Connecting a 3-Phase switch
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    Default Connecting a 3-Phase switch

    I'm converting my Hitachi bandsaw from singe phase to three phase using all factory parts. This is a 4-wire, 3 hots and ground. The push-button, non-momentary switch looks like this:

    pb290023.jpg

    On the back, you can see the terminals:

    pb290022.jpg

    This brings us to where I am a bit puzzled. You can see by the faintly visible numbers that the left side is marked 1-2-3, while the right side is 4-5-6. I have three leads coming in, and three leads coming out, but are you supposed to connect the leads to those middle (2 and 5) terminals? The three phase on/off switches I have used previously all had three pairs of connection inside, but this one appears to have two.

    I have the impression that the idea with this switch then is to switch only 2 of the three leads. That is, one lead going to the switch would connect through 1 to 3, and the second lead 4 to 6.

    But, if one of the leads is un-switched and is always sending current to the motor, with the other two off, then does this affect the motor in some way or not? I know it won't start with one lead, but not sure the affect otherwise.

    If connecting one of three leads to the motor and switching the other two would be bad for the motor, then how is one to connect three incoming leads to such a switch, and then three outgoing ones, using only 4 connection points?

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    Do you have an electrical diagram for the bandsaw? If not, how do you know what parts are needed?

    If this switch is the correct part then I would think it is only one piece of a multi part solution. Perhaps there is a 3 pole contactor that this switch operates.

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    Perhaps there is a 3 pole contactor that this switch operates.
    Indeed - that is what push button stations do - they operate a magnetic starter or similar

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    Quote Originally Posted by Bill Simmons View Post
    Do you have an electrical diagram for the bandsaw? If not, how do you know what parts are needed?
    No wiring diagram. i know what parts are needed by scanning the parts listing for the machine and selecting all the items which were specified/noted as '3-phase'. It's not a lot of parts, and includes the motor, the switch, the drive belts, the electrical supply cable, and the shroud for the drive belts.

    Quote Originally Posted by Bill Simmons View Post
    If this switch is the correct part then I would think it is only one piece of a multi part solution. Perhaps there is a 3 pole contactor that this switch operates.
    I'm sure the switch is the correct part, and it is completely different than the single phase switch and box it replaces. There are no other associated parts in terms of switching or contactors shown on the parts diagram. The motor electrical box has provision for connection of the 3 leads, plus ground. This is the motor side:

    pb260005.jpg

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    Given that an ac motor needs the induction of an electrical field in order to spin. I don't believe a field could be created if two of the three phases were switched off. But I'm no electrical engineer.

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    Quote Originally Posted by Chris Hall View Post
    On the back, you can see the terminals:

    pb290022.jpg

    This brings us to where I am a bit puzzled. You can see by the faintly visible numbers that the left side is marked 1-2-3, while the right side is 4-5-6. I have three leads coming in, and three leads coming out, but are you supposed to connect the leads to those middle (2 and 5) terminals? The three phase on/off switches I have used previously all had three pairs of connection inside, but this one appears to have two.
    It looks to me that the middle rivets are not connection points.
    The green switch is normally off and when you press it the connection between 1 and 4 will be made.
    The red switch is normally on and when you press it the connection will break between 3 and 6.
    To verify just use a ohm meter.

    And three wires are all you need. If you look at the many start/stop switch connections on the internet.

    Control Wiring - 3 Wire Control - Start Stop Circuit

    The first wire goes to the start button left side,
    The second wire goes to the common wire connecting the start button and the stop button.
    The third wire goes to the stop button right side.

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    You need the contactor that John pointed out and is show in the reference diagram from rons post.

    Tom

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    Quote Originally Posted by TDegenhart View Post
    You need the contactor that John pointed out and is show in the reference diagram from rons post.

