LVC Question

[Michael O'Sullivan]

What is the advantage to having an LVC (low voltage control) on a tablesaw or bandsaw? Will it shut down the motor in the event of a voltage drop? I am seemingly electrically illiterate.:o

[Bob Aquino]

Its pretty much standard equipment on anything with a motor 2hp or over. Heavier contacts, very fast close, equipment cant come back on if a power interruption occurs. Many lvc's include "heaters" that limit current and help to protect the motor.

[Tom Veatch]

Sounds like another name for what I've always heard as "magnetic starter" with the heaters or "magnetic switch" without the heaters. Where does the "low voltage" come into play? There is no reduced voltage anywhere in the starter for my 3HP 240v cabinet saw. There's not even a neutral conductor to the saw, nor any transformers that I can see, so even the contactor coil runs on 240v.

For the OP, if an LVC is the same as a mag. switch/starter, it contains switches and circuitry that prevents the motor from starting except when the "On" switch is pressed. That may sound a little strange, but an electric motor can't tell the difference between pressing the "Off" switch and a complete loss of electrical service. In either case, loss of electrical power or manual shutdown, a motor equipped with a mag. switch/starter will not restart, even if electrical service resumes, until the "On" button is pressed. That's a safety measure that's intended to prevent machines from suddenly and unexpectedly restarting when electrical service resumes after a power failure.

In the mag. starter configuration, the "heater" is essentially a circuit breaker that trips, opens the circuit, and turns the motor off if it sees a continued overcurrent condition. It performs the same function for the motor as the circuit breaker in the electrical wiring panel does for the circuit wiring. "Heater" equipped starters are particularly valuable for motors that don't contain internal overtemp protection.

[Dan Burstyn]

Hi Micheal,

First - The voltage to the switch is usually around 24 volts (e.g. the "low" part).

Most of the larger motor-ed tools (2 HP and above), run on 240 (220/230-ISH) Volts. To get a switch that will last, at the higher voltage (and amperage) would require heavy duty contacts.

So most tool manufacters put in a transformer, this will step down the voltage from the higher voltage to 24 volts.

My 5 HP unisaw has a 575/440/400/220/200 V to 24 volt transformer on it. I run the 220 connections to the line coming in. And the 24 volts out to the switch.
Also Note: That the switch wiring can be much smaller, as it is 24 Volts and low wattage. No need to run 10 or 12 gauge wire to the Low Voltage Switch.

Most of the larger machines also have a Mag (magnetic) controller on them. The electric magnet, in the case of a power failure breaks the contacts in the starter switch/controller, which requires you to hit the start button to restart after power is restored.

There is also the safety features of LV/Magnetic controllers/starters.

1) If something ever happened to the switch, you the person pushing the button, get a little 24 V zap :(. That is tolerable as compared to a hair curling, 220V or 440V (not to mention the 575V) zap:eek::eek::eek::eek::eek:.

I beleive that generally 220V will put a serious hurt on you, if not kill you.
I am not a par-a-metic, so do not quote me on the specific voltage (and amperage) needed to kill someone. All I know is that I take the 24V over any of the other voltages on my Unisaw.

2) Mag controlled - shuts the machine down, so that you can get whatever you are cutting out of the way (Safely), restart the machine and then restart the cut.
If the power restarted in the middle of your cut, mostly bad things happen.

Hopefully this helps.
Thanks,
Dan B.

P.S. - In the old shop, we use to have a wooden cain, that was there to pull someone that was getting zapped away from the zap-per, as you do NOT want to touch them.

[Steve Haley]

The LVC is just a motor starter as Tom states. The units made popular by Delta and other machine tool mfg. conatin a 24vac xformer for the control circuit. This way the operator pushbutton is only powered by low voltage. This came about as a safety feature b/c the pushbuttons on machine controls see some of the highest usage of anything on a machine. Buttons wear out and fall off and the operator is exposed to open wires. Somewhere in time OSHA and or NEC adopted a policy for control panel voltage. 24 vac or 24 vdc is pretty common. I have seen several panels constructed in the '50's that use full voltage from the power curcuit. This would mean the pushbuttons are energized with 480 volts AC. :eek: :eek:

[Fred Belknap]

Where I live power outages are as common as dirt. I find that when operating on a generator that my 220volt machines won't work. I don't normally use them on a gen but the other day I thought I would start the DC while running the generator and it failed to start. I tried the jointer and it would just hum (wasn't a tune I recognized). I suspect they have some kind of low voltage protection.

[Michael O'Sullivan]

Thanks for the replies. The safety angle makes sense.