# Sizing VFDs for optimal operating efficiency

## A new control strategy and implementation system optimize variable frequency drive (VFD) efficiency to meet cooling-air requirements.

**Learning Objectives:**

- Explain the impact of variable frequency drive (VFD) enclosure parameters on VFD performance and operational reliability.
- Analyze the VFD's efficiency and its ability to meet enclosure cooling-air requirements with the new control strategy.
- Outline the new control strategy and implementation system developed to optimize VFD efficiency.

The VFD enclosure cooling-air fan should be sized by the mechanical engineer by considering cooling-air load, temperature entering the enclosure, and the allowable operating temperature. The enclosure cooling load is caused by the drive inefficiency, resulting in increased air dry-bulb temperature. Required cooling-air volume should be calculated assuming that the VFD enclosure load will be carried out as the sensible load only.

The actual VFD enclosure cooling load during design and off-design conditions must be verified from monitoring data; otherwise, the enclosure cooling load could be substantially higher when compared with data from a VFD manufacturer. The actual enclosure-load deviation is due to the different patterns of the static and dynamic hydraulic-pressure losses in various systems. The design cooling-airflow rate (CAFR_{DES}) will be determined based on the maximum anticipated ambient temperature (T_{AMB MAX}) surrounding the VFD enclosure and by a manufacturer's allowable VFD enclosure maximum operating temperature (T_{OPR MAX}).

The following four equations determine major enclosure operational parameters in British system thermal units:

1. Q_{DES/OFF-DES} = Q_{TOT} = Q_{SEN} = 1.085 x (T_{EXT MAX} - T_{ENT MAX}) x CAFR_{DES} = 4.5 x ∆E x CAFR_{DES }

2. CAFR_{DES} = Q_{DES} / [1.085 x (T_{EXT MAX} - T_{ENT MAX})]

3. T_{OPR MAX }= (T_{ENT MAX} + T_{EXT MAX}) / 2

4. T_{EXT MAX} = 2 x (T_{AMB MAX }- T_{ENT MAX})

Where:

Q_{DES/OFF-DES}, Q_{SEN}, Q_{TOT} = VFD enclosure design/off-design, sensible, and total cooling load

T_{ENT MAX} = Maximum anticipated dry-bulb cooling-air temperature entering the VFD enclosure; depends on the location of the enclosure (i.e., in the air conditioned area, in the area with mechanical-free cooling, etc.)

T_{AMB MAX} = Manufacturer's allowable dry-bulb ambient air temperature surrounding the VFD enclosure, which represents the enclosure's maximum allowable operating temperature T_{OPR MAX}

T_{EXT MAX} - T_{ENT MAX} = Temperature differential between maximum cooling-air dry-bulb temperatures exiting and entering the VFD enclosure

∆E = Enthalpy differential for cooling-air exiting and entering the VFD enclosure.

Design cooling-air conditions entering the VFD enclosure are related to the design cooling-air dry-bulb temperature, relative humidity, air density, and humidity ratio.

Because VFD manufacturers allow high dry-bulb temperature and relative humidity of cooling air entering the enclosure, the moist loaded air is always present in the enclosure. As indicated by the first equation, the enclosure's total cooling load should be equal to its sensible load. However, if the VFD enclosure design's cooling-air flow rate is specified in the equations, the enclosure's optimal operating conditions can be satisfied without a moisture exchange. If these specifications aren't met, it might result in moisture deposition in the enclosure.

Figure 1 demonstrates the impact of T_{OPR MAX} on the design relative cooling-air (RCAFR_{DES}). The T_{OPR MAX} varied from 104° to 122°F. This is the conservative approach, because the actual T_{OPR MAX} might be higher due to the T_{EXT} increase associated with heat removal caused by a greater VFD power-loss factor. Compensating for the increase in T_{OPR MAX} will require employing an oversized VFD. The higher operating temperature of the VFD enclosure will lead to increased T_{EXT MAX}, which can be calculated in the last equation.

T_{ENT MAX} depends on the location of the enclosure and varies from 50°F for an air conditioned area to 100°F for mechanical-free cooling with a once-through ventilation system. The top graph in Figure 1 indicates that RCAFR_{DES} is drastically reduced when T_{ENT MAX} drops from 100° to 90°F, which coincides with VFD enclosure cooling-air temperature differentials increasing from 8° to 28°F. A further reduction of T_{ENT MAX} and the correlated increase in the enclosure's temperature differential leads to a lower reduction of the design's relative cooling-airflow rate.

Figure 1 also indicates that, for the considered conditions, the maximum design cooling-airflow rate of 100% occurs at T_{ENT MAX} = 100°F and T_{EXT MAX} = 108°F (i.e., at T_{OPR MAX} = 104°F). The increase in allowable VFD enclosure operating temperature from 104°F (shown in the top graph of Figure 1) to 113°F (shown in the middle graph of Figure 1) leads to the reduction in RCAFR_{DES} from 100% to 30.8%. It also causes the VFD enclosure's temperature differential to increase from 8°F (at T_{ENT MAX} =100°F and T_{EXT MAX} = 108°F; see Figure 1, top graph) to 26°F (at T_{ENT MAX} =100°F and T_{EXT MAX }= 126°F).

The further increase in allowable operating temperature from 104° to 122°F (shown in the bottom graph of Figure 1) leads to the additional reduction in RCAFR_{DES} from 100% (see Figure 1, top graph) to 18.2%. It also causes the VFD enclosure's temperature differential to further increase from 8°F (at T_{ENT MAX} =100°F and T_{EXT MAX} = 108°F; see Figure 1, top graph) to 44°F (at T_{ENT MAX} =100°F and T_{EXT MAX} = 144°F).

Therefore, RCAFR_{DES} could be noticeably reduced by lowering the cooling-air temperature entering the enclosure. The reduction of TENT from 100° to 50°F could lead to the RCAFR_{DES} decreasing from 100% to 7.4% (shown in the top graph of Figure 1).

Given equal load conditions, the reduction in cooling-air temperature entering the VFD enclosure leads to an increased exiting cooling-air temperature, causing an increase of the temperature differential and correlated reduction in cooling-airflow rate. Maintaining a lower operating temperature at the same load and cooling-air temperature entering the enclosure will require a lower cooling-air temperature exiting the VFD enclosure and a higher relative airflow rate compared to the design magnitude (shown in the first graph of Figure 1).

Further analysis demonstrates how the cooling-air temperature exiting the VFD enclosure impacts the enclosure's operating temperature of 70°F and flow rate at constant entering-air temperature. A wide variety of operating temperatures, from 75° to 100°F, might apply by adjusting the VFD enclosure's cooling-airflow rate. For instance, the increase in the enclosure operating temperature from 75°F to 85°F will allow the reduction in cooling-airflow from 100% to 33.3%. The increase in operating temperature from 75° to 100°F will reduce required cooling-airflow from 100% to 16.7%.