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 Mode of operation of a frequency inverter
A frequency inverter or AC-drive is a device to regulate/control the speed of a three-phase motor (asynchronous or squirrel cage motors).
Since the speed of the engine is determined by the frequency of the flowing current it would appear logical to regulate the motor speed via the frequency.
The frequency inverter not only changes the frequency but also the voltage applied to the motor. This ensures the required torque at the motor shaft without the motor overheating.
The block diagram of the frequency inverter drive is shown in the following:
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Modular mimic display of drive system
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Rectifier:
A 1-phase frequency inverter has 4 diodes and a 3-phase frequency inverter 6 diodes for the rectification of a fixed frequency input voltage to supply the DC-link.
The direct voltage DC-link:
The direct voltage for the inverter in the final stage is stored in several condensers with voltages of up to 800VDC. The surplus energy generated by regeneration is also stored in the DC-link. This is "generated" by the motor if a large centrifugal mass is suddenly brought to a standstill. Suitable transistors and braking resistances are needed to achieve high braking moments.
Inverter:
This part consists of 6 transistors which switch the DC-link direct voltage to produce a variable voltage and frequency to control the three-phase motor.
Control circuit:
The control units uses the information input by the user such as set speed to control the drive functionality and produce the necessary speed and torque at the motor shaft. It protects the drive whenever circumstances require and informs the user about the drive status.
The exact regulation of the frequency and voltage at the motor guarantees the user a high level of concordance between setpoints and actual values relating to the drive. Product features such as high standstill torques and positioning accuracy reflect the progress made over recent years.
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