As energy costs rise around the world, the incentive for facilities to operate their equipment more efficiently will multiply. There are mandatory means (regulations) that authorities use to enforce conservation, and compensatory means (special rate tariffs) that reward users for using less energy. In either case, decreasing energy consumption decreases the bottom line costs to the user.

The first step industrial facilities should take toward effectively reducing energy costs is to evaluate their motors — the number one energy consuming culprit. Motors in industrial facilities consume by far the largest percentage of energy of any electrical device used in the US infrastructure. Tens of billions of kWh are consumed by motors each year, accounting for more than 25% of all electricity sales in the US.

Polyphase induction motors, more than 90% of which are squirrel cage, are the most commonly used. Because of their prevalence throughout the industrial and commercial sectors, polyphase induction motors offer a great potential savings opportunity in both energy and operational costs during the motor’s useful life.

An adequate assessment of the impact that induction motors can have on an energy bill requires a detailed knowledge of the motor’s many operational and electrical parameters. Permanently installed monitoring devices are the most effective tool in the arsenal to reduce energy consumption, especially in motors. Knowing which parameters to monitor and evaluate helps you save energy.

Each motor within a facility operates with some level of distinctiveness from other motors. This distinctiveness may be due to a combination of factors, which include:

• Nameplate ratings
• Voltages
• Load/Application
• Duty cycle
• Environment
• Adjacent loads
• Impedances
• Age.

The more knowledge that can be accumulated about a motor and how it operates, the easier it is to reduce energy costs associated with that motor. Permanently installed monitoring systems are particularly useful because they are able to capture a great deal of information over the motor’s life — both real-time and historical.

Fig.1

A fundamental issue that can affect a motor’s energy usage is its suitability for the intended application. Motors are designed to operate most efficiently at their nameplate rating. Selecting the wrong motor for a particular application or operating the motor outside its recommended parameters decreases its performance by introducing additional energy losses into the electrical system. Monitoring systems identify many symptoms that result in reduced motor performance including deviations from various nameplate parameters (Fig. 1).

Fig.2

• Power factor
• Voltage variations
• Voltage unbalance
• Motor load (based on current)
• Harmonic distortion
• Frequency deviations.

Monitoring systems also have the ability to measure and record temperatures, number of starts, running time and even vibration through the use of I/O modules.

• Displacement power factor (total, and per phase)
• True power factor (total, and per phase)
• Distortion power factor (total, and per phase)
• Min/max power factor
• Reactive power and energy
• Real power and energy
• Apparent power and energy.

How does power factor relate to energy savings? Polyphase induction motors use current composed of both resistive and inductive components (Fig. 2). The resistive component includes the load current and the loss current; the inductive component includes the magnetizing current and the leakage reactance. It is possible to cancel out the inductive current component by using capacitance. A capacitor does not affect the magnetizing current or the leakage reactance of the motor. However, it offsets the inductive component at the point in the circuit where the capacitor is installed. As more capacitance is added, the power factor angle, è, becomes smaller until a unity power factor is achieved (è = 0). At a unity power factor, the electrical system is at its optimum performance for maximum power transfer (see “Example 1”). Placing excessive capacitance in the circuit causes a leading power factor condition (è is negative in this case), which can lead to serious complications.

Voltage unbalance is both a leading cause of motor failures and a major contributor to energy losses in motors. The subsequent current unbalance produces additional losses in the motor. Monitoring systems are typically used to quantify voltage unbalance for power quality purposes, but may also be used to provide information on the losses due to voltage unbalance at the terminals of three-phase induction motors (see “Example 2”). The cost of energy losses is substantial in this case and will be further multiplied by additional motors exposed to the voltage unbalance within the facility.

Other voltage quality issues adversely affect the efficiency of induction motors. Operating a motor at 90% of its rated nominal voltage results in roughly a 2.5% decrease in efficiency (Fig. 1). Harmonic distortion at the motor’s terminals produces additional currents including counter-rotational (negative sequence) currents that reduce a motor’s efficiency. Even variations in the system frequency result in energy losses for motors. Each of these dynamics contributes to energy losses and present untapped methods of reducing operational expenditures.

Historical and real-time data provided by monitoring systems is not only the key to locating motors that are operating uneconomically, but these systems can allow the user to more easily determine the root cause of the problem(s). Remedies that are employed can also be assessed on an “as needed basis” by the monitoring system, and modified to insure they are effective. Concurrently, the return-on-investment for a given solution can be easily established.

• Permanently installed monitoring systems collect vast amounts of data that can be scrutinized for motor savings.
• Actions taken to improve the motor’s efficiency will also increase the operating life of the motor.
• The payback period for improvements to the electrical system can be relatively short.
• Operating motors as closely as possible to their optimal parameters decreases capital expenses, lowers process downtime, lowers stress on the supporting infrastructure and reduces operating expenses — including energy bills.