Common industrial motors and how to drive them
In 2011, a paper published by the International Energy Agency stated that electric motor drive systems (EMDS) accounted for more than 40% of the world’s energy consumption. With the advent of power-hungry data centers, the percentages may have shifted by now. In absolute terms, the energy used by EMDS will still be around 10 petawatt-hours each year.
This huge number and its dollar and environmental costs drive efforts to make motors more efficient in industrial applications. OEMs must select the optimum motor and drive it in the most efficient way. The increasing use of automation, or Industry 4.0, also mandates more controllability of motors for maximum productivity. Regulations for efficiency are also being imposed. One example: IEC 60034-30-1 for line-operated AC motors, defining levels IE1 to IE4 with increasing efficiency.
There are four main categories of industrial motor types based on application:
- Variable horsepower/constant torque: Typically used for loads such as conveyors and cranes where the load and torque do not change but the application and horsepower required can vary
- Constant horsepower/variable torque: Typically used for loads such as winding drums where speed and horsepower are constant, but the load and torque vary as the drum fills and its diameter increases
- Variable horsepower/variable torque: Used for fans and centrifugal pumps for example, where both horsepower and torque requirements increase as speed increases
- Positional/torque control: Typically used for robots where precise positioning is needed, often with servo control
The most common types of DC motors
The first motors, invented in the early 1800s but still used today, operate from DC and use brushes. Carbon or graphite brushes make a friction contact with connections on the rotor winding. This arrangement wears relatively quickly and is noisy, both audibly and electrically.
DC motors however can have high starting torque, making them valuable for applications such as traction and constant horsepower/variable torque applications. Brushed DC motors can be universal, operating asynchronously from AC as well as DC. They can also operate as generators. Speed control can be achieved by varying the DC applied or controlling the current in the field windings in the rotor.
There are many variants of DC motors including separately excited types where the stator or field windings are powered separately to the armature, series wound, shunt wound, compound, permanent magnet, and brushless DC, considered later.
Types of DC motors
Many types of DC motors continue to be popular in certain applications due to their simple design, ease of use,
high starting torque and constant power.
The AC induction motor: the most common type of AC motor
After the industrial revolution was firmly established in the mid-19th century, through the efforts of George Westinghouse, AC supply generation became available.
The polyphase AC induction motor was patented by Nikolai Tesla and this became the most common type in use, being simple, requiring no electrical connection to the rotor, and no magnets, making them cheap and reliable.
Single-phase types commonly need a capacitor and switch arrangement to start, whereas three-phase types self-start. Speed is essentially fixed however, starting torque is low and efficiency and power factor are poor at light loads. These downsides act against the modern demands for energy saving and ability to control speed to match the application for maximum effectiveness. AC induction motors are ideal for constant power/variable torque applications such as conveyors.
Three-phase induction motors suit speed and torque control through Variable Frequency Drives (VFDs) where the AC supply to the motor is applied through pulse-width modulation (PWM) of DC, derived from rectified line AC. Although VFDs can in theory be added to a traditional three-phase line AC motor, in practice this can cause problems through insulation stress and unwanted bearing currents. Motors designed for VFD control are the best solution. A three-phase induction motor with a basic VFD can be used for variable horsepower/variable torque industrial load of fans or pumps.
Most induction motors are asynchronous and have slip, so run at a speed slightly below a value proportional to a fixed multiple of the supplied line AC frequency, depending on the number of motor poles. True synchronous types are available such as the Wound-Rotor Synchronous Motor (WRSM) and Synchronous Reluctance Motor (SyRM) but are not widely used in industry due to their cost and the limited value of being exactly synchronized to the AC supply, except perhaps in motor-generator sets.
Synchronous types can operate with unity or even leading power factor, which can be an advantage, compensating for lagging currents in other equipment in industrial environments. Synchronous types can have better starting torque than asynchronous so can suit constant horsepower/variable torque applications.
Other variants of AC motors
There are tens of variants of AC motors with different trade-offs in performance. Amongst synchronous types are single and three-phase excited types with powered rotor windings, and unexcited types such as reluctance and hysteresis motors. Variations of the asynchronous types include squirrel cage, slip ring, capacitor start/run induction motors, and commutated types - AC series, AC compensated series, and repulsion motors.
Types of AC motors
Synchronous and asynchronous AC motors have various configurations. Each type is better suited to specific applications.
What is a brushless DC motor?
Although it has “DC” in its name, a Brushless DC motor (BLDC) operates from multiphase AC, but this is at high frequency, generated from switching a DC rail with PWM through a VFD. This is similar to an AC induction motor with a VFD, but in a BLDC motor, the rotor has fixed magnets and the stator has typically three or six windings spaced around the circumference.
