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Feed your VFD with the right power

May 1, 2012
Certain precautions involving wiring, isolation, grounding, and shielding are relevant to all variable frequency drive (VFD) applications.

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, and why.

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Low-volt faults

A VFD reports a low-volts fault when the drive's dc link voltage drops below 62% of the nominal level for the high setting (480 Vac) and 50% of nominal for the low setting (400 Vac).

The +10% and -15% voltage tolerance in most manuals is the operating range recommended to allow the drive at hand to maintain premium efficiency and proper motor current. Drives can run below these tolerances, but reduced voltage can have unpredictable effects on motor current, motor temperature, and overall performance.

More specifically, if a drive's line volts parameter is set to high and 480 Vac is being applied, the drive will generate a low-volts fault when the dc link voltage drops to 62% of nominal — as 480 Vac·0.62·√2 = 421 Vdc. The nominal dc link voltage is 480 Vac · √2 = 679 Vdc.

Precautions when switching

Although switching a VFD from line power to a backup generator is an accepted practice in many applications, there are some limitations. Most importantly, many portable backup generators have a larger voltage swing on their phase-to-phase voltage input than a drive's recommended phase-to-phase voltage tolerances, which are generally less than 2%. To protect against voltage swings, VFDs are equipped with surge guards in the input rectifier circuit — but external protection (such as surge arrestors) may be required for severe input disturbances. Larger variances induce greater ripple on the dc bus capacitors, and this causes damage to both the capacitors and other power components over time.

Another potential problem arises when switching from line power to a standby generator. Most drives need a minimum of two minutes before power can be reapplied; disregarding this guideline can damage the charge relay circuit, or at the very least, blow the input fuses or trip the circuit breaker.

For cases in which input power exhibits moderate spikes in voltage that cause current surges, a 3% line reactor may improve the situation.

Another solution is to add a voltage monitor with a suitable time delay. These have a protection trip level that shuts the drive down in cases of under-voltage, over-voltage, loss of phase, and voltage imbalance between phases. These devices ensure that the VFD is never exposed to unbalanced input power or powered up until voltage is within tolerance and proper delay times are met.

Isolation transformers

A final suggestion — the most expensive solution — is to use an isolation transformer. 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 in close proximity to a substation.

Interposing a drive isolation transformer between the VFD and its power source offers several benefits. Isolation ensures that no direct electrical connection exists between source and load — but that's true for any transformer, other than an autotransformer. What makes the drive isolation transformer unique is the placement of grounded electrostatic (Faraday) shielding between and around primary and secondary windings. This shielding provides up to a million-fold decrease in the capacitive coupling involved in transferring common-mode voltage disturbance. Without such shielding, that capacitance allows passage of high-frequency noise and transient voltage spikes through the transformer.

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.

Consider one application in which a daily utility-company powerup of a substation capacitor bank in an industrial park causes a transient voltage spike — amplified by reflection from onsite capacitors in a nearby plant. Assume that one facility in the park has several small drives rated at 7.5 hp. Normal-mode transients can cause these drives to shut themselves off, resulting in costly process downtime. An isolation transformer can prevent such disruption.

When it's been sitting on a shelf

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, it must be reformed — to recondition the dc bus capacitors for service. Here, the designer must run the drive with no motor leads connected for at least eight hours before trying to run the drive under load. Why? 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.

Practical installation tips

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 kV-A rating of the drive.

  • 3% impedance line reactors 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 kV-A rating is greater than 10 times the drive kV-A 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.

  • Do use separate conduit for input power, output power, and control wiring. More specifically, when connecting the VFD's power and control wiring, the following guidelines should be followed:

    • Install the input ac power wiring in its own rigid steel conduit.

    • Install the output motor wiring in its own rigid steel conduit.

    • Install the control wiring in its own rigid steel conduit. Low-voltage dc control wiring and 120 Vac control wiring should be in separate conduits. Both twisted pair and shielded wire are sufficient when wiring to the VFD's control board. Two and three-wire connections are recommended. For many drives, the minimum wire size is 18 AWG.

