A careful review
of motor capabilities,
load requirements, power system characteristics, and applicable NEC rules leads to more successful installations
Whether you're working in an industrial, manufacturing, or institutional environment, there's no doubt that electronically adjustable speed drives (ASDs) have made life much easier for engineers and technicians, solving a wide range of motor control problems. But just because they're easy to buy doesn't necessarily mean they're easy to install.
An ASD installation typically includes the branch circuit supplying power to the ASD, a disconnect switch at the ASD, motor cables, and the motor. Specific installations may also require ASD input fuses, ASD input filters or reactors, and ASD output filters or reactors — all of which must coordinate with each other (and with the electrical supply system) to ensure a safe and reliable installation (Photo 1).
Although ASDs are commonplace, the technology is relatively new. Therefore, equipment capabilities and product standards include minimal requirements for ASD electrical ratings and construction. In fact, there are no industry standard ratings for ASD input/output current or ampere interrupting capacity (AIC) — some manufacturers don't even publish such information. Obviously, this creates challenges for electrical professionals. Avoid potential pitfalls by understanding a few key concepts.
Horsepower rating. Use the ASD horsepower rating for reference only — not for selection. The ASD should have a rated continuous output current of at least the maximum continuous current required by the motor. In many applications, the maximum continuous current required should be the full load current (FLC) specified in Table 430.250 of the 2005 NEC.
For low-speed motors (1200 rpm or less), the FLC may exceed Table 430.250 requirements. For those motors, the ASD-rated output current should be at least that of the motor nameplate current. For loads with high starting torque or fast acceleration requirements, you may need to size the ASD based on motor starting current rather than FLC.
Branch circuit conductor size. Conductors supplying the ASD must have an ampacity at least 125% of the ASD-rated input current [430.122]. The rated input current of an ASD may differ from its rated output current, and may vary with system AIC. There are no standard input ratings, and it's difficult to get rated input current from some manufacturers. If you are designing an ASD installation before buying the actual system, the purchase specification should specify maximum allowable input current and minimum rated output current — to allow for correct sizing of the branch circuit conductors.
ASDs have limited space for cables. Consequently, the maximum size conductor might be just enough to satisfy NEC requirements. For instance, one ASD has a rated input current of 185A and allows up to 2-1/0 copper cables per phase. The supply cable must have an ampacity of 232A (1.25×185A). Because the ASD cabinet does not permit a 250 kcmil conductor (255A), you must use 2-1/0 conductors (75°C rating of 300A). Raceway space is also limited, so you may need to install the 6-1/0 conductors in one raceway. You'd have a net supply circuit ampacity of 240A (0.8×300A).
Disconnecting device. The ASD is a controller. As such, it must have a disconnecting means within sight [430.102]. A remote, lockable disconnect does not satisfy this requirement. This disconnecting means must be rated not less than 115% of the ASD rated input current [430.128].
Short-circuit protection. Some ASDs include input fuses as standard equipment. Others require external fuses as a condition of installation. In some cases, the manufacturer requires a specific fuse. Where ASD manufacturers require external short-circuit protection, such protection must generally be fuses — circuit breakers are generally not acceptable.
AIC. The maximum available fault current at the ASD input terminals cannot exceed the ASD AIC rating. Unfortunately, many manufacturers don't publish this information. But, 430.8, paragraph 1 of the 2005 NEC includes a requirement that the controller shall be marked with the short circuit rating — which may lead ASD manufacturers to show this information on equipment and literature.
To ensure that an ASD AIC rating is suitable for a particular application, the ASD purchase specification should state the maximum expected fault current at the ASD location and should require the manufacturer to state rated AIC in its quotation. A particular ASD might need current limiting reactors and/or special fusing to meet AIC requirements.
Dimensions/interconnections. Physical configurations vary widely among ASD models. Components such as DC bus reactors, RFI filters, output filters, input filters, line disconnects, and line fuses may be integral to the ASD cabinet — or they may be separately mounted devices. Prior to purchasing a particular ASD, ascertain which components are inside and outside the ASD cabinet. For components external to the ASD cabinet, what are the mounting and interconnection requirements? Who supplies raceways and wires for interconnecting components? What are the dimensions and weights of the ASD cabinet and each separately mounted component?
Output cables. Many manufacturers specify recommended output cable types, cable sizes, and maximum cable lengths. What is the basis of this recommendation? Is it based on ASD capabilities or motor insulation capabilities?
Cables larger or longer than manufacturers' recommendations may require ASD de-rating due to higher cable capacitance and charging current. The manufacturer's recommended maximum cable lengths may not account for motor insulation capabilities. The length of an output cable determines the magnitude of voltage spikes at the motor — the longer the cable, the higher the spikes. The fact that the ASD has “been tested with output cables up to 1,000 feet” does not mean a general-purpose motor will operate satisfactorily in such an installation.
