A VFD cable gland has to do three jobs at once: clamp a cable screen to the enclosure around its full circumference, seal against humid plant air, and stay tight on a motor that never stops vibrating. Get one wrong and the failure rarely shows up at the gland — it shows up as an encoder that loses position at low speed, or a bearing that starts fluting after eighteen months.

This guide covers the application side: why the drive end and the motor end ask for different things, how to size a gland for a cable that is fatter than you expect, where the bond path breaks, and what a gland cannot fix.

Why a VFD Motor Cable Is Not Just a Power Cable

A drive does not deliver a sine wave. It switches a DC bus to synthesise a variable-frequency output, and those edges are fast. On a long motor cable they drive a common-mode current that returns through the screen, the motor frame and the building earth rather than the neutral. Above a few hundred kilohertz that current behaves like current on a transmission line, not in a wire. The screen contains it only while it is electrically continuous end to end — break that, or bond it through a thin wire, and the screen becomes an antenna instead of a shield.

For the mechanism inside the gland itself, see our explainer on what an EMC cable gland is and how it works.

What the Gland Actually Has to Do

An EMC gland earns its place with three mechanical features, and only the first is about EMC.

• A circumferential contact surface shaped to press against a braid or folded tape around the whole cable, as close to the enclosure sheet as the design allows. This is what keeps bond impedance low as frequency rises.
• A sealing insert that grips the cable jacket — not the cores — so the IP rating survives thermal cycling.
• A strain-relief grip that stays put under vibration, which otherwise works a loose cap nut until contact resistance climbs.

Nickel plating matters more than it sounds: bare brass oxidises in humid air, and a film of oxide on the contact face is a resistor in series with your bond. That is essentially what separates EMC from standard cable glands — a metal body with no shaped contact face is half the job.

The Drive End and the Motor End Are Different Jobs

Every guide says the screen must be bonded at both ends. Fewer point out that the two ends are mechanically different, and that the motor end is where installations usually fail.

At the drive you are working in a cabinet, with room to dress the screen and test it. At the motor you are working in a small cast terminal box that may sit on a gasket, may be painted inside, and may be plastic on smaller frames. The bond has to travel from screen to gland, gland to box wall, and box to motor frame — and every interface is a place it can stop.

The last row is the one people skip. If the terminal box is not bonded to the frame, the screen path ends at the box and the high-frequency current finds another route — often through a bearing.

Sizing the Gland for Drive Cable

VFD cable is not a standard four-core. It typically carries three phase conductors, symmetrical earth conductors in the interstices, and a copper tape screen under an XLPE or LSZH jacket. That makes it thicker for the same current rating, and the tape you fold back over the sealing cone adds thickness again where the gland clamps.

Measure the jacket outer diameter with a caliper at several points instead of trusting a nominal figure, then choose the gland by where that diameter sits inside its clamping range. Mid-range gives tolerance; sitting on the maximum means the seal insert is fully compressed before the cable heats up and expands.

Because the EMC range covers cable outer diameters from 3 mm to 38 mm across M12 to M63 and PG7 to PG48 threads, the practical question is whether the gland fits the knockout you already have. A step up in cable size often means a step up in thread size: a new hole, a bigger gland plate, or a reducer that adds another interface to the bond path.

Where the Bond Path Breaks After the Gland

Most EMC problems on drive systems are not gland selection problems. They are continuity problems in series with a gland that was chosen correctly.

Bond Both Ends — and Understand Why

The single-end rule applies to signal circuits, where bonding at both ends creates a ground loop that couples low-frequency noise into a sensitive measurement. A drive motor cable is the opposite case: the interference is high frequency and the return path is the screen itself, so a screen bonded at one end can only re-radiate it. Where a drive manual specifies otherwise, follow the manual. Both approaches are compared in our guide to matching cable glands to different screen constructions.

What a Gland Cannot Fix

• Bearing currents. Bonding the screen closes the cable’s return path but does not stop shaft voltage. Fluting is fixed with insulated bearings, a shaft grounding ring or drive-side mitigation — not a better gland.
• A cable with no real screen. Unscreened four-core, or foil with a drain wire and no circumferential path, cannot be rescued by any gland.
• Excessive motor lead length. Long runs without an output reactor or dV/dt filter generate reflected wave overvoltages regardless of the screen bond.
• An unearthed motor frame. The return path has to exist at both ends.

The Field Sequence at the Motor End

The order of operations matters more than the individual steps, because the screen has to be dressed before the gland is tightened, and tested afterwards. Strip the jacket to the length the gland specifies, fold the tape or braid back over the sealing cone without nicking strands, and seat the gland body so it makes metal-to-metal contact with the box wall.

Then measure continuity from the gland body to bare metal on the motor frame, and from the frame back to the drive earth bar. Record the readings before commissioning and repeat them at first service — a bond that deteriorates in service is the classic drive mystery fault. The full procedure is in our walkthrough of how to install an EMC cable gland.

Material and Protection for Motor Environments

Motor rooms bring heat, oil mist, coolant and washdown, and the gland has to survive them long enough for the bond to still be a bond in five years. Plating is a performance feature, not cosmetics.

Ingress protection is specified separately from EMC and should be read the same way at every entry. Our glands are tested to IP68 under IEC 60529, and the honest answer to “what does IP68 mean” depends on the declared depth and duration — see the breakdown in how to choose the right IP rating. Stainless steel bodies for harsher motor environments are listed in our stainless steel cable gland range.

Conclusion

A VFD cable gland is a small part doing an unglamorous job: keeping a screen a screen. Choose it by measured cable diameter and by whether the contact geometry presses against the screen around the circumference. Then concentrate on the two things that decide the outcome — bare metal at the gland seat, and a continuous path from screen, through the terminal box, into the motor frame.

If you are specifying glands for a drive panel or an OEM machine series, send us the cable construction and outer diameter per motor size and we will confirm the gland, thread and sealing range that fits, with the documentation your customer will ask for at handover. You can reach the team through our contact page.

الأسئلة الشائعة

Can I use a standard brass cable gland on a VFD motor cable?

Only if it has a circumferential contact face that presses against the screen. Most standard brass glands are shaped to seal and grip the jacket, with nothing designed to touch the screen. If the only way you can bond it is a pigtail, it is the wrong part.

Should the screen be bonded at both ends on a VFD?

Yes for the great majority of drive installations, at the cabinet entry and at the motor terminal box. The screen carries high-frequency common-mode current back to its source, and that path only works if it is continuous at both ends.

Can I use a nylon cable gland on a VFD motor cable?

Not on screened VFD cable. A nylon body is an insulator and cannot bond a screen under any circumstances. Nylon is reasonable for unscreened control and signal cables in the same panel, provided temperature and UV exposure suit the location.

Does an EMC cable gland stop bearing currents?

No. A correctly bonded gland reduces radiated interference from the motor cable and lowers high-frequency current in unintended paths. It provides no shaft current path, so bearing protection is a separate decision.

How do I check that the screen bond is actually working?

Measure from the gland body to bare metal on the enclosure, then to bare metal on the motor frame, and record the values. That proves continuity and catches the common failures — painted faces, gaskets, plastic boxes, missed earth washers. It does not show how the joint behaves at high frequency, so repeat it at first service.