    • Ensure that all ground connections are tight and properly grounded. The shield should be connected to ground at only one end of the cable to avoid ground loops. When connecting the shield at the VFD end, connect it to the chassis ground lug. Caution: Make sure to remove power from the VFD prior to connecting the shield to the VFD's ground lug.

    • Separate control and feedback wiring from power wiring by at least 12 inches.

      Caveat: In installations with multiple VFDs, input power wiring for all VFDs can be in the same conduit, and the control wiring can be in the same conduit, but the output wiring for each motor must be in a separate conduit. The only exception is that if one VFD is used to operate multiple motors, the output wiring for all of the motors can be in the same conduit.

    • 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.

Note: Certain legacy VFDs use a string of resistors between the dc bus and ground, which means common-mode noise isn't an issue. Some integral-hp drives also use a resistor string, so using the floating ground on these is probably okay. However, a floating-point system is not recommended for newer drive technology.

Approaches that spell trouble

  • 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.

    Either branch circuit protection via a circuit breaker or a disconnect switch and fuses must be provided to comply with the National Electrical Code (NEC) and all local codes. Consult Article 430, Section H, of the NEC handbook for more information. Select a circuit breaker or fuse rated at 1.5 times the input current rating for constant torque drives, and 1.25 times the input current rating for variable torque drives. The minimum rating should be 10 A, regardless of the input current rating — because a 10-A minimum accommodates in-rush during powerup. The VFD provides motor protection.

    Bussmann fast-acting current-limiting type fuses with low I2t values and 200,000 AIC rating (or equivalent) are recommended. Fuse types include:

    240/200 Vac models: KTK-R or JJN type, rated 250 Vac

    480/400 Vac models: KTK-R or JJS type, rated 600 Vac

    590/480 Vac models: KTK-R or JJS type, rated 600 Vac

  • Do not add a contactor between the drive and motor: A contactor or disconnect switch between the drive and motor is definitely not recommended. Operating a motor contactor or disconnect between the VFD and the ac motor while the VFD is running can cause nuisance tripping. Such devices should only be operated when the VFD is in a stop mode. There is also the possibility of noise from the output feeding back into the control board through the low voltage power supply — and damaging the control or driver board.

  • If the contactor is absolutely necessary, an early-break auxiliary set of contacts on the device should be interlocked with the VFD's external fault input or stop input. This way, if the device is opened while the VFD is running, it will stop the drive and immediately cut off VFD output power. In addition, use a minimum time-delay of 100 msec. Remember that if wired to the VFD's stop input, the stop method must be set to coast. Finally, allow the drive to completely stop the motor before restarting.

  • 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: Applying input power more quickly causes a buildup of voltage in the input pre-charge circuit, and eventually burns it out. Why? Here, the dc bus capacitors don't have enough time to discharge, and the input circuit needs time to stabilize. Otherwise, additional input can damage the charge relay circuit, or at the very least, blow the input fuses or circuit breaker.

    In other words, the pre-charge circuit allows a certain time limit for the inrush limiter to send current through to charge the dc bus capacitors. The inrush limiter resistance changes with temperature. The hotter the limiter gets, the lower the resistance value. When that pre-charge time ends, the relay cuts off and the capacitors hold the charge. When the drive is powered down, this voltage bleeds off through resistors in the discharge circuit. Power reapplied too quickly meets an inrush limiter that hasn't had time to cool down to an acceptable resistance level, so the current will be higher, and consequently, could blow the fuses or possibly damage the pre-charge circuit.

    One solution here is to install an external time-delay relay or voltage monitor circuit at the drive input. Then, when the voltage drops below a set level, the voltage monitor cuts off and allows zero power back into the drive until two to three minutes have passed — ensuring that voltage has stabilized to an acceptable level.

  • 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.

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