Characteristics under Current-source Inverter (CSI) Drive

34.4.4 Characteristics under Current-source Inverter (CSI) Drive

A CSI-driven synchronous motor drive generally gives higher e c c dynamic response. It also gives better reliability because of the

automatic current-limiting feature. In a variable-speed appli- cation, the synchronous motor is normally driven from a stiff current source. A rotor position sensor is used to place the FIGURE 34.35 Current waveforms in the dc-link current-source-driven phase current phasor I of each phase at a suitable angle with motor.

respect to the back-emf phasor (E f ) of the same phase. The

rotor position sensor is thus mandatory. Two converter schemes have generally been used. In one scheme, as indicated in Fig. 34.34, a large dc-link reactor

The inverter drives the motor with quasi-squarewave current (inductor) makes the current source to the inverter stiff. The waveforms as indicated in Fig. 34.35. The current waveforms

scheme is suitable for large synchronous motors for which are switched according to the measured rotor position infor- thyristor switches are used in the inverter. A current loop may mation, such that the current waveform in each phase has also be established by sensing the dc-link current and by using

a fixed angular displacement, γ, with respect to the induced

a closed-loop current controller that continuously regulates emf of the corresponding phase. Because of this, the drive the firing angle of the controllable rectifier in order to supply is sometimes referred to as self-controlled. The angular dis- the inverter with the desired dc-link current. It can be shown placement of these current waveforms (or their fundamental that the motor-developed torque is proportional to the level components) with the respective back-emf waveforms is indi- of the dc-link current.

cated in Fig. 34.35. Because of the large dc-link inductor, the phase currents may be considered to remain essentially con- stant between the switching intervals. The quasi-square current

DC Link

waveforms contain many harmonics, and are responsible for

Inductor I DCLlink

large torque pulsations that may become troublesome at low speed.

In the forgoing scheme, the motor can be reversed eas- T 1 T 3 T 5 ily by reversing the sequence of switching of the inverter.

It can also be braked regeneratively by increasing the firing

angle of the input rectifier beyond 90 ◦ while maintaining the dc-link current at the desired braking level until braking is no

Current T 4 T 6 T 2 longer required. The rectifier now returns the energy of the controller a overhauling load to the ac mains regeneratively.

I ref + In another scheme, which is preferred for lower capacity − T 1 −T 6 drives for which higher dynamic response is frequently sought,

phase currents are regulated within the inverter. The inverter I DCLink

typically employs gate turn-off switches, such as the IGBT, and Converter q Switching

E pulse-width modulation techniques, as indicated in Fig. 34.36.

Circuit

Motor currents are sensed and used to close independent current controllers for each phase. Normally, two current

FIGURE 34.34 Schematic of a current-source inverter (CSI) driven controllers suffice for a balanced star-connected motor. Three- synchronous motor.

phase sinusoidal ac currents are supplied to the motor, the

34 Motor Drives 939

+DC-Link

AC suppl y

Motor

i aref

T 4 −DC-Link

i bref

i c m FIGURE 34.36 Control scheme of the SPWM current-source drive.

amplitude and phase angle of which can be independently con- If the angle γ =0 ◦ is chosen, the familiar dc-motor-like trolled as required. The references for the current controllers torque characteristic is obtained. It should be noted from the are obtained from a three-phase current reference generator forgoing that the developed torque at any speed is independent that is addressed by the feedback of the rotor position. The of R since a high-gain (stiff) current-source drive is used. Note

rotor position is continuously measured by a high-resolution also that the ratio E f /ω at any operating speed is proportional encoder. In this way, the current references, and hence the to the amplitude of the stator flux linkage, λ f , due to rotor actual stator currents are synchronized to the rotor.

excitation. For fixed rotor excitation this ratio is a constant. Equation (34.65) indicates that the developed torque of a nonsalient-pole synchronous motor can be controlled by con-