Figure 1. Schematic representation of pulse-width modulation.
Over the last twenty years, variable-frequency drives (vfd's) have solidly established themselves as the method of choice for controlling the speed of ac motors used in hvacr systems. Initially applied exclusively to obtain energy and operating cost savings by taking advantage of the relationship between power and speed for centrifugal fans and pumps, they have since come to be accepted as the preferred means of control for most variable-flow systems.

During this period, vfd's have evolved from using analog-based electronics with limited flexibility and control capability to on-board microprocessors with advanced control and communications capability. Advances in power switching devices and power control algorithms have provided higher efficiency, improved accuracy, reduced noise levels, and bidirectional power flow capability. These benefits however, come at the price of a more sophisticated and complex device, requiring more knowledge for its effective application.

The basic principles of operation and energy benefits of vfd's are well understood by most hvacr designers and have been previously discussed at length in this (see "Basics of Vsd's" by Mark Ziemer, P.E. in the August 1999 issue of ES) and other publications. This article will instead focus on some of the application issues that are critical to successful integration of vfd's into the hvacr control system and the building electrical system.

State of the Art

Although there are many vfd products out there, with some legitimate differences between them, the commodity nature of the building systems market has created a power circuit topology and control capability that is common to almost all vfd's specified for hvacr application.

This configuration consists of a six-pulse diode-bridge rectifier and a microprocessor controlled, sine-coded, pulse width modulated (PWM) inverter using insulated gate bipolar transistors (IGBTs). In simple terms, the rectifier converts the ac input voltage into a dc voltage of constant amplitude and the inverter creates a sine wave output from the dc voltage by switching the IGBTs on and off to vary the width of a series of pulses sent to the motor (Figure 1). The microprocessor executes the control algorithms necessary to generate the pulse width modulation, includes on-board control capability such as current-limit, ramp rate, and PID control, as well as providing the external control interface.

While space does not permit defining all the terminology in that last paragraph, it is important to accurately establish the drive topology we are discussing because it determines the interaction between the vfd, the building power system, and the motor. Many of the issues we will discuss in this article are not applicable to, or must be addressed differently for, other drive topologies. In the case of certain system problems such as unacceptable harmonic distortion levels, one solution may be to use a different drive topology.

The other feature common to vfd's for the hvacr market is a move toward highly compact enclosures that consume the minimum amount of space in crowded mechanical and electrical rooms. This has led to the elimination from the standard drive package, of features and components that may not be required for all applications, such as disconnect switches, bypass contactors, and input fuses; and the enclosure supplied with a standard drive no longer has the space to accommodate additional components when they are required.

If a vfd application requires optional or custom features, most manufacturers provide an "engineered drive" package in which an open-frame drive is installed in a larger enclosure along with the extra components that are not integral to the drive. The engineered drive comes at a higher cost, larger space requirement, and longer lead-time than the standard package, which in many cases is literally available "off-the-shelf."

Benefits of Microprocessor Control

Once the microprocessor was introduced as an improved means of implementing power device switching algorithms, a host of other control benefits was obtained. While early drives were limited to a handful of control inputs and status outputs, microprocessor-based drives can provide a wide range of inputs and outputs, as well as the ability to monitor and output critical motor parameters. Drives now include on-board PID control capability permitting them to serve as standalone process controllers when provided with the appropriate process variables from external sensors.

In most hvacr applications in which the drive interfaces to the building energy management system, a major advantage of microprocessor control is digital communication capability. Virtually all drives have RS232 or RS485 serial communication capability as a standard feature, and a long list of optionally available protocols such as ModBus, Profibus, and LonWorks.

Many drives are also now available with drivers for common energy management system network protocols. Digital communication permits setup and modification of drive parameters from the energy management system as well as block transfer of drive and motor operating parameters to the system.

Making Sense of Specs

Reviewing the list of available performance features of any modern vfd is sure to leave many confused about what to specify. Many features such as slip compensation, vector control, dynamic braking, and S-curve acceleration were developed for demanding applications such as paper machines, steel mills, and conveyors, where ac drives were traditionally unable to compete with the superior performance of dc motors and drives, and are not required for typical hvacr applications.

The essential performance features for most hvacr applications are standard on major manufacturers' drives and require no special specification treatment. These include adjustable acceleration and deceleration rates, multiple critical frequency lockout, volts per hertz control with starting boost, adjustable carrier frequency, and current limitation. In addition, there are a number of specialized performance features relevant to fan and pump control that should be considered.

