Meeting efficiency requirements
Selecting the right motor and associated equipment for a project requires a close look at the options
WHILE DESIGNATIONS SUCH AS ENERGY-EFFICIENT AND PREMIUM-EFFICIENT have been used in engineering specifications and motor catalogs for years, facility professionals are now required to specify energy-efficient motors by federal law. The Energy Policy Act (EPACT) of 1992 mandates motor efficiency requirements for the types of motors facility professionals commonly specify.
EPACT requires that induction motors used for HVAC applications meet the full– load efficiency requirements of NEMA standard MG1-1993 if the motors run at 3,600, 1,800 or 1,200 rpm for motors powered at 230/460 volts. The EPACT requirements cover both totally enclosed fan cooled (TEFC) and open drip proof (ODP) motors up to 200 hp. For HVAC applications, totally enclosed motors typically are found in cooling tower applications with ODP motOrs in the majority of other applications, such as air handling unit fans and pumps. For HVAC applications, most motors run at 1,800 rpm. Some pump motors are specified at 1,200 rpm, especially for cooling tower applications where minimum net positive suction head is a problem. Motors specified at 3,500 rpm are used for some boiler feed pumps, but normally should not be used in HVAC applications due to noise problems.
At 1,800 rpm, a 1 hp ODP motor needs to have a minimum efficiency of 82.5 percent, while at 200 hp the efficiency requirement is 95 percent. Motors with these efficiencies are available in the catalogs of most major motor manufacturers. All efficiency testing must be performed in accordance with IEEE Standard 112, Test Method B. Facility professionals requiring specific efficiencies for a particular motor should consult Table 12-10 in the NEMA standard mentioned above for the motor types and speeds that the standard covers.
Facility professionals should verify that new motors have the required nameplate with the NEMA efficiency stamped on it. Facility professionals should state in specifications that the motor have at least the minimum efficiency in the NEMA standard for its size and type, when tested in accordance with IEEE Standard 112, Test Method B. Specifiers should not merely state that energy-efficient or premium-efficient motors be provided, as these terms in themselves do not meet the requirements of the federal law. Shop drawings for all motor– powered equipment must be checked to ensure that minimum efficiencies are met.
Motors are typically not a separate shop drawing item but are included with the fan or pump being submitted. Typically, only the horsepower and voltage characteristics of the motor will be included in the submittal; the facility professional must request the efficiency data separately.
Smaller fractional horsepower motors – which are typically less efficient – are not covered by the federal standard. These motors can amount to a significant building load if enough are used. An example is an office building using series-type fan-powered terminal boxes with many small motors powering fans that run constantly during the occupied cycle. Even though these motors are not covered by the federal standard, facility professionals should still specify the most efficient motors available.
Even though energy efficiency is a prime consideration when specifying electric motors, other factors are important. Consider the type of motor to be used, motor speed – which may not be the same as the fan speed the number of speeds the motor has, type of motor drive, overload capability, and coordination of motor requirements between the HVAC and electrical engineer.
MOTOR TYPE. Most motors used in HVAC systems are ODP, have ventilation openings and are installed in such a way as to keep water or other liquid from dripping into them vertically. This type of motor is not suitable for applications where the motor is exposed to significant amounts of water – for example, in a cooling tower. They are suitable, however, for most motor applications within a mechanical room.
Cooling tower fan motors are typically TEFC or totally enclosed air over with a fan rather than openings to provide cooling. These motors can survive the wet environment of a cooling tower. Special coatings are applied to the motor windings to aid in moisture protection. Some towers that use centrifugal fans have the motor located outside the wet airstream. These fan motors can be ODP.
Some environments – for instance, where combustible vapors are present or other hazardous conditions exist – require explosion-proof motors and other electrical components, such as junction boxes. Explosionproof motors have enclosures that prevent the ignition of gases or vapors. They are more expensive and should be specified only if the authority having jurisdiction over the project determines their use is required. That authority may be the local fire chief, the owner’s insurer or the architect. The electrical engineer usually investigates whether explosion-proof motors are required, as receptacles and lighting must be explosion-proof as well. The National Electrical Code articles 500 through 516 determines whether explosion-proof motors and equipment are required. These articles cover many different classifications of hazardous environments.
MOTOR SPEED. The great majority of motors used in HVAC applications spin at 1,750 rpm; these are the same as the 1,800 rpm motors mentioned in the standard above with an allowance for slip. Motors spinning at 1,150 rpm are used occasionally for condenser water pumps serving cooling towers, where a very low net positive suction head pump is required, as in the case of towers located a significant distance from the pumps or towers located below the pumps. Motors spinning at 1, 150 rpm are expensive and should not be specified unless required.
Motors spinning at 3,600, although commonly used for process applications, should rarely be used for HVAC applications because they can cause noise disturbances. An exception is some types of boiler feed pumps. Some small direct-drive motors for ceiling exhaust fans and fan coils have motors that spin at 1,050 or 1,550 rpm, the lower speed being preferable for acoustical reasons. Other rpms may occasionally be found in manufacturers’ catalogs for various types of small direct drive fans
Two-speed motors are commonly used in HVAC applications where occupied and unoccupied cycles are required for a fan, on fume-hood applications to save energy when a sash is lowered and as a means of capacity control for cooling towers. If the low speed cfrn of a fan is to be one-half of the high speed cfm, then the low speed rpm of the motor must also be one-half of the high speed rpm. Many two-speed motors have a low speed that is 67 percent of the high speed, which, if specified, will not produce the desired results.
