Long Motor Leads on a VFD: What to Check

Once the cable from a TECO drive to the motor runs past 82 ft (25 m), lower the carrier frequency to the TECO table. On longer runs, check that the motor meets NEMA MG 1 Part 31, use symmetrical shielded VFD cable, and add a load reactor at the drive output. Long leads act like a transmission line. Each fast drive pulse can bounce back and stack at the motor, up to about twice the voltage. That spike is what punches through motor insulation.

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Step 1: set the carrier frequency to the cable length

Cable length, drive to motor	Under 100 ft (30 m)	100 to 165 ft (30 to 50 m)	166 to 328 ft (50 to 100 m)	Over 329 ft (100 m)
Max carrier frequency (parameter 11-01)	16 kHz	10 kHz	5 kHz	2 kHz

Source: TECO Westinghouse A510 and F510 instruction manuals, table 3.14.1. TECO notes that past 25 m (82 ft) stray cable capacitance can cause overcurrent trips, more leakage current and a wrong current readout.

Step 2: check the motor insulation

NEMA MG 1 Part 31 sets the peak voltage an inverter duty motor (600 V and under) must take at its terminals, with a rise time of 0.1 µs or more.

Motor rated voltage	Withstand peak (Part 31)	Partial discharge free peak (Part 31)
230 V	716 V	513 V
460 V	1,431 V	1,025 V
575 V	1,789 V	1,281 V

Source: ANSI/NEMA MG 1-2016 (Rev 2018) Part 31, Table 31-3. Values are line to line, zero to peak.

TECO Westinghouse states its small VFD duty motors meet or exceed the Part 31 voltage levels. The MAX-E1 uses inverter duty magnet wire rated for spikes up to 2,200 V. The MAX-E2/841 is rated for inverter use per Part 31 at 20:1 variable torque and 10:1 constant torque (3:1 constant torque at 350 HP and up). See TECO Westinghouse motors.

Step 3: pick the cable, the reactor and the bearing fix

Item	What the maker says	On this site
VFD cable	TECO motor group best method: symmetrical 3 phase cable, 3 ground conductors grounded in both boxes, 100% coverage shield grounded at the glands	Tekima 365S-3XG (3C + 3 grounds, shielded, UL TC-ER 600/1000 V), Tekima 365S-1XG
Load reactor	Rex: a reactor alone on the drive output cuts long cable spikes from about 200% to typically under 150%, and cuts cable charging current of 10 to 20 A that trips small drives. Mount it next to the drive	Rex 3PR line/load reactors, Rex collection
Motor guarding filter	Rex: reactor plus resistors cuts peaks to under 125%	Quoted to order from Rex
Wire size	TECO: size input and motor wire for 2% voltage drop or less	Ask us for the drop on your run
Bearings, frames under 320	TECO motor group: shaft ground brush, or CoolBlue inductive absorbers	Specify with the motor
Bearings, 320 to 440 frames	1 insulated bearing at the non drive end plus a drive end ground brush, or CoolBlue absorbers	Specify with the motor
Bearings, 444/449 frames and up	2 insulated bearings plus a drive end ground brush, or CoolBlue absorbers	Specify with the motor

Sources: TWMC (TECO Westinghouse Motor Company) suggestions for the application and protection of small VFD duty induction motors; Rex Power Magnetics line reactor catalog, output reactor section; TECO A510/F510 manual section 3.13.

Worked example: 25 HP, 460 V motor, 300 ft from a 480 V drive

Step	Math or rule	Result
Carrier frequency	300 ft is in the 166 to 328 ft band	5 kHz max on parameter 11-01
DC bus	480 V × 1.35	About 650 V DC
Worst spike at the motor, no reactor	About 2 × the bus on a long cable	About 1,300 V, close to the 1,431 V Part 31 withstand and over the 1,025 V partial discharge free level
With a load reactor	Rex: typically under 150%	Under about 975 V, below the 1,025 V partial discharge free level
Reactor size	Rex 480 V table: 25 HP is a 34 A reactor	Rex 3PR-0034C3H at the drive output
Cable	Symmetrical, 3 grounds, shielded	Tekima 365S-3XG, sized for 2% drop or less

Math notes: a 3 phase rectifier gives a DC bus of about 1.35 × the line voltage. Reflected wave peaks of about 2 × the bus are the long cable case Rex shows. Real peaks depend on rise time, cable and motor. Send the drive, motor and cable data and we check it.

Common questions

Can I put capacitors or an LC filter on the drive output?

No. The TECO manuals say do not connect power factor capacitors or LC/RC noise filters to the drive output. NEMA MG 1 Part 31 also says power capacitors on the load side of an inverter are not recommended. Use a load reactor or a motor guarding filter made for drive output.

Why does my drive trip on overcurrent with a long cable?

Cable capacitance draws charging current on every pulse. Rex puts it at 10 to 20 A on long runs, enough to trip a small drive. Lower the carrier frequency first, then add a load reactor.

Is a load reactor the same part as a line reactor?

Often, yes. Rex 3PR reactors are rated for the drive input or output. Where it goes sets the job. See line reactor vs load reactor.

Does shielded VFD cable alone stop the spikes?

No. Good cable gives the high frequency current a clean path back to the drive and helps protect bearings. The spike at the motor is set by cable length, rise time and carrier frequency. A reactor or filter at the drive output cuts it.

What about bearing damage?

The drive puts a common mode voltage on the motor shaft. TECO motor group says to check shaft voltage and fit a ground brush, insulated bearings or inductive absorbers by frame size, as in the table above.

Get the package sized and quoted

Send the motor HP and voltage, the drive model, the cable length and how the cable is run. We send back the TECO drive and motor, the Rex reactor and the Tekima cable as one quote.

Request a Quote Book 30 Minutes to Review the Application Call 248-830-4850

NEMA is a registered trademark of the National Electrical Manufacturers Association. CoolBlue is a trademark of its owner. TECO Westinghouse, Rex Power Magnetics and Tekima data are from each maker's published manual, catalog or data. Check the current manufacturer datasheet before you specify.