ASD/motor compatibility. A modern pulse width modulated (PWM) ASD produces an output of rectangular voltage pulses. These pulses increase motor losses and motor operating temperature, as well as result in voltage spikes at the motor. If a motor has questionable insulation, don't use it with a PWM ASD. If such use is unavoidable, consider output filters.
Voltage spikes should not pose a problem for motors rated 208V or less and in good condition. De-rate motor horsepower by about 15% to allow for additional heating caused by the ASD. Voltage spikes may damage motors rated 460V or more. Where motors meet the requirements of NEMA MG1 Part 31, they should operate satisfactorily without thermal de-rating or the need for filters if the cable length is less than 100 feet. Longer lengths may be feasible. Contact the drive and motor manufacturer(s) for more information. Where motors do not meet the requirements of NEMA MG1 Part 31, thermally de-rate the motors and consider output filters.
Output filters. Analyze each installation to determine if output filters are desirable — filters may decrease system efficiency and increase cost and space requirements. ASD and filter manufacturers offer various types of filters (Photo 2). In addition, filter nomenclature is inconsistent, and filter selection is confusing. Not all filters are compatible with all ASDs, and some are usable only at certain carrier frequencies. Because the need for filters depends on ASD characteristics, have the manufacturer select and supply output filters. The manufacturer will need to know the cable length (feet), filter location, and motor capabilities.
Mounting. Many manufacturers have specific mounting requirements. Most ASDs must have clear spaces of 3 to 12 inches on each side, top, and bottom to provide for proper cooling. Some have a rear cooling fin design that requires mounting the unit on a smooth, flat surface. Some manufacturers do not allow use of spacers or channels to support the ASD. Where mounting space is limited, the ASD purchase specification should require the manufacturer to provide clear drawings showing cabinet installation requirements, required clearances, and available wireway entrances (Photo 3).
Auxiliary power. Cooling fans and controls for ASDs, ASD isolation transformers, input filters, and output filters often need a separate power source. The ASD specifications should clearly indicate whether this power source will be a remote source, or be a sub-feed from the ASD supply circuit. Address the responsibility for providing any required disconnecting devices, controls, and interconnection wiring for auxiliary equipment.
Control circuits. ASDs are often connected to remote controls. Most ASD manufacturers require that remote circuits be wired with shielded cables and kept separate from ASD power cables. Installing 120V stop-lockout cables in the same raceway as motor supply cables may void the ASD warranty. Where existing control wiring is already in place, but does not meet ASD requirements, you may need to install auxiliary relays or other isolating devices.
Pulse rating. ASDs come in 6-, 12-, or 18-pulse varieties. The 12- and 18-pulse ASDs produce fewer harmonics than 6-pulse ASDs. Therefore, they have a lesser impact on supply system power quality. These ASDs are supplied from special transformers that have 6-phase (12-pulse ASDs) or 9-phase (18-pulse ASDs) secondaries. When these transformers are not integral to the ASD, a 12-pulse ASD may need a 6-pole disconnect switch and six supply conductors. An 18-pulse ASD may need a 9-pole disconnect switch and nine supply conductors.
Programming. A modern ASD may have thousands of programmable parameters. You can usually leave most of these settings at factory default values. But review the ASD instructions to ensure you program all the parameters required for your specific installation. You can usually change phase rotation by ASD programming — always check phase rotation after making any changes to the ASD or setup programming. The carrier frequency is often programmable within the range from 1 kHz to 20 kHz. Manufacturers' instructions often provide little guidance on how to select the carrier frequency, yet it can significantly affect operation of the ASD system.
In general, program the carrier frequency for the lowest frequency that provides acceptable levels of audible noise and motor heating. High carrier frequencies:
Increase the potential for electromagnetic interference (EMI), radio frequency interference (RFI), and bearing damage caused by circulating high-frequency currents.
Lower ASD efficiency.
Increase the frequency of voltage spikes, placing more stress on motor windings. Some ASD output filters are incompatible with some carrier frequencies.
Sidebar: Output overload current rating
Most ASDs can supply a current above rated continuous current, for brief periods.
Loads with variable torque characteristics (e.g., fans and centrifugal pumps) will generally start and operate satisfactorily with an ASD that has a 1-minute overload capacity ranging from 110% to 120% of rated continuous current.
Loads with constant torque characteristics (e.g., conveyors) often need an ASD that has a 1-minute overload capacity equal to 150% of rated continuous capacity. Some ASDs are specifically designed for variable torque or constant torque loads. Others require de-rating when used with constant torque loads.
Cherry is a licensed professional engineer and technical director with O'Brien & Gere Engineers in Syracuse, N.Y.
Sidebar: NEMA Application Guide
Looking for some additional help on understanding ASDs? Check out NEMA's “Application Guide for AC Adjustable Speed Drive Systems.” The guide was developed to assist users in the selection and application of AC ASD systems, rated 600V or less, consisting of 3-phase induction motors, voltage source pulse width modulated adjustable frequency controls, and associated components. The guide is available as a free download on the NEMA Web site (www.nema.org).