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Variable speed drives, or VSDs, are dynamic speed drives. They are electric motors that are operated externally by controllers and can be used in the majority of manufacturing firms. The engines are massive in order to accommodate manufacturing processes. The motors are cost-effective because they assist businesses that need a large number of dominant electric motors in conserving electricity. They are commonly used in businesses for process management. VSD is described differently by different sources. The past, use, and function of VSDs in the marine industry are the subject of this article.
Start and Evolution of VSD
In the early 1980s, the variable speed drives (VSD) were only used in heavy industries, mainly for large motors. The application was expensive since the process involved very large speed converters (Gritter, Kalsi, and Henderson 2005, p. 349). The speed or frequency is controlled by the VSD by initiating the principle of AC motor_x0092_s speed. The speed of the AC motor is easy to control if it is supplied with a frequency and voltage that varies. In most cases, a motor runs with a constant speed if there is no frequency converter.
Historically, the VSDs were not available. Machines that required speed control for example sawmills conveyors used expensive hydraulic couplings and direct current (DC) motors to regulate their speed. In other applications, the opening and closing valves were being used to control the rate of the machine (Jayasinghe 2016). Other methods used traditionally, include the use of pulleys, gears and similar devices to change output speed. However, during such processes, the speed of motors always remained constant. The trend made researchers and engineers to find a solution that will try to regulate motor speed. In early 1980_x0092_s and 1990_x0092_s, the VSDs were invented and started to appear on the market (Giannoutsos and Manias 2015, p. 590). They offered an alternative method of motor speed control. The VSD in simple words is just an electronic power controller. The VSD has different names, which include _x0091_frequency converter,' _x0091_drive,' _x0091_inverter,' or adjustable speed drive. The VSD can adjust or control the electrical supply to an AC induction motor. The adjustment is in correspondence with the torque output and the speed of the motor. The control enabled a machine to have a close match of process requirements with the engine speed (Jayasinghe 2016).
The VSD technology is now live and mature. It enjoys the widespread of usage and adoption with AC induction motors. The VSD is extremely reliable and primarily offers a high degree of speed control in engines. This has allowed the motor speed to be varied from 0rpm to over 100% of the specified speed, while the required torque is adjusted to suit.
Application of VSDs
Variable speed drives have full application in marine industries. The system is used to control speed and frequencies of ships to save on energy. They help machines (motors) to use less power to start and run.
The sizes of the VSD tend to vary between 0.18kW to tens of MW. They are characterized as standalone devices and are mainly connected to the electrical supply of the motor. In most cases, for smaller units, the VSD is built as an integrated motor drive product attached to the motor (Rinaldi and Thaxton 2000, p. 731). The applications of VSDs in industries are only because of its advantages when it comes to the reduction of energy use. VSDs are mostly effective in pumps and fans applications. The use of the VSDs in fans and pumps tends to have replaced the traditional methods that were used for output control. The primary mechanism in the application of the VSD is the exponential relationship that is between the energy used and the machine speed (output).
Principle of Operation
VSDs have some variations in their designs. They have same functionality- converting incoming electrical energy with constant voltage and frequency that tends to be the output of the motor torque and speed. The speed can be varied from 0rpm to 100% of the full speed while the rated torque speed is achieved at a reduced speed (Shakweh and Lewis 2002, p. 969). The control of the motor can be in different directions. The standard VCDs applications are in AC induction motors. The design consists of four elements:
The element primarily converts Alternating current (AC) to Direct currents (DC). They are of different designs which are defined with the kind of the VSD application. The design of the rectifier influences the electrical harmonics induced on the power supply. Rectifiers can also be used to control power flow (Benelghali, Benbouzid, and Charpentier 2012, p. 554).
The rectified DC supply is directed to the intermediate circuit with the help of inductors and conductors. Most drives on the market use a more of constant voltage DC-link.
