Electric Motors Lecture B5 PDF

Dr Gianfranco Claudio & Dr Tim Harrison WSC353 INTERFACING FOR MECHATRONIC SYSTEMS Lecture plan Sensors (1) 31st Oct 14:00 Sensors (2) 7th Nov 14:00 Strain Gauges, Charge Amplifiers, 14th Nov 14:00 Sensor Networks & Relays Actuators (1)...

Dr Gianfranco Claudio & Dr Tim Harrison WSC353 INTERFACING FOR MECHATRONIC SYSTEMS Lecture plan Sensors (1) 31st Oct 14:00 Sensors (2) 7th Nov 14:00 Strain Gauges, Charge Amplifiers, 14th Nov 14:00 Sensor Networks & Relays Actuators (1) 21st Nov 14:00 Actuators (2) 28th Nov 14:00 Thermal and Noise considerations 5th Dec 14:00 Some practical examples 12th Dec 14:00 Part B: Practical Considerations Lecture B5: Electric motors Learning Outcomes Understand the range of different motors available Understand basics of how each type works Understand how to control motors Understand the advantages and limitations of the different types  Overview Microcontroller Analogue Conversion Filter / De- Glitcher Analogue Electronics Amplifier Actuation Actuator Gearing / Linkage Environment  Overview All types use an electric current in a coil to produce a magnetic field That magnetic field interacts with a second magnetic field to give a force/torque The force/torque accelerates the motor to give motion  Electric motors ACTUATOR SYSTEM Energy Power source supply Power Actuator amplifier From controller To physical system DC motor - introduction Speed proportional to voltage Torque proportional to current Small motors use a permanent magnet Larger motors use an electromagnet Shunt Series Compound Separately excited Science buddies https://www.youtube.com/watch?v=WI0pGk0MMhg DC motor Simple single pole motor N Not a practical motor! Not self-starting Torque not smooth N S DC motor Multi-pole motor - minimum 3 pole N (often more) S DC motor Multi-pole motor - minimum 3 pole N (often more) Self starts S Direction becomes certain & reverses with N power supply polarity S DC motor Multi-pole motor - minimum 3 pole N (often more) Self starts Direction becomes certain & reverses with power supply polarity S DC motor Multi-pole motor - minimum 3 pole N (often more) Self starts Direction becomes certain & reverses with power supply polarity S DC motor Multi-pole motor - minimum 3 pole N (often more) Self starts S Direction becomes certain & reverses with N power supply polarity Smoother torque S Need a way to switch the poles on and off with rotation  DC motor Multi-pole motor - minimum 3 pole (often more) Self starts Direction becomes certain & reverses with power supply polarity Smoother torque Need a way to switch the poles on and off with rotation Commutator & brushes GIF from electrical4u.com DC motor – sub types Small motors use permanent magnets Larger motors use electromagnets for the stator Series wound – rotor and stator coils connected in series Shunt wound – rotor and stator coils connected in parallel Compound – several stator coils, different series/parallel arrangements Separately excited – rotor and stator coils separately supplied and controlled. DC motor – series Rotor (Armature) and Stator (Field) are wired in series High starting torque, low torque at high speed Poor speed regulation speed will change as load changes will overspeed when not connected to a load Used in stop-start applications where high starting torque is useful, e.g. Winches, hoists etc. DC motor – shunt Rotor (Armature) and Stator (Field) are wired in parallel Speed is stable with varying load Used where continuous constant speed running is required e.g. Pumps Conveyor belts Machine tools – milling machines, lathes DC motor – compound Both series and parallel stator coils are present The two fields interact differently depending on current drawn (remember current is proportional to torque) Series coil with fewer thicker windings Parallel coil with more thinner windings Combine the advantages of shunt and series motors: Good starting torque good speed regulation Don’t have the no load overspeed problem of series motors DC motor – separately excited (Sepex) Separate power supply for the rotor and stator coils Much better control over the torque and speed characteristics of the motor Requires a more “active” control system Permanent magnet, shunt, series and compound motors can be turned on and will run in a manner that is a function of their physical characteristics/design Sepex motors require adjustment of the rotor and stator supplies for optimum performance DC motors – summary Motor type Power Speed Staring Control capability control Torque complexity Permanent Low Good Good Simple magnet Series High Poor Good Simple Shunt High Good Poor Simple Compound High Intermediate Intermediate Simple Sepex High Good Good More complex DC motor – basic control We most likely to want to control motor speed Apply a voltage N V = IR R is fixed, so current is proportional to voltage Magnetic field strength is proportional to current Force/Torque is proportional to magnetic field strength Motor accelerates and begins to turn 𝐹𝐹 = 𝑚𝑚𝑚𝑚 rotational form is 𝑇𝑇 = 𝐽𝐽𝜔𝜔̇ S Does it accelerate to infinite speed? DC motor – basic control Balancing speed As the motor accelerates, there is resistance to the rotation caused by friction, the load etc. N This is (mostly) proportional to speed Torque balance: 𝑇𝑇𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 = 𝑇𝑇𝑎𝑎𝑎𝑎𝑎𝑎 + 𝑇𝑇𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 As speed increases, the 𝑇𝑇𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 increases S Hence the torque available for acceleration (𝑇𝑇𝑎𝑎𝑎𝑎𝑎𝑎 ) reduces to zero (when 𝑇𝑇𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 = 𝑇𝑇𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 ) DC motor – basic control For a given set of motor properties Speed is proportional to voltage and inversely N proportional to load 𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 ∝ 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 For constant load, speed is proportional to voltage Open loop control is possible S If load is varying, then we need some form of closed loop speed control AC motor - introduction Speed