Constant-speed Compressor and MotorComponent

General Overview

A compressor is one of the main components of a chiller. A constant-speed compressor operates at a constant speed to raise the vapor pressure of the refrigerant and power the refrigeration cycle of the chiller.

Evaluation of Energy Transfer

The primary energy source for a constant-speed compressor is the electricity used to run the compressor motor. Table 2 provides a summary of measurements needed to quantify the annual power consumption and operating characteristics of the constant-speed compressor and motor.

Measurement Strategy

The measurement strategy for a constant-speed compressor and motor is to do a one-time Measurement Strategy. The measurement strategy for a constant-speed compressor and motor is to do a one-time measurement of the power draw and long-term measurements of the operational schedule of the motor. The approach assumes that the power draw stays constant throughout the measurement period. Because the motor runs at constant speed and is under a constant load, it is either operating at full power (when on) or it is drawing no power (when off).

A motor on/off data logger is used to record the operating schedule. The true RMS power is measured at main feed to the constant-speed motor. Measurement points are generically represented in Figure 1.

Measurement Equipment

Table 3. Measurement equipment.

Equipment	Description	Measurement (Units)
Onset HOBO Motor On/Off Data Logger (UX90-004)	Records when a motor is on and off, as well as runtime. Requires HOBOware software and a USB connection cable for programming and downloading data files.	Motor Runtime (minutes)
DENT ELITEproXC Portable Power Data Logger (EXCUNC) DENT 16” RoCoil Flexible Rope Current Transformers (CT-R16-A4-U)	Provides a measurement of true RMS power from voltage and current inputs and records long-term power (kW) and energy (kWh) measurements. Requires ELOG19 software and a USB connection cable for programming and downloading data files.	True RMS Power (kW)

Further details about measurement tools and equipment associated with measurements in Figure 1 can be found here .

Calculation Methodology

The general methodology for quantifying the electricity consumption of a constant-speed compressor and motor is to measure the electrical energy consumed by the compressor and motor. Depending on the variability of operations, daily or weekly models which incorporate cooling load or a proxy like outdoor air temperature as independent variables may be developed to better characterize the component. The typical energy use of a constant-speed compressor is estimated using the simulated yearly schedule of the compressor. A more detailed explanation of the calculation methodology can be found below.

Click the button below to go to the calculators for this component.

Pump Motor Energy ConsumptionCalculation

The equations and calculators in this page estimate the annual energy consumption of variable-speed and constant-speed pump motors.

Further Reading

• ASHRAE. (2020). HVAC Systems and Equipment. Atlanta, GA: ASHRAE.