Motor control applications
Where are motors used?
Motors are used in a wide range of industrial equipment, including pumps, fans, conveyors, robots, elevators, mixers, grinders and centrifuges. This widespread use means that industrial motor systems often account for around 70% of the electricity consumed by manufacturing in different countries.
What are motor control applications?
Motor control is used to alter the speed or torque of motors to meet the needs of the process or machine being driven by the motor. Motor control applications can include robot cells, handling equipment, HVAC systems, processing equipment in the food and beverage industry and stands for web-based products such as metal and paper. Control ensures the motor runs at the right speed for the process, saving energy, protecting materials and machines from damage and ensuring high quality production.
Types of motor control
There are several types of motor, each controlled in different ways.
Brushed DC motors
Brushed DC motors are one of the earliest and simplest types of motor developed. A simple DC motor consists of two major parts - a stationary set of magnets in the stator and a rotor consisting of an armature. DC motors usually provide better performance at low speed then a similar AC motor and can be controlled accurately down to 5-7% of the rated speed. Controllers for these motors generally consist of a processor, motor driver, analogue to digital converter and an encoder.
Brushless DC motor
A brushless DC (BLDC) motor has a rotor with permanent magnets and a stator that contains the windings. In a BLDC, electronics are used to switch the current to each coil on and off. The speed of operation of DC motors can be controlled by adjusting the voltage applied to the armature.
AC induction motor
AC induction motors make up over 80% of all motors. In an induction motor, the stator winding induces a current in the rotor in a similar way to a transformer. This motor is also known as an asynchronous motor because the rotor also turns at a lower speed than the field. The rotor will turn at a constant speed unless a Variable Frequency Drive (VFD) is used.
Permanent magnet synchronous motor (PMSM)
A permanent magnet synchronous motor combines the rotor from a brushless DC motor and the stator from an AC induction motor. The rotational speed of a PMSM can be varied by varying the frequency with a VFD, also known as a Variable Speed Drive (VSD). With permanent magnets the PMSM can generate torque at zero speed. PMSMs are usually used as part of high-performance and high-efficiency motor drives. They can achieve smooth rotation over their entire speed range, full torque control at zero speed, and rapid acceleration and deceleration.
Stepper motors have few moving parts, making them inexpensive and rugged. As their name implies, stepper motors are used to step to a number of discrete positions, rather than rotate continuously. They are readily controlled by a computer, with digital pulses being converted into fixed steps.
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 also requires a dedicated control module designed for use with servo motors. Servos are controlled via Pulse Width Modulation, which involves sending an electrical pulse of variable width through the control wire.
Motor drive applications
With the vast majority of motors used in industry being AC, most motor control applications will be performed by VFDs.
VFDs sit between the electrical supply and the motor. Power from the electrical supply goes into the drive, which then regulates the power fed to the motor.
Within the drive, there is a rectifier that converts the incoming AC to DC power. This is then smoothed by an array of capacitors and then goes to an inverter, which changes the DC power back to AC power to feed the motor.
This allows the drive to adjust the frequency and the voltage sent to the motor to match the demand of the process. AC motors can therefore be run at the correct speed or torque according to demand, potentially saving large amounts of energy.
A VFD will control either torque or speed in either “torque control” or “speed control” mode. When the VFD operates in torque control mode, the speed is determined by the load - when used in speed control, the torque is determined by the load.
Variable torque loads offer the most potential for energy saving and improved efficiency. These loads include pumps, fans, and air handling units.
The controllability of VFDs and their ability to receive inputs from sensors in the environment or within the process sees them widely used in many different types of industrial control applications.These can range from handling delicate food products such as tomatoes, to providing cooling and oxygen regulation in heavy industrial processes such as steel making. This ability to control processes by varying the speed of the motors powering them means they are often also called industrial automation drives.