    Tom

    Here is a picture of one. This is the ACTUAL three phase "switch"

    Your PB station discussed at length above controls the coil circuit in such a contactor / mag starter

    As such it would never be used to control a LOAD like a three phase motor
    Attached Thumbnails Attached Thumbnails p1000122sm.jpg  

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    As you asked, yes you can stop a 3 phase motor with just opening 2 leads. I have not seen it done and I would consider it bad practice as it leaves power connected where it is not expected. I would expect that switch to be used on a small single phase 240 Volt motor. What is the motor size and how much current is the switch rated for?

    Using some form of continuity tester tell us what connects to what on the switch when the green button is pressed and also when the red button is pressed. You will need this information even if a contactor is used. I suspect 1 connects to 3 and 4 connects to 6 when the green button is pressed and nothing connects to anything when the red is pressed.

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    Quote Originally Posted by Bill Simmons View Post
    As you asked, yes you can stop a 3 phase motor with just opening 2 leads. I have not seen it done and I would consider it bad practice as it leaves power connected where it is not expected. I would expect that switch to be used on a small single phase 240 Volt motor. What is the motor size and how much current is the switch rated for?

    Using some form of continuity tester tell us what connects to what on the switch when the green button is pressed and also when the red button is pressed. You will need this information even if a contactor is used. I suspect 1 connects to 3 and 4 connects to 6 when the green button is pressed and nothing connects to anything when the red is pressed.
    This motor is about 2 horsepower. Here's a video of the same saw as mine in the native Japanese 3-phase configuration:




    I have checked out the switch I have with a meter and figured out what connects to what relevant to the switch position. It works as a single pole switch for two leads.

    I managed to obtain photos of a similar machine in Japan wired 200v. 3-phase, junction boxes open and switch cover off, and from the description and photos it very much appears that they control the motor by switching on/off two of the leads. That's what I am planning to do with mine.

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    Updating this now as I finally completed wiring in my Phase Perfect and could put power out to an outlet and plug in my bandsaw. And it works perfectly. Yes, I am controlling the 3-phase motor by switching 2 of the 3 leads only. This is how Hitachi wired the machine, though it took a little detective work to confirm.

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    Quote Originally Posted by Chris Hall View Post
    Updating this now as I finally completed wiring in my Phase Perfect and could put power out to an outlet and plug in my bandsaw. And it works perfectly. Yes, I am controlling the 3-phase motor by switching 2 of the 3 leads only. This is how Hitachi wired the machine, though it took a little detective work to confirm.
    You're doing it wrong, and un safely too. You need to use that switch to control a 3 phase contactor. See above about 3 wire control of a contactor. Look at Automation direct for a contactor with adjustable overload. Your way may work, but it won't for long. Fix it right, do yourself a favor.

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    I respect your opinion, however it is a little short on specifics. I tern to think Hitachi has knowledge of how to design machine electrical circuits. What in particular do you think is unsafe? What conditions should be observable which will lead to failure or danger? Is the motor going to heat up and set on fire or give an electric shock?

    I'm aware of the safety feature that magnetic switches provide, but, okay, let's say I'm working in my 1-person shop and the power goes out while I am using the bandsaw. A power outage has never happened mind you as we have solar power, but let's say it does. I leave the machine switch in the 'on' position, and leave the phase Perfect disconnect also in the 'on' position for some reason. A while later the power comes on, the lights come back on and then the Phase Perfect comes on and goes through it's checking cycles for 30 seconds or so. I guess I remain oblivious to these developments. Then the PP provides output 3-phase and the bandsaw turns on. Unless I am resting my arm against the blade or have for some weird reason decided to take the blade off while the power was off and the lights out, how exactly is the machine turning on going to be dangerous?

    When the machine is off and the Phase Perfect is on, the current consumption is zero. When it is on, all of 2 amps is flowing.

    How long do you think it will go until it fails? Do I have hours, days, months or years? Be specific.

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    Watching your video Chris, I think you can hear the "snap" of a magnetic contactor closing when the machine is turned on. Or is that the sound of the push button switch?

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    Hi Richard. It's just the sound of the switch. Push the green button down and the red button pops up, and vice-versa. The only magnets are in the motor itself.

