Technical Article

How To Improve 3-Phase Motor Efficiency

May 29, 2024 by Bob Odhiambo

Learn how to improve the efficiency of three-phase industrial motors and explore how these improvements conserve energy.

Three-phase motors are important electromechanical components in today's industrial applications. They drive a range of machinery and equipment to ensure smooth operation in manufacturing plants and high-voltage alternating current systems, among many others. These motors are mainly used to generate consistent rotary power output due to their sustainability, efficiency, and reliability in handling heavy-duty applications.

According to industry estimates, these electric motors constitute about two–thirds of industrial electrical consumption and roughly 45% of the total electrical energy consumed around the globe. However, not all of these motors operate at the highest efficiency, which leads to wasted energy. This increased energy consumption affects industrial energy expenses and increases greenhouse gas emissions. It is, therefore, essential to improve the efficiency for the good of the environment and to meet energy conservation targets in industries. So, how can engineers improve the efficiency of three-phase motors in industrial applications with motor-driven systems?

 

Figure 1. Electric motors convert electrical energy to mechanical through electromagnetic induction. Image used courtesy of Pixabay

 

Three-Phase Motor Structure

The basic structure of a three-phase electric motor is composed of a stationary part that generates the needed magnetic field for rotary output. A rotating magnetic field results from three-phase AC power being fed to the windings or coils of the motor. The AC power supply is fed to three-phase motors that are in phase and spaced 120° apart, with each phase reaching its peak voltage, creating a continuous rotation of the magnetic field in the motor's stator. The stator’s core is made from laminated steel plates to reduce eddy current losses in three-phase motors. Therefore, the motor's turning effect is caused by current induction in the rotor, which transfers the mechanical energy to the load to be driven.

Three-phase motors form the backbone of most industrial systems. In manufacturing, three-phase motors power machine tools, conveyor belts, pumps, and fans. In process industries like pharmaceuticals and food and beverage, motors are used in grinders, mixers, and crushers.

In transport, electric vehicles (EVs) and electric trains benefit from the electric motor's mechanical torque output. Commercial systems like elevators, escalators, and high-voltage alternating current systems use AC motors to drive compressors in heating and provide reliable power for vertical transportation systems.

 

Efficiency in Three-Phase Motors

The efficiency of three-phase electric motors is crucial for enhancing overall performance, lowering operation costs, and improving energy consumption. Therefore, the following techniques are essential. 

 

High-Efficiency Design Improvements and Advanced Materials

A high-efficiency motor design can improve the efficiency of three-phase AC electric motors. Premium and super-premium efficiency motors use high-quality materials and optimized designs to enhance performance and reduce energy losses. Such motors adhere to the most stringent International Electrotechnical Commission (IEC) and National Electrical Manufacturers Association (NEMA) efficiency standards.

To improve the design of a three-phase electric motor, the magnetic material can be changed to rare earth magnets to reduce magnetic losses, otherwise referred to as core losses. In three-phase motors, these core losses can result from cyclic magnetization and the demagnetization of the core material in a phenomenon known as hysteresis. To determine the core loss in an electric motor due to hysteresis (Ph), the following formula is used where ( ) represents the Steinmetz coefficient, f is the magnetization frequency, (V) is the core’s volume, and (Bm) represents the maximum density of the magnetic flux.

\[P_{h}=\eta\times B^{1.6}_{m}\times f\times V\]

Other than using enhanced magnetic materials, other factors like improving the geometry of the stator and rotor and employing improved motor cooling systems can reduce losses due to heat and, in turn, increase the overall motor efficiency. Therefore, choosing a suitable motor-designed material is essential to minimize losses due to hysteresis, eddy currents, heat, and friction. This ensures the motor works optimally, reducing the overall energy consumption in industries.

