High voltage ac to dc converter.

[mooimooi]

Hi ,
I have 240VAC mains and i need to get this converted to 240 VDC at 60A. Is it possible to just use the high power rectifier diodes constructed in a bridge array, with just one electrolytic capacitor. Thanks

 

[zeitghost]

No.

I don't think that the utility company would thank you for taking enormous gulps of current (a lot more than 60A) every half cycle.

Not to mention the exploding electrolytic!

rgds
Zeit.

 

[mooimooi]

Hi Ziet,
Thanks for the fast response. Is there any other options available?
Thanks

 

[BrianG]

As stated by zeitghost, apart from the practicalities of drawing large current pulses from the supply, don't forget that a bridge rectifier/capacitor combination will give you a peak d.c. voltage of 1.414 x the input voltage, i.e. nearly 340 volts.

As this would necessitate using a step-down transformer to start with a lower voltage of about 170V the primary current would be reduced to 60* 0.707 - but that's still about 42 amps! What do you need that amount of d.c current at 240V for?

 

[zeitghost]

15kW is a bit outside my experience.

There are power factor correcting converters used in power supplies to reduce or eliminate the current spike caused by the simple diode bridge & capacitor approach.

These operate over the complete cycle (or very nearly).

I really don't know much about this sort of thing at the 15kW level, but something like that is probably available.

rgds
Zeit.

 

[mooimooi]

Hi,
This is to power the utilities of an old boat that uses 240vdc inputs when the boat docks
Thanks

 

[sdmays]

To rectify an AC source and to then apply the rectified source to a capacitor is to attempt to convert from one voltage source to another voltage source, which violates KVL. What you can do is rectify your AC voltage source, and then connect the rectified voltage to your capacitor THROUGH an inductor. The inductor will act like a constant current source allowing the rectifiers to stay on longer and have much smaller current spikes when recharging the capacitor. The inductor will also decrease the voltage seen by the capacitor, possibly as low as the 240VDC you require.

 

[logbook]

There are problems but I don’t see them as insurmountable. If you just bridge rectify the mains you get a nasty problem with potential. Both sides of your DC supply will be dangerous with respect to earth. Also, as previously pointed out, the voltage will be wrong.

If you need this much power then the output capacitor will necessarily be large and therefore expensive. At these power levels anything is going to cost more than a few pounds. So, you need a transformer. I would suggest a three-phase system, if you have that possibility. By using a three phase system and a three-phase bridge rectifier, the ripple in the capacitor will be minimised and the system has some chance of working. The voltage can be set where you need it, and one side of the supply can be safely earthed.

 

[mooimooi]

Hi all.
Thanks for your valuble inputs, probably converting the 3 phase inputs would be easier than single phase as mentioned by "logbook". Thanks old chap.

Joy to you.

 

[BrianR]

Utility regulations generally limit individual single phase 240VAC circuits to 15A. Higher currents need 3 phase.

A bridge rectifier is the most efficient rectifier to use, but run from single or 3 phase mains which have the neutral referenced to ground, it will give +/- DC out ref to ground.

As zeitghost said, a single phase rectifier will be gulping several times 60A each half cycle to average 60A out and as BrianG said, you'll get somewhere from 240V to 1.4x 240V appearing as +/-120VDC to +/-170VDC according to load and C.

This load needs 3 phase and a transformer.

 

[yvestensi]

You can use a Thyristor Controlled Rectifier (Half or Full Bridge and even double full bridge)

 

[boxxy]

Just to say that the transformer isolated 3phase route is the only one to take - however it's worth mentioning that 240VDC is extremely dangerous and you must take extra safety precautions when handling and using the supply. Anyone using the boat must be told about the hazard. If you get hold of 240VDC you will not be able to let go and at these power levels it will kill you.

Personally I would put the money toward a new boat.

Rgds, Boxxy

 

[whills]

The best way yo do this is to drive a 15kv 240 v dc generator .It could be driven by a 25hp electric motor but a petrol motor would more easier and cheaper
I have a 20kv dc generator it is out of an old world 2 electric crain.
whills

 

[mooimooi]

Hey guys,
All of you have been so wonderful in providing me plenty of options to go by. Thanks for the great illumination.
Joy to you
Mooimooi

 

[csharp]

Hello,

I have a question. Is either side
of the 250VDC supply on the boat tied
to the ground plate? If so you would
need to use an isolating transformer.

If the end user is protected from
the 240VDC and it's not grounded,
which I will assume for the moment
is the case. I might try an auto
transformer to buck the line voltage
down to where it needs to be, then
follow that with a choke input full
wave rectifier and smoothing capacitor.

Three phase would be best because the
power factor would be very good and
the ripple filtering capacitor could
be small.

If the branch circuit is sized properly,
there is nothing to prevent you from
running a single phase capacitor input
supply after the auto transformer.
A choke input supply would still be better.
But either would violate IEC standards,
so I wouldn't try to sell one of those
in the EU.

If you build any power supply with a 60A
250VDC output please use appropriately
rated fuses on both input and output
(This means DC rated fuses on the output.)
Include a disconnect switch to lock out
the AC line feeding the supply too. Wire
wound bleeder resistors with a 50% de rating
factor across the caps are also a must as
is a proper enclosure. A device to short
the output when the cover is removed isn't a
bad idea either.

I'd also want a GFI built-in to the supply
since it's unisolated and used on a boat.
Dockside power may not have this protection.

Most of my work is with 480-600 VAC input
switch mode power supplies with output
power from 10KW and up. Building a switching
power supply with a 250VDC 60A output is
pretty close to some previous designs that
I've worked on. They usually began with a
three phase choke input rectifier. If a single
phase input was required a boost converter
type power factor corrector was employed.

I hope this rambling has been of some help

Best regards,
csharp

 

[Warpspeed]

I like the three phase transformer/rectifier combination.

Obviously this boat has some sort of DC generator to supply the power when underway with the engine running, yes ?

Would it be possible to drive this generator that you already have with a three phase induction motor. It might just be a case of shifting a drive belt from the crank pulley of the boats engine to the electric motor when tied up.

 

[RodH]

I guess you won't be using 15kW for long ... have you considered charging a bank of 20x 12V lead acid batteries using a lower power rectifier, and then using the batteries to get you over the surge requirements?
HTH
Rod