Hands-on: Better ac drive installations

May 15, 2000
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Variable-frequency drives (vfd’s) are used to regulate flow on supply and return air-handling units, ventilation systems, cooling tower fans, and recirculating pumps, as well as some refrigeration compressors.

“These vfd’s have come a long way in technology and value since their troublesome introduction into variable air volume systems just 20 years ago,” said John Drews, director of hvac sales, AC Technology Corp., Uxbridge, MA.

“Most contractors, both mechanical and electrical, can remember the struggle to get those vintage vfd’s up and running,” he continued. “Fortunately, there have been huge gains on the technology front that result in very reliable controls.”

New vfd’s pose far fewer pitfalls to the contractors responsible for their installation, he said. But the technological advances may have left users, installers, and specifiers misinformed about what is necessary for a good installation vs. what equipment and features may now be a waste of money.

Drews offered the following tips to help you design and install your next vfd system.

1. Some specified accessories and features may not be appropriate.

Did the project engineer use an old master specification that addressed problems no longer applicable to modern vfd’s? Here are a few of the costly items that will require a modification to the vfd specification:

  • Drive isolation transformer (DIT) — “This was an excellent requirement back when state-of-the-art technology was current source and six-step VVI inverters,” said Drews.

    “These SCRs were not only sensitive to electrical noise on the power line, but also created noise, line notching, and a great deal of harmonics that fed into the power system and would raise havoc with other electrical equipment.”

    Isolation transformers reduced these problems by adding line impedance and also allowed for isolated control inputs and outputs. On the down side, DITs are large, heavy, expensive, and inefficient.

    “Today’s digital PWM design uses a diode bridge rectifier and electronic isolation, eliminating the need for these drive isolation transformers,” said Drews. Their one benefit, added line impedance (that protects the vfd from voltage disturbances, lightning, reduces harmonic voltage distortion, improves power factor, and protects power factor correction capacitors from this non linear load), can be duplicated with a small, lightweight, inexpensive, and very efficient ac line reactor.

    “Ac line reactors can often be supplied as an integral part of the vfd. Don’t confuse dc reactors with these devices,” he cautioned. “They don’t match all the benefits of an ac reactor.”

  • Harmonic trap filter — Trap filters, like the DITs, should be used with SCR controls but have limited value and pose potentially serious power system problems when misapplied to modern PWM devices, said Drews.
  • Twelve-pulse vfd’s — These expensive vfd’s are unavailable at low horsepower ratings typical in an hvac system, yet are sometimes specified. “They are of no value unless powered by a special dual three-phase DIT that merely eliminates 5th and 7th harmonics,” said Drews.

“The 12-pulse requirement seems to be of no value — other than driving up the project’s cost — when applied to modern vfd’s.”

2. Make sure that the vfd matches the motor to which it’s connected.

Check the motor and vfd nameplates for voltage and amp ratings. Confirm that dual-rated motors are properly connected for the required voltage.

Also, “Ensure that the motor rotates freely and in the correct direction. Reverse operation can often require more horsepower and may even cause damage to the machinery.”

3. Power factor correction capacitors don’t like vfd’s. “Motors without the benefit of a vfd will generate reactive current that is a burden to the electrical system,” said Drews. “Capacitors are often used to correct the poor power factor that results from this ac motor load.

“Vfd’s inherently correct for power factor, so the capacitor banks often become unnecessary. Make sure that any remaining power factor capacitors are 200 feet away from any vfd. If not, there should be an ac line reactor or transformer electrically buffering the two devices with added impedance.”

This should cure fuse blowing or premature failure of the power factor capacitors.

4. Remember, careful installation can prevent costly repairs.

“Heat, moisture, and construction dust can really do a number on electronic controls,” said Drews. “Costly repairs, arguments over warranty, and delays to project completion will haunt the careless contractor.

“But that’s just the beginning. An owner can be plagued for years if their vfd’s suffer from the constant deterioration of components brought on by these silent killers.” Here are Drews’ tips for a trouble-free installation:

  •  Allow plenty of space around the enclosure and be particularly aware of heat-producing devices, including other vfd’s, that could develop high ambient temperatures exceeding the typical 40°C (105°F) rating.

