Unit 335
How can you control the speed of a DC series motor?
A rheostat can be placed in parallel with the field winding to vary the field current and ultimately the speed.
What is a closed loop system?
Also referred to as a feedback control system. They record the output instead of input and modify it according to the need. It doesn't encounter any external or internal disturbances.
Advantages of ultrasonic proximity sensors
- Detect a variety of materials and surfaces - Can operate in a wide variety of climates
Applications of an image sensor
- Digital cameras - Camera phones - Medical imaging equipment - Thermal imaging devices
Application of an ultrasonic proximity sensor
- Distance measurement - Production line sensors - Tank levels
Applications of an open loop control system
- Electric hand drier - Washing machine - Toaster - Light switch
What is an encoder?
An electrical mechanical device that converts linear or rotary displacement into digital or pulse signals.
Disadvantages of SMPS
- Electrical noise: transient spikes can migrate into the rest of the circuit - Expert design required: ensuring it performs to the required specification - Cost: additional components required (capacitor, series switching element)
Disadvantages of positive feedback
- Error signal value is increased
Applications of a DC Shunt Motor
- Fans - Pumps - Small machine tools
Advantages of stepper motors
- Greater torque - Cheaper than servo motors - Offer a longer life - Excellent low speed torque - Excellent repeatability
Applications of stepper motors
- Hard disk drives - Robotics - Antennas - Telescopes - Toys
Advantages of SMPS
- High efficiency: little energy is dissipated - Compact: as a result of high efficiency and low levels of heat dissipation - Flexible technology: can be used to step up (boost) and step down (buck) voltage
Applications of a capacitive proximity sensor
- High/low liquid level - Material present/absent - Dry tank
Application of a closed loop control system
- Iron - Water level controller - Air conditioning system - Central heating
2 methods to identify current rating of connector
- Labelling - Diameter of the housing
What is a rotary encoder?
An electromechanical device that has two output signals, A and B, which issue pulses when the device is moved. The A and B signals indicate both the occurrence of and direction of movement. Does not indicate absolute position.
What is a motion sensor and the two different types?
An electronic device designed to detect and measure movement. 1 - Active motion sensor, 2 - Passive motion sensor.
What is a switched mode power supply (SMPS)?
An electronic power supply that incorporates a switching regulator to convert electrical power efficiently. Transfers power from a DC or AC source to DC loads
How does a permanent magnet stepper motor work?
When a stator is energised, it develops electromagnetic poles. The magnetic rotor aligns along the magnetic field of the stator. The other stator coil is then energised in the sequence so that the rotor moves and aligns itself to the new magnetic field. The rotor and stator are not teethed.
Operation of an offline UPS
When mains supply is ON - the load derives its power directly from the AC supply When mains supply is OFF - Inverter converts DC from the battery to AC which is fed to the load. This is done by the use of a static switch which immediately switches from one supply to another. The switching time is almost equal to 5ms.
How does a hybrid stepper motor work?
Combination of both permanent magnet and variable reluctance. Has a magnetic teethed rotor which better guides magnetic flux to the preferred location in the air gap. The magnetic rotor has two cups. One for the north poles and a second for the south poles. The rotor cups are designed so that north poles re arranged in an alternative manner. It rotates on the same principle of energising the stator coils in a sequence.
Line and phase voltages of a 3 phase 4 wire system
Line voltages = 415v (between each line conductor) Phase voltages = 230v (between line and neutral)
What is self excitation?
Produced by connecting the field winding with the armature.
Working principle of a PID controller
Proportional Tuning: corrects a target proportional to the difference. Target value never achieved because as difference reaches 0, so does the applied correction. Integral Tuning: increases correction factor from 'P' action. Drives cumulative error to 0, resulting in overshoot. Derivative Tuning: minimises overshoot by slowing correction factor applied as the target is approached.
What is a switched mode regulator?
