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  1. #1
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    Default Transformer question

    I have a transformer with on the primary side 0-220-380 volts Secondary 24 volts It is 100Va
    I need it for a low voltage circuit on a paper band filter But i also need 220volts on that circuit to power a small 50Watt motor to move the paper band
    It is connected with a special plug with no Neutral
    So I did some testing. If I connect the transformer to 380 volts on the 0 and 380 lead I measure 220Volts between 0 and 220 Obvious
    Can I connect that 50W 220volts motor to those 2 leads and still use the 24Volts side to control 2 contactors ???
    Also How to fuse it

    Peter

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    It depends on how much current total the items use. What you propose certainly will work if there is enough capacity, but a 100 VA transformer is not very big. You would use about a 1/3 amp fuse in the 380 line.

    Bill

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    Quote Originally Posted by 9100 View Post
    It depends on how much current total the items use. What you propose certainly will work if there is enough capacity, but a 100 VA transformer is not very big. You would use about a 1/3 amp fuse in the 380 line.

    Bill

    I plan to place a 0.315A fuse in the 0 line
    That protects both the 380 and 220 volts
    And a 3A fuse in the 24 V line
    Or do i need 3 fuses One for each voltage

    Peter

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    I have no idea what your code says, but I think that the fuse in the 380 line would give reasonable protection to the other two. Typically, instruments that might have a half dozen voltages from one transformer only fuse the incoming line because a short on any of them translates to overcurrent on the supply.

    Bill

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    You're essentially using the primary winding as an autotransformer. This has some issues, and can leave you with close to 380V across the motor in certain circumstances, for example if the 0V lead of the transformer becomes disconnected or a fuse blows.

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    Quote Originally Posted by SomeoneSomewhere View Post
    You're essentially using the primary winding as an autotransformer. This has some issues, and can leave you with close to 380V across the motor in certain circumstances, for example if the 0V lead of the transformer becomes disconnected or a fuse blows.

    We put a phase on the O lead and a phase on the 380 lead
    So if the O lead gets disconnrcted we have only 1phase connected to the transformer
    So basicly we have no circuit How can we get 380 volts to the motor then (leads O and 220) then
    Please educate me

    Peter

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    If you have an open 0 point, you will have 370 open circuit volts to ground, but as soon as the motor draws current, the inductance of the transformer will reduce the voltage so there is no problem.

    Bill

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    You'll never have 370V to ground because it's a 220V to ground, 380 L-L system. But you can have 380V across your 220V motor, depending on the impedance of the motor and the section of the autotransformer still in circuit. If the motor is small compared to the transformer, the transformer will more-or-less pass through full voltage.

    It's better than trying to supply an ELV load that isn't even insulated for mains voltage, but you've still got potential issues.

    I can't find a copy of the pretty pictures I got in sparky school online that show why this is a bad idea.

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    Quote Originally Posted by SomeoneSomewhere View Post
    You'll never have 370V to ground because it's a 220V to ground, 380 L-L system. But you can have 380V across your 220V motor, depending on the impedance of the motor and the section of the autotransformer still in circuit. If the motor is small compared to the transformer, the transformer will more-or-less pass through full voltage.

    It's better than trying to supply an ELV load that isn't even insulated for mains voltage, but you've still got potential issues.

    I can't find a copy of the pretty pictures I got in sparky school online that show why this is a bad idea.
    "Potential issues" is cute!



    Yahbut.. he can probably just fuse all three faster and cheaper than to add a second transformer or a larger different one. Shouldn't be a HUGE risk.

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    It's not *too* bad, but you want to put the fuses on the two supply lines, and definitely not connect the motor directly to one leg and then a fuse to the 0V of the transformer.

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    Not sure of overall connections but simple process should be in place.

    Any current supplying wire needs to fused or overcurrent protection as close to source of energy as reasonably possible.

    A corded device has this at the cord connection point where conduit fed may depend on the breaker in the panel.

    Transformers are "power converters" meaning voltage and current are converted by them resulting in different sizes of overcurrent protection which is optional but prefered as the input protection needs to consider power on surges in the transformer where output protection can better match the load.

    Protective devices usually cheaper than the equipment so build it best with every hot wire supplying a device protected at the source of power.

    Next consider what happens if any one fuse opens, if using odd wiring to get creative then the changed path needs to be such to not cause things to have resulting voltages in excess or their design.

    Sent from my SAMSUNG-SM-G930A using Tapatalk

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    my suggestion, buy a second transformer...... much simpler.


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