Roger Hayter asked:
An excellent question! I've no idea: my information about frequently switching the input on and off came secondhand from someone on a Unimat group who has done the same motor-replacement as I'm preparing for: he found the advice in the instructions which came with his PSU. Mine has no such warning.
Bert
I know you'll have your long hair tied back, won't wear a scarf, roll up your sleeves, take off your tie, etc but... Having a stop(*) button that you can reach for any bit of machinery can be a life saver. Also the "no volt release" feature can prevent nasty surprises. Not sure I'd like to be turning something and have the local supply auto-reclosure do it's few seconds off-on-off-on-off-on-lockout thing, machine starting and stopping, lights going on/off. The machine latching off straight away, is one less thing to worry about.
Also will the SMPSU object to the inrush current of a motor? Either by deciding there is an overload and shutting down (with/without restart attempts) or letting the magic smoke out? ISTR that you mentioned a speed controller, can that be "got at" to provide soft start on/off control as well as speed? Might just a switch across the terminals of the speed potentiometer with an RC ciruit to make the control voltage ramp up.
(*) Bear in mind that this "stop" button only removes power it doesn't apply a brake so the machine will still needs to run down or otherwise dissipate it's kinetic energy.
I do take the point that a small lathe potentially can inflict as much damage as a large one.
As it happens, the speed controller's (small) potentiometer does have a click-off position at its closing point, but the general experience of people who have also done this motor replacement (which is not an uncommon procedure) is that the lathe can be reluctant to start at the lowest speed setting: the usual recommendation is to leave the control partly open and not use it as an on-off switch. I don't know if switching on and immediately rotating the control to say quarter-full would overcome that.
My original idea of a simple on-off switch between the PSU and the speed controller, plus an overall on/off/safety cutout switch before the PSU might perhaps provide an ideal all-round solution?
One would really need to see how the pot and the pot switch are wired. It might be that you could just series connect another switch with the pot one to give a seperate on/off function.
Probably. Is the speed controller a PWM one or variable DC volts? I wouldn't have thought a variable voltage controller would be any good for a lathe as the torque would be too low at low speed settings (why it's reluctant to start at low settings...)
Apart from the difficulty in finding a suitably rated DC switch. Can't remember if you stated a power for the motor but I should imagine it's a couple of hundred watts which, at 24 V, is 10(ish) A.
24 V truck switches might be suitable or use a beefy 24 V truck relay. Just seems a bit "messy" to mechanically switch the DC when there is already something there doing it electronically.
It's PWM.
It's 150W. It's the standard electric scooter motor recommended for this project.
If you mean the speed controller, I can wire everything up and try it. I don't have a mains switch yet, apart, of course, from the switched wall socket where the PSU is plugged but I can hardly use that as an operational switch for the lathe, except for testing.
Thanks for the thoughts, Dave.
Bert
which effectively IS variable DC volts, especially if run in series with MUCH better than a potentiometer which wastes power and limits current at low voltage settings
Oh blimey. Its a crappy old speed 600 style motor then.
you can control that ever so easy from a DC source.
Its even easier to switch it with a semiconductor like a 2N3055.
You could add a switched extension in the mains lead, with the switch next to the lathe for emergency cutoff. Perhaps a big red button you can whack.
Thanks. this has been discussed higher in the thread, with opinion divided as to whether or not a big red slammable emergency button is rather over the top for a Unimat micro lathe.
Bert
But I think near unanimity that at least a small red cutout button would be a Good Thing.
What's the maximum current? Scooter motors typically draw 20A under heavy load.
I'd be inclined to switch the motor with a big-ass relay, controlled by a push-on push-off button switch. Use a relay with a 24V coil and you can power it from the existing PSU. I'd use a plug-in relay then if/when it gets unreliable it will be easy to change it.
Bill
The 33V DC psus that charge disability stairlift batteries are permanently powered, but after each operation of the lift they are abruptly connected to the DC-DC converter in the chair that charges the battery. Weirdly, although the supplied psus seem to be mostly rated at
1.2A, the initial current draw is usually about 1.85A.Bill
Dead man's handle in the form of a foot pedal?
Bill
opinion
The NVR feature certainly and if the switch is of "standard" appearance all the better. TBH neither the table saw or small pillar drill have nice big bashable "stop" buittons but rather fiddly little things, I'd rather have something easier to bash.
Starting to get complicated. In fact thinking about it wasn't the lathe orginally mains powered? So it surely has a mains NVR already... Maybe not Unimat have been around before things like NVR's or guards on belt drives where thought neccessary. B-)
Many thanks for all the latest replies. A switch across the mains supply, located near the lathe (which the PSU won't be) is clearly a good thing. And a panic cut-off does seem to be the majority recommendation.
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