Provides voltage regulation and can be used on its own or part of a power supply. Uses a series switching element to charge up a reservoir capacitor when the voltage falls below a predetermined level.
Disadvantages of photoelectric proximity sensors
Reflective - install at 2 points: sensor and reflector - slightly more costly than diffuse - sensing range is less than through-beam Through-beam - install at 2 points: emitter and receiver Diffuse - less accurate than through-beam or reflective - more setup time involved
Advantages of photoelectric proximity sensors
Reflective - slightly less accurate than through-beam - sensing range better than diffuse - very reliable Through-beam - most accurate - longest sensing range - very reliable Diffuse - only install at one point - cost less than reflective and through-beam
Unit substitution fault finding technique
Replacing a known faulty unit it with a working one
What is proportional band?
Represents the range of input over which the output is proportional to the input. Usually expressed as a percentage of the input between 0 and 100 percent.
Emergent sequence fault finding technique
The emergent problem refers to what has been changed or altered to cause other problems
DC Series Motor
The field and armature windings are in series. It has a high starting current which provides a high starting torque. As the speed of the motor increases, the armature current, field strength and torque reduce.
DC Shunt Motor
The field winding is connected across the armature winding. Torque only varies with changes in the armature current. The start up torque is less than an equivalent sized series motor. it is best employed where torque is not as important as speed.
Applications of temperature sensors
- AC systems - Food processing units - Medical devices - Chemical handling
Disadvantages of ultrasonic proximity sensors
- Blind zone at close proximity - Noise interference
Advantages of capacitive proximity sensor
- Can detect metallic and non-metallic targets - Lower cost - Has very good resolution and frequency response - High sensitivity and can be operational with a small magnitude of force
Disadvantages of inductive proximity sensors
- Can only detect metallic targets - Operating range may be limited
What documentation can be used to assist with fault finding?
- Circuit diagram - Fault finding charts - Service documentation - Operations and maintenance manuals
Applications of photoelectric proximity sensors
- Controlling doors, gates and corridors - Sorting by height detection - Counting and monitoring over long distances
Applications of a DC Series Motor
- Cranes - Lifts - Aircraft elevator controls
Advantages of negative feedback
- Decrease in output resistance - Bandwidth increases - Gain stability
Disadvantages of negative feedback
- Decrease in overall gain
What is an open loop control system?
Takes the input into consideration and does not react on the feedback to obtain the output. Non-feedback control system. There are no disturbances or variations in this system and it works on fixed conditions.
Applications of a TRIAC
- Light dimmer switch - Speed control for electric motors
Applications of a DIAC
- Light dimmers - AC applications - Starter circuit for flourescent lamps
Disadvantages of stepper motors
- Low efficiency - Torque declines rapidly with an increase in speed - Low accuracy - Can be very noisy
Applications of a displacement transducer
- Machine automation - Aerospace and aircraft - Power turbines - Hydraulics
Applications of SMPS
- Machine tool industries - Security systems - Support supplies with PLC's - Personal computers - Mobile phone chargers
Applications of an inductive proximity sensor
- Metal detectors - Traffic lights - Car washes
Advantages of a closed loop control system
- More accurate - Bandwidth range is large - Facilitates automation - Less affected by noise - Sensitivity can be reduced to make system more stable
Disadvantages of closed loop control system
- More expensive - Complicated to design - More maintenance - Overall gain is reduced due to presence of feedback
Applications of an SCR
- Power supplies for digital circuits - AC or DC motor speed controllers
Advantages of deadband
- Reduces false positive alarms - Easy to configure - Immediate activation of the alarm when deadband is only applied to the return to normal
Disadvantages of capacitive proximity sensor
- Sensitive to changes in environmental conditions such as temperature and humidity - Not as accurate as inductive proximity sensors
Examples of self excited DC motors
- Series motor - Shunt motor - Split field motor - Permanent magnet motor
Disadvantages of deadband
- Setting applied could be too large - Slow operator response on a real alarm when the deadband is applied to activation of the alarm.
