Symptom
Several instruments in a plant or a panel became noisy, offset, or intermittent after a drive was installed or replaced; the noise correlates with the drive running and changes with its speed or carrier frequency; instruments far from the drive are affected as well as near ones.
The short answer
Drive Causes Instrument Noise
A drive that disturbs several instruments at once is almost always a drive installation problem rather than a problem with each instrument. The drive needs a shielded or armored motor cable with the shield bonded 360 degrees at both ends, a short and wide ground connection to the panel ground, a motor ground conductor that returns to the drive, motor cable length within the manual limit or an output reactor beyond it, the carrier frequency set no higher than the application needs, the input filter connected as the manual specifies for the supply type, and separation between the motor cable and every signal cable in the plant. Work through that list in order, testing the worst instrument after each change, and most of the instrument problems disappear before any instrument is touched.
Key points
- Many instruments affected at once means fix the source, not the instruments.
- Shielded motor cable bonded at both ends is the single most effective change; a pigtail termination is not a bond.
- The drive ground is a high-frequency path and must be short and wide, not a long green wire.
- Motor cable length beyond the manual limit needs an output reactor or filter, for the motor as much as for the noise.
- Test after each change on the worst instrument; the order of the checklist is the order of payoff.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| Unshielded motor cable | Look at the cable between the drive and the motor. Individual conductors in conduit or an unshielded cable in tray return the drive common-mode current through the plant ground. |
| Shield terminated with a pigtail or at one end only | Inspect both ends. A shield that ends in a twisted drain wire to a terminal is an inductor, not a bond; the manual specifies a 360 degree clamp or gland at the drive and the motor. |
| Drive ground long or undersized | Trace the ground from the drive terminal to the panel ground bus. The manual specifies a short, direct connection; a few feet of wire routed through the wireway is not it. |
| Motor ground not returned to the drive | The motor frame must be grounded by a conductor in the motor cable back to the drive, not only to a local ground; check the motor terminal box. |
| Carrier frequency set high | Read the parameter. Drives often default to a high value for quiet motor operation; lower is less noise for instruments. |
| Motor cable longer than the manual allows without an output reactor | Compare the actual cable length with the manual table for the drive and motor cable type; long cables raise the capacitive current and reflected voltage. |
| Input EMC filter disconnected or absent | Some drives have a jumper or screw that disconnects the internal filter for ungrounded supplies; verify the setting matches the supply, and that a filter is fitted where the installation requires one. |
| Motor cable routed with signal cables | Trace the motor cable; every shared tray or conduit is a coupling path to every signal in it. |
| Drive enclosure door or panel section not bonded | Check the bonding strap on the door and the bonding of the drive mounting plate to the enclosure. |
Why the drive is the place to start
A drive produces common-mode current in proportion to how fast its output switches, how much capacitance the motor cable and motor present, and how high the carrier frequency is. Where that current goes depends entirely on the installation. With a shielded motor cable bonded at both ends and a short drive ground, the current circulates between the drive, the cable shield, and the motor frame and never leaves that loop. Without those, it returns through the plant grounding system, the building steel, the water piping, and the instrument grounds, raising the potential of every ground it passes and coupling into every signal circuit that touches one. One drive installed badly can disturb instruments in another building. Fixing that drive fixes all of them; isolating each instrument fixes them one at a time and leaves the current flowing.
The installation checklist
| Item | What the manual usually requires | Field check |
|---|---|---|
| Motor cable | Shielded or armored cable rated for drive use, with symmetrical ground conductors where specified | Cable type on the jacket; individual conductors in conduit fail this item |
| Shield termination | 360 degree bond at the drive and at the motor terminal box, with a clamp or gland | No pigtails; the shield should contact the clamp all the way around |
| Drive ground | Short, wide connection from the drive PE terminal to the panel ground, often a braided strap or a short large conductor | Length and routing; the ground bus close to the drive |
| Motor ground | A ground conductor in the motor cable from the motor frame to the drive PE | In the motor terminal box, a ground conductor from the cable, not only a local rod |
| Cable length | Within the table in the manual for the drive size and cable type; output reactor or filter beyond it | Measure or estimate; add a reactor if over |
| Carrier frequency | The lowest that the application accepts | The parameter value; set it down and test |
| Input filter | Connected for grounded supplies; disconnected or omitted only where the manual says so for ungrounded supplies | The filter jumper or screw, and the supply grounding type |
| Routing | Motor cable separated from all signal and control cables | Trace the run; shared trays and conduits |
| Enclosure bonding | Drive mounting plate and door bonded to the enclosure | Bonding straps present and tight |
| Control wiring | Shielded twisted pair, shield grounded where the manual specifies, routed away from the motor leads inside the panel | The reference and feedback wiring path inside the drive panel |
Working through it
- 1
Pick the worst instrument
The one with the clearest noise, and trend its raw input at a fast rate. This is the meter for the whole exercise.
