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Thread: 3-phase power design. WHY?

  1. #1
    Associate Engineer
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    Jan 2018
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    Lightbulb 3-phase power design. WHY?

    Hello all,

    At our shop we have a benchtop size piece of mixing equipment that uses a 3 phase motor (Machine 1). The equipment's power entry is 220V single phase, but inside the machine (from what I've been told) there is a Variable frequency drive that converts this single phase power into 3-phase power for the motor.

    We have another very similar machine (Machine 2) that performs the same function and also has a 3-phase motor. The difference is that Machine 2 requires a 3-phase supply directly from the outlet. Machine 2 is a much higher quality, German engineered, and overall better machine compared to the Chinese built Machine 1.It seems to me like the much nicer option is to have a VFD inside of the of the machine to convert from single phase into 3-phase power, than to need an outlet for 3-phase power which requires an electrician to install and is costly to the consumer. So my question is:

    Why does Machine 2 require a 3 phase outlet connection instead of converting the power internally like Machine 1 does? Can anyone tell me the benefits to this design? It seems like there is a big disadvantage for the consumer to have to hire an electrician to use this type of equipment, and therefore would be limiting to sales.

    Thanks

  2. #2
    Principle Engineer Cragyon's Avatar
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    Single Phase power is a two wire Alternating Current (AC) power circuit. Most people use it every day because it’s the most common household power circuit and powers their lights, TV, etc. Typically there’s one power wire and one neutral wire and power flows between the power wire (through the load) and the neutral wire.

    Used for low power applications

    Three Phase power is a three wire Alternating Current (AC) power circuit. Most US commercial buildings use a 3 Phase 4 Wire 208Y/120V power arrangement because of its power density and flexibility. Compared to single phase, a 3 phase power arrangement provides 1.732 (the square root of 3) times more power with the same current and provides (7) power circuits.


    • Low power loads (Lights, etc.) powered using any of (3) 120V single phase power power circuits
    • Medium power loads (Water Heaters, etc.) powered using any of (3) 208V single phase power circuits
    • High power loads (HVAC Systems, etc.) powered using (1) 208V three phase power circuit


    Most US industrial facilities use a 3 Phase 4 Wire 480Y/277V power arrangement because of its power density. Compared to 208V 3 Phase, 480V 3 Phase provides 2.3 (480 /208) times more power with the same current or 43% (208/480) less current with the same power. This yields additional benefits.

    • Reduced construction costs with smaller electrical service, wiring, conduits, and electrical devices.
    • Reduced energy costs will less energy lost as electrical current resistance (converted to heat).
    • Used in high power and flexible applications

  3. #3
    Administrator Kelly Bramble's Avatar
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    A "Variable frequency drive" does not convert single phase to three phase. Changes in frequency changes the rotational speed of the variable frequency motor.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  4. #4
    Associate Engineer
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    I must agree with Kelly on this one. A VFD (variable frequency drive) is there essentially for speed control. Some VFD's can take in 3PH (three phase) power, produce DC from that, then convert the DC back to a single phase pulse train of variable frequency (and duty factor).
    Other VFD's can operate from single phase power (1PH), to produce a similar output. There are obvious differences of the power capabilities of the unit VFD based on type of feed.

    Quick answer, even if the VFD is designed originally for 3PH power input, one may can get away with feeding single phase power at a much reduced overall power capability. IF 1.732VI is maximum power for all three legs operating when delta configured, and leg-leg potential is V, and leg-leg current is I, then if the same V and I were employed from a single phase source, then the power achievable maximum would be ~58% that of the three phase case, but with the caveat that the conductor must be able to handle the current.
    Last edited by jastewart; 01-30-2018 at 02:14 PM.

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