 

Proper Sizing and Load Matching

Correct sizing of a three-phase AC motor is essential to ensure the motor runs efficiently with reduced energy wasted. Oversized motors expose their components to higher mechanical stress that leads to wear and tear, which reduces the motor's lifespan and causes frequent on/off cycles. Under-sized motors may also experience excessive wear and lower efficiency levels due to motors' inability to handle the required load. This leads to high energy consumed by the motor for the same work output, resulting in overheating. Proper motor sizing is therefore essential for load matching, and to best determine the power requirement for a three-phase motor, consider the torque (T) of the load that the motor is required to handle and the motor’s angular velocity (ω), typically expressed in revolutions per minute. The required power (P) is, therefore, expressed using the formula:

\[P_{required}=\frac{T\times \omega}{9550}\]

Where 9550 is a mathematical constant that converts the angular velocity and torque product into power in kW.

Considering the graph below, insight can be drawn to help select the right size motors and implement load-matching strategies by dynamically adjusting the motor's speed and torque. This helps to reduce the energy wasted by ensuring the motor operates at optimal efficiency. During motor sizing, it is essential to consider the power required to push RPM limits for a range of torque values in a given load requirement.

 

Figure 2. Graph of power in kW against RPM under varying torques. Image used courtesy of Bob Odhiambo

 

Power Factor Correction

Power factor (PF) correction in three-phase AC motors significantly contributes to its efficiency and reduces energy consumption. Looking at the concept of PF, three-phase motors experience a lagging reactive power due to induction in their windings. This means that a high amount of current can be unnecessarily drawn by the motor for a given amount of work due to a low PF. The high energy dissipated in the form of heat results in losses not only in mechanical energy but also in the electrical supply system of the industrial system. To solve this, capacitor banks can provide the motor with reactive power to counteract the lagging reactive power resulting from the inductance of the three-phase motor.

PF can also be improved in three-phase motors using a modern variable frequency drive (VFD) to vary the voltage and frequency of AC power for speed and torque control. These VFDs reduce the strain on the motor by allowing for the possibility of adjusting its parameters with the variation of loads. Besides improving process control and PF correction, VFDs enable the motor to start smoothly by reducing the inrush current, which is seven times its rated current in a direct online connection (DOL).

Example: 

Consider a three-phase motor with an initial PF of 0.7 that runs with a real power (P) of 100 kW. Capacitor banks are to be used to correct the motor's lagging reactive power. Find the apparent power (S2) when the PF is corrected to 0.95. 

P = 100 kW

PF1 = 0.7

find initial reactive power Q1

\[S_{1}=\frac{P}{PF1}=\frac{100}{0.7}\approx 142.86\,kVA\]

\[Q1=\sqrt{S^{2}_{1}-P^{2}}\]

\[Q1=\sqrt{142.86^{2}-100^{2}}\approx 102.06\,kVAR\]

find the new reactive power when PF is corrected to 0.95 

\[S_{2}=\frac{P}{PF2}=\frac{100kW}{0.95}\approx 105.26\,kVA\]

\[Q2=\sqrt{S^{2}_{2}-P^{2}}\]

\[Q2=\sqrt{105.26^{2}-100^{2}}\approx32.59\,\,kVAR\]

find the capacitive reactive power Qc

Qc = Q1 - Q2

\[Qc=102.06-32.59\approx69.47\,kVAR\]

Therefore, adding a 69.47 kVAR capacitor bank, the PF is corrected to 0.95

The apparent power after correction is:

\[S_{corrected}\approx 105.26\,kVA\]

 

Motor Efficiency and Energy Reduction

An efficient three-phase motor contributes to direct energy savings, lowering the overall operation costs in industrial plants and reducing the load on infrastructure like power grids by lowering overall energy demand. With the reduction in wear and tear, operators and engineers can enjoy reduced industrial wastage as a longer motor lifespan means less replacement, reduced power consumption, and lower environmental impact; it is, therefore, important for engineers to consider efficiency techniques to optimize the operation of three-phase industrial AC motors while saving electrical energy.