Electric Motor Control Explained

By R.W. Hurst, Editor

Efficient and reliable electric motor control (EMC) is essential in industrial, commercial, and institutional power systems. It encompasses the management and regulation of electric motors, such as AC servo motors, brushed DC motors, and brushless DC motors, to achieve desired performance outcomes like speed, torque, and direction. This article delves into various aspects of EMC, including systems, components, and applications.


An example of EMC in an industrial setting is the operation of conveyor belts, pumps, or HVAC systems. The coordinated and precise movement of these machines relies on an array of control components, such as motor control centers (MCC), motor control circuits, programmable logic controllers (PLC), and electronic devices like variable speed drives.

There are four primary types of motor control methods: Direct-On-Line (DOL) motor starters, soft starters, variable frequency drives (VFD), and closed-loop control systems. Each method has specific applications and benefits, depending on required performance, energy efficiency, and system complexity.

  1. DOL starters: Simple and cost-effective, DOL starters provide full voltage to the specific motor upon startup. However, they can generate a high inrush current, potentially causing mechanical stress on the motor and power system.
  2. Soft starters: By gradually ramping up the voltage, soft starters reduce inrush current and offer a smoother startup.
  3. VFDs: VFDs enable precise control of speed, torque, and direction by modulating the frequency and voltage supplied to the motor. VFDs are used in applications requiring varying speeds and loads and stopping the motor, such as HVAC systems and pumping stations.
  4. Closed-loop control systems: These systems utilize feedback mechanisms to monitor and adjust motor performance based on set parameters. PLCs often coordinate multiple motors within a system and ensure seamless integration.

Designing an EMC circuit involves selecting appropriate components and configuring them to meet the specific application's requirements. This process includes choosing the right control method, protection devices such as circuit breakers, and control devices like contactors, relays, and PLCs. Additionally, engineers must adhere to relevant electrical codes and standards.

MCCs are crucial in organizing and managing control components in industrial settings. An MCC is a centralized panel housing starters, overload protection devices, and control components like variable speed drives. These panels streamline electrical power distribution, facilitate system monitoring, and enhance safety by providing a single point of control and maintenance.

VFDs are instrumental in improving EMC. By allowing precise speed control, VFDs enable energy-efficient and highly adaptable control of motor performance. This technology is essential in energy conservation and control flexibility applications.

A motor control panel houses key components like starters, circuit breakers, and PLCs. These panels provide a centralized location for controlling and protecting motors, ensuring efficient operation and maintenance.

PLCs are a vital part of EMC systems, serving as the "brain" of the operation. They process input from sensors and send commands to various components, coordinating multiple motors within a system. As a result, PLCs are integral to implementing closed-loop control systems and ensuring optimal performance.

EMC is vital to industrial, commercial, and institutional power systems. Proper selection, design, and maintenance of components can optimize system performance, enhance the energy efficiency, and prolong equipment life. By understanding the various types of motor control methods and their applications, engineers and system operators can fully harness the potential of EMC technology in their facilities.

How can motor control be used to improve energy efficiency and save my company money?

EMC can be used to improve energy efficiency and save your company money by:

  1. Implementing variable frequency drives (VFDs): VFDs allow precise control of speed and torque, enabling them to operate at optimal efficiency for different loads and demands. This reduces energy consumption and minimizes wear on the motor.
  2. Upgrading to energy-efficient motors: High-efficiency ones use less energy and produce less waste heat, resulting in lower energy bills and reduced maintenance costs.
  3. Utilizing soft starters: Soft starters reduce inrush current and mechanical stress on the motor during startup, leading to energy savings and prolonged life.
  4. Employing closed-loop control systems: These systems use sensor feedback to monitor and adjust performance, ensuring optimal efficiency and minimizing energy waste.
  5. Conducting regular maintenance: Proper maintenance, such as lubrication and alignment, helps ensure efficient operation and prevents energy loss due to mechanical issues.
  6. Optimizing sizing and selection: Choosing the right size and type for each application ensures motors operate at peak efficiency, preventing unnecessary energy consumption.

By implementing these strategies, you can enhance the energy efficiency of your motor-driven systems, resulting in reduced energy costs and improved overall operational efficiency for your company.