Capacitor Continuous Current Rating?

Electrolytic

The electrolytic rectifier[4] was a device from the early twentieth century that is no longer used. A home-made version is illustrated in the 1913 book The Boy Mechanic [5] but it would only be suitable for use at very low voltages because of the low breakdown voltage and the risk of electric shock. A more complex device of this kind was patented by G. W. Carpenter in 1928 (US Patent 1671970).[6]

When two different metals are suspended in an electrolyte solution, direct current flowing one way through the solution sees less resistance than in the other direction. Electrolytic rectifiers most commonly used an aluminum anode and a lead or steel cathode, suspended in a solution of tri-ammonium ortho-phosphate.

The rectification action is due to a thin coating of aluminum hydroxide on the aluminum electrode, formed by first applying a strong current to the cell to build up the coating. The rectification process is temperature-sensitive, and for best efficiency should not operate above 86 °F (30 °C). There is also a breakdown voltage where the coating is penetrated and the cell is short-circuited. Electrochemical methods are often more fragile than mechanical methods, and can be sensitive to usage variations which can drastically change or completely disrupt the rectification processes.

Similar electrolytic devices were used as lightning arresters around the same era by suspending many aluminium cones in a tank of tri-ammomium ortho-phosphate solution. Unlike the rectifier above, only aluminium electrodes were used, and used on A.C., there was no polarization and thus no rectifier action, but the chemistry was similar.[7]

The modern electrolytic capacitor, an essential component of most rectifier circuit configurations was also developed from the electrolytic rectifier.

Click to expand...

cachehiker said:

how much current is the generator generating?

can you measure the inductance of the motor winding?

and what is the resistance of the motor winding?

what frequency is the IGBT being switched at?

Click to expand...

cachehiker said:

motor = generator (in reverse). sorry for the confusion.

The energy stored in the winding ½LI² will be converted to energy stored in the capacitor ½CV² minus resistive losses incurred while charging the capacitor. The rectifier diode will limit ringing but the resistance of the winding is probably very low so, in its current configuration, the capacitor surge current charging the capacitor is effectively the same amount of current flowing in the winding. At 40A, the capacitor will charge in about 1 millisecond. When the IGBT switches back on, it will discharge through the load in the same amount of time.

Electrolytic capacitors don't stand up to such treatment very well.

I somehow don't believe the 2A ripple current measurement can be accurate. A power resistor is usually put in series with the snubber capacitor to limit these surge currents and damp oscillations in whatever LC tank is present. Even with one in place, you may need more ripple current capacity than a reasonably sized electrolytic can deliver. I'd try four 100µF 400V caps wired 2x2 to provide the equivalent of an 800V 100µF with double the ripple current capacity if I had them but I still have my doubts.

A smaller setup here involving a motor less than 1/10th that size required a 50W 50Ω wirewound resistor and a 400V 10µF film capacitor to run at a relatively low 140V with a sub-1KHz switching frequency. The sub-1KHz switching made it obnoxiously loud.

I think 100µF of capacitance is probably about right for snubbing but a basic 400V 100µF electrolytic won't cut it. Think hundreds of Watts of resistance and an enormous 800V film capacitor if you're taking that path.

An alternate method used here: charge a 10,000uF 400V of electrolytics with the generator, use the IGBT/heater to discharge it at 4.88KHz, and place a much smaller snubber around the IGBT alone. The snubber will no longer be absorbing the energy stored in the collapsing field of the generator winding, just the much smaller field from the inductance that's present in the heater. With 10,000µF, ½LI² won't add up to much V in ½CV².

Click to expand...

ronv said:

All those questions and I still missed a couple:

Probably the hardest -- and that is the inductance of the generator.
And the winding resistance.

Extra credit!

What is the ripple rating on your caps?

Do you have any 250 - 300 watt loads laying around?

Click to expand...