Elevator with bad capacitors (again)

[cheapie]

See here (not my video, but I found it interesting): https://www.youtube.com/watch?v=6VdFyuZaVKE

Also, I'm not entirely sure what that circuit at about 1:15 in is doing (as far as resistors RB3 and RB4).

[Longbow]

Re: Elevator with bad capacitors (again)

An interesting u-toob video to be sure. I'm not sure why an elevator technician would want to post this kind of thing, but I'm glad this is not an elevator that I have to use. I'm going to assume that the brake coil shown in the schematic and discussed in the video is actually a brake solenoid winding. For this size motor, electric braking is the primary means of slowing it down, with a mechanical brake (looks something like a set of automotive drum brakes) locking the motor once it is nearly stopped. Guess what happens when electric braking fails???

Anyway, from the small hunk of schematic that is visible, I think we are looking at the solenoid pull-in and hold circuits. Pull in requires a larger surge of current. Once the solenoid is activated, a smaller hold current can keep it closed. Notice the resistors...250 watts each. Almost out of sight at the top are what looks like contactors (relays) which would be timed to operate the pull-in and hold modes. The resistors in series with the capacitors are most likely surge limiters.

What bothers me is the damage to the capacitors. That evidence tells me that the caps were subjected to overvoltage and/or to AC if they do in fact work in a d.c. filter environment. AC across a d.c. capacitor would cause that kind of venting and the culprit would most likely be rectifier leakage. Notice that the setup uses a typical rectifier bridge hooked up as a simple half wave diode. 2 sets of series diodes in parallel to handle the higher current and higher voltage. I've never seen that arrangement, but I think it is a poor design. There are lots of high current rectifiers that are stud mount that would avoid the series/parallel components that you see in the schematic.

I think the technician called it a day a bit too early. The new caps may have worked ok at first. What do you bet he had to return to finish the job?

[budm]

Re: Elevator with bad capacitors (again)

The caps are as shown in the diagram are AC caps CB1, CB2: 124~149uF 330VAC as shown on the print and on the cap itself (CDE 149uf 330VAC 50/60 HZ, +65c), so it can take AC or DC since it does not have polarity.
220VAC feeding the half wave will charge up the caps to peak voltage of about 310VDC, as you notice, there is not suppression device for kick back voltage from the brake coil when brake is released (relays open) which can be way higher than the supply voltage and opposite in polarity which will exceed the cap voltage rating of 330VAC, those two 25 Ohms in parallel (12.5 Ohms) are not going to do much on high kick back voltage spike.