If a two-speed motor is specified, a two-speed starter also must be specified. Two-speed starters have two contractors and are close to double the cost of single-speed starters. As more facilities use variable frequency drives (VFD) and their costs come down, twospeed motors will be used less. The energy savings for a VFD are significantly greater than for a two-speed motor, assuming the motor load can vary continuously downward. In some cases where only two-speed operation is desired, such as an occupied unoccupied cycle for a fan motor, a variable speed drive (VSD) is still purchased to allow for greater flexibility of adjustment.
MOTOR DRIVES. Motor drives consist of pulleys and belts, and is not variable. HVAC fan motors have belt or direct fan drives. With a belt drive, a series of V-belts connecting the motor pulley and the fan pulley drive the fan. The motor pulley has an adjustable sheave that may be adjusted during the balancing procedure to obtain the proper airflow through the fan. This pulley may be replaced with a fixed sheave when the balancing procedure is complete. With a direct-drive fan, the fan wheel is connected directly to the motor without possibility of direct adjustment. Smaller directdrive motors may have their speed adjusted with silicon-controlled rectifier speed controllers.
Belt-drive motors are generally preferred for fan applications due to the ability to adjust the drive. This is important, as the installed static pressure in the ductwork is often different from the engineer’s original calculation. Sometimes, the fan does not perform according to its fan curve, and its speed must be adjusted in the field – although this is more unusual than static pressure variation. On small fans under 200 cfm, only direct-drive motors may be available. These should always be specified with speed controllers to make air flow adjustment possible.
OVERLOADING. Most motors have a service factor of 1. 15 to 1.35, which represents the ability of the motor to work at a continuous overload without failure. Motors should not typically be sized to operate in the service factor. Specifiers must carefully check submittals to ensure the motor supplier is not trying to use an undersized motor to save money. Specifiers of fan and pump motors must be careful to size the motors so the operating point of the fan or pump does not exceed the brake horsepower curve of the selected motor or circuit breakers will trip or cause fuses to blow.
Coordinating motor and motor starter requirements between the HVAC and electrical design engineers can be a significant challenge on design projects if not done properly. Motors for HVAC equipment are typically purchased by the mechanical contractor, usually with the equipment. The electrical characteristics of these motors – voltage and phase – should appear on the HVAC schedules. The electrical engineer designs the circuits that supply power to these motors based on voltages available in the building. If the two engineers do not communicate, the wrong type of motor or starter can arrive, and a significant change order may result.
Motor starters can be purchased by the mechanical or electrical contractor. It is not uncommon for starters to be specified in both sections of a job specification, resulting in unnecessary project costs to the owner.
The HVAC engineer specifies starters for larger chillers and purchases them with the chillers. The HVAC engineer must coordinate with the electrical engineer to determine whether reduced voltage starter is necessary. The data provided by the chiller manufacturer will include the required size of the overcurrent protection device. This information must be passed to the electrical engineer so circuiting can be properly designed. Some very large chillers run at 2,400 or 4,160 volts. This happens when the voltage drop caused by the large chiller would be too large for the electrical system to handle. When medium-voltage starters are used, they may be located remotely from the chillers, as these systems have inherently low voltage drop. If medium– voltage starters are located near or on the chiller in the mechanical room, clearances must be increased.
Packaged HVAC equipment, such as rooftop air conditioning units or air cooled chillers, typically have single point power connections and do not have separate starters. Motor contractors are provided within the unit’s control panel. The HVAC engineer should transmit to the electrical engineer the unit’s required minimum circuit ampacity and the maximum overcurrent protection device rating or maximum fuse size. This information is provided in the HVAC equipment catalog and should be verified by both the electrical and HVAC engineer on the shop drawing submittal.
VARIABLE SPEED DRIVES. VSDs vary motor speed based on an input control signal. Significant energy savings are available as speed is varied downward. While a complete discussion of VSDs is outside the scope of this article, several issues relevant to HVAC applications are discussed here.
VSDs have the potential for the largest energy savings for fan and pump operation with modulating loads. Drives can save more energy than fan inlet vanes, two-speed motors or having a pump ride up its curve with two-way control valves throttling down. For these energy savings to take place, the load must truly vary.
An office building’s air flow, for example, may vary from 60 to 100 percent of maximum load, depending on how many people occupy the space and whether the sun is shining. Office buildings are appropriate places to use variable-volume systems and VSDs are typically the most efficient variable volume system available. Many large chillers have traditionally required constant flow and are not a good application for variable-volume pumping, although modern microprocessor chiller controls allow some variation in chiller flow to take place, so this situation may change. Secondary pumping to the building system is generally variable volume and is best accomplished with VFDs.
The energy savings due to drives also requires appropriate control strategies. For pump applications, drives are controlled to maintain the setpoint of a differential pressure transmitter located remotely in the hydronic system. The setpoint for these transmitters is often 25 to account for pressure drop through the open control valve, the coil and pipe fittings. System conditions vary, however, and the setpoint must be tailored to specific conditions. If variable volume pumping is employed, two-way rather than threeway control valves must be used so the flow actually varies.
For air systems serving office buildings, the supply fan variable frequency drive is almost always controlled to maintain a static pressure sensor setpoint with the sensor located two-thirds of the way down the supply ductwork. The return fan variable frequency drive can be controlled in a number of different ways. Most commonly, the supply airflow is measured, and the return fan drive is tracked to maintain a return airflow equal to a constant differential with the supply cfm.
VSDs are more commonly used in fume-hood exhaust applications. In these situations, the drive varies the speed of the exhaust fan to maintain a constant face velocity over the open hood sash area.
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Henry Shir is an associate at Fitzemeyer & Tocci Associates Inc., a Mass., engineering firm.
Copyright Trade Press Publishing Company Nov 1999
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