The inverter does the opposite of a rectifier. It converts the rectified voltage and the DC back into an AC supply of the variable voltages and frequency. Generating a high-frequency pulse facilitates the outcome that goes along with the modulated variable frequency and the effective voltage. The output is created by the help of the semiconductors switches such as the _x0091_Insulated Gate Bipolar Transistor (IGBT) (Giannoutsos and Manias 2015, p. 1909).
The group assists in the regulation of the whole operation of the VSD. It controls and monitors the rectifier, the inverter, and the intermediate circuit. The control facilitates the delivery of the correct output in response to outward control signals.
Elements of a VSD
A VSD is an application of great semiconductors. It has features which include generator, rectifier, controller, motor, feedback, load side filter, inverter (Jayasinghe 2016).
The motor converts power from electrical to mechanical.
Converter regulates the power electronic converters to control the engine speed.
Filters suppress the harmonics of the system.
PWM-VSI Type VSD
These filters primarily help in the injection of harmonics in the power system and generator. The rectifiers generate the harmonics. This results in power loss in the windings of the generator. The effect leads to insulation degradation and rises in temperature. In most cases, the harmonics cause adverse effects to the connected sensitive load in the power system (Shakweh and Lewis 2002, p. 970).
The purpose of the feedback system is to sense the speed or current of the motor and then feeds them to the system converter to complete a closed loop control.
The function of the controller is to compare the real speed and monitor the output frequency of the inverter and the voltage to bring the error to 0. It resets the motor speed to point.
The diode bridge rectifier can not control the DC-link voltage. The rectifier only injects the harmonics into the power supply of the system. The power level of the bridge rectifier is high. The rating can even go up to tens of megawatts (Jayasinghe 2016).
Capabilities and Restrictions of the Diode Bridge Rectifier
The SCR can control the DC link voltage by controlling the firing angle. As a result, the supply voltage tends to vary with a limited range output voltage that can be regulated. The harmonic distortion needs a larger filter to sustain it when the line current increases. The level of power is same as that of the diode bridge rectifier (Jayasinghe 2016).
IGBT rectifier controls the dc link by switching _x0091_on_x0092_ and _x0091_off_x0092_ pattern. The trend is called pulse modulation.
There is a possibility to regulate the dc link with filter inductance. This is done by boosting the dc link voltage when the supply voltage drops. A small filter is required in this process since the current during harmonic distortions is low. At this point, the IGBT power level is always not high. The aspect makes it possible to achieve power up to several megawatts (Jayasinghe 2016).
Diode Fault in PWM- VSI Type VSD
The system has a graph representing speed and torque which shows that the open circuit diode failure does not affect the output. It is not always recommended to continue as it increases the level of stress and temperature in other diodes. The effects lead to failure of the diodes. It also leads to an imbalance in the supply that is not good for the generators.
In a current waveform, the A-phase does not assist in determining faults in diodes. It primarily helps in determining open circuit fault in diodes.
Power Converters in Electric Ship Propulsion
In traditional electric ship propulsion, the synchro converters and the cyclo converters were being used. Today, the PWM-VSIs are used.
The voltage source converter has various advantages. They include;
Less vibration, smooth torque, and less noise.
Drives both induction and synchronous motors.
Low harmonic injection into the load side and supply side; less filtering effort. The aspect assists in the lowering of the size of the filter as compared to that of the synchro converters and cyclo converters.
Operates at a wide range of the input voltage. The rectifier still boosts the dc link regardless of the drop in supply voltage. It works with a set value and mainly maintains the desired operation.
The output frequency is mostly higher than the system supply frequency. Demerits
High failure rate in dc links, which reduces the reliability of the converters.
Requires complex converter_x0092_s control for high, multilevel converters
Electromagnetic interference that results from high-frequency switching
Low power and voltage levels as compared to the cyclo converters and synchro converters.
Current Applications in Electric Propulsion
The electric propulsions find applications in electric ships that have different speed profile and wide loads. In cruise ships, the podded motor in electric propulsion is ideal (Jayasinghe 2016).
