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24V vs. 115V For Contactor Control on Lathe

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EPAIII

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#1
My first question here. I have a SB-9 lathe and have been using a drum switch for control: forward, reverse, off. I am in the process of re-installing it in my new garage shop and am finding that there is a problem with the drum switch being in danger of accidental activation by someone passing by. The garage is a common path to the vehicles so the entire family goes through there often. For some time I have also wanted to add Emergency Stop switch(es) for additional safety while working with it.

So, I want to install a contactor with the appropriate On, Off, and E-Stop controls. I have noticed that the contactors most commonly come with either a 24 V or a 115 V coil. Now, I have worked with and even designed and built a lot of electronic equipment that has remote controls and, for the most part, those items would use a low Voltage for the control circuit: 24 V, 12 V, and more recently with the increased use of logic ICs, 5 V. These controls have always worked with little or no trouble and any problems were usually not due to the low Voltage being used. On the other hand, much machinery does use 115 Volts or an even higher Voltage for controls. So I am asking, is there anything different about mechanical machinery that makes the higher Voltage desirable or even necessary? Or is this just a hold-over from older use? I would prefer to use the 24 Volt control circuitry as it would be safer to run it to the E-Stop switches around the machine.
 

GLCarlson

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#2
115vac has fewer parts, thus lower cost to build. You're going to run the wiring properly (conduit, enclosures). You must control your power voltage (110-220) anyway, so it's going to be in the control box, as is power to a transformer to make 24v. E-stops on my machines all control 220v relay coils. If you've got 24v relays, etc, use 'em. Ditto, 115. If you're buying, your call. Properly built, I see no difference in safety.
 

tq60

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#3
If you have the contactor then 120 vac is okay but lesson learned with another project is 120 vac coil contactor ate not common meaning lots more cost and fewer choices.

Most common are 24 volt versions and ac or dc not much difference.

Least parts and most safe is use 24 vac coils since a doorbell transformer is this voltage and there are often adaptors and transformers of correct voltage at yard sales cheap and door bell transformers not too much new.

Lower voltage safer in switch wiring as well

Sent from my SAMSUNG-SGH-I337Z using Tapatalk
 

British Steel

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#4
Yup, and 24v AC is easier on switch and relay contacts too - note that for DC the contacts get derated by a factor between 5 and 10 on current in most cases!

Dave H. (the other one)
 

tq60

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#5
Only coil is rated as ac or dc.

Do not cheap out and get a properly sized unit.

Sent from my SAMSUNG-SGH-I337Z using Tapatalk
 

markba633csi

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#6
24 volt is easier on the switch contacts and is safer as the 115 to 24 volt step down transformer gives isolation from the line.
Mark S.
 

mksj

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#7
A coil is a coil, so it is just a factor of the voltage available and if you want your switch gear to operate at lethal voltages (120+VAC) or non lethal voltages 24VAC/VDC or less. In general thee is no or little difference in the cost of the contactors/coils for different voltages, about $17 no matter what you pick.https://www.automationdirect.com/ad...ic_Miniature_Contactors_(3-Pole)/22_Amp_(AC3)
The primary reason for "low voltage" controls is safety. The issue with very low voltage control is noise and possible triggering due to induction from adjacent wiring. If you look at many machines, the standard on smaller lathes mills is 24VAC, larger 3 phase machines you often see 110/120VAC, both single phase that is usually taken off of a transformer. I do see 240VAC contactors when used in single phase applications where everything is housed in one cabinet, but can't say it is commonly used where the controls are in another panel. So something like my compressor use a 240VAC coil contactor with a separate on/off switch and a pressure switch all operated at 240VAC. There are sometimes other situations where DC allows you you to use logic diodes and do some more creative things than AC, so I use 24VDC in my control systems.

If you have a 120VAC machine, then probably fine with using a 120vAC contactor and associate switch gear. KISS. The drum switch is also switching 120VAC so I do not see it warrants a separate low voltage power transformer or supply. In many cases the contactor has an auxiliary NO contact which is used in a latching circuit to the coil, it the power drops for any reason, the coil is de-energized so it cannot restart if the power comes back on. You always have the issue of the drum switch on a lathe getting knocked into the on position, most newer lathes have a power start/on circuit where the power contactor/relay will only latch on if the spindle switch is in the stop position. It may be possible to do this with a drum switch, haven't really looked into it.
 

EPAIII

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#8
I hadn't thought about having a Start lock out if the drum switch is on. Since my drum switch probably dates from the 1940s, I doubt that there is any provision for that. It is basically a On/Off on two pairs of contacts and the third pair reverses the phase of the line fed to them.
 

Glenn Brooks

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#9
Aside from the emergency off switch, an a preventative measure might be to install a simple, inline on/off 110v electrical disconnect box upstream from your lathe. These are often required for isolating sub panel electrical boxes. In the off position you would have no power running to the lathe or the three way switch. Hence no chance someone could accidental power it up when walking by.

IMG_1467.JPG
 

Blackjackjacques

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#14
In your case, you can use either 120V or 24V without issue. The upper voltage limit across manual controls is 120V. So you are fine either way and can use any inventory you already have on hand. However, a frequently occurring error involving relays and contactors is "duty cycle" - that is, make sure that whatever contactor you are using is rated continuous.
 
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