Inverters & Variable Frequency Drives

Three stages, three failure families

An inverter rectifies incoming AC into DC, smooths it across a bank of bus capacitors, then switches that DC with IGBTs to synthesise a variable-frequency, variable-voltage output. Motor speed follows frequency; torque follows the voltage-to-frequency relationship the drive maintains.

Each stage fails differently. Rectifier faults show as low or unstable DC bus and often as a blown input fuse. Bus capacitor ageing shows as ripple, nuisance undervoltage faults, and bulged cans. Output stage faults show as one weak or missing phase, unbalanced motor current, and overcurrent trips on acceleration.

• Expected DC bus on a 230 V single-phase input: roughly 320 VDC
• Expected DC bus on 400–480 V three-phase input: roughly 560–680 VDC
• Low bus with correct input = rectifier or bus capacitor problem

Bus capacitors hold lethal charge for minutes after disconnect. Wait the manufacturer's stated time and measure the bus before touching anything.

Measuring a drive output correctly

Do not judge inverter output with an ordinary averaging meter. PWM output is a train of high-frequency pulses; averaging meters report meaningless values and some meters are damaged by the switching transients. Use a true-RMS meter rated for the application, or better, judge output by current balance with a clamp.

Current balance is the practical field test. Clamp each output leg with the motor running at a steady speed: the three legs should be within a few percent of each other. A leg that is significantly low or zero points to the output stage or a motor winding, and comparing against motor winding resistance tells you which.

• Output legs should balance within about 5 percent
• Unbalanced with balanced motor windings = drive output stage
• Balanced current but no torque = load or coupling, not the drive

Reading drive fault codes as evidence

Drives log why they stopped, and those codes are more precise than most appliance codes. Overcurrent on acceleration usually means the ramp is too fast for the load or the load is dragging. Overvoltage on deceleration means regenerated energy has nowhere to go — a braking resistor or a longer ramp is the fix. Undervoltage means supply, not drive. Overtemperature means cooling: blocked heat sink fins, a failed fan, or an enclosure that has been closed up.

Clear the code only after recording it. The code plus the moment in the cycle when it appeared is often the entire diagnosis.

Installation faults that kill drives

Repeat drive failures usually trace to the installation rather than the drive. Long unshielded motor cables produce reflected-wave voltage spikes that erode winding insulation. Missing or shared grounds create common-mode currents that damage bearings and confuse control signals. Poor ventilation cooks the output stage.

Check supply quality too: voltage imbalance between phases above about 2 percent produces disproportionate current imbalance and heating, and is a common cause of drives that fail seasonally when the building load changes.

How an inverter actually controls a motor

An inverter does not vary voltage the way an old speed control did. It synthesises a waveform. The output stage switches the DC bus on and off thousands of times per second, and by varying the width of those pulses it produces an average that traces a sine wave at whatever frequency the control asks for. Motor speed follows that frequency almost exactly; slip accounts for the small difference on induction motors, and there is effectively none on the permanent-magnet motors used in modern compressors and direct-drive washers.

Torque comes from the relationship between voltage and frequency. The drive raises output voltage in proportion to frequency so magnetic flux in the motor stays constant; that is the classic volts-per-hertz behaviour. Below a few hertz the drive adds a voltage boost, because winding resistance eats a disproportionate share of a small voltage. Above base frequency the drive runs out of voltage and torque falls off — which is why an overspeed request on a loaded machine produces a stall, not more work.

Permanent-magnet motors add a further requirement: the drive must know where the rotor is before it can energise the right winding. It learns this from hall sensors, from a resolver, or by sensing back-EMF in a sensorless design. Lose that feedback and the drive will not commutate, which the machine reports as a start failure or a motor fault even though the motor is fine.

• Speed follows frequency; torque follows the volts-per-hertz relationship
• Low-speed torque depends on the drive's boost setting, not on the motor alone
• Permanent-magnet drives cannot start without valid rotor position feedback

A drive that refuses to start a healthy motor is usually protecting the machine because feedback or a permissive is missing. Prove feedback before condemning the drive.

