VFD HVAC troubleshooting: why the drive trips when the motor is fine

I got called to a rooftop unit that was faulting about once an hour. Supply fan, 480 volt, nothing exotic. The tech had already megged the motor and it read clean, so the drive was getting the blame. We put the same model in from the shelf with the same parameters and the unit ran. Then the drive we pulled sat on a bench the next day turning a smaller motor and never tripped once.

That is most of VFD HVAC troubleshooting. The drive is usually reporting something true about the cable, the load, or a default nobody changed. It just is not reporting the thing people read into the code.

The ground fault that is not a ground fault

Start with the code everybody trusts too much. ABB spells the problem out in the ACH550-UH User's Manual, publication 3AUA0000081823: fault 16 EARTH FAULT watches for ground faults while the drive runs and while it sits idle, and "detection is more sensitive when the drive is not running and can produce false positives." That is the manufacturer warning you, inside the fault description itself, that this code cries wolf.

Then look at what the drive shipped with. Parameter 3028 EARTH FAULT LVL sets the threshold, and value 1 (LOW) trips on the smallest ground current. Most sensitive of the three, and it is the US default.

Two ordinary things on a VFD HVAC install push it over: a delta grounded supply, still common in older commercial buildings, and motor cable capacitance, which mostly means a long run. ABB names both as causes of erroneous reports, for fault 16 and again for fault 35 OUTPUT WIRING.

The ACH580 covers the same physics under another name. Its HVAC control program firmware manual, publication 3AXD50000027537, lists 2330 Earth leakage and calls it load unbalance, typically from an earth fault in the motor or its cable. That manual also notes 2310 Overcurrent can come from an earth fault or a lost supply phase. One physical problem, three codes that can announce it.

You can turn the sensitivity down. 3023 WIRING FAULT kills the not-running check on an ACH550, 3017 EARTH FAULT kills the whole function, and 31.20 and 31.23 do the same on an ACH580. I touch none of them until both the motor and its cable have been megged separately, drive disconnected. Disabling a nuisance ground fault is housekeeping. Disabling a real one is how you learn what a rewind costs.

What is actually happening	PowerFlex 525	ABB ACH550	ABB ACH580
Output current above the limit	F012 HW OverCurrent	1 OVERCURRENT	2310 Overcurrent
Current path to ground on the output	F013 Ground Fault	16 EARTH FAULT	2330 Earth leakage
DC bus too high	F005 OverVoltage	2 DC OVERVOLT	3210 DC link overvoltage
DC bus too low	F004 UnderVoltage	6 DC UNDERVOLT	3220 DC link undervoltage
A motor phase gone	F021 Output Ph Loss	34 MOTOR PHASE	3381 Output phase loss

One physical failure, several fault codes. The ones I look up most on VFD HVAC calls, from Rockwell 520-DU001 and ABB 3AUA0000081823 and 3AXD50000027537.

Yaskawa's A1000 uses its own shorthand for the same three: oC for overcurrent, GF for ground fault, LF for output phase loss.

Overvoltage on a fan is a load problem, not a line problem

F005 on a PowerFlex 525 says "DC bus voltage exceeded maximum value," and in publication 520-DU001 that is all Rockwell gives you. Same event is fault 2 DC OVERVOLT on an ACH550 and 3210 DC link overvoltage on an ACH580. Nobody reads that and thinks about the fan. On VFD HVAC gear the fan is usually it.

Say you have a return fan on a rooftop unit with the drive set for flying start. That fan is already turning before anyone hits start, because the exhaust pulls through it or a damper got left open. The drive grabs a spinning load, the motor generates into the bus for a moment, and the DC voltage climbs. I have watched a bank of blower drives do exactly this at startup and behave perfectly the rest of the shift.

Flying start onto a windmilling fan: a spinning load pushes energy back up the DC bus before the drive has control of it.

 

What I try, in order: stretch the accel ramp until it is actually achievable, back the flying start current off its default, or drop flying start and let the fan coast down first. On an ACH580 the overvoltage controller lives at 30.30 Overvoltage control, and it wants to be on.

The other cause is meaner. I had an F005 once, not even on HVAC, that came down to a chafed spot in the motor cable four feet out, buried in a tray nobody would look in. The damaged shield let common mode noise back into the drive and pushed the bus up. Fault code said overvoltage. Actual fault was a wrecked cable.

Then there is the default that hides all of it. On an ACH550, parameter 3105 AR OVERVOLTAGE ships ENABLED, with 3101 NUMBER OF TRIALS at 5 and 3103 DELAY TIME at 6 seconds. The drive resets its own overvoltage faults and carries on. Meanwhile 3104 AR OVERCURRENT ships DISABLED. Overcurrent stops the unit and somebody calls you. Overvoltage restarts quietly and nobody hears a word. So when a VFD HVAC unit is acting up and the log looks empty, that proves nothing.

