Distributed Energy Engineering (IKE1002) Part5: Frequency Converter
Energy growth 2007-2030 by IEA
World average Energy efficiency potential Electrical energy needed to produce 1 USD in GNP Midle-East China USA India Brasil Russia Germany Japan
Energy efficiency Only 20 % of primary energy is used to do the actual work
Global wind power market 2007-13 Source: MAKE Consulting March 2009
Global solar power market 2007-13 Source: EPIA 2009
The technology The 3-phase voltage in the national electrical grid connected to a motor creates a rotating magnetic field in it. The rotor of the electrical motor will follow this rotating magnetic field. The speed is controlled by changing the frequency of the electrical supply to the motor using frequency controller An frequency controller converts the frequency of the network to anything between 0 to 300 Hz or even higher, and thus controls the speed of motor proportionally to the frequency.
The technology Rectifier unit The AC drive is supplied by the electrical network via a rectifier. The rectifier unit can be uni- or bidirectional. When unidirectional, the AC drive can accelerate and run the motor by taking energy from the network. If bidirectional, the AC drive can also take the mechanical rotation energy from the motor and process and feed it back to the electrical network. DC circuit The DC circuit will store the electrical energy from the rectifier for the inverter to use. In most cases, the energy is stored in high-power capacitors. Inverter unit The inverter unit takes the electrical energy from the DC circuit and supplies it to the motor. The inverter uses modulation techniques to create the needed 3- phase AC voltage output for the motor. The frequency can be adjusted to match the need of the process. The higher the frequency of the output voltage is, the higher the speed of the motor, and thus, the output of the process.
The benefits Adjusting speed as a means of controlling a process Smoother operation Acceleration control Different operating speed for each process Compensate for changing process variables Allow slow operation for setup purposes Adjust the rate of production Allow accurate positioning Control torque or tension System stress Reducing the start-up current, which allows use of smaller fuses and supply connections and reduces peak loads on the electrical network Reducing the mechanical shock in start and stop situations
The benefits Adjusting speed as a means of controlling a process An AC drive often uses less energy than an alternative fixed speed mode of operation. Fans and pumps are the most common energy saving applications. In these applications, energy savings are typically 20-50% Fans and pumps Using an AC drive to control the fan or pump output rather than using dampers, vanes, valves or on/off control brings substantial energy savings, if the required output is less than nominal most of the time. The AC drive controls the speed of the pump and fan by changing the electrical energy supplied rather than damping the air- or water flow. It is like reducing the speed of a car by pressing less on the accelerator instead of using the brake to slow down the speed. The payback time of an AC drive is typically one year or less. Graph 1: Electrical power consumed by pump at partial loads is significantly less with an AC drive than with valve or on/off control. Other benefits : Smooth ramp up and ramp down causes less stress to the mechanics of fans and pumps and to air ducts and water piping Slowing down the speed rather than damping the output will result in lower noise levels
The benefits Compressors Compressors in HVAC are often used in chillers for cooling water, which again is used for cooling air. Utilising AC drives in compressor applications will potentially bring energy savings compared to on/off control. Reduced number of starts and stops reduces the wear of the compressor Possibility to use high speed compressors
The benefits Windturbines Modern windmills can be used to convert energy from wind into usable electricity for the electricity grid. Frequency controller includes Yaw control and pitch control Allows low voltage ride-throughs and micro-grids
The benefits Solar power inverter Photocells produces DC-voltage. This energy is fed to grid using inverter Several units are working parallel in order to achiev required power output Only the necessary amount of units are run at the same time
Motor torque charasteristic From previous lectures we know the torque charasteristic of motor Depending on the value of slip the motor can act as a Motor, Brake or generator Synchoronous speed depends on polepair number The maximum torque is reached just below nominal speed. Speed variation is only 0-5% depending on load As brake M Motor Generator M L 2 1 0 n M -1 s -n s n ML M 0 M L M +n s +2n s n n
Motor torque charasteristic Load torque T L usually increases with speed. Depending on the application it can be linear or quadratic. The motor will automatically accelerate until the load torque and motor torque are equal. This point is shown on the graph as the intersection of Tm and T L.
Motor torque charasteristic In principle torque characteristic depends only on slip(difference between supply frequency and rotor speed. Characteristic is about the same what ever the supply frequency is. By changing the supply frequency also the motor speed is changed.
Modules in frequency converter Incoming unit Fuses and disconnector for connection and protection against the grid. Filter and charging unit - to filter harmonics and to load DC-bus capacitor. Supply unit - six pulse diode unit to rectify grid 3 phase voltage to DC voltage. With electrical vehicles the dc voltage comes from accumulator. No supply unit is needed Chopper and resistor to allow active braking, if supply back to grid is not available Inverter to generate 3-phase voltage from DC voltage. Photovoltaic systems generate directly DC-voltage. Motor side inverter is not needed, but supply unit (diode bridge) is changed to inverter unit.
Switch model Inverter is 3 switches, which can be connected to + or side of the DC-voltage 1. Diode conducts Switch is at - side 2. Upper IGBT is opened diode is going to shut down itself 3. IGBT conduct alone switch is at + side
Fourier series Each periodical signal can be represented as a series of sine waves http://www.hersheyenergy.com/harmonics.html
One phase inverter Pulses approximate sine wave more pulses means more accurate sine wave (less harmonics) Amplitude and frequency can be controlled with pulse widths Current waveform depends on load (R, L, C)
Frequency converter V R, V S and V T get values +U d /2 or -U d /2 if they are connected to + or - side Main voltage is voltage between connection points. For example U RS =V R - V S At star connection phase voltage is 1/3 or 2/3 of Ud depending which phase is alone
Frequency converter At star connection phase voltage is 1/3 or 2/3 of Ud depending which phase is alone. Delta connection can be handled as equivalent star
Ac-circuit Impedance -Flux For inductance and flux we got U = L*ω * I Φ = L* I U = ω *Φ U Φ = ω Nominal flux requires that ratio is kept constant U ω
Inverter operating principles Required supply frequency is generated with proper pulse width modulation Nominal torque is kept available by keeping nominal flux. Ratio is kept constant U ω Voltage is kept as close to sine wave as possible. This is achieved by having switching frequency khz level. Clean sine wave means also that the generated current is close to sine wave.
Voltage regulation Required voltage level is achieved by switching also zero voltage to the motor. This occurs, when all switches are connected to same DC-level, + or side U ω At nominal frequency full voltage is supplied. There are no more any excess zero voltages. With higer frequencies the nominal flux can not be sustained
RI compensation At speed 1% of nominal also the voltage is in principle only 1 %, but current is nominal if required torque is nominal Voltage losses at stator resistance are nominal. U=R*I This voltage drop has to be compensated in order to keep the flux constant.
Examples of waveforms Motor phase voltage at 5 Hz Motor phase voltage at 50 Hz
Motor current Motor phase current at 50 Hz Current drawn from the grid. Diode bridge supplies the DCcircuit