Fan Powered Terminal Unit

When selecting a fan powered terminal unit (FPU) as a component in the overall building HVAC system, it is important to understand the differences in the motor technologies that are available. The type of motor–permanent split capacitator (PSC) or electronically commutated motor (ECM)–should be evaluated in terms of energy consumption characteristics and the cost differential/payback period due to energy savings is an important component of that selection.

Several local codes have adopted rules that limit the motor type in series FPUs to only ECM, but fail to address the motor type in the parallel FPU.  At the national code level, there is some discussion on making the ECM motor the default type for use in series FPUs as well.

Motors 

Fan powered terminals and their motors are rated as an assembly using Standard UL 1995, Heating and Cooling Equipment. This standard only applies to equipment manufacturers and not international or local codes, nor field issues. Since the FPUs are rated as an assembly, the nameplate rating on the fan powered terminal unit takes into account the motor current draw and power consumption of all integral accessories such as electric reheat or controls. Due to over-current protection requirements in the UL code, always use the nameplate rating of the fan powered terminal unit, not the motor nameplate rating, when sizing the supply circuit wiring and fusing. This over-current rating requirement is intended to provide the safest possible operation of the fan powered terminal unit and any electrical accessories.

Permanent Split Capacitator (PSC)

A PSC motor is a type of single-phase AC motor and a type of split-phase induction motor. The PSC motor uses a capacitor as a component of the start windings in the motor. This provides a greater starting torque than a standard split-phase induction motor. The capacitor allows the motor to be operated at variable speeds by the use of a thyristor (speed controller).  This modification of the AC Sine Wave reduces the RMS value of the voltage supplied to the motor, resulting in a reduced torque and lower rpm. As long as the motor and blower are sized properly and not operating near an extreme on the fan curve, the modified current draw is very similar to the unmodified current draw with just a somewhat higher amount of current being used. Note that at some speed settings significant power may be dissipated by the thyristor (speed controller),which will lower the overall efficiency of the fan powered terminal unit as this power will be dissipated as heat.  Due to the heat dissipating nature of the speed controller,the overall motor efficiency is considered to be around 35% when the motor is operated below 70% capacity.

Electronically Commutated Motor (ECM)

The ECM is a brushless DC permanent magnet motor. The ECM has been on the market since 1985 and is a proven technology. ECMs are microprocessor controlled programmable motors that incorporate a built-in power inverter (converts AC to DC). In fan powered terminals the ECM is typically of higher efficiency (lower power consumption) at most operating conditions.  Control of the rpm of the ECM is achieved by varying the DC voltage supplied to the motor.  As a result of the type of control for the ECM very little heat is dissipated, which gives an overall motor efficiency around 65% when the motor is operated below 70%.

In selecting a motor type, unless the code mandates a type, the question is a matter of energy consumption verses initial cost.  The ECM is more expensive than the PSC motor, but since there are now at least two suppliers of the ECM type, the cost may drop over time.  With energy savings typically being the driving force between selecting an ECM over a PSC motor, the question becomes how fast can I get a return on my investment for the more expensive motor?

For the sake of discussion, l am going to set the additional cost for an ECM over that of a PSC motor at $350.  This is a conservative amount, and the actual cost will vary based on the terminal unit size (motor size) and controls selected. Based on the maximum 26 cents/kWh and minimum 7 cents/kWh for the United States in May 2010, with the average cost being 10 cents/kWh [EIA 2010], two different loading profiles were developed and are shown in Figures 1 and 2.  The payback period is based on the number of years required to return a savings of $350 and 10 cents/kWh, and is shown in Figure 3.  The actual payback period will vary based on the actual cost differential for the ECM over the PSC and terminal unit motor size and actual motor operating point.  A note of caution: in these calculations, no attempt was made to account for demand based energy surcharges. Also, the terminal size (motor hp), operation point, and schedule of operation will all impact the energy consumption.

Tech Tips Figure 1

Figure 1: Power consumption and energy savings of ECM over PSC motor for a 24-hours-per-day, 365-days-per-year operation with 0.25 in. w.c. discharge static pressure.  This chart is for a ½ hp motor in a size 40 Price FDCG series terminal unit.  Power consumption and energy savings for other terminal unit sizes and models will vary based on motor size, fan powered terminal unit model, inlet static and discharge static pressures.

Tech Tips Figure 2

Figure 2: Power consumption and energy savings of ECM over PSC motor for a 12-hours-per-day, 260-days-per-year operation with a 0.25 in. w.c. discharge static pressure. This chart is for a ½ hr motor in a size 40 Price FDCG series terminal unit. Power consumption and energy savings for other terminal unit sizes and models will vary based on motor size, fan powered terminal unit model, inlet static and discharge static pressures.

Tech Tips Figure 1

Figure 3: Number of years to pay back the initial cost differential through energy savings.  Calculations are based on 10 cents/kWh and do not include any demand or other charges. This chart is for a ½ hp motor in a size 40 Price FDCG series terminal unit.  Other terminal units will vary based on motor size, fan powered terminal unit model, inlet static and discharge static pressures.

Summary

The payback period for a 24-hours-per-day, 365-days-per-year operation cycle is less than 1.7 years for all air flow volumes for this size and model of series fan powered terminal.  For a 12-hours-per-day, 260-days-per-year operation cycle, the payback period is less than 3.5 years at the normal selection point for this size and model of series fan powered terminal unit.

It is suggested that the building HVAC designer evaluate the energy consumption profile of the building design using the power consumption characteristics for the fan powered terminal unit to assist in making the proper motor type selection.

Jerry Sipes, PhD, P.E.
Vice President of Engineering

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