Azionamenti elettrici - Controllo di corrente

Facoltà di ingegneria

Università degli studi di Pavia

Corso di laurea magistrale

Control diagram

Fig.1

• v- is a triangular voltage which sets the switching frequency of the inverter.
• S.C. is a static comparator.

Note that the signals ua, ub, uc command the static switches of the inverter by connecting the phase of the motor to the positive value of voltage if u = 1, or to the negative value if u = 0.

• Trend of current i_a, i^*_a
• Current error e_ia

In the Field Oriented Control is possible to realize a current control system in the rotating reference d, q instead of in the fixed system a, b, c or 1, 2, 3.

Control diagram in F.O.C. case

Fig.2

• i_a, i_b, i_c instantly measured with transducers.
• Transformation from a, b, c, to d, q - compare with the values of i^*_d and i^*_q imposed.

Obtained values of v_d and v_q are transformed back into the main reference system and the scheme is the same as the fig.1.

Lastly, it should be noted that with this scheme you can make error phase independent of the frequency of the supply voltage inverter.

Control diagram – Single-phase loads

Fig.3

The transition of the transistors occurs when the error changes its sign and exceeds 0 of a value Δ.

If output at the controller is a signal high (1), are controlled in closing T1 and T3, and T2 and T4 in openings. Vice versa if the signal is low (0), are controlled in closing T2 and T4, and in the opening T1 and T3.

Consider the case in which the current signal I is an ideal sinusoidal and the load to be supplied both RL with a time constant very long.

• Trend of the sinusoidal current into an RL load

Consider now the case in which you want to power the load RL with a constant current I_o.

• Trend of the constant current into an RL load

Control diagram – Three phase loads

Suppose we have a three-phase load predominantly inductive and with the neutral connected directly to the central point of the battery.

Fig.4

Control diagram – Three phase loads

Fig.5

Representation of bandwidth HB (i_a)

Limit of the vector Is^*

Supposing to carry out the control, no longer in the reference system a, b, c, but in the fixed system α, β, we introduce the space vector of the current Is. This vector also presents limits of variation, which are presented in fig.6 hexagon marked.

Fig.6

Problem of this method

The control scheme in the Laplace domain is:

Block diagram of the Adjusting system with hysteresis in the d, q δd and δq can be alone or > 0 or < 0; analysis of the sign represents the method for choosing the voltage to be