Selecting the right motor and associated equipment for a project requires a close look at the options.

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 all 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 oil voltages available ill 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 three-way control valves must be used so the flow actually varies.

For air systems serving office buildings, the supply fan variable frequency drive 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 commollly 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.