Do rotary phase converters need a breaker box?
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    Default Do rotary phase converters need a breaker box?

    Not my phase converter but I was recently asked a question maybe you guys could help with. This old 3 phase converter has been in use without any issues for years. At the time it was pieced together with a bunch of industrial scrap wire, hence the 480v breaker box (600vac fuses inside) and various sizes wires.

    Should a proper breaker that would actually work if needed be installed? The input to the converter is just 220v out of a 50a breaker (if I remember right) The goal I think is to get the converter tucked away somewhere that won't take up as much space and if it can do without a breaker box that would be great.
    20190405_110954.jpg20190405_110858.jpg20190405_110831.jpg

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    Quote Originally Posted by setlab View Post
    Not my phase converter but I was recently asked a question maybe you guys could help with. This old 3 phase converter has been in use without any issues for years. At the time it was pieced together with a bunch of industrial scrap wire, hence the 480v breaker box (600vac fuses inside) and various sizes wires.

    Should a proper breaker that would actually work if needed be installed? The input to the converter is just 220v out of a 50a breaker (if I remember right) The goal I think is to get the converter tucked away somewhere that won't take up as much space and if it can do without a breaker box that would be great.
    20190405_110954.jpg20190405_110858.jpg20190405_110831.jpg
    If you have it connected to a 50a breaker then it is already breaker protected. How many hp is the motor? A 50a breaker should be good enough for up to @15 hp. I usually run my wiring size and breaker size questions through "The Engineering Toolbox" Engineering ToolBox
    Last edited by Joe Miranda; 04-05-2019 at 10:52 AM. Reason: clarification

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    It looks to be 5 HP, so about 15A FLA (the plate is very hard to read). The 50A is a bit large, a 20 should do it based on 125% FLA, just for the idler. But then there is power for the load which presumably comes through that as well

    To do it right, a motor controller with proper overloads should be used for the idler.

    Does this power a particular load, or many loads?.

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    A breaker in the box is primarily to protect the wire. Not the item at the end of the wire. Something like a motor at the end should have a Starter with heaters or what ever to provide motor protection which is in a disconnect box with breakers or fuses on the machine sized for the need of the machine. Anything needing protection plugged in or connected to the line will have protection so your Breaker at the panel should be sized to the wire you are using!

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    If the unit is fed from a 50 amp breaker then *all* the wiring downstream of that needs to be at least number 6
    until you reach a smaller overcurrent device. This is possibly the best reason to feed a subpanel from this
    idler, and then feed the individual machines from that.

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    So no harm in not having a knife switch /breaker box in between the converter and machines?

    I'll pass along the subpanel advice, that seems like a good idea.

    This converter never runs more than one motor at a time, a Drill press and a bridgeport, definitely less than a 3 hp motor.

    I'll have to look into motor controllers, not entirely sure what that is (vfd of some sort just for 3 phase?)

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    A motor controller, or "motor starter" * of the simplest type is a switch or contactor, which has a current sensor as part of it. The sensor has somewhat the same characteristic as the motor, so it reacts to the same current vs time conditions as the motor, but trips before the motor is damaged. When it trips, ir opens the switch or contactor and removes power from the motor.

    Old ones had a "heater" arrangement that would, if the overcurrent was large enough and/or lasted long enough, melt a low melting alloy in a tube, which would act to open the contactor or switch. Newer ones are electronic, and may be settable to a range of currents, but the electronics gives a similar timing to the heater heating the alloy to melting. The old style would reset when the alloy re-solidified and the switch or contactor was again closed.

    There are "manual motor starters" which are a manual switch type, and there are ones that operate from the usual pushbutton setup with a contactor.

    As for VFDs, some are rated as motor controllers, others are not. But the term does NOT imply a VFD.

    * The term "motor protector" can refer to just a temp sensor (often a black plastic case with a red button) in the motor that itself breaks the circuit, or to the current sensor which is incorporated in the motor controls such that it opens the contactor or itself breaks the circuit to stop the motor when it trips. It has to be "reset" to start the motor again.

    "Motor controller" or "motor starter" generally means the complete system to start and stop a motor, and includes the current sensor type of "motor protector'.

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    Just to add to JST's list of over current devices there was another type that was similar to the "thermal" type described. It had a Bi-Metallic strip that held a switch in the closed position. Heat would cause the ratchet to release and open the contact. When cooled a single button or individual tab could be pushed when the strip cooled. This is the most common type encountered and I think still available. A separate block was available that did not have a coil or contacts and was to be used with contacters without overloads. I have seen this type block used on machines with more than one motor. A single contacter capable of the combined amperage was used and individual overload blocks were connected to each motor, their contacts wired in series with the contacter's coil. Any one would de-energize the contacter if tripped.
    If you are interested in protecting individual motors this or the type JST mentioned should be used.

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    [QUOTE=setlab;3340030]So no harm in not having a knife switch /breaker box in between the converter and machines?

    There is harm. If you do not have a subpanel with breakers after the converter then ALL THE WIRING TO ALL THE MACHINES
    NEEDS TO BE RATED 50 AMP. Does not matter what the overcurrent device at the load motor is.

    This means you must run number six wire to all the machines and I doubt you want that.


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