PLC DC motor control
DC motors are most easily operated via relays. An electromechanical relay (EMR) is essentially a switch operated by an electromagnet. The relay turns a load circuit on or off by energising the electromagnet, which in turn opens or closes contacts connected in series with a load. Relays are generally used to control small loads of 15A or less.
A relay has two circuits, the coil input (also known as the control circuit) and the contact output (the load circuit). In motor circuits, EMRs are often employed to control the coils in motor contactors and starters.
A relay will usually have only one coil, but it could have many different contacts. EMRs have both stationary and moving contacts, with the moving contacts attached to the armature. Contacts are designated as normally open (NO) and normally closed (NC). When the coil is energised, an electromagnetic field is set up, causing the armature to move, closing the NO contacts and opening the NC contacts.
Coils are usually designated by a letter, with M used for a motor starter, while CR is used for control relays. Control relay contacts are small because they only need to handle the small currents used in control circuits, allowing them to contain numerous isolated contacts.
A similar device to an EMR is a contactor, with the main difference being the size and number of contacts. Contactors are intended for direct connection to high-current load devices. Devices switching more than 15A or in circuits rated at more than a few kilowatts are generally described as contactors.
Variable frequency control offers a number of benefits:
- Better operational efficiency
With drives, the control of production systems can be automated as the drive can form part of a closed loop control system. This reduces the need for manual labor, saving in man-hours and labor costs
- Savings on energy costs
By running the motors at partial load to match demand, VFDs use only the amount of energy needed, giving up to a 50% reduction in energy consumption
- Savings in capital expenditure
With an immediate reduction in electrical consumption, rapid payback times can be achieved, in some cases within months of installation
- Savings in maintenance and spare part costs
Using VFDs causes less stress on mechanical equipment during startup and operation, giving a longer equipment lifetime
Fan control applications
One of the major uses of VFDs is in industrial fan speed control. Fans are widely used in applications ranging from heating, ventilation and air conditioning (HVAC) in commercial, industrial and residential property, blower speed control to cool clinker in cement production and regulating the temperature of ovens for the production of baked goods.
Using VFDs in industrial fan speed control offers a number of benefits. As well as accurate air flow control, VFDs also help produce lower fan noise and help prolong fan lifetime. In addition to reducing energy consumption, VFDs also allow fans to be started while spinning, avoiding the need to bring the fan to a complete stop before starting it again. They also offer power loss ride through. During a dip in the supply voltage, the drive commands the fan to slow down and uses regenerative power to stay alive and maintain control of the fan.
A VFD can be used to vary a compressor’s speed to meet demand, and also react quickly to demand to avoid the need to keep a high discharge pressure in reserve.
Controlling motors for pumps
Pumps are used across industry, from processes such as petrochemical production, to oil and gas, food and beverage and water and wastewater treatment. The nature of these pumped fluids varies greatly in composition, density, volume flow rates, and pressure levels, requiring different performance and control parameters from the pumps used to move them.
Applications that require variable flows, such as meeting water demands from consumers at various times of the day or matching the process conditions and controlling water quality in a water treatment plant, will require a VFD. However, some pump motor applications may not require variable speed and can instead combine a higher efficiency IE3 motor with a direct online starter, a star-delta starter that can limit inrush current or a soft starter.
Soft starters can also reduce the danger of water hammer, a condition in which pressure surges occur in the fluid when the pump is turned on or off. These pressure surges can damage joints and ultimately lead to leaks from pipes or vessels. VFDs, with their ability to ramp up pump motor speeds, are also a very good way of avoiding water hammer.
Industrial automation motor control
Industrial automation systems are used in applications ranging from food and beverage, packaging, logistics systems, paper production, machine tools and robots. Because VFDs can form part of a closed loop system, taking in data from sensors on the machine, processing this and outputting a command to the motor, they can form the heart of an automation system. They can either perform a particular standalone control function on one machine or cell or form part of a larger control system, receiving data from other parts of the line or from human operators in the control room.
