Inverter Compressors and DC Fans: What a Meter Cannot Tell You
Why this matters
On a conventional single-speed refrigerator, an ohmmeter at the compressor pins and a look at the relay settle most no-start calls. On a variable-speed refrigerator the motor is driven by a board, and the same habits mislead. The windings read fine on a compressor that will never run again because its drive board died, and they read fine on a compressor that is mechanically locked and needs a sealed-system job. A brushless DC fan gives no ohm reading worth having at all. Techs who carry the old method across condemn inverter boards that were good, order compressors for board faults, and, worst of all, try to start an inverter compressor straight off line voltage and finish it off. This card says exactly what a meter proves on this equipment, what it cannot prove, and where the useful readings are.
What is different about the motor
An inverter compressor is a three-phase brushless permanent-magnet motor, sealed in the shell like any other hermetic compressor. It has three terminals of equal standing, often labeled U, V and W, not common, start and run. There is no start winding, no start relay, no overload in the conventional sense, and no run capacitor. The drive (inverter) board rectifies line voltage to a DC bus, on a 120 V refrigerator typically around the 170 V peak of the line, or about twice that where the board uses a voltage doubler, stores it in capacitors, and switches it out to the three terminals in a rotating sequence at whatever frequency gives the speed the main board is asking for. The board also works out rotor position, usually from the motor's own back voltage, so it knows when to switch each phase.
Two consequences follow, and both change the test.
First, the motor cannot run without the board. A permanent-magnet synchronous motor has to be commutated to turn. Line voltage applied across two of its terminals does not start it; it pushes current through two windings with nothing to limit it but their resistance, which can overheat the windings and weaken the magnets. Never apply line voltage directly to an inverter compressor, and never fit a hard-start kit or use a direct-start test cord on one. Those tools exist for single-phase compressors with start windings and they do damage here.
Second, the board is half the motor. A no-start can be the compressor, the inverter board, the signal from the main board telling the inverter to run, or the power reaching the inverter. The meter readings that separate them are on the board side, not the compressor side.
Compressor families and how to tell which one a unit has are in 'Refrigerator Compressor Types Reference'. The rating plate and the tech sheet name it; an inverter unit's tech sheet will have a drive board test section.
Safety before any reading
Unplug the refrigerator and verify absence of voltage at the inverter board's input with a meter proven on a known live source before and after, per 'Appliance Electrical Safety Check Before Service'. Then deal with the stored energy: the DC bus capacitors on an inverter board hold charge after the cord is pulled. Wait the time the tech sheet gives, then measure DC volts across the bus test points it names and confirm the reading has fallen to a few volts or less before touching the board or unplugging the compressor harness. Do not short the capacitors with a screwdriver.
Live readings on the drive board are taken only at the test points the tech sheet names, with the board's cover or shield in place where the sheet allows, and with the compressor harness connected and secured before power goes back on. Never reconnect a compressor harness to a powered board. The compressor terminal cover stays on whenever the compressor can be energized, for the reason 'What Is Inside the Sealed System Line' gives: a damaged terminal can blow out of the shell.
What the ohmmeter proves on an inverter compressor
With the harness unplugged at the compressor or at the board, read the three phase-to-phase pairs: U to V, V to W, W to U.
- All three equal, within the small spread the tech sheet allows, and matching its stated value. The three windings are continuous and not shorted to each other in a gross way. That is all.
- One or more pairs OL. An open winding or a broken internal lead. The compressor is bad, which is a sealed-system job.
- One pair markedly lower than the other two. A winding short between turns. The compressor is bad.
- Any terminal to a clean, bare spot on the shell or a copper line reading anything other than OL on the ohms range. A winding to ground. The compressor is bad, and the board that was driving it should be suspected too.
What equal readings do not prove: that the rotor turns (a seized bearing or a mechanical lock reads perfectly), that the magnets are healthy, that the sealed system is charged and unrestricted, or that the board can drive the motor. Equal readings narrow the fault to "not an open or shorted winding." They do not clear the compressor.
What the ohmmeter proves on a brushless DC fan
Evaporator, condenser and many dryer and range cooling fans are now brushless DC motors with the drive electronics in the hub. The connector usually carries three or four wires: DC supply, common, and one or two signal lines (a speed command, a speed feedback pulse sometimes called FG, or both). An ohm reading across those pins measures the input of a small circuit board, not a winding. It may read anything, it may change with the meter's polarity, and it proves nothing about whether the fan works.
