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Welcome to basics of drives training module, looking at process control and various control methods. To view the presenter notes as text, please click the Notes button in the bottom right corner.

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After completing this module, you will be able to:

list the main process control methods

describe why process control is needed

identify various control methods

describe principles of speed, torque and position

recognize common motor starting methods and

list general benefits of AC drives

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There are numerous rotating machines and moving equipment in the world. Machines and equipment can be connected to an electrical supply network.

Electrical energy from that network is converted into mechanical energy using the following components:

Electrical supply, for example an AC drive

Electric motor

Gear box

Axis and

Driven machine, such as a pump or a conveyor

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Process control is carried out either by:

Controlling directly a physical quantity like flow (measured in liters per second) or pressure (measured in bars or Pascals). For example in pumps, flow can be controlled by way of throttling, and in fans, pressure can be controlled by way of a damper.

OR

Controlling directly the speed of the driven machine or the rotating speed of an electric motor in order to control the physical quantity.

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Let us take, as an example, water flow control:

A motor that is directly connected to a 50 Hz supply network runs at constant speeds of either 750, 1000, 1500 or 3000 rpm.

A constant speed causes a constant flow of water (measured in cubic meters or liters per second or liters per minute), which, in some cases, is undesirable.

For example, residential water use varies over time and thus the flow rate must be adjusted accordingly.

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The common flow control methods are: on-off control, throttling by valve, by-pass and variable speed control of pump motor.

With on-off control, the motor runs at constant speed and is started and stopped according to the demand. Benefits include pumping system simplicity while the disadvantages are inaccurate control, low energy efficiency and frequent starts and stops which cause stress in the electrical and mechanical equipment such as pipes and pumps.

Throttling by valve ensures the flow and the motor speed are constant. Benefits of throttling include its simplicity and its ability to stop the flow completely.

Disadvantages are that it increases the pressure in the pumping system and causes unnecessary energy losses.

By-pass is seldom used for centrifugal pumps but is more suitable for positive displacement pumps. The method uses a valve in a feedback pipe that runs from the pump discharge side to the pump suction side. Opening the valve reduces the flow. Benefits include simple methodology and suitability for high pressure pumps. Disadvantage is its low energy efficiency.

Variable speed control of a pump motor is an increasingly common method for flow control. It fully utilizes the affinity feature of pumps. The flow is proportional to the speed, efficiency stays in the optimal range, and power consumption is the lowest possible. Benefits are the ability to soft start and stop pumps, high energy efficiency and easy control using electronics. Disadvantage is higher capital expenditure at the investment phase, but thanks to substantial energy savings, the payback period is short.

Next we familiarize ourselves with variable speed control.

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Examples of mechanical techniques for controlling pump speed include mechanical gearing using a variator, hydraulic coupling and hydraulic motor.

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Mechanical gearing using a variator:

This control method is based on a component called a variator. The pump speed is controlled by changing the gear ratio of the gear (variator) continuously.

A variator is a mechanical power transmission device that is capable of changing its gear ratio steplessly. This enables pump speed to be varied.

Mechanical variable-speed drives were probably the first types of drive to make their way into the industrial environment.

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With a hydraulic coupling, the hydraulic fluid is pumped into the coupling from the hydraulic fluid reservoir.

The pump is driven by an AC motor that is connected direct-on-line. The flow of the fluid is controlled with a valve. The fluid circulates in a closed circuit system.

The hydraulic coupling is driven by another, direct-on-line connected AC motor.

With a hydraulic coupling, the more the valve is open, the higher the pressure in the coupling, the higher the speed on the output drum of the coupling

and the higher the pump speed.

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In this control method, the pump speed is controlled by adjusting the flow of the hydraulic fluid into the hydraulic motor.

The constant speed, direct-on-line connected AC motor operates a hydraulic pump. The pump builds up the necessary operating pressure in the system to allow the hydraulic motor to develop power. The speed of the hydraulic motor is controlled with a control valve. This valve operates like a water faucet—the more the valve is open, the more fluid passes through the system and the faster the speed of the hydraulic motor and the greater the flow of the pump.

Hydraulic drives have been, and continue to be, the workhorse of many metals processing and manufacturing applications.