Advantages of including VFDs in the automation loop are firstly the energy savings they can bring. With the precise speed control they can bring to motors, VFDs can also improve quality by ensuring that equipment such as bottling lines are run at the correct speed to prevent damage. They can also feed material at the correct rate to avoid backlogs or a shortage of work in progress items, helping achieve productivity goals.
An example of productivity improvement is a solution for a tomato grower that used VFDs from ABB. The company needed to pack tomatoes more quickly and accurately while also inspecting the tomatoes for quality.
A conveyor system was designed using servo motors and high-performance machinery drives. This system controlled the speed of the tomato handling conveyors, matching the speed of the packing machine. This ensured that the tomatoes are packed quickly and accurately. A simple drive runs the rollers on the conveyors, which allows the tomatoes to be turned automatically and inspected for quality.
The feeder conveyors are each driven by a high-performance machinery drive. Arranged in master-slave configuration, the master receives an encoder signal from the wrapping station. The drive knows what stage the wrapper is in its cycle and controls the motors on the conveyor to ensure the tomatoes arrive at the wrapper at the correct time.
The solution allows an average packing rate of 70 to 80 packs per minute, double the number achieved by the purely mechanical system.
In fact, conveyors are a very common control application using VFD controlled motors. Conveyors may need to move in one or two directions, start or stop frequently or stop at exact positions to allow precision tasks like decanting drugs into vials. VFDs can achieve all these motion requirements.
Pumps and fans are also often part of industrial automation motor control. For example, with temperature feedback from a sensor, a VFD can switch a fan on or off to achieve the correct firing temperature in a kiln. Similarly, level sensors can feed the VFD with data on the level of fluid in a tank, allowing the VFD to switch a pump on or off to keep the level within specified limits and maintain the correct amount of water or other fluids to feed a process.
Robotics motor control
Robots are obviously big users of motors to achieve precise motion in a range of applications that include pick and place, material handling, component inspection, painting and precision welding.
Movements required can be rotary or linear and robot motor drivers rely on four types of motor to achieve these various motions.
Robot servo motor
Servo motors are essentially rotary or linear actuators, used by robot applications to rotate or push parts of the robot structure with high accuracy and precision. Servos make use of regular motors but with the addition of a sensor to provide feedback on its position. A robot servo motor will form part of a closed-loop system with components such as a shaft, gears and a control circuit.
Linear motors are essentially induction motors that produce linear motion rather than rotational motion. They employ an AC power supply and a servo controller, often the same as those used in rotary servo motors.
Spindle motors are small, high precision electric motors used to rotate a shaft or spindle. They usually take the form of a stepper motor with a hollow shaft and in robotics applications are used for tasks such as drilling, routing, engraving and deburring.
Stepper motors bring a high degree of precision accuracy to robotic movements with their ability to step through a precise angle that is a subdivision of one complete rotation of the motor shaft. Very precise positioning can be achieved with stepper motors using digital, computer-controlled stepping, making them ideal for controlling camera platforms, X-Y plotters and other subsystems that find use in robotic applications.
Elevator motor control
Elevators are big users of energy but, using the right type of motor control, much of this energy can be reused for other escalators or other electric loads on the network.
When an elevator ascends carrying a light load and descends carrying a heavy load, the system is generating more power than is used. In a traditional elevator drive, this excess energy is lost as heat. By contrast, using a regenerative drive captures that energy for reuse – when the elevator cab travels down, the motor that lifted it up acts as a generator, transforming the mechanical power into electrical power for other loads. Regenerative drives can reduce the energy used by a building’s elevator systems by as much as 70%.
Electric vehicles and HEVS
Electric vehicles (EVs) and Hybrid Electric Vehicles (HEVs) are a growing market for motor control. Although batteries are constantly being improved, range anxiety remains a concern for drivers and so any system that can save battery life is useful. As with elevators, EVs can make use of regenerative braking, converting the mechanical movement of the spinning wheels into electrical energy by using the motor as a generator. This also slows the car down as energy is consumed by the wheels rotating the shaft of the motor.
