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issue: October 2006 APPLIANCE Magazine - Part 2: Motors & Air-Moving Devices

Motors and Air-Moving Devices
Evaporator Fan Motor Energy Monitoring


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by Angelo Karas, research engineer, Fisher-Nickel, Inc.

Researchers at the Food Service Technology Center (FSTC) recently did a test to compare the use of induction-type fan motors to electronically commutated motors (ECM).

Table 1. Fan Energy Usage and Operating Cost

Evaporator units are integral components of commercial walk-in refrigerators and freezers. The small fans housed within these units continuously move air across the evaporator coils and circulate it throughout the cooled space. Most come equipped with fans driven by relatively low-efficiency, shaded-pole (SP) or permanent split capacitor (PSC) induction-type electric fan motors. Individually, these small induction-type fan motors may seemingly not consume a great deal of energy, but the energy consumption and operating cost of multiple evaporator fans combined and running continuously may accumulate to substantial levels, leading some users to conclude that a more efficient motor alternative would be beneficial.
The ECM combines the inherent efficiency of a DC motor design with an electronic drive control in a package small enough to fit in a typical evaporator unit. In evaporator fan applications, ECMs have the potential to dramatically reduce the energy consumption over that of lower efficiency shaded-pole and permanent split capacitor motor equivalents. Researchers at the FSTC installed and monitored two GE ECM™ motors in a walk-in freezer to document the energy savings potential.
An ECM utilizes a permanent magnet, three phase, brushless DC motor combined with a built-in, electronic control module that is used to drive it. The particular ECM tested is tailored for evaporator fan use and programmed to maintain a constant speed of 1,550 rpm. It has a maximum shaft output power of 90 W and is designed to replace a range of SP or PSC motors rated up to 1/8 horsepower (hp). The rated life span is 15 years.

Figure 1. Power and Temperature

Test Procedure and Equipment

The FSTC’s laboratory walk-in freezer evaporator unit was originally equipped with two shaded-pole 1/15-hp fan motors. Before the retrofit was performed, the existing shaded-pole motor power was measured for 1 day, and the rotational speed and airflow were also measured and recorded. They were then removed and replaced with the ECMs while retaining the original fan blades. Upon replacement, fan speed and airflow were measured and confirmed to be the same as in the original SP motor configuration. The combined energy consumption of both ECM fans was then monitored for 2 weeks. Evaporator exit air temperature across the motors and the bulk internal air temperature were also recorded.
The fans’ rotational speeds were measured with an electronic stroboscope and airflow measurements were done with a vane anemometer. Energy data recording was performed with the use of a Dent Instruments ElitePro data logger with a Magnelab 5 A current transformer to record voltage, current, power, and cumulative energy at 15-minute intervals. For temperature monitoring, a Hobo H8 temperature data logger, with a remote temperature probe placed directly in front of an evaporator fan guard, was used to record the evaporator air exit temperature. The walk-in freezer wall-mounted thermometer was used to monitor the bulk air temperature.

Results

The initial measurement of the original SP motor fans’ input power was 271 W (two fans combined). Following the retrofit, the average measured input power for the ECM fans throughout the 2-week monitoring period was 88 W (two fans combined). The average supply voltage throughout the test period was 207.4 V. The freezer maintained an average bulk air temperature of 0°F (-17.8°C), and the average evaporator exit temperature was -5.1°F (-20.6°C).
Based on the as-tested FSTC freezer data, the daily energy consumption and estimated annual energy cost and cost savings were determined. The ECM equipped fans used 67 percent less energy and would yield an annual operating cost savings of approximately U.S. $104 per fan. The results are summarized in Table 1. Figure 1 shows a graph of a 5-day period of the combined power of the ECM fans and also the evaporator exit air temperature.
The zero-value power points shown on the graph signify the freezer defrost cycles when the fan motors were switched off and the defrost heating coils were energized. Because the timer-controlled defrost cycle frequency and duration can vary from one installation to another, these brief off periods were disregarded, and a duty cycle of 100 percent was used in the average power and energy calculations.
The intermittent highest power values (above 95 W) are indicative of periods when the evaporator coil was operating under heavily frosted conditions, which came during peak operation periods when extended loading and/or frequent door openings would allow moist air to enter the freezer and cause frost. This would create airflow restriction across the evaporator, thereby slightly increasing the load on the fans until the next defrost cycle would commence and then clear the frost from the coil.
It should also be noted that the input power for any given fan motor can vary with different fan blade size and design, as well as operating conditions such as the pressure drop across the evaporator and the air density across the fan. In this example, where the installation was a freezer as opposed to a refrigerator, the colder, denser freezer air temperature resulted in a slightly higher load on the fan and a corresponding increase in input power.

Table 2. Energy Usage and Cost Comparision w/Heat Load Reduction Factor

Refrigeration Cost Comparison

An additional benefit of using an evaporator fan motor that consumes less electrical energy is the resultant internal heat load reduction within the refrigerated space. Since all of the electrical energy that the evaporator fan consumes is ultimately converted into heat, a reduction in fan energy will translate into an equal reduction in internal heat load. This ultimately results in less energy required by the system’s condensing unit to maintain the same temperature within the space.
A common unit of reference used to indicate a refrigerator or freezer’s overall efficiency is the coefficient of performance (COP). It represents the ratio of the amount of heat energy added to the cooled space divided by the amount of electrical energy that is required by the condensing unit to reject that heat. The particular COP and amount of energy used depend on whether the system is a freezer or a refrigerator and also on the system’s specific design, size and operating conditions. Freezers can require 50 percent to 100 percent more energy than refrigerators to remove the same amount of heat. For example, a small walk-in freezer can have a COP of 1.5, while a similarly placed and sized refrigerator can have a COP of 2.5 or 3.
For simplicity, a COP of 2 will be used for the example below. For every 2 W of evaporator fan power reduction, there will be one additional watt of condenser unit power reduction, for a net reduction of 3 W. The energy consumption and calculated annual operating costs factoring in the refrigeration power are summarized in Table 2.

Conclusions

FSTC concluded that the monitoring results of this study show a great opportunity for energy and operating cost savings through the use of ECMs. A 67-percent power reduction, for an overall reduction of 137 W and 3.3 kWh/day per fan, resulted in approximately $156-per-fan yearly savings. Initial purchase pricing, installation costs and any local utility company purchasing incentives or rebates need to be factored into the life cycle cost or return on investment calculations, but the annual energy cost reduction alone is a strong incentive for evaporator fan motor retrofitting.
Although the purchase cost of an ECM is generally higher than that of a replacement SP or PSC induction motor, the difference is largely dependent on the induction motor’s size and type. In the example presented above, using an installed retrofit cost estimate of $250 and an annual operating cost reduction of $156 per fan, the simple payback period for this installation would be about 19 months. In the event of an existing induction-type motor failure, replacement with ECMs would result in a more immediate savings benefit.

References:
Energy Solutions: EC Motors. http://www.plugloads.com/ecm.html

 

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