Scheme of an asynchronous electric drive with AIT - Electric drive

Scheme of an asynchronous electric drive with AIT

AITs possess the properties of a current source, feeding them from a current source. If the AIT is powered by a controlled rectifier, a choke with a large inductance is installed at its output. When AIT is used, the control actions on the AD are the stator frequency and current. In Fig. 5.41 presents a closed system of frequency-current control of AD with AIT.

Fig. 5.41

SUV - control scheme for SC thyristors; IMS - control circuit of thyristors AIT; RT - stator current regulator; DT - stator current sensor; DS - speed sensor; VO is an amplifier-limiter; FP - functional converter; - the reference signal - determines the switching frequency of the thyristors AIT VS1-VS2 and the stator current frequency. The signal is subtracted from the speed feedback - the signal is obtained, proportional to the relative frequency rotor , (fl - absolute slip),

The signal |, passing through the UO together with the signal , arrives at the output of the MIS. The frequency at the output of the MIS is determined by the signal The MIS is configured so that while the VO operates in the linear zone, the frequency at the AIT output is constant and independent of the load . The engine has rigid mechanical characteristics.

The signal after passing through the AF is also the master signal for the current control loop. Since is proportional to the absolute slip, then the current of the AD at all frequencies will be proportional to

When overloading or abrupt changes, The UO enters the zone of limiting the output signal , limiting the current. The engine operates at any speed in this mode with constant current and absolute slip, that is, the mechanical characteristic becomes absolutely soft (Figure 5.42).

When braking AD with energy recovery to the network, the AIT switches to rectifier mode, and the HC to the network driven inverter mode. In DC with a DC link, inverters with pulse width modulation (PWM) are used that allow wide regulation of voltage and frequency and provide an output current of sinusoidal shape. When using inverters with PWM in the IF circuit, an uncontrolled rectifier can be used.

Fig. 5.42

These PWs also use pulse width voltage regulators (WIRs) that are installed between the DC power source and the inverter. The BIR scheme provides a wide range of voltage regulation over the entire frequency control range. The SHIR circuit is shown in Fig. 5.43: VS 1 - the main thyristor; VS2 - auxiliary thyristor; - the limiting reactor; - switching circuit; VD - an uncontrolled diode.

When a control pulse is applied to the KS4 from the SIPF, it opens and the load of the power supply is applied to the load . To disconnect the load from the power supply, an impulse is applied to the K52 and is removed from the VS 1, then the switching circuit thyristor VS is closed. The average voltage on the load is proportional to the duty ratio of the thyristor VSI.

Thus, voltage is regulated within wide limits.

In the open IF-AD systems, the speed control range is 5-10, and in closed (with feedbacks) reaches 1000 or more. Application of IF-AD: high-speed systems (electrospindles, aerotube fans, etc.).

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