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Thread: Does a Rotary Phase Converter Actually Make Real 3-Phase Power?

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    nova_robotics's Tools

    Does a Rotary Phase Converter Actually Make Real 3-Phase Power?

    I have an inverter-based phase converter in my shop. It passes through the 240v from the utility untouched on two wires, and synthesizes a third phase. I always believed the synthesized phase created a close enough approximation to actual 120 symmetrical 3-phase that it just didn't matter. Nope! This is super interesting.


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    carloski (Dec 6, 2022), Frank S (Dec 6, 2022), metric_taper (Dec 8, 2022)

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    Supporting Member Frank S's Avatar
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    My RPC according to anyone you want to talk to is undersized to run even my mill. This is both true and untrue at the same time.
    the General accepted policy is to size your RPC at 125% of the motor on the machine you need to power.
    When I bought my 3 hp 17-54 LeBlond lathe is came with a 2 HP RPC made by American Rotary I asked the guy about it, and he said that was how it was configured when he bought it and had run it without issues for many years it just didn't instantly start or reverse instantly. So, I ran it like that for 3 or 4 years until I started constructing a power panel for the machine shop 200-amp 24 circuit 120/240v split phase panel added a magnetic starter in an enclosure for the RPC allowing me to switch it on or off from a wall switch. a 12 circuit 3ph 200-amp breaker panel with a main breaker ran the 2 lines to the main breaker and pulled the 3rd leg from the RPC. Start the RPC and have 3 legs of power to the bus bars below the main and 3 pole breakers to each machine.
    Now you might think all that is well and good, but you can still only run 1 machine at a time. NOT ALWAYS TRUE. each motor brought online in a sense once it is started and runs up to RPMs becomes and idler adding to the 3rd or wild leg of my panel. I can have my lathe running and switch on the mill and it will start almost instantly. or have the mill running and switch on the lathe and it starts faster. The other day I wanted to bring my 7HP lathe online to check and see if it is going to be a good lather or if I should just sell it Naturally the RPC by itself could not start the much larger motor it just tripped the breaker to the RPC. So I started the mill let it run as a idler motor started the lathe I always use and allowed it to do the same, then made sure the spindle clutch was disengaged on the 22" Leblond the motor spun up as if it were receiving power company 3ph power I reversed it several times engaged and disengaged the spindle a few times and it never faltered so to simulate a load I engaged the spindle clutch and it the start button, it spun right up, not that I would ever start that lathe with the spindle engaged but my 17" lathe does not have a spindle clutch so switch it on and the spindle runs all the time the motor is running just like my mill.
    I am building a second PRC to parallel in circuit but don't have all of the Start and run capacitors connected up to assist in starting it and balancing out the line outputs, the second RPC is an 8 hp motor. Once this one is online then I can finish connecting my 9KV step up transformer and have enough 460v to power my layout drill, plus, if need be, I have 2 more of the 8hp motors to put together a 3rd or even a 4th RPC.

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    I have a 20HP (15kW) Phase Perfect inverter to drive a couple CNC machines in my shop. Apparently it has a 200% overload capability for short periods (~30 seconds or so). You're totally correct that the manufacturers will always WAY oversize their recommendation on equipment. They did the same to me, which is why I ended up with this gigantic inverter in my shop. I think it's just a way of eliminating their own headaches and issues with tech support. My 3 axis mill has a 15 HP spindle, plus a servo for each axis, plus another horsepower or so for the high pressure coolant pump. So the "rating" for all of these things is about 20HP. In reality that machine it barely ever uses more than about 2 kW when running. It doesn't even flex the phase converter's muscles. I really do like the inverter though. It only uses 70 watts at idle, so I just leave it on all the time. Rotary Phase Converters use a ton of power at idle so it's not really economical to do that.

    But that said, do yourself a favor and buy one of these for your lathe:

    https://www.amazon.ca/Variable-Frequ.../dp/B08D94RXS5

    That'll also let you run your 3-phase lathe directly off of 240v single phase, and you get infinitely variable speed control of the motor (awesome for threading) plus electronic braking. It is a MASSIVE upgrade to your lathe. It's like night and day. You have a little dial and you just turn it up and down to get any RPM you want. Plus it might take enough load off your existing RPC so you don't have to build a second one and you can avoid all that extra wiring of your shop.
    Last edited by nova_robotics; Dec 6, 2022 at 09:36 PM.

