Siemens Frequency Converter (Micro Master 6SE3116-8CB40) teardown - Page 1
Forum section: electronics / general

author: D Straney
author_rank: Frequent Contributor
timestamp: January 24, 2020, 03:31:42 am
content: Came across a spare in my possession at work, and since the cover comes off so easily with a single screw, I couldn't resist having a look.According to what I could find about it online, it's actually supposed to be used as a motor drive most of the time, although the sticker only mentions motors in one place. That explains some things, as we'll see later.One screw gone, and the cover lifts right up...There's two boards stacked on top of each other (top one control/UI likely, bottom one power), with the input/output/control cables coming in through rubber inlets to a sea of messy connections to terminal blocks:...and everything sits on a large heatsink (half the total height of the unit) with a fan in the middle of it:Nothing too interesting on the top board; it has the display and controls, along with a serial port for debugging or automated control probably. There is a crystal......which makes me think there must be a microcontroller somewhere (probably underneath the display/button panel) for running the direct display+buttons interface as well as the remote interfaces (there's the serial port on the front, and another serial cable connected to it as well; don't know if they're actually separate). There's also a connector to the power board below:Nothing much else going on here, so here's the maze of twisty traces (all alike) on the bottom side, and some colorful MELF resistors - don't see those much anymore even in industrial equipment:Now with the top board removed, the power board underneath is really where all the action happens:There's some input EMI filtering and over-voltage protection at the left side where the input power enters, some serious bulk capacitance for the DC link at the top, an auxiliary supply at the top-right for creating the logic and gate drive supply voltages, control circuitry at the right side, and gate drivers in the middle. The one power device is a single large module sandwiched between the power board and the heatsink:Normally the way these things work is as AC-DC-AC converters; the incoming AC power goes through some bridge rectifiers and large capacitors (which we can see here) to convert to DC, and then a separate inverter stage chops the DC link voltage to the correct frequency of output AC power, which then gets fitered to smooth either the PWM or the fast edges to cut down on radiated EMI. There are direct AC-AC converters that eliminate the need for a DC link and its large expensive/comparatively unreliable capacitors, but the controls and filtering are more involved. Here though, that single bridge module must contain both the 3-phase input rectifier, and also the inverter's 3 half-bridges. In the photo above that one you can also see a relay and some large power resistors (standing upright near the bulk capacitors): I'm guessing those are left in series at turn-on to limit the inrush current into the huge DC link capacitance, and then after a short time, once inrush is over, the relay is closed to bypass them and operate normally.The auxiliary supply uses aPower Integrations SMP220flyback converter, which either runs straight off the input or from the DC link voltage. The big red box next to it is probably the flyback inductor (VAC stands for Vacuumschmelze, who a co-worker once accidentally called "Vacuumschnitzel"; a large magnetics manufacturer).The control section seems to consist of a custom Siemens chip (probably either some kind of microcontroller with specific peripherals like 3-phase frequency generator+PWM, an ADC, and a serial interface, maybe some control loop acceleration), a ROM chip for program code probably (the M27C256B), and a few optocouplers for isolated comms to the front-panel controls.You can also see the gate drivers here, with theHIP2500low-side + high-side chip (one for each output half-bridge). As is usual with low-to-medium power converters, instead of the high-side driver being completely isolated, the high-side gate driver "floats" at the high-side transistor's source/emitter voltage, with a high-voltage tolerant level shifter circuit to bring in the gate drive signal, and a "bootstrap capacitor" circuit for power which charges when the low-side transistor is on (search something like "gate drive bootstrap", various application notes will explain it far better than I could).After putting everything back together, I suddenly realized I had no idea where the output filtering was. I looked back through the photos and saw no signs of any inductors or common-mode chokes either large enough to take the output current, or sitting anywhere near the output - you can even see in that last picture where two of the power module pins connect directly to the output terminal block. However, the fact that it's meant as a motor drive means that this matters far less than if it was meant to power, let's say, aircraft equipment at 400 Hz, or any random piece of electronics that might want a non-50/60 Hz frequency. First, the heavily-inductive motor windings will do a lot of their own filtering on the output signal (even if the high dV/dt on the cable leading to the motor does want to capacitively couple to everything - the cable may be shielded though). Second, when it's meant for use in an industrial setting anyways, everything is pretty much expected to be electrically noisy; the motors themselves will put out a ton of noise, any induction heaters or capacitive spot-welders or arc furnaces etc. will put out a ton of noise, and in the end the amount a small-ish motor drive would add isn't proportionally very much.