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Registered Member #54278
Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367
There is an old 'rule-of-thumb' relating capacitor size (capacitance), current draw, and voltage drop that, even if not directly applicable here, may be of use to some readers.
It is mainly for voltage smoothing of rectified sinusoidal line voltage. It states that for every Ampere drawn on a full-wave rectified input, voltage ripple is reduced by one volt for every 10,000uF of filter capacitance. Note, this if for 60Hz full wave rectification--if you are only using half-wave rectification, then you have to double the capacitance for the same effect. For 50Hz it is 8300uF.
I think these C values given here are a little conservative (just to be on the safe side)--I will derive the exact values and post later.
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
IamSmooth wrote ...
I just ordered 8 10000uf 450v capacitors for about $25/cap
I can make a voltage doubler using 4 of them with a ripple of about 5% @ 50A output.
Maybe I'll make another induction heater one day.
This doesn't seem like a very good idea. It will draw about 150A RMS from the mains line if running from 120V AC, and each capacitor pair will see almost 100A RMS of ripple current. Why do you need so little ripple in an induction heater? I run mine from unsmoothed rectified mains, and I get perfect power factor.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
A few improvements;
1) if possible use 3-phase line as lower value smoothing/bus capacitors can be used for similar dc ripple. 2) regardless of number of phases, some inductance before the smoothing/bus cap drastically reduces peak currents, hence I^2.R heating. you can quickly simulate using the link that I posted above. ..... the inductance can be on the ac line side (one per phase) or between rectifier(s) and bus caps. 3) i think that you will need a soft-start, a surge rated resistor (location similar to above) to limit inrush currents ..... shorted by a contactor (large relay) when about 50/66/75/80/90 % dc voltage is achieved on the bus. ..... Triacs if on the a.c. side and thyristors if on the dc side (less common) are also common for inrush bypass ..... for testing purposes a manually operated high current switch will do. ..... a filament lamp per phase shows any faults at power on and gives protection during early testing of other modules e.g. output bridge NTCs are used at low power but not useful at high power. All old stuff but worth remembering.
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voltage doubler
