Magnetic Starter?

If they want to scrap it then are they willing to guarantee the protection you just threw away?

The overload part of the starter allows the high inrush currents that happen when the motor first starts up, but will open up when extended high currents are going to toast something. The contacts are made for switching this kind of load and will last a very long time and not weld themselves shut eventually, unlike most switches you could use. You might look and see if a manual starter might work for you. No control wiring required.

Joe, The way a magnetic starter works is this: It is a relay that uses a low current to close the contacts which handle the large current of the device being powered. They also, usually, have a feature where they only need momentary power applied to stay on. Once the relay contacts close they also supply power to the relay coil along with powering the device. This is good because if power is lost for some reason the relay contacts open and the device will not turn itself back on once power is re-established. Your compressor may be wired so that the pressure switch supplies power to the magnetic starter. If so, then the starter probably does not power its own coil so that when max pressure is reached the pressure switch shuts off power to the coil and the compressor motor turns off. In any case, if the magnetic starter is functioning properly why remove it? You can be sure that if it's original equipment it wasn't put there just for fun. ERS

It may have an "overload" connected to it as well so that if the motor draws too much current for any reason, it cuts it out before it cooks the motor windings

Tom

JB

You really dont want to accept the advice of the "couple of individuals" who suggested you discard the magnetic starter. If the magnetic starter has included in it, overloload switches that match your 5 HP 3 phase motor, you have an excellant switch for your motor. It surprizes me that you have a manual switch to replace the magnetic starter.

If you choose to keep the magnetic starter and wish to actuate it with push bottons, I will give you a couple used, high quality Allan Bradley pushbuttons. I'd even throw in a couple indicator lights.

You can get all the help you need for woring that magnetic starter from qualified RCM guys. Ugly's Electrical Referances booklett (Home Depot) has all the information you'd need for wiring the magnetic starter.

Jerry

Keep the mag starter in the circuit. For openers, it has overload heaters that will sense a problem like a locked rotor far faster than the circuit breaker at the panel - usually fast enough to save the motor. This is doubly critical on a 3-phase unit, because the circuit breaker doesn't care that one phase is dead - the motor sure does!

(If you want double insurance on 3-Phase you can hook a simple $20 electronic phase monitor up after the starter contactor, which will detect a dropped phase, rotation reversal, or big voltage imbalance and open the contactor even before the overload heaters do, in well under a second. Cheap insurance.)

When you get above 1 Horsepower, the switching contacts in some pressure switch models are rated for and /can/ handle the motor load directly, but they /shouldn't/. They slowly burn themselves up and fail open, and occasionally they weld closed and the compressor fails to shut down. With a 5-horse it happens much faster.

And having a mag starter makes it extraordinarily simple to put an "Off" switch on the compressor so it doesn't run at night, or when your shop is not in use. You can use any simple switch (or even a timer!) because it's a pilot-duty device switching 50 milliamps, not the full 50 amps.

I have a 4-horse compressor without a starter, and I'm going to ADD a mag starter to it as soon as I find one for the right price. (Which is "Free to a good home." I run into them at work occasionally.) Does that tell you something?

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Here's something I hadn't heard of. Are they readily available from a place like Grainger or McMasterCarr?

Would it be a good idea to wire one of these into the Estop circuit on large machines? If this device drops, open a NC contact in front of MCR? (Master Control Relay)

I had a weird one where my generated leg dropped out after my hydraulic pump started on my huge CNC lathe. (pump comes in on Estop reset) The pump kept going on two legs (as all three phase motors will) until I tried to bring in my 20 hp spindle. It popped the main circuit breaker in the machine. No big deal, but this means the control lost power also.

Karl

Those who suggest starting a 5 HP motor without a mag starter are working from a vast background of ignorance. The starting currents are too high for pressure switch contacts. They will quickly burn and pit. The running current protection is there to protect the motor. Have you priced a 5 HP single phase motor lately? 'Nuff said. Bugs

Magnetic starters are a good idea on any heavy equipment, because you can design any kind of control logic into the system that you want. Ladder logic is used to work up time delayed starts & stops, machine sequencing, limit switches, etc, etc. etc. Bugs

Grainger carries the Square-D starters with Motor Logic (tm) electronic overload, which has a phase monitor built in. (Cat 395 PP

ICM makes separate wire in phase monitors.

If you're talking a manufactured leg from a phase converter, you are probably better off with a plain old starter with three heaters. Or a phase monitor that can be adjusted to rather wide tolerances of "OK".

The voltage on the manufactured leg is probably going to vary enough to drive an electronic monitor nuts - it's expecting to see nice clean and rock steady utility-fed 3-phase that doesn't vary more than a few volts, at worst a motor generator plant...

And yes, you could tap into the control loop of any motor starter and have it drop out the main control power to the equipment - hell, you can do about anything with relays. Give me 10,000 relays (and a few other critical components), and I can build you a complete telephone exchange switch. One that will take a whole lot of daily maintenance to keep running, but they worked for 75 years...

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