If a appropriate Ac induction motor is linked directly to the mains furnish has the following limitations:
Uncontrolled speed
Very high starting current
Uncontrolled torque
Uncontrolled acceleration
No braking (stopping control)
The function of an Ac inverter drive is to overcome these limitations.
Speed control
You can't change the frequency of the incoming 3-phase power furnish as this is fixed by the rotational speed of the generator that produces it, but by using an Ac inverter, you can change the frequency of the furnish to the motor.
The rated speed of an Ac induction motor is the rotational speed of the stator field less the slip at rated load. A appropriate 4 pole motor run from a 50Hz furnish with 2% slip will rotate at 1470rpm. Ac inverters operate the speed of the motor by varying the furnish frequency to the motor. If you halve the furnish frequency, you halve the speed of the motor, and so on.
Input bridge rectifier & Dc Link
An Ac inverter is a variable frequency 3 phase furnish generator. The incoming fixed frequency furnish is linked to a 3 phase diode bridge which converts the Ac furnish into a Dc voltage. This Dc voltage on the production of the diode bridge is linked over a large high voltage capacitor. The capacitor is called a Dc link and acts as a stockroom of charge. Payment accumulates in the capacitor until the voltage over it is equal to the peak of the voltage in the middle of any pair of 3 phases, nearly 600Vdc for a 415Vac supply.
Inrush circuit
The inrush circuit is used to limit the current, at power up, which flows into the Dc link capacitors. These capacitors look like a short circuit to Dc at power up, so without the inrush circuit, a huge current would flow from the supply, through the bridge rectifier into the capacitors. This current could cause fuses to blow and could cause damage to the bridge rectifiers and capacitors. Once the Dc link capacitors have charged, a relay over the resistors closes to short out them out. If the resistors were left in circuit, they would get highly hot or even burn out when the drive is being loaded by the motor.
Dynamic Brake Chopper
When the rotor of a motor turns faster than the synchronous speed set by a drive output, the motor is transforming mechanical vigor from the motor shaft into electrical energy. This health is referred to as 'regeneration".
Regeneration may be caused by a drive ramp to stop or a discount in commanded speed on a high inertia load or an overhauling load that causes the shaft speed to be greater than the synchronous speed.
If the motor is regenerating, the Dc bus voltage will increase. Unless a recipe of dealing with the regenerative vigor is provided the drive will safe itself with a Dc Bus overvoltage trip, thus stopping the regenerative condition. The braking chopper allows a braking resistor to be linked over the Dc bus of the inverter thus dissipating the regenerated vigor as heat rather then the drive tripping on a Dc bus overvoltage.
Output transistor bridge
Connected over the Dc furnish is a circuit which turns the Dc back into Ac. This process is called inversion hence the name 'Inverter'. The inversion circuit consists of 6 high speed, high voltage transistors arranged as a 3 phase bridge. The arrangement is the same as for the input diode bridge but with transistors instead of diodes, except now the input is Dc and the production is Ac. The transistors are used to join together the + and - of the Dc link to the 3 production phases. The transistors are switched on and off in a constantly repeating pattern to produce a synthesised Ac waveform.
Current measurement
Either on the production phases of the drive or on the Dc bus, or even a combination of both, current determination devices are fitted to part the current being taken by the load and feed this data back to the operate hardware. This data can be used to make sure that the production current from the drive stays within the drives rating and also to safe the drive if a short circuit occurs either within the drives production stage or on the drives production terminals, motor cabling or motor itself.
Pwm - pulse width modulation
The production voltage waveform looks nothing like a 3 phase supply. The pulses are in groups alternately certain and negative as the Dc link is linked first one way round and then the other way round. Within each group, the pulses gently vary in width production the connections for longer or shorter times. If a motor is linked to the inverter then the production voltage waveform of the inverter will be impressed over the motor stator windings. Because the motor is an inductive load, the current waveforms that supervene are almost sinusoidal. This kind of inverter in which the production pulses are of constant amplitude (the Dc link voltage), but vary in width, is called a Pulse Width Modulation Inverter (Pwm).
Switching
The rate at which the pulse pattern alternates in the middle of certain and negative and then certain again is the frequency of the output. Within each cycle of certain and negative, the pulse widths average to a sine voltage waveform with narrow pulses for low production volts and wide pulses at the peak. The more pulses in a cycle for a given production frequency, the good the average voltage. During a pulse, the Dc link is linked to the production so the motor current rises, in the middle of pulses the motor current falls. The closer the pulses are together, the smaller the current steps and therefore the current approximates good to a sine wave. Deviations from a excellent sine wave cause power losses in the motor so it gets hotter, make the motor noisy and create interference.
Switching wastes power
Each time a transistor switches, it has to dissipate some power as heat. To get more pulses, you need to switch faster so switching is minute to the type of transistor used and the maximum power it can dissipate. Most drives now used Igbts - Insulated Gate Bipolar Transistors which are fast switching, high frequency, low loss devices. The switching pattern is generated automatically by the electronics within the drive which controls which transistor is switched, when and for how long. All the user has to do is tell the drive what production frequency is required and the inverter electronics does the rest.