Electric motors have undergone numerous transformations since their inception in the 19th century. While it was relatively easy to control the speed of a direct current (DC) motor, alternating current (AC) induction motors were largely tied to the frequency of the power supply. This article will examine the developments in motor and variable frequency drive (VFD) technology with an emphasis on the past three decades.
Induction motors produced a minor speed adjustment due to slip, which was not controllable in a practical way. The only way to control speed in the early 1900s was to include a different number of poles. This meant physically rewiring the internal windings of the stator to influence the speed of the rotor.
The 1920s to 1940s brought about the invention of the wound rotor motor. By adding resistance to the rotor circuit, starting torque was boosted without drawing large currents. This motor was used in hoists, mills, conveyors and a host of other applications.
In addition to the high starting torque, this motor offered controlled acceleration and adjustable speed control when required. However, wound rotor motors were maintenance heavy. The use of brushes and slip rings in wound rotors meant regular servicing due to these parts wearing out. Furthermore, these motors relied on external resistors to control their speed, wasting energy due to heat dissipation.
Multispeed motors were another way to primitively control speed; however, these motors presented discreet steps in speed versus continuous and dynamic control. There was no way to vary speed smoothly.
The discovery of the transistor in 1947 paved the way for modern control of motor frequencies. High-power, bipolar junction transistors (BJTs) presented the opportunity for switching and control. The rise of power electronics led to the development of metal-oxide-semiconductor field-effect transistors (MOSFETs) and insulated gate bipolar transistors (IGBTs), which were both crucial for motor control. These electronic inventions eventually led to the development of modern VFDs.
A Quick Primer on Motors & Drives
Motors convert electrical energy into movement, or kinetic energy. VFDs control the speed, torque and direction of motors by limiting the power supply and frequency available to the motor. Modern motors and drives are ubiquitous in industries ranging from manufacturing and heating, ventilation and air conditioning (HVAC) to renewable energy and robotics. Over the past three decades, motor and drive technology has advanced dramatically.
The 1980s to 1990s
In the early ’90s, the most common motor was the three-phase induction motor. While robust, the induction motor was mostly operated at constant speed and controlled by gearboxes and contactors. The need for speed control was ever growing.
VFDs existed before the ’90s, but they became more affordable, compact and reliable in this decade largely due to the invention of the IGBT in the ’80s. Benefits of using VFDs included reduced energy consumption, lower mechanical wear, improved process control and soft starting and stopping, which reduced stress on the overall system. These improvements led to widespread adoption in HVAC systems, conveyors, pumps and fans.
In the ’90s, another type of motor—the permanent magnet (PM) motor—was quickly gaining popularity. PM motors had permanent magnets embedded in the rotor, which meant current did not need to be induced there. In PM motors, the rotor speed synchronized with the rotating magnetic field of the stator due to internal permanent magnets. They came with enhanced power density, allowing for significant size and weight reduction.
However, PM motors could not be started direct on line (DOL) due to the lack of slip. They needed to be operated with a VFD, which would ramp up the motor, bringing it up to speed in a controlled manner.
The 2000s
The digital revolution of the 2000s was brought about by advancements in microcontrollers and digital signal processors (DSPs). This allowed for the more precise algorithms required for motor control.
Pre-2000s, motors typically required mechanical feedback sensors for precision control. Sensorless control existed but was limited to basic voltage/frequency (V/F) control or scalar control. The 2000s brought about additional sensorless motor control techniques, such as:
- Field-oriented control (FOC) used mathematical models and measured stator currents/voltages to estimate rotor position and speed.
- Model reference adaptive systems (MRAS) compared two models of motor behavior (one adaptive and the other being the reference) to estimate rotor speed.
- Back electromotive force (EMF)-based estimation detected the back EMF generated by the motor to estimate rotor position.
An improvement in user interfaces for drive programming brought about text-based liquid-crystal displays (LCDs), allowing menus, parameters and fault codes to be shown completely. Manufacturers began implementing logical menu structures inspired by mobile phone user interfaces (UIs). Software tools with USB or RS485 let users configure, monitor and log drive data directly from a laptop. Modular human machine interfaces (HMIs) and remote panels were introduced, allowing OEMs to detach and mount these interfaces. Actual fault messages such as “overvoltage” were now being shown vs. obscure error codes.
Growth of industrial fieldbus applications became more common due to growing demand for diagnostics and remote configuration. Commercial buildings and industrial facilities began using VFDs with motors to regulate centrifugal pump systems.
The 2010s
The 2010s brought a strong push toward energy efficiency. Governments worldwide began introducing regulations mandating the use of high-efficiency motors. Metric motors now required IE3 (premium efficiency) and IE4 (super premium efficiency) classes defined by the International Electrotechnical Commission (IEC). To meet these standards, manufacturers turned to brushless DC and PM motors, which offer better efficiency and performance over AC induction motors. PM motors integrated with advanced VFDs became a popular choice for numerous applications, ranging from fans and pumps to electric vehicles. VFDs developed features for:
- Energy regeneration feeding power back into the grid for regeneration
- Predictive maintenance capabilities using integrated sensors for monitoring conditions
- Integration with smart grids
The 2020s
The current decade builds on the developments made in the past with an emphasis on connectivity, intelligence and sustainability.
Modern drives are becoming smarter and are capable of auto-tuning with adaptive control. Most motors now offer the capability to measure the electrical characteristics of the motor and tune the drive for more efficient operation. In addition, with the rise of AI and machine learning, modern drives are now capable of adaptive control, fault prediction and diagnosis, energy optimization and real-time process adjustments.
This decade brings about a movement toward industrial Internet of Things (IIoT) and connectivity where motors and drives are no longer isolated components. This connectivity enables remote monitoring and data analytics across a fleet of motors.
Additionally, there have been unique developments in specific sectors. Robotics and computer numerical control (CNC) machines are now more likely to rely on high precision servo motors and drives with multiaxis motion.
In HVAC systems, smart drives can now optimize fan and pump performance based on load. Wind turbines and solar trackers can now use high-efficiency motors with rugged, weatherproof drives.
As the industry evolves, there are risks to not upgrading equipment. These risks are not just technical in nature—they can impact productivity, profitability, competitiveness and even compliance with industry standards. Higher operating costs are directly associated with inefficient motors and outdated drives. With the rising costs in energy prices, legacy systems are at a major disadvantage. Companies that do not use drives that regenerate energy during braking or deceleration are essentially wasting recoverable energy, reducing the overall system efficiency.
Motors and drives that have not been upgraded can lead to faster wear and tear and increased downtime. Due to the lack of preventative maintenance, they require more frequent service to avoid potential downtime. Besides lower productivity, older equipment cannot keep up with variable production demands, real-time adjustments and integration with smart manufacturing platforms.
The Future
Pump applications have traditionally been major energy consumers. Modern VFDs let the user adjust the motor speed to match actual demand with precise flow and pressure control. This is especially valuable in closed-loop systems, where consistency is crucial.
Developments in silicon carbide and gallium nitride are leading to smaller, faster and more efficient drives. In addition, the movement toward sustainable manufacturing practices and energy reduction is expected to continue, leading to even more energy savings. From the surface, it may be difficult to see obvious trends, but the past indicates a strong movement toward higher efficiency and improvements in overall control, intelligence and motor interfaces.
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