Industrial motor control
A guide to industrial motor control
Electric motors are widely used across many industrial applications, from driving rollers for web-based products such as paper and steel sheet, to grinding and mixing ingredients in the food and beverage industry. They are also widely employed to drive fans and compressors in HVAC and chilling applications, as well as to operate production and handling machinery such as CNC machining centres and conveyors.
This ubiquitous use means that industrial motors currently consume almost 50% of the world’s electricity. With energy supplies under increasing price pressures combined with the need to reduce energy consumption to reduce carbon emissions, motors are a prime candidate for measures to improve their energy efficiency.
Motors also need to be controlled to improve the precision of their movement and to protect them from potentially damaging inrush currents on start up. Motors connected Direct On Line (DOL) are wired directly to their supply. When switched on, the motor draws a large amount of current, typically six to eight times the full load current of the motor. This produces a large peak torque that can cause damage such as broken conveyor belts, as well as less obvious damage to gear boxes and other drive train components. This type of starting also produces thermal stress on the motor, potentially shortening its life. Ramping up the current gradually via a motor controller avoids these shocks.
Industrial motor controllers overcome the challenges of starting by limiting the current and reduce the energy use of a motor during operation by running it at the appropriate speed for the application conditions.
For example, a cooling application using a fan can be managed using closed loop control. The air temperature can be measured, and data sent back to the controller, which selects the correct speed to run the fan. This ensures the temperature set point is maintained while running the fan at a speed that saves energy. This contrasts with constant speed operation where the fan will run at maximum speed whatever the cooling demand.
Motor controllers are therefore an important part of any industrial system, controlling motors and allowing greater precision in production.
What is industrial motor control?
Industrial motor control involves using a device to control the speed and operation of an industrial motor. Controlling a motor’s speed allows it to meet the demands of the process while using minimum energy. A motor controller can also protect the motor and provide more precise control of the application.
Types of industrial motors
Essentially, an electric motor is a machine that turns electrical energy into mechanical energy. This is achieved through the interaction between a magnetic field in the moving part of the motor – the rotor – and an electric current in the coil winding in the fixed part of the motor – the stator.
There are three main types of industrial motors:
- AC induction
- AC synchronous
DC motors are most often employed in toys, electric vehicles, hoists and lifts. For industrial processes, they have largely been superseded by AC motors.
A simple DC motor consists of two major parts – a stationary set of magnets in the stator and a rotor consisting of an armature that has one or more windings of insulated wire wrapped around a soft iron core. This arrangement is designed to concentrate the magnetic field.
The ends of the windings are connected to a commutator, a rotary switch that periodically reverses the current direction between the rotor and the external circuit. This allows each armature coil to be energized in turn. It also connects the rotating coils to the external power supply through contacts called brushes.
The coils are turned on and off in sequence to produce a rotating magnetic field. These magnetic fields interact with the magnetic fields of the magnets in the stator, which can be either permanent or electromagnets. This in turn creates a torque on the armature, causing it to rotate.
An alternative to brushes is to use brushless DC motors, which switch the current to each coil on and off using electronics.
The speed of operation of DC motors can be controlled by adjusting the voltage applied to the armature.
The stable flow of energy of DC motors makes them highly suitable for applications that need constant speed and torque, such as steel mill rolling equipment and paper machines.
AC induction motors
An AC induction motor, also known as an asynchronous motor, uses the magnetic fields in the stator winding to induce an electric current in the rotor winding. These induced currents in the rotor in turn create magnetic fields in the rotor.
An induction motor's rotor rotates more slowly than the stator field, hence the term asynchronous. The magnetic field of the stator therefore changes relative to the rotor, inducing an opposing current in the rotor.
To oppose the change in rotor-winding currents, the rotor will start to rotate in the direction of the rotating stator magnetic field. This induction effect means that an induction motor requires no electrical connections to the rotor.
An induction motor’s rotor can be either of two types, wound or squirrel-cage. Three phase squirrel cage motors are widely used in industry for their reliability and efficiency. They also offer the benefit of being self-starting.
AC synchronous motors
A synchronous motor is an AC electric motor where the shaft rotates at the frequency of the supply current. This means that the rotation period matches an integral number of AC cycles.
The stator carries a number of multiphase AC electromagnets. These consists of a 3-phase winding provided with a 3-phase supply, creating a magnetic field that rotates in synch with the oscillations of the line current. The rotor has either permanent magnets or electromagnets and is provided with a DC supply.
A synchronous motor operates because of the interactions of the magnetic fields of the stator and the rotor. The 3-phase stator winding carrying 3-phase currents, produces a 3-phase rotating magnetic flux. The rotor locks to this rotating magnetic field and rotates along with it. In this state, the motor is said to be in synchronization.
Once the motor has started, its speed depends only on the supply frequency. For example, at 120 Vac and a frequency of 60 Hz, an AC synchronous motor will rotate at 72 rpm. This rotational speed can be varied by changing the frequency with a Variable Frequency Drive (VFD), also known as a Variable Speed Drive (VSD).
As they exhibit continuous energy changes, AC motors are the preferred choice for applications such as compressors, hydraulics and irrigation pumps.
Two other types of motors are commonly used in industrial applications including the servo motor and stepper motor.
A servo motor takes the form of a rotary or linear actuator. It can be commanded to adopt a precise, angular or linear position, velocity or acceleration. A servo motor consists of a motor coupled with a sensor to give feedback on its position and requires a dedicated control module designed for use with servo motors.
