Replacing An Old DC Filter Capacitor in a 400Hz AC Generator

[elec_mech]

Hi All,

I'm trying to help a customer replace an old, discontinued capacitor for a portable power supply. The unit runs from a gas engine that spins a custom generator at about 3600 RPM to get 400Hz, 3-phase AC power. This is then put through a series of diodes (bridge-rectifier) and a capacitor to get 250VDC or 500VDC at 2A or 1A, respectively.

Unfortunately, the unit is 30 years old and the capacitor is no longer made.

It is a CBE (formerly Mallory) TCG200T450N1C with the following ratings:
20uF
450VDC
8.722ohm ESR
341mA Ripple Current

I can find equivalents from the same manufacturer, but nothing off the shelf or at least readily available.

I found a Illinois Capacitor 226TTA450M with similar specs:
22uF
450VDC
18.839ohm ESR @ 120Hz (though this will used at 400 x 2 = 800Hz)
160mA Ripple Current

I can't say I know much about filtering, but from what I read having a slighter higher capacitance should be okay, but I worry about the higher ESR and lower ripple current rating. I don't know if the old capacitor was selected with those parameters in mind by design or if it was simply a convenient one to use at the time.

Any suggestions or assistance is greatly appreciated. Thank you in advance.

[goontron]

Re: Replacing An Old DC Filter Capacitor in a 400Hz AC Generator

Assuming you haven't confused the exciter cap for the filter cap:

It would be OK to go up in uF rating, normadigly these caps are 20% rated, so stay within the 20% esr wise, go up current wise or down to 20% current wisr, and you will be fine.

[stj]

Re: Replacing An Old DC Filter Capacitor in a 400Hz AC Generator

take a look at the Rubycon datasheets - they have a large range of 450v caps

[elec_mech]

Re: Replacing An Old DC Filter Capacitor in a 400Hz AC Generator

Thank you both. The old capacitor is rated -10%, +50%.

Per stj's suggestion, I looked at Rubycon and found a 22uF with a 292mA ripple current. Unfortunately they have no ESR data. These are physically smaller than the existing capacitor 16 x 22mm (radial) vs 25 x 28mm (axial - old one).

Think this would be okay? At only -10%, it sounds like I should be looking for a cap with a ripple current over 307mA, so I'm unsure whether the 292mA rating would be okay per goontron's advice.

At +50%, perhaps I can use a 33uF instead? Rubycon offers several with ripple currents from 312 to 392mA.

If it helps, I've attached the wiring diagram.

[stj]

Re: Replacing An Old DC Filter Capacitor in a 400Hz AC Generator

BXW series:
18x16mm 340ma

all the ones starting with "BX" are worth a look

[elec_mech]

Re: Replacing An Old DC Filter Capacitor in a 400Hz AC Generator

Thank you stj. I can't find any readily available 22uF or 27uf caps in the higher ripple currents, so I'll go with the 33uF 10x40mm, 360mA in the BXW series. Thanks again!