62261

John Coates

Participant

@johncoates48577

Posted by LADmachining on 11/01/2011 17:55:44:

The idea of using an industrial plug to switch between the two machines is probably the most feasible idea, but then careful choice of the type of plug should be given to ensure the integrity of the screeing is maintained.

Ah – good point. I suppose the screening could be soldered to a wire that was connected to an earthing point 

#62262

John Coates

Participant

@johncoates48577

Posted by KWIL on 11/01/2011 19:08:19:
Are both your motors exactly the same? If not, how are you going to setup the internals of the inverter to suit both?

 Lathe is a 1hp motor (0.75kW up from the original which I guessed was 3/4 hp as it was so old the plate was indistinguishable so I went with the same motor as on  Myford) and mill is 0.55kW. I have a remote control pod made by Drives Direct which has Fwd/Stop/Rev, variable speed and Run/Jog so I can set the frequency (Hz) before switching to Fwd or Rev with a 1 sec ramp up to full speed. With the lathe I have worked out the RPM from 1Hz to 50Hz in 2.5 increments, printed this out and laminated it to put up in the garage as a quick reference. I will do the same for the mill and put that up next to the mill.

#62263

John Coates

Participant

@johncoates48577

Posted by _Paul_ on 11/01/2011 21:12:18:
A switch would be nice.

Looks like your reply from Drives Direct supports the individual plugs option rather than the switch. Had bought the industrial plugs and a wall socket from Screwfix so will go with that option

 

#62264

Steve Garnett

Participant

@stevegarnett62550

Posted by WALLACE on 11/01/2011 19:49:08: 

I should also add that the inverter is an electronic one – but very basic – it was built from a design published in Elektor magazine a few years ago and doesn’t have any fancy settings to adjust for different characteristics – just speed, direction and ramp up and down times. And the cable feeding each machine is armored (i.e. screened) to try and keep any RF escaping.

   

 

One of the reasons that this works for you is probably because this is a simple inverter. It’s simply supplying switched current into the three phases, and the motor is responding accordingly. On most of the more modern inverters, the situation is somewhat different, because they are using reverse emf feedback from the motor windings (which gives a reasonable loading indication) to control what the inverter is sending to the motor. This is how improved motor running characteristics at low speeds are achieved, and it’s a very significant reason for most modern inverter installation instructions suggesting quite strongly that you shouldn’t run motors via switchgear, etc. Anything that potentially increases the resistance of the winding path between the inverter and motor represents a problem to a system like this.

 

Relays are particularly bad, because on almost all of them, the contact resistance can vary quite a bit, and the inductive loads motors present appears to cause rather more contact damage than a straightforward resistive load does. Compared to this, a plug and socket arrangement is a considerable improvement, as long as the contacts are kept clean, and not a loose fit. Switches are somewhere in between, but unless they are significantly over-rated for the application and very well made, they are little better than relay contacts.

 

So if you really insist on using the same inverter for different motors despite the warnings, then your best bet is to use proper, maintained plugs and a single socket. Anything else is likely to land you with worse control, and possibly an expensive repair bill for some burned-out switching components. And if you want to play safe, then make sure that the socket is under a protective cover, with a switch which only allows a supply to the inverter primary side when the cover is firmly closed.

#62311

Billy Mills

Participant

@billymills

Would strongly advise against setting any drive to run at very  low frequencies such as 1-20 Hz. excepting for a few seconds with the motor unloaded. The motor is a system of components which include the windings, magnetic paths and cooling fan designed to work at a nominal voltage,  frequency and output shaft load. 

 

Under the rated conditions the windings will be cooled to be well within the limits of the enamel coating on the windings. Run the motor slow under load and the cooling will be greatly reduced. If the coil temp rises above limits the enamel insulation will soften then char as the winding shorts.Stops any further machining and starts a trip to a rewinder.

 

If you do want to run the motor slow under load then at least make sure that there is sufficient air from an external fan and ducting to keep the windings cool but do be mindful that the poor motor designer did not design an electronic gearbox but a fixed frequency motor.

 

regards,

Alan.

#62314

Steve Garnett

Participant

@stevegarnett62550

Alan’s absolutely right – and with more modern controllers of the type I was talking about the effect is even worse; they often provide torque-compensation to produce rather more power at lower revs than you would get from a simple controller, and that makes the heat dissipation issue even more critical.