The propulsion is also applied in icebreakers and the floating platforms of offshore oil. It is also common in the shuttle tankers, car ferries, research ships and the cable laying ships.
Centre of gravity management
Reduces propeller cavitation
Electric propulsion is however not common in cargo ships. The reason behind is that the long sea routes tend to sail at a steady speed. The efficiency of the electric propulsion mostly depreciates with a full load. This is due to the losses of energy involved in the conversion of the mechanical power into electrical power then back to mechanical energy.
In cargo ships, there is no need of pleasing passengers, and also there is no need for high power burst (Jayasinghe 2016).
Power Converters in Shaft Generators
The inverters have on-controlled thyristors. Anode current is usually reduced to zero when turning off the thyristor. The power factor is 1 when a voltage is zero crossing. The current flow continues in thyristors when power lags with 0.8 power factor. This happens even after the voltage has gone through the zero crossing. The sequence of switching in inverters is affected by the trend (Boldea and Nasar 2016, p. 129).
The purpose of this system is to make current to be in phase with voltage in the thyristors. The shaft generator mainly provisions the load with the active power.
The synchronous motor provides the load with reactive power. The reactive power runs basically as a capacitive element. The reactive power is characterized with over-excitation in the fields (Jayasinghe 2016).
The synchronous compensator facilitates the commutation of the thyristors. It supplies the reactive power of the load. In this model, the sequence entails the synchronous compensator then follows the adding of the shaft generator to the bus (Giannoutsos and Manias 2015, p. 591).
Other systems build converters with controller devices. They include GTOs, IGBTs, and IGTCs. The advantages of building converters with such parameters include;
There is the possibility of boosting and regulating dc link.
It is easy to switch controlled devices on and off at very high frequencies of up to 20khz for low power devices and 2khz for the high power appliances.
There is reduced filtering and distortion harmonics due to high-frequency switching.
Synchronous compensators are not ideal in these settings since the reactive and active power to the load.
Converters with IGBT are always bidirectional. The system allows reverse power flow.
Power and voltage level in IGBT are not high
Multi-level converters topologies achieve high power levels.
Controlling modulation and multilevel converters are complex.
The system requires more IGBT in building the converters. The aspect increases costs due to power loss.
Power Converters in _x0093_Hybrid Shaft Generators_x0094_ (HSGs)
They are two types; variable speed system and fixed speed system. The power losses at a zero pitch at the speed of 145rpm in an engine with a constant speed is 900kW. The rate of the HSG drive can be reduced up to 90rpm, which brings power loss to approximately 200kW and thus saves 700Kw. The variable speed operation tends to improve the efficiency of the system and also reduces the fuel consumption and the emission (Jayasinghe 2016).
The system needs the HSG drive consisting of AC-DC-AC electronic converters. It also needs to use the PWM-VSCs for both the DC-AC converters and the AC-DC converters. The bi-directional power supply of the converters facilitates the power flow combinations in the system.
Power Flow Combinations in Hybrid Electric Propulsion
Diesel electric mode - This ideal for efficient lower speeds.
Consists of two auxiliary gensets that run at 50 percent power.
Part of the power is used in hotel loads, and the rest is used for the propulsion.
The shaft generator at this mode runs as a motor with the help of the HSG that controls the shaft speed.
_x000c_Parallel mode _x0096_ suitable for excess power
Efficient for running two engines; ideal when power required for the hotel load and propulsion exceeds the available supply from the gensets alone.
The auxiliary generator is in parallel with shaft generators, which feeds the electrical power supply.
The frequency of the system is kept constant at 60Hz even when the main engine is running at half power with the variable speed.
Ideal for optimum efficiency
Optimizes the propeller efficiency for the speed required
The main engine always runs at variable speed with shaft generators that supply the ship_x0092_s electrical needs.
The latter point enables the auxiliary generators to shut off.
Shore Connection Mode
Best for low fuel consumption
This is capable when the ship is at the coast.
