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Thread: Is my circuit overloaded?

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    Is my circuit overloaded?

    I recently purchased a 3Hp Ryobi air compressor.
    I initially thought the unit was faulty as the motor would randomly run but the motor would never get up to full speed.
    I took it back to the retailer where is ran without a problem.

    I later identified that the compressor would only run properly on certain electrical outlets and only when there are no other high wattage appliances running on the same electrical circuit.
    I used a power meter to test and the motor draws around 5 Kw at start-up and then the current draw goes down to the rated amperage when running at full speed.

    Does this mean that none of my houses electrical circuits is capable of delivering enough start-up current to run my compressor?
    The breakers does not trip. What could be the cause of this? What needs to happen to get this fixed?

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    Diamond Member Justloadit's Avatar
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    I am not a sparkie, but I reckon the wiring on the plug circuits you have tested the compressor on, may not be of the correct wire size, which causes the wire to act as a resistor to your compressor motor, and causes a volt drop at the compressor motor terminals, not allowing the motor to generate sufficient magnetic force to get the motor at full speed. Continuing the running in this way will cause the motor to fail, as it will heat up very quickly and damage the motor windings.
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    Diamond Member AndyD's Avatar
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    Quote Originally Posted by debtcollectorZA View Post
    .........I used a power meter to test and the motor draws around 5 Kw at start-up and then the current draw goes down to the rated amperage when running at full speed.
    I wouldn't be surprised to see a high start current but the start current is only momentary, as soon as the motor speeds up the current should drop convincingly. Start current could easily be 6 x the run current. I'm guessing the run current could be around 15-16 Amps so the start current could be 90Amps momentarily so expect the lights to go dim and other possible interference issues. We don't really worry too much about start currents when designing motor circuits, we usually size the circuit with the run current in mind and allow for the nature of a highly inductive load then just make sure the protective device (circuit breaker in your case) will allow the very brief start current without tripping.

    3HP is around 2.2kW which is a fairly big motor for single phase domestic use. The most common causes of the type of starting problem you describe is low voltage.

    In answer to your question, it's doubtful you're 'overloading' your circuit because a 20 Amp socket radial circuit can deliver the 15Amps your motor requires.....but....the circuit you're using might not be suitable for several reasons such as if it's a long cable run it might not be able to accomodate the start current without significant volt-drop. Also your incoming house supply might be at the lower end of the permissible voltage range which would also add to the unsuitability of the supply. Finally there may be an underlying fault with your electrical installation such as a poor termination that's causing low voltage.

    I'd suggest you just get a decent sparky out for an hour to test your installation and rule out the fault possibility and he can check the line voltage under load etc. If your installation checks out okay then I'd probably suggest you may need to install a 4mm or even 6mm dediacated supply circuit for the compressor depending on the cable length etc.
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    I agree with AndyD.

    Because of the lower voltage at the motor terminals, the motor doesn't get up to its rated speed. This is because the built in centrifugal switch in the starting circuit will only open at a certain speed to allow the running winding to take over and get the motor up to the rated speed. If you are not getting to the speed needed for the centrifugal switch to open, then the motor will constantly run at a low speed.

    As AndyD suggests, you may have a low incoming voltage to your premises and adding a 3kW load will drop this voltage even further, causing problems. Remember, the voltage is the driving force behind your electrical system. Just like a car battery that is beginning to fail, you just won't have that oomf needed to get the motor turning.


    Good luck!

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    Diamond Member AndyD's Avatar
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    I have similar problems with my 2.2kW home-made compressor at my house, it's also single phase and it struggles to start when the voltage drops below 210v which is just about most days during the peak load periods.

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    Another thought did occur to me, it would be worth checking the off-loader is working properly, it's the valve that makes the ''pssssst' noise every time the compressor motor stops and it lets the air out of the pipes so when it restarts it's not under too much load.
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    cool home made compressor . whats the extra loops in the copper pipe for? cooling?
    is that a gas bottle for a tank?

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    Diamond Member AndyD's Avatar
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    Yep, the extra loops are for cooling so the check (one-way)valve at the tank doesn't get too warm during long run times. The bottle is an Afrox type oxygen bottle. Not sure where the compressor head came from originally, bought it very second-hand for a hundred bucks. Someone who once saw it said they thought it was from a train and it was for charging the emergency brake air tank but I never found out if this was true or not.
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    nicely done. now you just need a cover for the fan belt

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    Diamond Member AndyD's Avatar
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    I'll take the fifth on the pulley cover, it's only accessible by me and I know where fingers shouldn't go so I never bothered making one. The pics were taken when I was still building it so there's a few bits not fitted like the start contactor, air filter, drain valve etc. Nice project if you're interested, using secondhand parts which are readily available total cost was around R750.00 for a pretty capable compressor, it's run every air tool and spray gun I've thrown at it without problems.
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    Hi Guys,

    Thanks for all the replies. I suspect you are right with regards to the voltage being too low.
    Testing the wall socket with no load the voltage varies between 215V and 222V. As soon as I start the compressor the voltage drops to below 210V.
    I need to get bigger wires installed to feed my garage. While we are at it get it running on a separate breaker.
    I estimate the wires from the main distribution box to the garage is around 20m or so.
    What gauge wire should I go for?

    I've got a Vodacom base-station in my garage running from the same circuit.
    The Vodacom kit draws around 1 Kw 24/7.
    Maybe I should get them to pay for the upgrade.

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