Advantages of an open loop control system
- Simple in construction and design - Economical - Easy to maintain - Generally stable - Convenient to use as output is difficult to measure
Advantages of permanent magnet motors
- Small in size - Increased efficiency due to no copper losses as there is no field circuit
Advantages of positive feedback
- Stable - Maintain constant speed
Half split fault finding technique
- The expected input is examined for correct operational parameters - Should this prove okay then tests are made at the 3/4 stage of the system - Should this prove faulty then the system is checked at the 1/4 stage - At each test the system is further halved until the error is isolated
Disadvantages of permanent magnet motors
- The magnetic field is present at all times - Large armature currents can lead to demagnetisation of the poles
Types of temperature sensors
- Thermocouples - Resistance temperature detectors - Thermistors - Infrared sensors - Semiconductors - Thermometers
Disadvantages of an open loop control system
- They are inaccurate - They are unreliable - Any change in output cannot be corrected automatically
Applications of a permanent magnet motor
- Toy industries - Computer drives - Portable electric tools
Applications of an IGBT
- Variable frequency drives - Electric cars - Air conditioning units
Advantages of inductive proximity sensors
- Very accurate - Have a high switching rate - Can work in harsh environmental conditions
Six Point Fault Finding Technique
1 - Collect the evidence: observe machine running use senses consider environment 2 - Analyse evidence: think logically 3 - Locate fault: isolate using fault finding techniques 4 - Determine and remove cause: fault will reoccur if not 5 - Rectify fault 6 - Check system or machine: check the operation of the system prior to returning it to service
Methods to increase resolution of a permanent magnet stepper?
1 - Increase number of poles in rotor 2 - Increase number of phases 3 - Increase number of coils per phase
Methods to increase resolution of variable reluctance stepper
1 - Increase number of teeth in rotor 2 - Increase number of phases 3 - Increase number of coils per phase
What are the 3 types of stepper motor?
1 - Permanent magnet 2 - Variable Reluctance 3 - Hybrid
What are the steps to fault finding?
1 - Symptom: clue as to what has gone wrong 2 - Fault: a defect that causes the symptom to occur 3 - Cause: the reason why the fault has occurred 4 - Fault Location: the process that leads to the isolation of the fault or area of the fault 5 - the activity involved in the identification of the cause of fault
What are the connector ratings and index protection
16A = IP44 or IP67 32A = IP44 or IP67 63A = IP44 or IP67 125A = IP67
What is a stepper motor?
A brushless, synchronous motor which divides a full rotation into a number of steps. When the stators are energised, the rotor aligns itself with the stator or moves to have a minimum gap with the stator.
What is a temperature sensor?
A device that collects data about temperature from a particular source.
What is an uninterruptable power supply?
A device that provides an interruption free supply to sensitive electronic equipment.
How does a permanent magnet motor work?
A permanent magnet is used to create the magnetic field. The armature is placed inside the magnetic field of the permanent magnet and rotates in the direction of the generated force. Each conductor of the armature experiences a force and the compilation of those forces produces a torque which rotates the armature.
How does an image sensor work?
A sensor that detects and conveys information used to make an image. It converts the variable attenuation of light waves, as they pass through or reflect off objects, into signals, small bursts of current that convey the information.
What is separate excitation?
A separate external DC supply is used to provide exciting current through the field winding
How can the direction of rotation be controlled on a DC motor?
A split field motor is a reversible DC electric motor that has 2 separate field windings. One for clockwise rotation and the other for anticlockwise. The switch determines the direction of current through the coils. When the switch is one position, the polarity of the field will be in one direction and reverses when the switch is in the other position.
What is proportional integral derivative control?
A well-established way of driving a system towards a target position or level. Uses closed loop control feedback to keep the output from a process as close to the target or setpoint output as possible.
working principle of a capacitive proximity sensor
Consists of a high frequency oscillator and a sensing surface formed by two metal electrodes. When an object comes near the sensing surface, it enters the electrostatic field of the electrodes and changes the capacitance of the oscillator. The oscillator circuit starts oscillating and changes the output state of the sensor when it reaches a certain amplitude.