proportional to AC frequency (mostly!) Torque proportional to current Universal motor Small motors use permanent magnets Synchronous Larger motors use electromagnets Synchronous DC field windings Asynchronous/Induction 3-phase Single phase AC motor - Universal What happens if we apply AC to a DC motor? Oscillation! N But if we use an electromagnet, then the magnetic field alternates in phase and the motor will work The series wound DC motor is often called a universal motor and will work on AC and DC It is the most common AC motor type for low power S single phase motors, e.g. Hair dryers, vacuum cleaners Similar properties to a DC series motor Speed in proportion to voltage High starting torque, but poor speed regulation AC motor - Synchronous 1 Needs a 3 phase AC supply S Each phase connected to separate coils Gives a rotating magnetic field N The rotor is a permanent magnet which follows the rotating field 1 Rotor speed is equal or synchronous with the AC supply frequency But at high loads, “slip” or “cogging” can occur as the rotor drops behind the field rotation AC motor - Synchronous 1 The rotor can be an electromagnet, but this must be supplied with DC S Starting can be a problem Field instantly rotates at supply frequency N Small motors should be ok, but, 1 If the rotor has a high inertia or is loaded, then it will not get in sync with the magnetic field Used where constant speed is required AC motor – Synchronous single phase Synchronous motors can be made to run on single phase AC, but They will not self start so a starting circuit is required: Split phase Capacitor start Shaded pole The starting circuit kick starts the motor in the required direction before cutting out and leaving the main circuit to run the motor  AC motor – Asynchronous / Induction 1 Rather than relying on a magnetic rotor, induction motors rely on the rotating magnetic field inducing a current and therefore a magnetic field in the rotor 1 1 The strength of the magnetic field induced in the rotor depends on there being relative 1 motion between the rotating field and the rotor large speed difference leads to stronger core field which leads to high torque  AC motor – Asynchronous / Induction 1 So: High starting torque, but As the rotor speed increases, torque reduces As the rotor speed approaches the field 1 1 rotational speed, the torque approaches zero 1 The rotor speed will never quite match the field speed, hence asynchronous Rotor speed typically >95% of the field speed Speed difference often called slip  AC motor – Asynchronous / Induction 1 Advantages: Self starting No permanent magnet & no electrical connections to the rotor 1 1 Disadvantages Precise speed control can be difficult. At high load the “slip” between field and rotor speed will be 1 greater High “inrush” current on starting Uses Any high power application where precise speed control is not important  AC motors – summary Motor type Single or Power Speed Staring Self 3 phase control Torque Starting Universal Single Low Intermediate Intermediate Yes Synchronous Either Medium Good Low Yes for 3 (permanent magnet) phase Synchronous Single Medium Good Low No (single phase) Synchronous Three High Good Low Poor for (3 phase) large motors Asynchronous Three High Poor High Yes  Motor speed control DC motor AC motor Speed proportional to voltage Speed proportional to AC frequency Needs some form of variable Needs some form of variable voltage control system frequency control system Motor speed control Earliest systems used DC AC power transmission is more efficient and became more common. AC can be transformed to any voltage and then rectified to DC From 1960s onwards, higher power variable frequency drives have advanced and become available  DC Motor speed control - resistive M M Switched resistors Variable resistor Simple but both methods waste power by dissipation through resistors No longer used for high power applications Variable resistors still very common for low power applications – e.g. model railways  DC Motor speed control – series parallel Only works when we have multiple motors to run at the same conditions e.g. a train M M M M Start with all motors in series. ¼ voltage with full current gives low speed and high starting torque Switch to series parallel. ½ voltage and ½ current to all motors. Higher speed, lower torque M M M M Switch again to full parallel. Full voltage and ¼ current. High speed, but low torque No power dissipation through resistances only 3 speeds (more speeds available with M M M M more motors) Complex relay network to do the switching  DC Motor speed control – tap changer Rectify AC to DC AC ~ M As AC power distribution becomes more prevalent Different “taps” on a transformer give different AC voltages Rectify to DC for the motor Reduced power loss due to dissipation Stepped control with several discrete voltages DC Motor speed control – PWM As high power electrical switching becomes commonplace (1960s onwards): A DC voltage is switched on and off (a few KHz to a few MHz) Apparent voltage is proportional to duty cycle Fully variable voltage Some power dissipation by the power electronics  DC Motor speed control – Summary Motor type Variability AC or DC Power Suitable Used for supply dissipation for: today Switched Stepped Both High Any power Rarely resistive Variable Variable Both High Lower Low power resistor power Tap changer Stepped AC Low High power High power PWM Variable Both Medium Any power Any power  AC Motor speed control – Variable Frequency Early AC motors were limited to operating at the frequency of the AC power supply As high power electrical switching becomes commonplace (1960s onwards): AC input rectified to DC Variable Frequency Drive (VFD) DC voltage is switched on and off Fully variable frequency and voltage (a few KHz to a few MHz) to Some power dissipation by the power replicate the AC waveform electronics Thyristors (GTO Gate Turn Off) Transistors (IGBT Insulated Gate Bi- polar Transistor)  Motor speed control DC motor AC motor Speed proportional to voltage Speed proportional to AC frequency DC timeline AC timeline 150 years ago 100 years ago