A multi-phase high carrier frequency square wave, often around 10kHz, is applied to the stator windings, and the pulse width is modulated with a lower variable frequency which sets the speed, while the depth of modulation or maximum pulse width per switching cycle sets the torque. Feedback of rotor position and winding current can be applied to precisely control the motor performance, making it the preferred solution for many applications. Rotor angle may be determined by hall effect sensors, encoders, or in a “sensorless” scheme, derived from winding voltages.
Inside a brushless DC motor
A brushless DC motor has fixed magnets and three stators, which are exciting using a waveform controlled with pulse-width modulation. The motor shown has three stators and six phases.
An overview of servo and stepper motors
Servo and stepper motors are used for precise angular positioning. Servo motors can be AC or DC powered and are used in robots for example, with some feedback mechanism to achieve smooth, controlled motion. Stepper motors are permanent magnet types and are used to generate movement to an exact angular position and hold it under load. Stepper types can be open loop, where a desired number of steps are programmed by applying a set number of electrical pulses or they can incorporate angular position feedback to avoid over- or under-stepping.
Performance summaries
A summary of the performance of the different motor types for high-power operation and their torque characteristics is shown below.
A summary of AC and DC motors
Motors can be categorized based on key parameters. This includes speed, horsepower and starting torque. Efficiency and operational lifetime are also important factors to consider.
Comparing types of AC and DC motors
Speed and torque are useful parameters to use when comparing types of motors and considering how they will support your application’s requirements.
What are the main methods of motor control?
AC induction motors can be controlled by varying the supply voltage within a limited range, sometimes by phase control of the waveform with thyristors or triacs to modulate the RMS value of the AC supply, or even by using a motorized Variac for slow variations.
Torque peaks at a rotational speed and falls to zero at a speed limit. Feedback control is required to keep the speed from fluctuating. Alternatively, a VFD is used to apply speed and torque control through frequency and pulse-width modulation if the AC motor is rated for VFD use.
Brushed DC motor speed control can be achieved in three ways: by adding variable resistance to the armature (rotor) winding, to the field winding, or by varying voltage to the armature. These are relatively inefficient methods, but speed can be controlled over a wide range.
Widely popular brushless DC motors are used with VFDs. Various PWM modulation schemes have been proposed to maximize performance. The simplest drive method is to use trapezoidal or six-step modulation of the high-frequency drive, shown below.
In this arrangement in a three-phase motor, there will be one winding at any moment that is not driven. Either the zero crossings of the back EMF on that winding, or sensors, can be used as an indication of rotor position, for speed and torque control. This method does however produce some torque ripple as shown in the diagram.
Another method is to use a sinusoidal modulation, which largely eliminates the torque ripple issue. However, all windings are now continuously driven, so rotor position sensing must be achieved by sensors or a sensorless method of monitoring winding voltages and interpreting angular position. The windings for sinusoidal operation are wound in a more complex way, distributed more evenly around the stator, making construction more expensive. The method also has lower peak torque and power density than the trapezoidal implementation.
Comparing modulation methods and position sensing in BLDCs
Trapezoidal (left) and sine wave (right) modulation can be used to drive BLDCs. Position sensing can be achieved by measuring the back EMF in the windings or by using sensors.
How field-oriented control optimizes motor performance
A way to optimize performance with sinusoidal operation is to implement “vector” or Field Oriented Control (FOC). This technique provides smooth operation over the whole speed range, full torque at zero speed, and superior dynamic performance with fast acceleration and deceleration.
The principle of FOC is to continuously optimize the direction of rotor and stator fields so that they are at 90 degrees to each other under all conditions, thereby providing maximum torque. In practice, this requires fast digital processing of voltages, currents, and rotor position.
Dedicated IC controllers are typically used. With FOC, rotor position information on start-up is typically not available, so some schemes start with trapezoidal modulation and then switch to FOC as the motor spins up and angular position information becomes available.
The key factor in motor selection
The selection of an industrial motor depends on many factors including load characteristics, controllability, cost, and efficiency. Basic DC and AC induction motors still have their place.
As industry automation continues, the efficiency and controllability of technologies such as brushless DC motors with variable frequency drives become more attractive. Adopting advanced methods can show payback in terms of energy savings and production efficiency, despite higher initial outlay.
There is no single perfect motor type for all applications, so research continues to incrementally improve each technology. One example is spoke BLDC motors. Innovators are always looking for better concentration of magnetic field and higher power density. Hybrid construction methods for permanent magnet motors are also being investigated to reduce magnet size and weight along with maximized average output torque.