How does an optical transducer work?
Converts light rays into an electronic signal by measuring a physical quantity of light that is then translated into a form that is readable by an integrated measuring device.
Operation of single phase full wave rectifier circuit
D1 conducts during the positive half cycle and D2 conducts during the negative half cycle. This causes a phase inversion of the negative half cycle which provides a 100Hz pulse output. The filter capacitor charges the peaks of these pulses, resulting in a DC output.
How does an active sensor work?
Detects motion by measuring changes in the amount of sound or radiation reflecting back into the receiver. When an object interrupts the sensor's field, an electric pulse is sent to the embedded computer. Most common type of active motion detector uses ultrasonic sensor technology.
Working principle of a photoelectric proximity sensor
Detects objects directly in front of them by detecting the sensor's own transmitted light reflected back from the object's surface. There are 3 basic configurations: 1 - Reflective sensor: light beam is sent from an emitter, and bounced off a reflector back to a detector. No object present when light beam is able to reflect back. 2 - Through-beam sensor: light beam is sent from an emitter to receiver. Object detected when beam of light is broken. 3 - Diffuse: light beam is emitted and the object to be sensed functions as the reflector.
How does a passive sensor work?
Does not have a transmitter. Detects motion based on the increase of radiation in its environment. Most common type of passive motion sensor is a passive infrared (PIR) sensor. Designed to detect the infrared radiation emitted from the human body. The receiver is contained in a filter that only allows infrared to pass through it. When a person walks into the PIR sensors field of detection, the difference in radiation creates a positive charge within the receiver.
Differences between open and closed loop systems
Effect of output - OL acts on the basis of the input - CL considers output and alters it to the desired condition Stability - OL are mostly stable - CL are quite unstable Effect on gain - OL no affect on gain - CL no linear change in system gain Implementation - OL easy to construct and therefore easy to implement - CL complex structure, often difficult to implement Cost - OL overall cost is low. Less components needed, need good calibration but lesser power rating. - CL overall cost is high. More components needed, less calibration but higher power rating.
Starter protection for a DC series motor
High armature current is drawn on start up. Add a variable resistor in series with the armature winding for starter protection.
How is the speed of a DC shunt motor affected by the load?
In the absence of a physical load, the motor will not accelerate to destruction. As the field winding is connected in parallel with the armature winding, the field current and magnetic field will be constant and not vary with motor speed.
How are UPS systems classified?
On the basis of output power: - Low Power Rating UPS - High Power Rating UPS On the basis of working: - Online UPS -Offline UPS Line Interactive
Fault finding aids
Permanent: - Ammeters - Flow meters - Variety of sensors and indicators Ancillary: - Multimeter Fault finding charts: - Flow diagram (logical sequence steps) Historical documentation: - Planned preventative maintenance reports and results
How is the speed of a DC series motor affected by the load?
Small load = Higher speed Bigger load = Lower speed No load = Self destruction of the motor
Visual Examination fault finding technique
Smell - Burning Look - Visible faults Listen - Abnormal noise Feel - Vibration and heating effects
What is signal conditioning?
The manipulation of an analogue signal so that it meets the requirements of the next stage for further processing. It can include: - amplification - filtering - attenuation - converting - range matching - isolation
What is deadband?
The range through which an input can be varied without initiating an observable response. An interval of a signal domain or band where no action occurs.
Principle of operation of SMPS
The regulation is done by a switching regulator. A series switching element turns the current supply to a smoothing capacitor ON or OFF. If the voltage on the capacitor is higher than required then the switching element is turned off and vice-a-versa. This way the voltage is maintained.
How does a variable reluctance stepper motor work?
