Q12Electrical Machines and Drives

Current Source Inverter (CSI) Fed Induction Motor Drive

In a current source inverter drive, the AC supply is first rectified by a controlled (phase-controlled thyristor) rectifier and then fed through a large series inductor (Ld) into the inverter DC link, so that the DC link behaves as a nearly constant current source (rather than the constant voltage source of a VSI drive) - the large series inductance smooths and stiffens the DC link current, making it essentially independent of short-term variations in the load-side inverter voltage.

The inverter itself, typically built using thyristors (with forced commutation circuits, since load-commutation is only reliably available above a certain minimum speed where the motor's back-EMF-generated leading power factor assists commutation) or, in modern designs, GTOs/IGBTs, switches this constant current sequentially into the three stator phases, producing a quasi-square-wave (six-step) current waveform in each phase rather than a voltage waveform - this is the defining characteristic that distinguishes a CSI from a VSI, and it gives the CSI drive inherent short-circuit protection (since the current is actively limited by the DC link inductor regardless of any fault at the inverter output) as well as simpler regenerative braking capability (since reversing power flow only requires reversing the DC link voltage polarity via the controlled rectifier, without needing an additional line-side inverter).

Speed control is achieved by simultaneously adjusting two independent quantities: the firing angle of the input controlled rectifier sets the DC link current magnitude Id (and hence, indirectly, the motor's torque-producing stator current amplitude), while the inverter's own switching frequency directly sets the output frequency, and hence the synchronous speed, of the three-phase current waveform applied to the stator. In practice, the V/f (or more precisely here, the current-frequency) relationship is coordinated so that the stator flux is kept approximately constant across the operating speed range, exactly analogous to the constant-V/f principle used in VSI-fed drives, ensuring the motor develops adequate torque capability at every commanded speed without excessive saturation at low frequency or torque deficiency at high frequency.

A significant practical drawback of the CSI drive is that the quasi-square current waveform, being rich in low-order harmonics (5th, 7th, 11th, 13th, etc.), causes pulsating torque components and additional harmonic copper and iron losses in the motor, along with audible noise, considerably more pronounced than in a well-filtered PWM-VSI drive; furthermore, the large DC link inductor is bulky, heavy, and expensive compared to the DC link capacitor used in a VSI, and the drive's dynamic response is inherently slower due to the difficulty of rapidly changing current through a large inductor. For these reasons, CSI drives, once common in large, high-power induction and synchronous motor drives (owing to their simpler thyristor-based construction, straightforward overcurrent/short-circuit protection, and simple regeneration), have been largely displaced in most modern applications by PWM voltage source inverter drives using fast-switching IGBTs, though CSI topology remains relevant for certain very-high-power drive applications and specialized load-commutated-inverter (LCI) synchronous motor drives.