I. External Main Circuit Structure
The external main circuit of the frequency converter is shown in Figure 1. The three-phase AC power supply is connected to the power input terminals R, S, and T of the frequency converter via circuit breaker QF and AC contactor KM. The output terminals U, V, and W of the frequency converter are directly connected to the motor, and the motor protection is handled by the frequency converter. The circuit breaker serves two purposes: first, it disconnects the frequency converter from the power supply when it is not in use or under maintenance; second, it provides overcurrent and undervoltage protection, offering some protection to the frequency converter. The contactor allows for convenient control of the frequency converter's power supply via push-button switches, and it automatically disconnects the power supply to the frequency converter in case of a fault in the frequency converter or related control circuits.
Figure 1 External main circuit of the frequency converter
II. Selection of relevant components
Whether an AC contactor is needed between the inverter output and the motor depends on the specific application environment. Generally, when one inverter controls one motor and switching between inverter and mains frequency is not required, a contactor should not be used between the inverter and the motor, as shown in Figure 1. However, when one inverter drives multiple motors, each motor must have its own dedicated contactor, and a suitable thermal relay FR should be selected to protect the motor, as shown in Figure 2. Sometimes, although one inverter drives only one motor, it may switch between inverter and mains frequency operation. In this case, a contactor KM3 and a thermal relay FR should be configured between the inverter and the motor, as shown in Figure 3. Contactor KM3 is used to disconnect the inverter output from the power supply when the motor is running at mains frequency; the thermal relay FR protects the motor when running at mains frequency.
Figure 2 shows a frequency converter driving multiple motors.
Figure 3 Switching between frequency converter and mains frequency
III. Permissible Distance Between Frequency Converter and Motor
While the output voltage of a frequency converter is advertised as sinusoidal alternating current, it actually outputs a sequence of voltage pulses with a frequency equal to the carrier frequency (approximately several kHz to 20 kHz) and an amplitude equal to the average voltage of the DC circuit. When the connection line between the frequency converter and the motor is long, the distributed inductance and capacitance of the wires become significant. The inter-wire capacitance and the motor's leakage inductance may approach a resonant point, potentially causing the motor's input voltage to be too high, damaging the motor or causing vibration during operation. Therefore, the permissible distance (permissible wire length) between the frequency converter and the motor is limited. Because different frequency converters employ different internal technologies, their permissible distances also vary.
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