240v AC to 24v DC 3hp supply

Discussion in 'Power Electronics' started by JBASport, Jan 15, 2019.

  1. JBASport

    Thread Starter New Member

    Jan 15, 2019
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    I have a hydraulic platform that uses a bank of batteries supplying a 24v DC 3 HP motor that drives the hydraulics. I want to be able to introduce a 240AC to 24AC transformer, then rectify the current to 24v DC. I have acquired the transformer & rectifier (hopefully advice on the sizing/rating when I bought them will be sufficient for the application) but no idea how to connect them! I also want to be able to switch the current to charge the batteries when not operating the machine. Any help will be graciously received. Thanks, Terry
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Do a search for transformer and bridge rectifier, you should end up with enough diagrams and information.
    I properly sized double pole double throw switch with centre off can be used to switch over.
    Max.
     
  3. drc_567

    AAC Fanatic!

    Dec 29, 2008
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    ... similar thread was posted recently:
    https://forum.allaboutcircuits.com/threads/24vac-to-12vdc-converter-circuit.155887/unread

    ... The only difference is that following the diode bridge rectifier, you will want a voltage regulator with different specifications ... 24 volts out and more current.
    When calculating transformer numbers, check the primary and secondary sides to see that they conform to the transformer V*I rating ... that is, the product of AC volts *amps on the primary side is equal to the AC volts*amps on the secondary side, and neither value exceeds the manufacturers suggested V*I rating. In fact, it is usually recommended to only operate the transformer at less than the rated value. If you observe that the transformer is excessively hot, you might want to reduce the input or output volts or amps, since heat is a sign of inefficiency.
     
  4. JBASport

    Thread Starter New Member

    Jan 15, 2019
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    Many thanks for your reply. If it is ok with you, I will take some pictures of the current setup and the transformer & rectifier I have bought to install as described in my intial post. kind regards, Terry
     
  5. JBASport

    Thread Starter New Member

    Jan 15, 2019
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    Here are pictures of the transformer and rectifier along with a picture of the current setup.
    Appreciate on any advice on connecting it up.
    IMG_5271 (1).jpg IMG_5274.jpg IMG_5276.jpg Many thanks, Terry
     
  6. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    I know you've already bought some expensive parts, but you really should consider an industrial switching power supply. If your system expects "clean" (no ripple / very low ripple) DC like what comes from a battery, you will need massive filter capacitors and a voltage regulator circuit designed for at least 30 A. 3 hp is over 750 W, and that will be one huge linear power supply.

    A single output 1 kW switcher will be smaller, lighter, and cheaper, not to mention way more efficient.

    ak
     
  7. drc_567

    AAC Fanatic!

    Dec 29, 2008
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    ... Do you have any schematic or diagram or model number for the bridge rectifier. The transformer is designated as 3.3 KVA, so that would place it in the 3 Hp range. The only question is whether that is to be used for continuous duty or intermittent.
    ... If not too much trouble, post a picture if the 240 VAC source.

    *** The one thing to keep in mind is to shut down the 240 volt AC source when making any kind of wiring connection or modification.
     
  8. drc_567

    AAC Fanatic!

    Dec 29, 2008
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    The nominal current demand of the 3 Hp 24 volt electric motor is 93 amps DC.
    The 24 volt sine output of the transformer will be converted to 34 volts DC after it passes through the diode bridge.
    As noted be AK above, there is some question as to how much ripple current will be tolerated by the intended application, as well as the question of voltage regulation ... that is, bringing it down to the specified 24 volt electric motor voltage.
     
    Last edited: Jan 17, 2019
  9. jhovel

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    Jul 9, 2016
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    3Hp is actually 2.235kW - so no, a 1kW switcher is only one third the size required...
    I've never found a reliable switching power supply in that output range. I'm sure they exist, but used transformer and rectifier is likely to be a lot cheaper.
    An electric motor driving a hydraulic pump won't need any smoothing of the ouput - only good overload protection!
    I've had a chinese made switching power supply of1.5kW - it had 7 parallel switching ICs and lasted about two hoursm, beforone of them went belly up, closely followed by lots of sparks form the others, as they were wildly and progressively overloaded. That was my brief experience... back to batteries for now. Keeping looking for transformer and rectifier.
     
  10. drc_567

    AAC Fanatic!

    Dec 29, 2008
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    Would it be possible to obtain some form of a reasonably accurate measurement for the actual DC current that is used by the 24 volt DC motor?
    There is a method of voltage regulation, using zener diodes, which requires the use of a non-standard current limiting resistor. Such a resistor would necessarily be required to be custom made and fashioned out of nichrome or a similar material. Knowing an actual measurement for the motor peak DC current would facilitate the calculation of the current limiting resistor.
     
  11. AnalogKid

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    Math fart. After remembering the conversion from hp to W, I dropped the 3. oops.
     