The tests that do mean something on a DC fan:
- Does the blade turn freely by hand, with no ice, no rubbing on the shroud, and no grinding? A seized blade is a fan fault whatever the electronics read.
- Is supply voltage present at the fan connector when the board calls for the fan? The tech sheet gives the value, which on many refrigerators varies with speed. Back-probe it with the fan connected, per 'How to Back-Probe a Control Board Connector Without Shorting Pins'.
- Is the feedback line pulsing when the fan runs? Many boards ramp the fan, wait for feedback, and shut it off for a minute or more if none arrives, then retry. A meter left on DC volts with min and max capture, or watched through a retry, catches a few seconds of supply that an instant reading misses.
The procedure for the evaporator fan, including telling an AC shaded-pole fan from a DC one, is 'How to Test and Replace an Evaporator Fan Motor'.
Where the useful compressor readings are
The tech sheet's drive board section usually gives four checks in this order, and the order matters because each one only means something if the one before passed.
- Line voltage into the inverter board, with the unit calling for cooling.
- The run signal from the main board to the inverter. Often a low-voltage DC level or a frequency signal on one pin to a common; the sheet says what to expect and how to read it.
- The board's own diagnostic, usually an LED blink code or a service-mode code distinguishing "no signal", "failed to start" and "overcurrent".
- Output phase-to-phase during a start attempt, compressor connected, read on all three pairs. The values and the meter setting come from the sheet, because a basic meter reads a switched waveform unevenly and the sheet's figure is what that waveform reads on a typical meter. What matters is balance: three similar readings, or one pair well off the other two.
Worked example: same complaint, a fork at the output
A bottom-freezer refrigerator, variable-speed compressor. Fresh food at 50 F, freezer at 28 F by the tech's thermometer. The evaporator fan runs, the coil is not frosted solid, and the condenser fan runs. The compressor is silent.
Ohms. Unplugged, power verified absent, bus confirmed discharged to under 2 V at the sheet's test points after the stated wait. Compressor harness unplugged at the inverter board. Phase-to-phase readings of 9.8, 9.9 and 9.8 ohms, against a tech sheet figure of about 10 ohms with no pair to differ by more than a small fraction. Each terminal to a scraped spot on the suction line: OL. The windings pass, which clears nothing yet.
Board inputs. Harness reconnected and secured, cover in place, power restored. Line voltage at the inverter input: 120 V. Run signal from the main board: present and matching the sheet's expected reading. The inverter's LED gives the "failed to start" code.
Output. With the sheet's meter setting on the output pairs during a start attempt: two pairs read similar values and the third reads well off them. The inverter's output is unbalanced, which means one of its three phase legs is not switching properly. A compressor cannot start on that, and no compressor fault produces it, since the output is read at the board with the compressor's balanced windings as the load.
The call. Inverter board. It is outside the sealed system, so the uncertified tech replaces it, unplugging first, verifying absence of voltage, discharging and confirming the old board's bus before handling it.
The other branch. Had all three output pairs read balanced and the compressor still failed to start, with the windings already proven, the remaining cause is mechanical: a locked compressor. That is a compressor replacement and a sealed-system job for an EPA Section 608 Type I or Universal certified tech. The board in that case is fine, and replacing it would have been a wasted part and a second visit.
Finish test and the question after. New board, harness connected, cover on, power restored. The compressor starts within the board's start delay, the condenser coil warms over the next few minutes, and the freezer begins pulling down; the ticket notes a return check or customer temperature call after several hours. Then the question an inverter board failure always raises: why did it fail? A surge, a condenser fan that had been dead long enough to cook the board, or a compressor drawing high. Here the condenser fan ran and the coil was clean, so the ticket records "board failed, cause not found, condenser airflow verified."
What changes the method
- Linear compressors in some refrigerators are a different design again and have their own test procedure in the tech sheet; do not apply the three-phase pair check to one.
- Units without a separate inverter board carry the drive on the main board, so the run-signal check disappears and output is read at the main board.
- Intermittent no-starts need the board's fault history, if it stores one, more than they need a meter reading taken while it happens to be working.
References
- Manufacturer tech sheet drive board and compressor test section for the specific model; the source of every value on this card
- EPA Section 608, 40 CFR Part 82 Subpart F, for any compressor replacement
- See related: Refrigerator Compressor Types Reference
- See related: What Is Inside the Sealed System Line
- See related: How to Test and Replace an Evaporator Fan Motor