A PowerFlex 525 back on the bench. F005 and F013 are the codes that send these off a fan.

 

On VFD HVAC retrofits, switching frequency is a cable length budget

HVAC has a reason to raise carrier frequency that industrial work does not: motor noise in an occupied space. Push the switching frequency up, the whine goes away, the tenants stop calling. Then you find out what you spent to get there.

The ACH550 lets you pick 1, 4, 8 or 12 kHz. Its maximum motor cable length depends on which one you picked.

Frame size	1 or 4 kHz	8 or 12 kHz
R1	330 ft (100 m)	330 ft (100 m)
R2	650 ft (200 m)	330 ft (100 m)
R3 to R4	650 ft (200 m)	330 ft (100 m)
R5 to R6	980 ft (300 m)	490 ft (150 m)
R6 (600 V)	330 ft (100 m)	330 ft (100 m)
R7 to R8	980 ft (300 m)	Does not apply

Maximum motor cable length on an ACH550 by frame size and switching frequency, from ABB 3AUA0000081823. On a VFD HVAC job, raising the carrier frequency can halve the allowed run.

Go from 4 kHz to 8 kHz on an R2 frame and the allowed run drops from 650 feet to 330. On R7 and R8 frames the higher carrier frequencies are not rated at all. ABB is blunt in that same manual: a motor cable longer than the table allows "may cause permanent damage to the drive." Not nuisance trips. Damage.

ACH550-UH. Its rated cable length changes with the switching frequency you pick, which is easy to miss on a VFD HVAC retrofit.

 

One drive, a whole wall of fans

Fan array units broke a habit of mine. I used to size a drive off summed nameplate current and move on.

The ACH580 startup wizard asks whether you have a single motor or multiple motors, and when you say multiple it asks how many, anywhere from 2 to 18. Answer honestly and two things change. Parameter 99.06 wants the total current of every motor, and a single motor current value gets capped at the drive's maximum rating divided by the motor count.

Control mode matters more. 99.04 Motor control mode has to be Scalar on a multimotor system if the load is not shared equally, if the motors are different sizes, or if any motor is getting swapped after the ID run. ABB goes further and rules vector control out with multiple motors on one drive, period. Whatever tight speed control you were counting on is gone the moment a second motor lands there.

Starting is its own problem. Parallel motors look like a very low impedance at start and the inrush is nothing like one motor's. The number I work to is roughly 150 percent of summed full load amps for continuous output. I would rather be over than exactly right.

Two more that bite here. The drive's electronic overload protects the total, so one motor out of nine pulling wrong is invisible to it. Every motor still needs its own. And do not bundle motor cables from several drives into one conduit: a pulse down one set of conductors couples into whatever lies against it. I have seen fan arrays eat motors and drives at a rate that makes no sense until somebody opens the conduit and counts.

One drive summing nine fan motors. The overload sees the total, not the one motor in trouble.

 

What I check first on a VFD HVAC trip

1. Read the fault log first. Count the trips and note the spacing: once an hour and once a week are different problems.

2. Meg the motor and the cable separately, drive disconnected.

3. Walk the cable: trays, the transition out of conduit, the roof curb penetration.

4. Add up the real full load amps on that output, then compare to the drive rating.

5. Check whether auto reset has been hiding repeat trips.

6. Swap hardware last, not first.

VFD HVAC questions I get asked most

What does a VFD do in HVAC?

It varies output frequency and voltage so a fan or pump turns at the speed the load needs, instead of running flat out against a damper or a throttling valve. It is also a soft start.

What is a VFD on an RTU?

The same drive, mounted on or near a rooftop unit, running its supply fan and sometimes the condenser fans. What differs is the environment: wider temperature swings, a long motor cable back to the electrical room, and nobody checking that cable for years.

Can one VFD HVAC drive run more than one fan motor?

Yes, and fan array units do it as a matter of course. You give up vector control, enter summed motor current, keep an overload on every motor, and size the drive with headroom over the total. The ACH580 supports up to 18 motors on one drive.

Why does a VFD HVAC ground fault come back when the motor megs clean?

That detection looks for current imbalance, not insulation resistance, and it is most sensitive while the drive is stopped. A delta grounded supply and a long motor cable can produce it on their own. Check the threshold setting and the cable before condemning the motor.

Keeping the right spare on the shelf

The honest ending on that rooftop unit is that swapping the drive fixed the symptom and taught us very little. Sometimes that is where you land. What made it a two hour job instead of a two day one was having the same model on the shelf.

If you keep VFD HVAC equipment running, the drives worth stocking are the ones whose lead time you cannot absorb. We carry ABB's HVAC line, including the ACH580-01-052A-4 and the smaller ACH550-UH-08A8-4, plus PowerFlex 525 drives like the 25B-D017N104 and Yaskawa A1000 units.