  12. JBASport

    Thread Starter New Member

    Jan 15, 2019
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    You have probably worked out by now that my knowledge is very limited, so appreciate you all being patient!
    I have checked through the manual and it says the motor is rated to 3kw, but as the motor is only operated when elevating or lowering the platform, very intermittent draw on the power. I have attached some pictures of the rectifier (it has 24v 120amp written on the bottom along with other printed labels).
    Hope this will help you. Once again, appreciate your patience! Terry
     
  13. drc_567

    AAC Fanatic!

    Dec 29, 2008
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    Does that last picture read 24 volt ... 120 Amp ?
    That's what it looks like.
     
  14. JBASport

    Thread Starter New Member

    Jan 15, 2019
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    Yes, that is what appears to have been written, it was on their when it arrived. Terry
     
  15. drc_567

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    Dec 29, 2008
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    The one remaining obstacle to your project is building a voltage regulation circuit that can withstand the current and heat generation that are by-producrs.
    The scheme given here is a zener diode based regulator, which requires an extremely low value series resistor, labeled Rs. Two 12 volt, 50 watt, zener diodes will be used, in series, rather than the ones in this example diagram.:
    ... where it says zener diodes in series:
    https://www.electronics-tutorials.ws/diode/diode_7.html
    [​IMG]
    The two 12 volt zener diodes are these:
    https://www.digikey.com/product-detail/en/microsemi-corporation/1N3311B/1N3311B-ND/4377736

    The series resistor can be made from short lengths of 17 gauge nichrome wire. The nichrome resistor is still under review ... don't want it to melt.
     
    Last edited: Jan 17, 2019
  16. drc_567

    AAC Fanatic!

    Dec 29, 2008
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    The initial calculation for the Rs nichrome resistor shows that two 200 mm lengths of 17 gauge nichrome wire, clamped side by side between copper, should approximate the series resistance required to supply 125 amps to the rest of the circuit ... 120 amps DC to the motor, and about 4 amps to the series connected 12 volt zener diodes. The two nichrome wires should dissipate about 778 watts of heat each, and should not melt.
    You will have to locate some square cross-section pure copper stock and fashion upper and lower end pieces to securely clamp the nichrome wires. When measuring the length of the nichrome wire, allow 2 cm extra on each end for clamping purposes. The 200 mm dimension is measured from the beginning exposed wire portion to the end exposed wire portion.

    ... this is all a first pass calculation, and will have to be reviewed checked, and verified in a day or two. Note: the circuit described here is specifically for running the 24 volt electric motor from the 240 VAC supply. A circuit for re-charging the 24 volt batteries would be different, requiring activation by throwing a knife switch, as suggested previously.
     
    Last edited: Jan 17, 2019
  17. jhovel

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    Jul 9, 2016
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    For a large DC motor, I don; believe voltage regulation is required at all.
    These hydraulic units are used in trucks a lot and the voltage is quite variable with engine going or not etc...
    I would just connect the bridge rectifier to the transformer, put in a 120A fuse or circuit breaker and get on with it....
     
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  18. drc_567

    AAC Fanatic!

    Dec 29, 2008
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    ... that sounds reasonable. The extremely large current draw by the motor makes any sort of fine tuned voltage regulation very questionable. When trying this out for the first time or two, check the electric motor for unusual heat build up, since it will be operating at about 34 volts DC, rather than the original 24 volts. As suggested, try adding a 120 A circuit breaker or fuse ahead of the motor, and see what happens. At the very worst, the fuse will blow ... maybe you can buy a slow blow fuse.

    ... another option, if you can obtain a 12 in. length of 1/4" carbon rod, use this as a kind of series resistance in line with the motor. It will drop the voltage by 12 volts of so ... depending on its operating temperature, and will limit the motor current. If you get too much current restriction, just break off an inch or two, until the motor voltage is at 24 volts. ... used this product on a battery test load bank ...
     
    Last edited: Jan 17, 2019
  19. JBASport

    Thread Starter New Member

    Jan 15, 2019
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    Great advice from you all, really appreciate your advice. I was really struggle with the way forward, so your input is priceless to me. Thanks again.
     
  20. jhovel

    Member

    Jul 9, 2016
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    That's the theory. The reality is that such a large load will knock the tops right off the rectified sine waves and the mean voltage will be right back to 24V - or a little less...
    PS: I checked my "3hp" hydrualic pump today - and it too is actually 3kW I made the same memory mistake as you... So I'll be looking for a larger transformer than I had looked for previously as well..... until then, batteries will do.
    I found and bought a 200A bridge rectifier a few months ago - ready for when I can find a 3kVA 24V transformer....
    @JBA Sport: do you still need a circuit drawing for your connections to replace the battery setup or is it all clear to you now?
     
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