The hydraulic motor’s small size makes it ideal for situations where high power is needed in very tight locations.

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Examples of electrical ways to control the pump speed are: eddy current coupling, Ward-Leonard drive system, DC drive and AC drive.

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The electromagnetic, or eddy current coupling, is one of the oldest and simplest of the electrically controlled variable speed drives and has been used in industrial applications for over 50 years.

In this control method, the pump speed is controlled by adjusting the DC current in the eddy current coupling.

An eddy current drive consists of a constant speed, direct-on-line connected AC motor and an eddy current coupling. The coupling contains a fixed speed rotor, the so-called input drum and an adjustable speed drum, the so-called output drum, separated by a small air gap. A field exciter feeds the field coil of the coupling with DC current.

The higher the current, the greater the magnetic field that is produced and therefore the stronger the attraction between the input and output drums – and the higher the pump speed.

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The Ward-Leonard drive system dates back to the mid-1940s.The system also gained the name motor–generator set, or M-G set.

In this control method, the pump speed is controlled by adjusting the DC voltage of the DC motor fed by the DC generator.

The constant speed, direct-on-line connected AC motor rotates the DC generator. The armature of the generator is connected to the armature of the DC motor. The generator produces a DC voltage for the DC motor. The speed of the motor is directly proportional to the level of the voltage.

There are two field exciters, one for the DC motor and the other for the DC generator. The purpose of the field exciters is to create the magnetic field.

The field exciters feed DC current into the field windings of DC motor and DC generator. The field exciter that feeds the DC generator field windings determines the level of DC voltage fed into the DC motor. The field exciter that feeds the DC motor field windings creates a magnetic field. The DC motor magnetic field is usually kept at full strength, although in some cases, the field will be weakened to produce a higher speed than the nominal speed.

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DC drives have been the backbone of the industry for decades. In this control method, the pump motor speed is controlled by varying the DC voltage fed to the motor.

The DC drive consists of a DC motor and a controlled rectifier bridge, the so-called converter. The motor speed is controlled by varying the motor voltage with the converter.

In a DC motor, there is always a separate magnetic circuit called the field winding. The winding creates a magnetic field. The strength of the magnetic field is determined by the separate motor field exciter. In newer DC drives the field exciter is included. The DC motor magnetic field is usually kept at full strength, although in some cases, the field will be weakened to produce a higher speed than the nominal speed.

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In this control method, the pump motor speed is controlled with an AC drive.

The basic objective of an AC drive is to change a fixed incoming mains voltage (V) and mains frequency (Hz) to a variable voltage and frequency output.

The output frequency of the AC drive will determine how fast the motor rotates. AC drive enables use of reliable, low-cost squirrel cage induction motor.

Frequency reference, speed reference or rotation speed reference is set either manually using, for example, a potentiometer, or set from the automation system, such as a PLC (programmable logic controller).

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There are many names and acronyms that are used for electronic variable speed drives.

These include VSD (variable speed drive) and ASD (adjustable speed drive),

VFD (variable frequency drive) and AFD (adjustable frequency drive),

frequency converter, inverter and just drive.

The different names and acronyms are often used incorrectly, so it is important to find out the proper meaning of the names and acronyms to secure correct use and understanding. Some of the terms are particularly used in North America, for example ASD and AFD.

The frequency converter is an electronic device that converts alternating current (AC) of one frequency to alternating current of another frequency. In North America, the word ‘frequency converter’ is not in use, so ABB widely uses the word ‘AC drive’ to mean a frequency converter. International standards, however, are basically using the term ‘frequency converter’.

An inverter is an electrical device that converts direct current (DC) to alternating current (AC). An inverter is one part of frequency converter.

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What exactly is an AC drive? An AC drive is an electrical supply device also known as a frequency converter.

An AC drive system is a combination of an electrical supply device and a motor plus a transmission system, for instance a gear box.

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Let us take a brief look at the principles of speed, torque and position.

The controlled quantity depends on the process control needs:

Speed needs to be controlled in pump, fan, conveyor, centrifuge and compressor applications for example. Accurate speed control enables accurate process control offering benefits such as reduced energy consumption, improved quality of end products and reduced need for maintenance of mechanical components.