[John CoatesParticipant @johncoates48577]

Posted by LADmachining on 11/01/2011 17:55:44:

The idea of using an industrial plug to switch between the two machines is probably the most feasible idea, but then careful choice of the type of plug should be given to ensure the integrity of the screeing is maintained.

Ah – good point. I suppose the screening could be soldered to a wire that was connected to an earthing point 

[John CoatesParticipant @johncoates48577]

Posted by KWIL on 11/01/2011 19:08:19:
Are both your motors exactly the same? If not, how are you going to setup the internals of the inverter to suit both?

 Lathe is a 1hp motor (0.75kW up from the original which I guessed was 3/4 hp as it was so old the plate was indistinguishable so I went with the same motor as on  Myford) and mill is 0.55kW. I have a remote control pod made by Drives Direct which has Fwd/Stop/Rev, variable speed and Run/Jog so I can set the frequency (Hz) before switching to Fwd or Rev with a 1 sec ramp up to full speed. With the lathe I have worked out the RPM from 1Hz to 50Hz in 2.5 increments, printed this out and laminated it to put up in the garage as a quick reference. I will do the same for the mill and put that up next to the mill.

[John CoatesParticipant @johncoates48577]

Posted by _Paul_ on 11/01/2011 21:12:18:
A switch would be nice.

Looks like your reply from Drives Direct supports the individual plugs option rather than the switch. Had bought the industrial plugs and a wall socket from Screwfix so will go with that option

 

[Steve GarnettParticipant @stevegarnett62550]

Posted by WALLACE on 11/01/2011 19:49:08: 

I should also add that the inverter is an electronic one – but very basic – it was built from a design published in Elektor magazine a few years ago and doesn’t have any fancy settings to adjust for different characteristics – just speed, direction and ramp up and down times. And the cable feeding each machine is armored (i.e. screened) to try and keep any RF escaping.

   

 

One of the reasons that this works for you is probably because this is a simple inverter. It’s simply supplying switched current into the three phases, and the motor is responding accordingly. On most of the more modern inverters, the situation is somewhat different, because they are using reverse emf feedback from the motor windings (which gives a reasonable loading indication) to control what the inverter is sending to the motor. This is how improved motor running characteristics at low speeds are achieved, and it’s a very significant reason for most modern inverter installation instructions suggesting quite strongly that you shouldn’t run motors via switchgear, etc. Anything that potentially increases the resistance of the winding path between the inverter and motor represents a problem to a system like this.

 

Relays are particularly bad, because on almost all of them, the contact resistance can vary quite a bit, and the inductive loads motors present appears to cause rather more contact damage than a straightforward resistive load does. Compared to this, a plug and socket arrangement is a considerable improvement, as long as the contacts are kept clean, and not a loose fit. Switches are somewhere in between, but unless they are significantly over-rated for the application and very well made, they are little better than relay contacts.

 

So if you really insist on using the same inverter for different motors despite the warnings, then your best bet is to use proper, maintained plugs and a single socket. Anything else is likely to land you with worse control, and possibly an expensive repair bill for some burned-out switching components. And if you want to play safe, then make sure that the socket is under a protective cover, with a switch which only allows a supply to the inverter primary side when the cover is firmly closed.

[Billy MillsParticipant @billymills]

Would strongly advise against setting any drive to run at very  low frequencies such as 1-20 Hz. excepting for a few seconds with the motor unloaded. The motor is a system of components which include the windings, magnetic paths and cooling fan designed to work at a nominal voltage,  frequency and output shaft load. 

 

Under the rated conditions the windings will be cooled to be well within the limits of the enamel coating on the windings. Run the motor slow under load and the cooling will be greatly reduced. If the coil temp rises above limits the enamel insulation will soften then char as the winding shorts.Stops any further machining and starts a trip to a rewinder.

 

If you do want to run the motor slow under load then at least make sure that there is sufficient air from an external fan and ducting to keep the windings cool but do be mindful that the poor motor designer did not design an electronic gearbox but a fixed frequency motor.

 

regards,

Alan.

[Steve GarnettParticipant @stevegarnett62550]

Alan’s absolutely right – and with more modern controllers of the type I was talking about the effect is even worse; they often provide torque-compensation to produce rather more power at lower revs than you would get from a simple controller, and that makes the heat dissipation issue even more critical.

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