    “Direct sunshine will convert the drive enclosure into a solar oven that can bake the life out of solid-state components. Rooftop installations can produce the worst problems.”

  •  Use factory conduit knockouts. Cutting into the sheet metal enclosure may produce fine metal particles that short out microcircuits in the control and may cause intermittent problems.
  •  Protect against construction dust. “That sheetrock dust looks harmless,” said Drews, “but it can coat electronic circuits and later pick up moisture. Then the coating becomes conductive and short circuits may cause failures.

    “If dust is found, always vacuum the dust; blowing it out with compressed air can make the situation worse by lodging debris in the electronic circuits.”

    5. Electrical wiring between the vfd and motor is critical. “Alternating current produced from these very efficient, very high-speed, solid-state switching devices is nothing like the smooth sine wave that comes from your local utility.”

    Improper wiring can bring EMI (electro-magnetic interference), RFI (radio-frequency interference), and circulating ground current problems to the system.

  •  Install a dedicated steel conduit. “Plastic conduit or cable trays should never be used for wiring between the motor and its vfd. The steel conduit must be well grounded at the steel vfd enclosure, at the motor’s junction box, and through all conduit splices.”

    Painted surfaces must be well penetrated by the fasteners. “This grounded steel encasement will contain the RFI that might otherwise broadcast from these wires,” said Drews. “Don’t run any other wiring in this conduit, and that includes other motor leads, as they will pick up noise through transformer coupling.”

  •  Avoid long leads between the vfd and motor. “State-of-the-art transistors in today’s vfd’s have reduced power consumption and lowered audible motor noise,” said Drews. “Unfortunately, the dv/dt — rapidly changing voltage — also interacts with long motor leads to induce high peak voltages at the motor that could be destructive.”

    Most new motors are well protected against this phenomenon, and it poses no problem with 208- or 240-V power systems, but some caution should still be taken on 460- and 575-V installations, he said. If these leads exceed 60 ft in length, consult the motor supplier to ensure that the motor is corona resistant and suitable for use on a vfd.

6. Carefully install control wires. Low-voltage control wires should be shielded and kept away from high-voltage wires.

The shielded cable will prevent electrical noise from disturbing the proper operation of a variable-speed system. “These same shields, if improperly grounded, can actually attract noise into the system, causing instability and nuisance fault trips,” said Drews.

“These shields should be carefully grounded on both ends of the cable when digital signals are involved. Leave one end ungrounded if the signal is analog, such as a 0- to 10-vdc or 4- to 20-mA signal. If both ends are grounded, a ground loop could generate additional voltage that would corrupt the analog signal.”

Serial communication requires further coordination between manufacturers to ensure that all devices within the system speak the same language or protocol.

“Great care is required so that any problems are resolved before the various components are even purchased,” he said. “Freeze stat, fire stat, smoke purge, and run signals to vfd bypass starters may still have to be hard-wired. How does that all coordinate with the serial communication? Maybe the startup technician can help!”

7. A factory startup can wrap it up. “Most projects call for factory-trained startup assistance and training,” commented Drews. “This can be an expensive and time-consuming process if the vfd’s aren’t ready when the technician arrives.

“That technician is bound to be more helpful and understanding if the contractor helps him get the job done efficiently. This should be a team effort.

“A cooperative technician will often save the day when specifications, products, or installations are slightly out of order. Startup quotes for future projects will often reflect the cooperation on previous jobs. A good working relationship between technician and contractor can really pay dividends.”

Technological advances in solid-state power devices, microprocessors, and improved production techniques will help ensure the proliferation of variable-frequency drives. Two-speed motors and motor starters will soon be replaced by more cost-effective vfd’s, Drews predicted.

“As manufacturing costs decline, the vfd may replace single-speed fans and pumps as the norm,” he said.

“Contractors and engineers will become more familiar with these devices, the manufacturers will make them easier to live with, but the seven suggestions discussed here will probably remain appropriate for a successful installation.”

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