Torque needs to be controlled for example in winders and test rigs. The torque reference is typically given by an external controller. For instance, in a winder the tension of the roll is of utmost importance and the torque reference is given to the control device (such as an AC drive) by an external tension controller.

Position control means that the place of an object is exactly known at any time and the object can be moved to a desired location at any desired time. An example of this is a bottling line, where each bottle needs to be stopped at an exact place for filling and cork insertion.

With an AC drive it is easy to control speed, torque and position

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Speed can be defined in two ways: linear speed or rotational speed.

Linear speed (v) is the measurement of straightforward movement. Typical machines with linear speed are conveyors, elevators and escalators.

The units of measurement for linear speed are meters per second (m/s) or meters per minute (m/min).

Often, technical specifications with linear movement requirement are transformed into rotational movement, for example linearly moving conveyor belt is in practice driven by rotationally moving rollers.

Rotational speed (n) Is the measure of rotational movement. Typical machines with rotational speed are pumps, fans, centrifuges and mixers.

The units of measurement for rotational speed are revolutions per minute (rpm or 1/min) and radians per second (rad/sec).

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Torque is a twisting or turning force that tends to cause an object to rotate.

A force applied to the end of a lever, for example, causes a turning effect or torque at the pivot point.

Unit of torque (T) is Newton-meter (Nm)

Torque (Nm) = force (N) x radius (m).

Force (N) = mass (kg) x acceleration (m/s2). For example, if a 10 N force is applied to a one meter long lever arm the result is 10 Nm of torque.

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Position control means that the place of an object is exactly known at any time and the object can be moved to a desired location at any desired time.

Units of measurement include meters (m), increments (for example pulses, degrees or distance), and angle (rad, degree) from starting point to the desired location.

Pick and place applications are typical as well as rotating tables, automatic filling lines, machine tools, flying shears and automated warehouses.

The animation shows what position control means and why it is important.

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In this e-learning module, we have discussed the basics of process control. Now we will familiarize ourselves with the common methods to start an electric motor.

The most common starting methods for squirrel-cage motors are: direct-on-line (DOL) starting, star-delta starting, motor soft starter and starting with an AC drive.

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Direct-on-line starting is the most common starting and operating method of low power AC motors.

In direct-on-line operation, the motor frequency is the same as the mains frequency. However, when starting the motor from zero frequency the motor draws from the mains, current that is 3 to 7 times the nominal motor current causing a voltage drop, poor voltage quality, uncontrolled shutdown of plant supply network, and mechanical stress on the motor and the driven machines.

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The star-delta starting is used when there is the need to reduce the starting current and achieve smoother starting compared to direct-on-line starting.

In star-delta starting, both ends of each phase of the stator winding are brought out and connected to a star-delta starter circuit.

For starting, the stator windings are connected first in star to reduce the starting current, but once the motor is running at full speed, the change over into the delta connection is done manually or is controlled by a time relay. The delta connection is meant for normal operation. The phase voltages and currents of the motor in star connection are reduced to 30 percent of that of the direct-on-line starting current. A disadvantage of this method is that the starting torque is reduced.

This starting method only works when the application is light loaded during start-up. The starting time depends on the characteristics of the load and on the starting method.

Large inertias of the load will cause long starting times, which in turn can cause overheating of the motor.

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A motor soft starter is used when there is the need to reduce starting current and achieve smoother starting compared to direct-on-line starting.

A motor soft starter is an electronic device that can control the motor voltage during startup. The motor voltage is raised from zero to full voltage. Acceleration time of the ramp can be set. In the beginning, the voltage increases rapidly until the motor’s torque overcomes the load torque and the motor starts to rotate. During start-up the voltage and the torque increase so that the machinery starts to accelerate. One feature of the soft starter is the soft stop function, which is very useful when stopping pumps where the problem is water hammer within the pipelines.

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Starting a motor with an AC drive is the most sophisticated starting technique. During starting and entire operation, the motor speed and torque can be controlled accurately.

Benefits offered by an AC drive include soft and controlled motor starting and stopping, and reduced mechanical stresses, meaning fewer repairs and lower maintenance costs. An AC drive enables low starting current, and the mains current is proportional to the motor shaft power.

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An AC drive does not cause any current peaks in the network, meaning less stress to the supply network.