- 2
Confirm the drive
Stop and start the drive, and change its carrier frequency; the trend should follow. If it does not, the source is something else.
- 3
Lower the carrier frequency
To the low end of the manual range. Note the improvement. This is a test and a partial fix, not the answer.
- 4
Fix the grounding
Short, wide drive ground; motor ground back to the drive; enclosure bonding. Test.
- 5
Fix the motor cable
Replace with shielded drive cable and terminate the shield at both ends with 360 degree clamps. This is the change with the largest effect and the most labor. Test.
- 6
Add output filtering if needed
An output reactor or dv/dt filter where the cable is long or where the noise persists after the cable is fixed. Test.
- 7
Fix routing
Separate the motor cable from the signal cables where they share a path. Test.
- 8
Then the instruments
Whatever noise remains is handled per loop: shielded twisted pair, one shield ground, isolation, input filtering. There should be much less of it.
- 9
Record
The changes, the carrier frequency, the cable type, and the raw trend before and after, in the drive file and on the drawings.
What good looks like
A properly installed drive is quiet to the instruments around it: the trend of a nearby 4-20 mA input shows no difference between drive stopped and drive running at full speed, and the voltage between the drive panel ground and a remote instrument ground does not change when the drive starts. The shield of the motor cable carries the common-mode current, which can be measured with a clamp meter around the shield alone, and the ground conductors elsewhere in the plant carry none of it. Drives meet that standard when they are installed the way the manual says, and the manual is specific because the manufacturer has seen every other way fail.
Frequently asked questions
- The drive has been in for years and the noise just started. Is it still the drive?
- Possibly: shield terminations corrode, a ground strap loosens, a motor cable is replaced with plain conductors during a motor change, or a carrier frequency is raised to quiet a motor. Walk the checklist again; something in it changed. If nothing did, the new instrument or the new cable is the victim of a drive that was always noisy but never mattered.
- Can I use armored cable instead of shielded drive cable?
- Continuous corrugated aluminum armor bonded at both ends with the right fittings works well and is common in drive installations. Interlocked steel armor is less effective at high frequency. Either way, the termination must be a full 360 degree bond, and the cable should include ground conductors as the drive manual specifies.
- Does the reactor go on the input or the output?
- An input reactor reduces harmonics on the supply and protects the drive from line transients; it does nothing for output noise. An output reactor or dv/dt filter slows the edges going to the motor, which reduces reflected voltage at the motor and reduces the common-mode current. For instrument noise, the output side is the one that matters.
- How low can the carrier frequency go?
- To the minimum the drive manual allows, often around 2 kHz, at which the motor is audibly noisy and current ripple is higher. For pumps and fans in a plant that is not a concern. Many installations settle on a value near the low end that keeps the motor tolerable and the instruments quiet.
Related topics
- VFD Noise on Analog SignalsAnalog readings that jump or wander when a drive runs, and speed references that make a drive hunt: how the drive output couples into signal wiring, why the input turns kilohertz noise into a slow wander, the tests that pin it on the drive, and the fixes.
- Common-Mode NoiseNoise that rides on both conductors of a signal pair together, relative to ground: where it comes from, why a differential input rejects it only up to a limit, how to tell it from normal-mode noise with a scope, and the fixes that work.
- Cable Routing ProblemsNoise that comes from where a cable runs: signal cables sharing a conduit, tray, or wireway with power or drive cables, long parallel runs, and shallow crossings. How coupling works, the separations that prevent it, and how to find the shared path.
- Variable Frequency Drives in Pump PanelsWhat a drive adds to a pump panel and what it costs: speed control, soft start, reduced inrush, and diagnostics against heat, harmonics, cable and motor stress, bypass, and the minimum speed below which a pump does no work.
- Drive Trips on Ground FaultA ground fault trip on a variable frequency drive: how the drive detects it, the motor and cable insulation faults behind most trips, the submersible cable problem, the capacitive leakage on long cables and filters that produces nuisance trips, wiring errors, and the insulation test that decides it.
- Ground Loop SymptomsHow a ground loop shows itself: hum on analog signals, offsets that change when large loads switch, serial links that error, and readings that differ by location. The mechanism, the measurements that prove it, and fixes that never lift a safety ground.
Direct contact
Have a controls question?
Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.