The rotor is teethed. The teeth of the rotor are designed so that when they are aligned to one phase, they get misaligned to the other. When the next stator is energised, the rotor moves to align its teeth with the next stator.
How can you control the speed of a DC shunt motor?
The strength of the magnetic field needs to be changed. A rheostat can be placed in series with the field winding to vary the current of the field.
Working principle of ultrasonic proximity sensors
They emit waves at a frequency too high for humans to hear. They then wait for the sound to be reflected back, calculating the distance based on the time required. 1 - Diffuse or Reflective sensor: When a target enters the sensing range of the device, the ultrasonic waves are reflected back to the sensor. 2 - Opposed or Through-beam: When an object enters the sensing range of an opposed signal, the transmitted signal is blocked. The trigger is activated when the signal is broken.
Operation of line interactive UPS
This is a combination of online and offline UPS. The inverter plays a dual role of charging the battery when the mains supply is present as well as regulating the output voltage and working as a normal inverter in the absence of the mains supply.
Operation of an IGBT (Insualted Gate Bipolar Transistor)
Turns on or off depending on if the gate is activated or deactivated. Constant voltage across the gate and emitter maintains IGBT in ON state. Current flows from collector to emitter. It switches off when the input signal is removed.
Definition of a fault
Underlying error that has caused the malfunction
Applications of a PID controller
Used in industrial control applications to regulate: - temperature - flow - pressure - speed
How does a displacement transducer work?
Used to convert the motion of an object or machine into electromagnetic, magnetoelectric or electrostatic signals which are read and interpreted into data.
Input to Output fault finding technique
Used to test known parameters. For example, inputting 100 units to a transfer unit which should multiply it by 2 and having an output of 175 units.
Delta circuit voltages and currents (formulas)
Vline = Vphase Iline = root 3 x Iphase
Star circuit voltages and currents (formulas)
Vline = root 3 x Vphase Iline = Iphase
Operation of a TRIAC
When MT2 gets a positive voltage with respect to MT1 and the gate gets a positive trigger then the left SCR of the triac completes the circuit. If the polarity of the terminals change and a negative pulse is applied to the gate then the right SCR completes the circuit. When the gate current is removed, the triac switches off. A minimum holding current must be maintained at the gate to keep the triac conducting.
Operation of an online UPS
When the mains supply is ON - the rectifier circuit converts AC to DC, which is fed to the battery and the inverter. The inverter converts DC to AC which is fed to the load. The load is isolated from the mains so there is no affect of interrupted power supply on the load. When the mains supply is OFF - the rectifier stops its operation as there is no incoming AC. The bank of batteries provides power to the inverter. Inverter converts DC to AC which is fed to the load.
What is positive feedback?
When the signal fed back from the output is in phase with the input signal. The gain of the positive feedback is increased and bandwidth is decreased. This feedback is used to make an oscillator.
What is negative feedback?
When the signal fed back is of opposite polarity or out of phase by 180 degrees with respect to the input signal. The gain of the negative feedback is decreased and the bandwidth is increased. This feedback is used in an amplifier circuit.
Voltage ranges for coloured connectors
Yellow = 100v - 130v Blue = 200v - 250v Red = 380v - 480v
working principle of an inductive proximity sensor
a coil is wound around an iron core within an electromagnetic field to form an inductive loop. When a ferromagnetic material is placed within the eddy current field, the inductance changes. The proximity sensor detects this change and produces an output voltage.
Definition of a malfunction
anything that is outside of normal operation of plant
Operation of a DIAC
conducts current after a certain amount of breakdown voltage is exceeded. When it occurs, the resistance of the component decreases which leads to a sharp decrease in voltage drop across the diac and an increase in current. The diac conducts until the current flow drops below the holding current. The diac returns to its high resistance, non-conducting state.
Operation of an SCR
conducts when forward biased (cathode negative and anode positive). When there is a positive clock pulse at the gate, the SCR turns on. Can only be turned off by either removing the supply voltage or by reducing the anode to cathode current externally (switch).