The DC bus as evidence

The DC bus is the most informative single measurement on a drive, and each way it misbehaves points somewhere specific. A bus at the expected value with the drive faulted tells you the front end is healthy and the problem is in the output stage, the control, or the load. A low bus with correct incoming voltage points to the rectifier or to bus capacitors that no longer hold. A bus that rises during deceleration is regeneration — the motor is being driven by its load and pushing energy back — and needs a longer ramp or a braking path, not a new drive.

Ripple matters as much as level. Ageing capacitors let the bus sag between line cycles; the drive sees momentary undervoltage and trips at unpredictable moments, usually under load, usually described by the customer as random. On a three-phase input, a lost phase produces heavy ripple and a bus that reads plausibly on average while the drive trips constantly.

None of this is worth guessing at. The value, the stability, and the behaviour of the bus during acceleration and deceleration are three separate observations, and together they usually name the failing stage before you open anything.

• Correct bus, drive faulted: look at output stage, control, or load
• Low bus, correct input: rectifier or bus capacitors
• Bus rises on deceleration: regeneration, needs ramp or braking path
• Heavy ripple on three-phase input: suspect a lost phase before suspecting the drive

Bus capacitors store lethal energy. Observe the manufacturer's discharge time and verify the bus is safe with a rated meter before contact — this is not a step to compress.

Communication between control and drive

On residential inverter platforms the main control and the inverter are separate boards that exchange commands and status over a dedicated link. Most 'inverter' fault codes are really communication codes: the main control asked for a compressor start and heard nothing back within its timeout. The machine then reports a compressor or inverter fault, and technicians replace one or both boards while the actual fault sits in the harness between them.

The order that works: confirm supply and ground at the inverter board, confirm the ground reference is shared and solid, confirm link continuity with the harness flexed, and read whatever the inverter board itself indicates — most have an LED pattern or a service display that reports its own state independently of the main control. An inverter board that is powered, grounded, and reporting a healthy state while the main control claims it is absent is a link fault, not a board fault.

Commercial VFDs communicate too, over Modbus, BACnet or a proprietary building bus. The same logic applies with higher stakes: verify address, baud rate and termination before assuming a device has failed. A drive that runs correctly from its keypad and ignores the building control is a network problem.

• Most inverter fault codes are communication timeouts, not power-stage failures
• Read the inverter board's own indication before trusting the main control's account
• Runs from the keypad but not from the building control = network, address, or termination

Residential inverter compressors versus commercial VFDs

The physics is shared; the service reality is not. A residential inverter compressor is a sealed permanent-magnet motor driven by a matched inverter board, with no keypad, no parameter set, and no user-accessible fault log beyond the appliance's own codes. Your diagnostic instruments are winding resistance and insulation to ground at the compressor terminals, the inverter's own indication, the DC bus, and the machine's service mode. Winding balance and a clean insulation reading with a healthy bus and an inverter that will not drive is a strong case for the inverter; low insulation to ground condemns the compressor and predicts that a new inverter would fail too.

A commercial VFD is a configurable industrial device. It has a keypad, a parameter set, a fault history with timestamps and often with bus voltage and current captured at the moment of the trip, and it can be run locally to separate a drive problem from a control-signal problem. Its faults are frequently installation faults: cable length producing reflected-wave stress, missing or shared grounding, inadequate ventilation, or supply imbalance between phases.

The judgement difference matters. On residential equipment you are deciding between two sealed assemblies with limited evidence, so you gather every reading available before committing. On commercial equipment you have abundant evidence and the discipline is to read it — fault history first, parameters second, hardware third.

• Residential: winding balance, insulation to ground, bus, inverter indication, service mode
• Commercial: fault history with captured values, local keypad run, parameters, then hardware
• Repeat commercial drive failures are usually installation, supply, or cooling — not drive quality

Failure modes and what confirms them

Test procedures

Manufacturer differences

Safety and professional boundaries

• Bus capacitors remain lethal after disconnect. Observe the stated discharge time and verify with a rated meter — no exceptions, no shortcuts under schedule pressure.
• Three-phase drive panels require arc-flash rated protection, lockout/tagout, and the qualification your jurisdiction demands. Decline work outside that qualification.
• Opening a sealed system requires refrigerant certification and recovery equipment. If you cannot recover legally, the job stops and gets handed to someone who can.
• Do not raise protective trip thresholds to keep a machine running. Protections exist because the alternative is hardware damage or a fire.

Expected readings and what they mean

Field scenarios

Knowledge check