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    This may be allowed by the NEC, at least in the older versions, but I wouldn't do it. See NEC 422 & 430. You will need to have a separate disconnect within site of the machine that DOES break all the ungrounded conductors.

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    And, at 2 HP, the NEC wants a "motor controller" complete with an overload sensor that cuts power if an overload beyond it's cut-off rating occurs.. Normally all that is wired to a contactor. At 1 HP and below, the NEC has accepted an overload in the motor, with a simple switch for power.

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    Chris,

    I can assure you that Hitachi wouldn't use a switch the way you describe. And yes, this is because "Hitachi has knowledge of how to design machine electrical circuits".

    First, you MUST disconnect all three hot legs (phases) for safety reasons. I think it's pretty obvious, isn't it? Do you know why your regular lighting switches or circuit breakers are installed on the hot side (phase) vs. neutral?

    Every single 120V device that uses an ON/OFF switch will have it installed into the hot side. And every "single phase" 240V device that uses two hot wires will have a double-pole switch to disconnect BOTH hot wires. Can the devices be turned ON and OFF if the switches were in the neutral or any one side of a 240V circuit? Of course they can, and they will. But try to touch any exposed conductor inside this turned off device and tell us how you feel. You better put rubber boots, stay on a thick dry rubber mat, use single hand and not lean against a plumbing pipe. Shortly speaking, an ON/OFF switch should not only turn the device off, but also de-energize it. And this can only be done when all hot wires (phases) are disconnected.

    As far as directly switching inductive loads such as motors, you should remember that at the moment of turning a motor off, you can expect arcing across the contacts. Therefore the contacts should be designed to deal with this and last under this condition. Contactors are much more reliable in this respect than switches, and when you deal with really powerful motors, you'll need to use them.

    Your switch may be perfectly sufficient for switching a 2HP motor (it's hard to judge based on a photo alone), but it definitely must be a 3-pole one for a 3-phase motor unless it's used to operate a contactor or a control device such as VFD, etc.

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    Quote Originally Posted by MichaelP View Post
    Chris,

    I can assure you that Hitachi wouldn't use a switch the way you describe. And yes, this is because "Hitachi has knowledge of how to design machine electrical circuits".
    The problem with this assertion is two-fold: the 3-phase specific switch Hitachi provides for the machine has only 4 terminals, as shown above. Secondly, a friend in Japan who works in a shop with the same bandsaw, with the same 3-phase motor and switch, took photos for me of the connections at the motor's junction box and at the switch, and it is clear from those photos that Hitachi wired the bandsaw so that the switch operates two phases. Now that I have wired it up, the machine operates fine. And there is no amperage draw when the machine is plugged in but turned off. When the bandsaw is running, draw is 2 amps.

    As to the rest of your piece, you seem to presume that I would open up machine electrical parts, like the motor junction box, without first unplugging the machine, which is hardly something I would do. With the machine unplugged, your points are moot in my view.

    I didn't pose a question of: is there some type of switch that might be better than this one?

    I didn't ask people to relate to me how magnetic switches are safer for all kinds of machines.

    I only asked whether a small 2 hp 3-phase motor could be controlled by switching 2 phases, as at the time I posted the question, I only had the information of the switch sitting in front of me.

    I do not have time for people insisting that there must be parts missing.

    I think there are plenty of folks who can tell you what they have done and what they have seen, and what current code requirements might be, but someone who can clearly and accurately answer the question posed seem to be non-existent. And if you truly understood the matter, and felt the switch would fail or damage the motor, I have yet to see someone provide a clear mode and time frame by which that will occur. You know, something that I can observe or test.

    The machine is from the late 1980's or early 1990's, so it is entirely likely that switch requirements in the US were different then. Lot's of old machines lack magnetic switches, like most of the Bridgeports out there for example. I'm perfectly aware of the safety issues magnetic switches address, just not seeing the need for that with a small bandsaw.

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    I just spoke to a retired electrical engineer with wide experience. He said my set up is fine, and is a type used with open phase delta systems. These systems are uncommon in the US, but he has worked on them. He is coming up to visit me in the near future and I'll have him inspect my set up.


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