65 Frequency Converters The speed setting ranges associated with the individual FC types are roughly as follows: • Simple (scalar) without compensation 1:15 • Scalar with compensation 1:25 • Space vector 1:100(0) • Flux (fi eld-oriented) open loop 1:1000 • Flux (fi eld-oriented) close loop 1:10.000 • Servo 1:10.000 The torque control performance can be classifi ed as follows:
• The reaction time may be defi ned in the same way as for speed control • The accuracy is determined in relation to the motor’s rated torque Please note that FC’s that rely on a simple control method cannot be used for either open-loop or closed-loop control of the motor torque 2.7.1 Simple control method This type of control is rarely used today. The control is in principle a fi xed relationship between desired motor speed and a motor voltage. The model can be more or less refi ned, but the major disadvantages are:
• Unstable motor speed • Diffi cult start of the motor • No protection of the motor The only advantage of simple control could be the low price, but since basic components for sensing motor are relatively low-cost, very few manufacturers now pursue this method.