Control modes of the inverter

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Control modes of the inverter

1. General overview

  • Control modes:
    • V/F . control mode
    • Vector control mode
    • Torque control mode

Control modes of the inverter.

2. V/F . control mode

  • Most inverters have the default control mode of V/F. In this mode, the inverter will control the frequency linearly with the voltage.
  • In V/F control mode the inverter outputs voltage and frequency to control the speed of the motor. The inverter has no control over the actual speed of the motor.
  • Frequency control inverter in range 0Hz — Fmax. And corresponding voltage range 0V — Vmax. In practice, the frequency-to-voltage ratio is divided into three basic speed ranges:
    • From 0hz to F starting (Hz): Low input voltage, low starting current. The engine does not have enough torque to start. So voltage compensation is done resulting in a non-linear V/F ratio.
    • From starting F (Hz) to rated F (Hz): the V/F ratio is linear so the motor torque is stable.
    • Operating F (Hz) > Rated F (Hz): Voltage is limited at rated voltage to avoid breakdown of insulation between windings, and motor bearings.

Control modes of the inverter.

  • For asynchronous motors, the speed on the motor shaft always has a slip coefficient s. When the motor is idling, the slip coefficient s is very small so that the motor speed is close to the speed of the rotating magnetic field. When the engine is running under load, the coefficient of slip is large. V/F control mode will not be accurate so slip compensation is applied to improve the above factor.

3. Vector control mode

  • Open loop vector control and closed loop vector control have higher accuracy than V/F mode.

See more :

Torque control
Electric motors and applications
Synchronous system load balancing solution

3-1. Open loop vector control mode

Control modes of the inverter.

  • Open-loop vector control mode:  It is a control method based on the feedback current from the motor. The inverter will simulate a rotating magnetic field vector and control the frequency and voltage parameters. For precise control of motor speed.

3-2. Closed-loop vector control mode

Control modes of the inverter.

  • Closed-loop vector control mode: is a control method based on building the rotating magnetic field vector inside the motor. Determine the magnetic field speed based on the actual speed on the motor shaft through the encoder. Based on the feedback encoder signal, the inverter will control the frequency, voltage, and carrier frequency parameters to accurately control the motor speed.

4. Torque Control Mode

4-1. Formula setting

  • The formula for calculating Torque generated by the engine:
    • M = P/ω
  • Where M : motor torque (N.m), P : motor power (kW), ω : motor speed (rad/s)
  • The inverter drives the motor to run Torque, which means that the motor torque is controlled exactly as required. To do that, the inverter needs to calculate the load pulling force and acceleration/deceleration time so that the power P is constant. So when the Torque force is inversely proportional to the speed, it means that the heavy load runs at low speed and the light load runs at high speed.

4-2. Open loop torque control mode

Control modes of the inverter.

  • Open-loop Torque control mode: The inverter based on the motor’s feedback current and voltage will automatically calculate and interpolate the torque value at the motor shaft. Based on the initial torque setting value, the inverter will adjust the desired motor torque. Due to the nature of torque interpolation, the accuracy is not high. This mode only applies to applications that do not require high accuracy.

4-3. Closed-loop torque control mode

Control modes of the inverter.

  • Torque closed loop control mode: the inverter will rely on the motor’s feedback current and voltage and the speed based on the encoder signal from which it will accurately adjust the motor torque. This mode applies to applications requiring high accuracy.

5. Contact Control modes of the inverter

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