66 Frequency Converters 2.7.2 Scalar Control with Compensation + - f stator u stator f REF u 0 Δ f Ramp Voltage generator Δ U Load compen- sator I wirk Slip compen- sation Current compens. calculation Fig. 2.22 Structure Scalar type Frequency Converter with compensation When compared with simple control, FC with compensations adds three new control function blocks as illustrated in Fig. 2.22 Structure Scalar type Frequency Converter with compensation. The load compensator uses the current measurement to calculate the additional voltage (ΔU) required to compensate for the load on the motor shaft. The current is typically measured by means of a resistor (shunt) in the intermediate circuit.
It is assumed that the power in the intermediate circuit is equal to the power consumed by the motor. If several active switch positions are combined, these can be used to reconstruct all the phase current information. Basic features: • Voltage/frequency [U/f ] control with load and slip compensation • Control of voltage amplitude and frequency Typical shaft output: • Speed setting range 1:25 • Speed accuracy ±1% of rated frequency • Acceleration torque 40-90% of rated torque • Speed change response time 200-500 ms • Torque control response time Not available Inverter control