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    Quote Originally Posted by Frank S View Post
    Once this one is online then I can finish connecting my 9KV step up transformer and have enough 460v to power my layout drill, plus, if need be, I have 2 more of the 8hp motors to put together a 3rd or even a 4th RPC.
    Wait a second. I was busy writing stuff about VFDs and phase converters and totally glossed over the fact that you're adding a 9kV step up transformer to your shop. What on earth do you need 9kV for?

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    Quote Originally Posted by nova_robotics View Post
    Wait a second. I was busy writing stuff about VFDs and phase converters and totally glossed over the fact that you're adding a 9kV step up transformer to your shop. What on earth do you need 9kV for?
    OOPs the "a" was missed, the 9 KVa transformer weighs about 300 lbs it was with the Cleereman layout drill when I bought it at a Lockheed auction about 25 years ago and was the only way I could boost my then 208 3ph up to the 480 required to run the drill press I could rewire the many motors in the machine to run on the lower voltage but all of the magnetic starter coils would have to be replaced so simpler to buck up the voltage I have a 600v 3ph panel mounted with circuit breakers fed from the transformer for any 480 machine I might need to eventually wire in and 600v disconnect boxes to be mounted near each machine requiring 480 v just like 240v 3 pole disconnects at the lathes all wiring is run in steel conduit with a ground wire for each machine 120/240 single phase in in its conduit runs 208/240 3ph in its own conduit runs and the 480v 3ph in its own runs I don't crowd conduits with conductors and don't mix 3 ph with single phase stuff.
    If I ever decide to have 3ph hit my building by the power company, it will be a simple matter to set a meter and hit a 500-amp 3ph disconnect I already am using with 2 200 amp fuses reverse wire a couple panels and it will be good to go
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    Oh okay. You should have made up a lie because I was looking forward to hearing what wild and crazy project you were doing with 9kv.

    You guys have 600v down there? I thought 600v was only a Canadian thing.

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    Don42's Tools
    A rotary phase converter (RPC) does indeed make real three-phase power. I've been using a shop-made RPC in my shop for more than 20 years.

    However: perhaps I blinked or had a senior nap during the good part, but I saw nothing in this video about rotary phase converters. A rotary phase converter uses a 3-phase induction motor as an "idler" to create a third phase. This motor need not have any useful mechanical load and usually doesn't; it just needs to be in the circuit, sit there and spin. The rotating part is the rotor or armature in the motor. The idler motor is actually being powered by a single phase supply driving one phase winding. The motor runs as a single-phase induction motor. They're called "induction motors" because current is induced in the rotor by the field winding(s), which results in a magnetic field from the rotor that interacts with the field(s) produced by the field winding(s). The idler motor must be "kick started" either mechanically or with phase shifting start capacitors that get switched out once the motor is running. As the rotor spins, the magnetic field that spins with it generates second and third phases in the other two phase windings. Phase shifts occur because the phase windings are angularly displaced symmetrically around the spin axis.

    You don't need to understand how they work to build one that'll work for you. Here is what i regard as an excellent practical reference on making rotary phase converters by Jim Hanrahan:

    https://raremetalblog.com/metalwebne...v/ph-conv.html

    Further: a "wild leg" delta configuration does NOT produce three phase from single phase. It merely produces a single phase of 208V from a delta-connected 120V 3-phase source. It's useful when a three-phase transformer is needed anyway, so adding a center tap to one winding gets 208V for free.

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    Quote Originally Posted by nova_robotics View Post
    Oh okay. You should have made up a lie because I was looking forward to hearing what wild and crazy project you were doing with 9kv.

    You guys have 600v down there? I thought 600v was only a Canadian thing.
    For reasons of electrical safety practices, disconnects, panels and any electrical components associated with voltages in or above the 440/460v ranges are required by the NEC to have the higher voltage rating. This is because higher voltages have the potential to produce more powerful arc blow when breaking contact, and can fail to properly disconnect if lower voltage rated components are used.



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