Whether it's stock VFDs in conveyors, fans, and cooling towers, or specialized units in presses, extruders, roll-forming machines, lathes, and routers, their proper use follows specific guidelines. Often, these requirements are outlined in drive manuals — but here we review when critical warnings and precautions are applicable. Following are some dos and don'ts when installing VFDs.
- Do add a line reactor when line power source is more than 10 times the kVA rating of the drive. A line reactor of 3% or 5% impedance should be used to reduce power line transient voltages caused by capacitor switching, line notching, dc bus over-voltage tripping and inverter over-current and over-voltage conditions. Line reactors improve the true input power factor and reduce crosstalk between drives. The input line reactor offers some protection to the drive in short-circuit conditions. If the supply transformer kVA rating is greater than 10 times the drive kVA rating, then a line reactor is recommended to minimize damage to the drive, in case the supply transformer shorts out. This line impedance depends on the drive's short-circuit rating, and on the supply power distribution transformer. Specifically, the line impedance must be greater than or equal to the ratio of the supply source transformer's rating to the drive's short circuit rating.
- Interposing a drive isolation transformer between the VFD and its power source offers several benefits This ensures complete isolation of grounding and noise-related input power problems that can affect the drive. Will an isolation transformer provide better protection than a line reactor? Yes. When does an application require an isolation transformer? Incorporating such a transformer is recommended when an installation is near a substation. Common-mode transients are those appearing between ground and neutral of the ac system. Although those two parts of the circuit are normally bonded together at one point, they cannot be presumed to be at the same potential throughout an entire power system. Common-mode transient disturbances arise from switch-mode power supplies, drive operation, arc welders, lightning, or even from normal operation of such equipment as stepper motors. Some isolation transformers can also block “normal mode” transients appearing between line and neutral.
- Do use separate conduit for input power, output power, and control wiring. Use braided shield cable for the drive and motor wiring or run the wiring through a metal conduit. Keep wiring as short as possible. Ground the shield on both the drive side and the motor side. Make sure the protective earthing conductor complies with technical standards and local safety regulations.
- Use the drive on a grounded system. Never use a floating ground. Some manufacturers do not recommend operating with a floating input on any sub-micro or newer designed drives. If there are no disturbances on the line, the drive should run fine — but serious common-mode noise could cause nuisance tripping or worse.
- Do not use time-delay input fuses. If fuses are time-delay, the designer will have problems, because these are not made for protecting solid-state rectifier front-end equipment like VFDs. Time-delayed breakers allow the MOV (metal oxide varistor) to continue drawing current — to the point of causing the drive to burn up or the MOV itself to blow before the breaker ever trips. Bussmann fast-acting current-limiting type fuses with low I2t values and 200,000 AIC rating (or equivalent) are recommended. Semiconductor protective type fuses are preferred.
- Do not add a contactor between the drive and motor: A contactor or disconnect switch between the drive and motor is not recommended.
- Do not use a ground-fault circuit interrupter (GFCI) if the drive is equipped with a filter. Installation of these devices can cause nuisance tripping — from parasitic capacitance producing leakage currents between the motor power cable lines during VFD operation, connecting multiple drives to the same input source, and using RFI filters on the input side.
- Do not cycle the input power more than once every two minutes. In fact, drive manuals specifically warn that switching a drive off and on without waiting two to three minutes is detrimental.
- A VFD can sit unused and without power for a short time without service, but if a VFD has been stored for one or more years, the DC bus capacitors must be recondition by running the VFD with no motor leads connected for at least eight hours before trying to run the drive under load. The electrolyte inside the bus capacitors changes state when not used for a long period of time; repowering the drive under no load brings the electrolytic charge back to its proper charged state. If possible apply a reduce voltage initially then gradually raise it to the operating voltage.