Pump control algorithms are available that match acceleration and deceleration rates to hydraulic performance of the pump to prevent pressure surges and fluid hammer. These can be of particular advantage on pump shutdown, preventing slamming of the check valve caused by rapid decrease of pump discharge pressure. Adaptive algorithms use feedback of motor current to adjust the deceleration rate and provide improved performance over preset parameters when the hydraulic conditions of the system are variable.

Drive specs should also address how the system is expected to perform in the event of both extended power failure and momentary interruptions. A sustained loss of power will result in an undervoltage trip of the drive, which may be automatically or manually reset. Automatic restart is a selectable option on most drives, and is generally preferred over requiring an operator to visit the drive. If the vfd is setup for automatic restart, the temperature control system will also require the proper setup and programming to ensure that the overall system start sequence is properly executed and that the drive does not simply start and run in isolation from dampers, valves, etc.

The duration for which a vfd can continue to deliver power to the motor on loss of input voltage depends on how heavily loaded it is, but is generally very short, less than 20 msec at full load and less than 100 msec at light load. Therefore, control of the motor at the desired speed cannot be maintained for most of the momentary outages expected on the typical electrical system. Vfd control power, however, can be maintained for as long as a second or more, providing the basis for an orderly resumption of control at the end of the momentary dip.

Various methods of "coasting motor restart" or "flying restart" are available as optional features through which the drive will determine the rotational speed of the motor when power returns and resume control at that speed, accelerating back to the speed required by the system. This can be a particular advantage for high-inertia loads such as large fans, where waiting for the motor to come to a stop would entail a complete system restart sequence. It is of less value for low inertia loads such as centrifugal pumps because they are likely to have coasted to a complete stop before the drive is able to detect their operating speed and initiate a restart.

Can IGBTs and Motors Play Nicely Together?

From the beginning, it was known that motors required certain characteristics for successful application with vfd's. The primary concern was the effect of operation at reduced speed and the harmonic content of the vfd output voltage on motor heating. The early "inverter duty" motor, developed to address this concern, was designed for effective cooling down to a minimum of 10% of rated speed for variable-torque application or 16% of rated speed for constant-torque application. It had a minimum of Class F (155?C) insulation and was designed for the heating effects of the high frequencies contained in the drive output waveforms. For many years, this was adequate to ensure a successful application.

Introduction of the IGBT drive, however, has sent us back to the drawing board on the issue of vfd-motor interaction, as a number of new problems have been found to stem from the fast turn on capability and higher switching frequencies that are characteristic of this device. These problems include motor winding insulation failure from sustained overvoltages and fast voltage rise times, bearing failures from leakage currents between the motor rotor and ground, and electromagnetic interference (EMI) with other equipment.

There are a number of advantages to the fast switching capability of the IGBT. An electronic switch dissipates the most power while it is turning on and turning off. In the off state, the voltage across the switch is high, but there is very minimal leakage current so power (product of voltage and current) is low. Similarly, when the switch is on, the current is high but the voltage drop across the device is very low. Only during the actual turn on and turn off periods are both significant voltage and significant current present, resulting in high power dissipation.

The faster you can turn on and off the pulses that make up the PWM voltage waveform, the more efficient the drive, and the use of IGBTs has pushed published vfd efficiencies as high as 98% at full load. If you can turn the PWM pulses on and off faster, you can also use a higher pulse rate or carrier frequency, which allows a closer approximation to an ideal sine wave, reducing harmonics and associated motor heating.

The price to be paid for the advantages of fast turn-on shows up in the damaging effects of the fast voltage rise times on the interturn insulation in the motor winding. This can be worsened by something called "transmission line effect" in which the voltage pulse from the vfd travels down the circuit to the motor and is reflected at the motor terminals due to the difference in electrical characteristics between the cables and the motor winding. The reflected wave adds to the original pulse to produce a surge of voltage that may be as high as two times the nominal value2. This effect is more pronounced the longer the length of the circuit from the vfd to the motor, and it is recommended that motor lead lengths be kept as short as possible.

Another advantage of higher carrier frequencies is reduction of audible motor noise produced by the vibration of the laminations of the steel stator core and rotor under the magnetic forces generated by the applied voltage. Higher frequency vibrations are attenuated more effectively by the mass of the steel and are also less audible to humans. To take advantage of this, drives are available with carrier frequencies as high as 20,000 Hz.