It can connect to the standard shore power at a 50Hz frequency.
HSG power converter primarily converts the frequency to 60Hz which is the rating for ship power system.
This helps in avoiding blackout during the changeover from the shore to the ship power.
No need to run the auxiliary gensets, thus fuel-saving and reduces emission.
The model also reduces vibration and noise levels in the ship.
Boost mode _x0096_ ideal for maximum speed.
Harnesses most of the ship's power that includes the auxiliary generator sets and output for propulsions as a motor. The auxiliary engines run parallel with the main diesel engines.
This is an element of a VSD. It facilitates the conversion of the AC to AC direct power. The system has thyristors that point upwards, which assist in the provision of the positive output voltage. Those pointing downwards tend to provide a negative power output. The output frequency is lower than the frequency supplied. Cycloconverters find applications in low-speed machines with high power use (Jayasinghe 2016).
The system has side converter that controls the supply current to adjust the torque. The current flow is made smooth by the DC link inductors. The motor side converters are the first frequency controllers. In synchronous motors the thyristor fire in line with the rotational speed. In the load converters, thyristors turn off by the effects of the back e.m.f.
Two Level Voltage Source Converters (VSC)
This is a type of VSDs applicable in marine industry. They have Thyristor Bridge or Diode Bridge as line converters. It has DC link capacitors that smooth the DC link voltage. The load controllers control the frequency. The model has the pulse width modulation (PWM) that reduces the supply current harmonics as well as the motor current (Bucknall and Ciaramella 2010, p. 1508).
_x000c_Three Level VSCs
The two levels VCSs are not reliable when handling high voltages. The aspect makes it necessary to use multi-level VSCs. The most common are the diode-clamped 3-level converters. They are significantly used in high power appliance.
Note: the output current of a 3-level converter is smoother than that of the 2-level converter.
The thyristor is a general name that covers semiconductors such as triac, diac, and SCR.
The thyristor is just a solid device with two layers of N and P materials.
Mostly conducts current in one direction.
The only difference between diode and thyristor is that the latter has two purposes, unlike diodes that only control the direction of current flow in a circuit. Thyristors can be used as open switches and same time controls the flow of current in one direction. Thyristors can be utilized as rectifying diodes. The two roles are only achieved depending on how the gate of the thyristors is triggered (Jayasinghe 2016).
Thyristor has three terminals; anode, cathode, and gate. The gate controls the device. An electrical indicator applied to the gate terminal controls the current flow in the device.
The anode and the cathode act as power terminals. They handle the large voltage applied and conduct current to the instrument.
Thyristors primarily act as an on-off switch of a circuit. They switch on and off on a preset interval. The advantage of the thyristor to other switches is that it can withstand large voltages. Applying a positive polarity to the anode terminal of the cathode, then no current will flow. The condition is known as the forward blocking state. Current only flows when the ideal signal is applied to the gate terminal. The small potential in the anode-cathode terminal allows the device to be in a forward conduction state (Jayasinghe 2016).
Uses of Thyristors
They are used in controlling power in the output circuits.
They are used in VSC to convert a.c. power into power with different amplitudes and frequency. They do this process in converters part of a VSC.
The Silicon Controlled Rectifiers (SCR)
Shockley diodes have little application in engineering. They are mainly used for amplifying devices. They work in an on and off mode. Engineers refer them to SCR. Shockley diode is only made to be SCR by adding a third wire to the PNPN structure (Jayasinghe 2016).
Source: (Jayasinghe 2016).
An SCR behaves typically as a Shockley diode if its gate is disconnected. That is achieved by use of the break over voltage between the cathode and anode. The transistors tend to fall into a cutoff mode when the current is reduced. Another way of latching the Shockley diode is to connect directly the gate terminal to the base of the transistor on the lower side (Jayasinghe 2016).
An SCR, is primarily a Shockley diode fixed with an extra connection of the terminal, called gate added. The gate is used to trigger the appliance into conduction; a process called latching by the application of a low voltage.