Servo motors are often used in applications such as robotics, CNC machinery and automated manufacturing. Servo motors can be powered with DC or AC voltage.
Stepper motors are a type of brushless DC electric motor. As their name suggests, these divide a full rotation into several equal steps and the motor can be commanded to move to and hold at one of these steps.
Computer controlled stepper motors are typically digitally controlled as part of an open loop system for use in applications requiring holding or positioning.
Stepper motors are today most commonly used in applications such as floppy disk drives, flatbed scanners, computer printers, plotters, CNC machines and 3D printers. In most industrial applications, their role has been superseded by servo motors.
Industrial motor controllers
Industrial AC motor control
There are several options for controlling motors, with the simplest being a soft starter. This is a device used with AC motors to temporarily reduce the load and torque experienced by the powertrain as well as the electric current surge drawn by the motor during the start-up. Soft start reduces mechanical stress on the motor and shaft as well as electrical stress on the cables and connections, helping prolong service life.
A soft start can be formed of either mechanical or electrical devices. Mechanical devices can include clutches or couplings to limit torque, while electrical soft starters can be used in any control system that reduces the voltage or current input temporarily and thus cuts torque.
A more modern and capable solution, which also offers great flexibility of control, is the Variable Speed Drive (VSD) or Variable Frequency Drive (VFD). VFD motor control varies the frequency of the AC supply to the motor. As the speed of an induction motor depends on the supply frequency, the VFD can be used to vary its speed. They can also be used with synchronous motors.
A VFD is a power converter that uses electronics to convert a fixed frequency and fixed voltage into a variable frequency and variable voltage. They will usually have a programmable user interface that allows easy monitoring of the speed of the electric motor.
As drives reduce the output of an application, such as a pump or a fan, by controlling the speed of the motor, this can often cut energy consumption by 50% and by as much as 90% in extreme cases.
As well as energy saving, the controllability provided by VFDs can also bring other benefits. For example, on an extruder, a VFD may not save much energy, but the control and speed regulation it offers would result in a higher quality output.
VFDs have been used for many years to control motor speeds and torques, managing line speeds to alter production parameters including thickness, grain formation and winding tightness. They also find use in pumps, where they solve problems such as water hammer, cavitation and shaft shear at starting. All these problems can be mitigated by running the motor with a VFD.
The number of motors designed for use with VFDs has increased dramatically over the last few years. Drives have also proliferated, with many special versions designed to specifically work with pumps or fans. This makes it challenging to achieve the right drive and motor combination. Matching the proposed drive to the motor will result in getting the right size solution, avoiding oversizing that can result in increased costs, greater space requirements and a bigger environmental footprint.
DC motor controllers
DC motor controllers differ according to the characteristics of the motor and the type of control they provide, such as the motor’s type and operating voltage, the type of power regulation, the type of control signal, the motor power and type of control.
For example, a brushless DC motor (BLDC) has an electronic commutator without brushes. A BLDC motor controller for this motor uses sensors to detect the motor’s position and switches the current in the winding using transistors.
A brushed DC (BDC) motor controller regulates the speed and torque by changing the power to the motor, using either a linear or switching voltage regulator. A linear regulator provides a stable output voltage, independent of the input voltage supplied to it by a power source. A switching regulator uses pulse-width modulation (PWM), supplying voltage in pulses. This means we can regulate the speed of the motor by adjusting pulse duty cycles. Offering higher efficiency and low power loss, PWM is widely used in the speed control of DC motors.
Industrial servo motor control
In a servo motor, control is applied through a feedback loop between the motor and the controller. The position and speed of the motor is sensed with encoders integrated into the motor.
Servo motors are driven by a pulse width modulation (PWM) signal sent through the control wire while power is provided to the motor. The rotor in the servo motor will turn through a particular angle depending on the pulse width. This means that the duty cycle will determine the final position of the shaft.
One of the big advantages of a servo motor is its ability to hold its position between actuation steps. If an external force pushes the rotor away from its commanded stationary position, the encoder will sense this deviation and cause the controller to drive against the external force, holding the rotor’s position steady.
Industrial stepper motor control
A stepper motor driver controls a stepper motor by controlling its exact position without using a feedback system. These types of drivers usually employ variable current control as well as several step resolutions.
They include fixed translators to allow the motor to be controlled with both direction and step inputs. These are the main signals and can be readily supplied by an inexpensive microcontroller such as an Arduino or Raspberry Pi.
The direction signal, when set at logic high (+5V), tells the motor which direction to move in. Depending on the leading phase, the motor will rotate clockwise or counter clockwise. When the signal is set low (GND), the motor will move in the opposite direction.
The step signal determines the step resolution. If it is set to full step, the motor will move from one step position to the next. However, if set in half step mode, the motor will take a half step.
As the workhorse of industry, electric motors play a vital role in manufacturing, processing and transportation of goods and materials. From grinding and mixing to conveyors, chillers, compressors and many more applications, motors are the mainstay of modern production.
As well as providing the ‘brute force’ for simple movements, motors also need to be controlled to make the most effective and efficient contribution to a manufacturing process. Controlling a motor with the right drive helps it achieve maximum energy efficiency, ensures an extended service life and contributes to the accurate control of processes, enabling high quality production.
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