INTRO
0:00-0:30
ON-SCREEN: A33-inverter-drive-chain.svg, title visible
You hook gauges on a two-year-old variable speed system and the suction pressure will not hold still. 142, then 121, then 96. Superheat says three different things in two minutes. Nothing is broken. The compressor is changing speed on purpose, and every fixed speed instinct you own is lying to you. This is the inverter module: how the drive works, why the rules change, and the one safety step that is never optional on this equipment.
MAIN
[0:30-1:30] The drive chain: AC to DC and back
ON-SCREEN: A33-inverter-drive-chain.svg, camera pans left to right across the three stages
Follow the power. Single phase AC comes in from the panel at a fixed 60 Hertz. A motor's speed follows the frequency feeding it, so to vary speed, the drive throws the incoming frequency away. Stage one: a rectifier, a bridge of diodes, turns the AC into lumpy DC. Stage two: a bank of big capacitors smooths it into the DC bus, a high voltage reservoir. On a 230 volt supply that bus sits around 310 to 340 volts DC, and boost designs run higher. Stage three: fast transistors chop the bus into three output legs using pulse width modulation, fast pulses whose average looks like smooth AC at any frequency the control wants.
ON-SCREEN: text overlay "Every inverter compressor is a THREE PHASE motor"
And that is why every inverter compressor is a three phase motor, even in a single phase house. The drive manufactures three phase power from scratch. No start winding, no run capacitor, and from D26, the winding check adapts: all three phase-to-phase readings equal, often under 2 ohms.
[1:30-2:20] The DC bus is the hazard
ON-SCREEN: A33-dc-bus-safety.svg
Now the sentence you memorize before anything else. Those DC bus capacitors hold a lethal charge after the power is off. Pulling the disconnect stops the refill. It does nothing to the charge already stored. Bleed resistors are supposed to drain it in minutes, and bleed resistors fail open with no warning. So the rule is layered: disconnect locked out, wait the labeled time, then measure the bus yourself. Below 50 volts DC and still falling before your hands go anywhere near that board. Never short it with a screwdriver. Verify, every single time.
[2:20-3:20] Ramping: why your gauges mislead
ON-SCREEN: A33-ramping-vs-fixed-speed.svg, then the gauge needle clip
Here is what variable capacity does to your diagnostics. A fixed speed compressor is a step: off, then one hundred percent, settle, read. An inverter soft starts from near zero and ramps for minutes while it hunts for the output the load needs. During that hunt, suction, head, superheat, subcooling, all moving. The numbers are not wrong, the question keeps changing. So the discipline: force or wait for a known speed. Use the manufacturer test mode where it exists, hold a fixed commanded speed, give it 10 to 15 minutes, then compare to the manual's targets for that mode. And know the deliberate behaviors: a sudden high speed burst after hours of low speed running is an oil return cycle sweeping oil back to the compressor, and readings within about an hour of a defrost are unstable by physics. Features, not faults.
[3:20-4:15] Codes first, and the new diagnostic order
ON-SCREEN: A33-inverter-diagnostic-flow.svg, camera follows the flow top to bottom
Inverter equipment self-reports, so the call order changes. Fault codes first, before you cycle power, because some platforms erase the history on reset. Photograph them, read them in order, oldest code is often the cause. Then the service tool: live compressor speed, bus voltage, sensor data. Then electrical work in safe order: supply proven, bus verified discharged, sensors compared against your own instruments, compressor windings checked equal with the drive disconnected. The board gets condemned by elimination plus evidence, never by default, because a shorted compressor kills the new board at first start, and most condemned inverter boards were never bad.
ON-SCREEN: A33-ecm-family-and-signals.svg
Same discipline indoors. An ECM blower needs high voltage AND a command, 24 volts at a speed tap or serial data from the board. A motor nobody asked to run is not a failed motor. Power, command, then split motor from module: windings under 20 ohms and equal points at the module, not the motor.
OUTRO
4:15-4:30
ON-SCREEN: text overlay "Codes first. Bus verified dead. Stable speed before judgment."
Codes first. Bus verified dead before you touch. A stable known speed before any refrigerant verdict. Read the article, take the quiz, and bring your meter discipline to the practical, because Darrel will be watching the discharge check before anything else.