Voltage is applied between the cathode and gate to fire the SCR, negative to the cathode and positive to the anode.
Testing SCR, a connection is linked between the anode and the gate that is sufficient in intensity, duration, and polarity.
Triggering of the gate terminals by initiating excessive voltage between the anode and the cathode may cause firing of the SCR.
Current falling can be used to turn off the SCR. The effect cam also is achieved by applying a negative voltage to the gate.
Reverse firing tends to be effective when the gate current is high.
Reverse triggering helps in turning off the Gate-Turn-Off thyristor (GTO). The GTO is a variant for SCR.
The terminals of the SCR can be identified by the help of a continuity meter.
SCR are ideal rectifiers. They only allow current to flow unidirectional. In this case, they are not ideal in full-wave AC power control unless they incorporate other devices.
TRIAC and DIAC
Triac consists of two transistors. They are connected in parallel but opposite directions. The same gate links the transistors. Triac has three main terminals; MT1, MT2 and the gate terminal (G). The generating pulses are usually applied between the gate terminals and the MT1.
Diagram of Triac
Source: (Jayasinghe 2016)
Construction of Triac
Triac is a three terminal device. The purpose of the gate terminal is that it acts as a control terminal. The current in triac can travel in both directions. The SCR is used an s a switch on both sides whereby it is connected anti-parallel.
Gate terminals and the MT1 terminals are always located close to each other. The triac can obstruct the polarities of both terminals when the gate terminal is open across the MT1 and MT2.
Characteristics of TRIAC
The Triac has two SCRs. They are fabricated in the opposite direction.
The operations of Triac in the first and third quadrants are similar but only for applied voltage and the flow of current.
Features of the V-I in a Triac in the third and first quadrant are same to that in the first quadrant in SCR.
Gate current defines the switching on of the triac_x0092_s supply voltage. The principle allows the regulation of the power in the load to change from zero to full power in a permanent and smooth manner and records no loss in the device control.
Source: (Jayasinghe 2016).
This is a bidirectional semiconductor switch. The device can be turned on both polarities. DIAC means _x0093_alternating diode current._x0094_
Zenner diodes are used to connect DIAC back to back. DIAC is used primarily for activating the TRIAC in AC switches, fluorescent lamps, and the dimmer applications.
Construction of DIAC
Source: (Jayasinghe 2016).
DIAC has two terminals with a combination of parallel layers of semiconductors. This allows activation in one direction. DIAC activates the TRIAC. The symbol of DIAC is the same as that of a transistor.
DIAC conducts after a _x0093_break-over_x0094_ voltage is exceeded. After it surpasses the break-over voltage, DIAC goes into a negative dynamic resistance of the region.
Source: (Jayasinghe 2016).
Characteristics of DIAC
The characteristics are in a letter z form. The shape is due to its symmetrical switching feature applied to each polarity of the voltage applied.
The performance of disc seems like an open circuit before its switching is exceeded. DIAC at that state performs until there is a decrease of current towards zero. DIAC has an abnormal construction that does not allow it to switch like Triac or SCR. It does not turn to a low voltage or current condition once in transmission (Hoevenaars, Evans, and Lawson 2010, p. 17).
DIAC often preserves a negative resistance characteristic. The feature means that the voltage tends to reduce by increasing current. DIAC does not maintain the low voltage as in triac and SCR unless the current falls to the point that level of holding current is an issue.
Source: (Jayasinghe 2016).
In summary, VSDs have wide applications in the marine industry. They assist in the control of the motors and thus make it easy for the systems to maintain required speed. They incorporate the use of thyristors such as the SCR and GTO to achieve the required results. The thyristors have functions such as acting as switches in high voltage system. VSD are economical since they save on power used in the power converters as compared to that of the traditional means of controlling speed and frequency in marine engines. VSD consists of elements that make it functional, they include inverters, converters, filter, and motor.
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