Modern vehicles also use motors in many other applications, including power steering, automatic doors, windows and mirrors.
Other industrial motor control applications
As well as the above examples, industrial motor control is used in a wide range of other applications. These include cranes, where VFDs can now be used to control all the motorized parts of the crane, including the hoist, bridge, trolley and hook rotation. Using VFDs avoids abrupt stops and starts on the bridge and trolley, avoiding excessive wear to components such as couplings, gearboxes, wheels, and structural supports. One particular VFD technology that is highly beneficial for crane control is Direct Torque Control (DTC). This allows the maintenance of full torque at zero speed to keep a suspended load at a set height. It also responds more quickly than other technologies to keep swinging loads under control.
CNC machines are also major users of motors. Many general CNC machines that are used for a wide range of tasks such as drilling, reaming, and milling will feature motors in up to six axes. This allows a cutting tool to perform a wide variety of operations along three linear axes and three rotational axes.
Motor control is also widely used in the military and avionics spheres. For example, Fly-by-Wire involves the activation of flight control surfaces via motors that are commanded to operate by electrical signals from the pilot’s controls rather than the traditional method of physically connected wires and pulleys. This allows a much faster response, with less pilot effort. It also allows the introduction of techniques such as artificial stability where the airframe is designed to be deliberately unstable. The onboard computer commands the control surfaces to adjust to maintain stability, greatly improving the response times and the agility of the aircraft.
Another major use of motors and their controllers is in the medical field. Infusion pumps for delivering fluids such as nutrients and medication into a patient’s body are common and other major uses include the movement of medical scanning equipment such as MRI and CAT scanners.
The importance of industrial motor control
With so many motors in use in industry, performing so many tasks, controlling them properly is of vital importance.
Proper motor control can achieve a wide range of benefits for the process and the machine being controlled. One of the chiefs among these is energy efficiency. By controlling the speed of the motor to match the demands made by the process, energy use can be drastically reduced. This compares to no speed control, where motors are left to run at maximum speed, using full power whatever the actual demands of the process. Regenerative technologies are also useful in certain applications, allowing mechanical braking to run the motor as a generator and thus provide extra power that would otherwise be wasted as heat.
Certain motors such as stepper motors also offer a high degree of precision, bringing the ability to advance in small, precise increments to achieve accurate positioning. This is particularly useful in robotics applications, where grippers have to be precisely placed above components and the gripper itself must be moved to grip the components, some of which may be delicate. Servo motors can also be used to move camera focusing mechanisms to ensure accurate capture of visual information for the robot.
Motor driven applications come with some risks of injury to operators or damage to equipment or material, so safety is of paramount concern in motor control. For example, a motor may experience an interruption in power, bringing it to a halt. If power is then unexpectedly restored, the motor may start in an unsafe position or injure people working on it. To avoid these outcomes, VFDs can employ safety functions such as Safe Torque Off or STO, which brings a drive safely to a no-torque state and prevents an unexpected start-up of the driven machinery.
Similarly, PLCs contribute to motor safety through their ladder diagram programming. This allows the PLC to be programmed to respond to inputs from switches and sensors. In this way, the programmer can ensure that the PLC will only provide the motor switch on signal if certain safety conditions are met.
Motor control also plays a major role in protecting both the motor and the driven equipment from damage, contributing to increased reliability and safety, while maximizing production time and minimizing maintenance effort and cost. For example, using VFDs causes less stress on mechanical equipment during startup and operation, giving a longer equipment lifetime. VFDs can also be programmed to avoid driving equipment such as grinders and mixers beyond set torque limits, protecting motors and the machine itself from damage.
Choosing the correct control method for the motor and the process thus plays a crucial role in successful operation of industrial processes, ensuring a long life, safe operation and optimized costs.
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