WEG CFW300 V1.3X, Programming Manual

Page: 63 / 157

V/f Scalar Control
CFW300 | 9-1
9
9 V/f SCALAR CONTROL
This is the classical control method for three-phase induction motors, based on a curve that relates output
frequency and voltage. The inverter works as a variable frequency and voltage source, generating a combination
of voltage and frequency according to the configured curve. It is possible to adjust this curve for standard 50 Hz,
60 Hz or special motors.
According to the block diagram of Figure 9.1 on page 9-2 , the frequency reference f* is limited by P133 and P134
and applied to the input of "V/f Curve" block, where the output voltage amplitude and frequency imposed to the
motor are obtained. For further details on the frequency reference, refer to
Chapter 7 LOGICAL COMMAND AND
FREQUENCY REFERENCE on page 7-1 .
By monitoring the total and active output current, and the DC link voltage, compensators and regulators are
implanted so as to help in the protection and performance of the V/f control. The operation and parameterization of
those blocks are detailed in
Section 11.2 DC LINK VOLTAGE AND OUTPUT CURRENT LIMITATION on page 11-3 .
The advantage of the V/f control is its simplicity and the need of few settings. The start-up is quick and simple
and the factory default, in general, requires little or no modification. In cases whose objective is to reduce losses
on the motor and inverter, the "Quadratic V/f" may be used, where the flow in the motor air-gap is proportional to
the output frequency up to the field weakening point (also defined by P142 and P145). Thus, the result is a torque
capacity as a quadratic function of the frequency. The great advantage of such control is the capacity to save
energy when driving loads with variable resistant torque, due to the reduction of motor losses (especially losses
in the air-gar, magnetic losses).
The V/f or scalar control is recommended for the following cases:
„
Drive of several motors with the same inverter (multi-motor drive).
„
Energy saving in the drive of loads with quadratic torque/frequency relationship.
„
Motor rated current lower than 1/3 of the inverter rated current.
„
For test purposes, the inverter is turned on without motor or with a small motor with no load.
„
Applications where the load connected to the inverter is not a three-phase induction motor.
„
Applications that aim at reducing losses on the motor and inverter (Quadratic V/f).