The power factor of an AC drive is high, which means that it draws only active power from the network.

An AC drive also ensures a stable supply network during start-up, and causes no mechanical stress on the motor and the driven machines.

Although an AC drive is designed for continuous operation of motors, it is also a superior starting device. When using an AC drive, the actual motor speed follows exactly the given speed reference.

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When starting a motor, an AC drive takes low starting current from the network compared to other starting methods.

During starting and stopping, motor speed can be accurately controlled with AC drive.

With AC drive, motor torque can also be accurately limited according to mechanical or other needs – the starting torque can also be set above that of a typical direct-on-line connected motor.

The starting torque or constant torque can be anything between zero and the maximum torque of an AC induction motor. When necessary, an AC drive is the only device (of those mentioned in the drawing) that is able to produce a starting torque higher than that of a typical direct-on-line motor.

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By far the biggest benefit of using an AC drive is its ability to reduce the energy consumed by the motor driven load. AC drives reduce the output of an application, such as a pump or a fan, by controlling the speed of the motor, ensuring it runs no faster than it needs. Many motors are oversized to cope with a maximum demand that rarely or never occurs. The drive brings the motor speed down to match the actual demand needed by the application. This often cuts energy consumption by 50 percent.

The relationship between a pump’s speed and its energy requirement is known as the cube law, because the need for power increases with the cube of the speed. This means that a small increase in speed requires a lot more power, but also that a modest speed reduction can give significant energy savings.

A pump or a fan running at half speed consumes only one eighth of the power compared to one running at full speed. Lower energy consumption means lower CO2 emissions.

An AC drive has a high power factor compared to a direct-on-line connected motor, meaning less reactive power – several power companies charge for reactive power consumption.

The payback time of an AC drive is often less than 12 months on energy savings alone.

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AC drives deliver accurate control and less mechanical wear, reducing maintenance and extending the life expectancy of the system. AC drives provide soft and controlled starting, stopping and operation of motors. This reduces mechanical resonances and stresses on the motor and its driven load thereby prolonging the lifetime of conveyors, fans, pumps and other mechanical equipment.

In pump applications, being able to vary the speed of a pump helps alleviate sudden pressure surges that can lead to effects such as water hammer. Water hammer can ultimately crack pipes, causing leakage. High pressure in the pipeline can lead to leakages. Using an AC drive can help maintain the correct water pressure and reduce leakages.

AC drives use standard squirrel cage induction motors. Although very simple in design, these motors have a good power to weight ratio and are extremely robust, needing very little maintenance. Because they are standard motors, several manufacturers provide motors all with the same dimensions, thereby giving a wide choice of suppliers. For applications requiring high protection classes, such as IP54, standard induction motors offer a cost-effective solution.

Finally, the driven machine can be operated in its most efficient operating point or area, meaning that the machine runs cooler and thus its lifetime is extended.

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An AC drive delivers accurate speed control and dynamic torque control. This combination brings a high degree of precision to many processes which, in turn, can enhance the quality of end products. The accurate speed control obtainable with an AC drive results in process optimization. The optimal process control leads to the best quality end product, which means the best profit for the customer.

Process equipment is usually designed to cater for future productivity increases. With the AC drive, speed can be increased from 5 to 20 percent and above.

AC drives also have several internal features and functions which are sometimes required for accurate process control, such as application macros, inputs and outputs, reversing function, ramp times for acceleration and deceleration, energy optimizer, torque boosting, load limits to prevent nuisance faults, power loss ride-through, stall function, and flying start.

With extensive inputs and outputs, for example, different kinds of process information can be fed to the drive and it will control the motor operation accordingly. The load can be limited to prevent nuisance faults and to protect the working machine and the entire drive system.

The two animations are examples of processes where an AC drive offers benefits such as enhanced quality of end products.

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AC drives provide fast and accurate control and ability to operate a machine at its optimal operating point leading to benefits such as reduced noise and less vibration, both of which enhance the health and safety of employees.

In heating, ventilation and air conditioning (HVAC) applications, an AC drive can control climate by varying temperature, humidity, air change rate or CO2 emissions.

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Thank you for your attention. You may now go ahead and move on to the next e-learning module.