Unfortunately, a number of adverse effects are also linked to high carrier frequency operation. Motor winding insulation damage has been shown to be related to cumulative surge exposure as well as surge amplitude, and this risk, therefore, increases with increasing carrier frequency1. EMI impacts other systems through coupling to motor circuits and increases with carrier frequency. Bearing currents resulting from the generation of voltage between the motor rotor and ground by capacitive coupling across the air gap can result in premature failure, as early as several months of operation; the capacitive coupling is more pronounced at higher frequencies resulting in higher voltages and a greater likelihood of damaging currents.

Both drive and motor manufacturers are working to resolve these issues and although no single solution has been developed, a number of options are available for specific applications. Drive manufacturers offer output filters and reactors to reduce high-frequency components of the waveform, common-mode chokes to reduce EMI and bearing currents, and motor circuit terminators that match the characteristics of cables with the motor winding to eliminate reflected waves.

Motor manufacturers are developing surge-resistant winding insulation, insulated bearings, and electrostatically shielded motors to reduce bearing currents, but unfortunately none of these features are universally available yet. Specially constructed motor lead cable with shielding and multiple symmetrically distributed ground conductors is being promoted as a means of reducing both EMI and bearing currents. Because many of these solutions involve significant added cost, development of analysis techniques to accurately predict their need for specific applications is also desirable.

Hopefully, continued research and development will soon result in a better understanding of how to select appropriate mitigation measures and competitive availability of those solutions that are proven effective. Until this occurs, there are some basic rules that can reduce the likelihood of problems in typical applications.

  • Keep motor circuit lead length as short as possible and definitely less than the drive manufacturer's published limit for the type and rating of drive.
  • Specify energy-efficient motors that comply with NEMA MG1 Part 31 requirements for adjustable-speed operation.
  • Use shielded cable or cable installed in steel conduit for motor leads.
  • Operate the vfd at the lowest carrier frequency available.
  • Consult with both the vfd and motor manufacturers on requirements for applications where any of the above are not possible.

Don't Forget the Extras

Finally, don't forget that your vfd application may require some of the features or components that have been relegated to optional status in order to shrink the standard enclosure to miniscule dimensions. Some things to be aware of include:

  • Vfd's are considered motor controllers under the National Electrical Code (NEC) and are therefore required to have a disconnecting means within sight of the drive. If the switch or circuit breaker serving the vfd is not within sight, and an integral disconnecting means is not included with the drive, a separately mounted switch must be provided by the installing electrician with associated cost and real estate requirements.
  • Many vfd's require fast-acting current-limiting fuses to protect the drive components from failure when the motor develops a short circuit or ground fault or when the supply voltage experiences a transient. Standard circuit breakers and fuses used in electrical distribution systems to protect the circuit supplying the drive do not meet these requirements. If these fuses are not integral to the drive, they must also be separately installed.
  • Input line reactors may be required to limit the harmonic distortion created on the distribution system by the drive, or to protect the drive from high, available fault currents on very high-capacity electrical systems. If the electrical power system supplying the drive is not solidly grounded, an isolation transformer with a solidly grounded secondary is recommended. These devices can either be included within an engineered drive enclosure or installed in a separate enclosure similar to a standard dry-type transformer.

The benefits of vfd's in reducing energy consumption and providing controllability of modern hvacr systems are well established and easily taken for granted by designers. Proper care and attention to the many issues that must be addressed for a successful vfd application will help ensure that the owner and operator can also take these benefits for granted for the life of the facility. ES

Works Cited

1. Hodowanec, Mark M.,"Proper Application of Motors Operated on Variable Frequency Control,"IEEE Industry Applications Magazine, September/October 2000, vol. 6 no. 5.

2. Saunders, et al., "Riding the Reflected Wave - IGBT Drive Technology Demands new Motor and Cable Considerations," IEEE IAS-Petroleum and Chemical Industry Conference, September 1996.

3. Ziemer, Mark, P.E., "Basics of Vsd's" ES, August 1999, vol. 16, no. 8, pp XX.

Erdman, et al., "Effect of PWM Inverters on AC Motor Bearing Currents and Shaft Voltages," IEEE APEC Conference, March 1995.

MG1 - Motors and Generators, National Electrical Manufacturer's Association (NEMA).