ECE 8830 - Electric Drives
Introduction
Current-fed inverters requires a “stiff” constant current source input - thus are sometimes referred to as CSI (current source inverters or current stiff inverters).

A large inductance can be used to change a variable voltage input to a variable current input.

VSI-inverters and CSI-inverters are dual to each other.
Introduction (cont’d)
Power semiconductor devices used in CSI inverters must be able to withstand large reverse voltages. Therefore, power MOSFETs, BJTs, IGBTs, MCTs, IGCTs and GTOs.

Symmetric blocking GTOs and thyristors can be used in CSI inverters.

Generally CSI inverters are now used in very high power applications.
General Operation of a 6-Step Thyristor Inverter
General Schematic of Thyristor Inverter

General Operation of a 6-Step Thyristor Inverter (cont’d)
Initially, ignore commutation considerations.
Induction motor load is modeled by back emf generator and leakage inductance in each phase of the winding.

The constant dc current Id is switched through the thyristors to create a 3 6-step symmetrical line current waves as shown on the next slide.
General Operation of a 6-Step Thyristor Inverter (cont’d)

General Operation of a 6-Step Thyristor Inverter (cont’d)
The load or line current may be expressed by a Fourier series as:

where the peak value of the fundamental component is given . Each thyristor conducts for radians. At any instant one upper thyristor and one lower thyristor conduct.
General Operation of a 6-Step Thyristor Inverter (cont’d)
The dc page link is considered harmonic-free and the commutation effect between thyristors is ignored.

At steady state the voltage output from the rectifier block = input voltage of inverter.

For a variable speed drive the inverter can be operated at variable frequency and variable dc current Id.


General Operation of a 6-Step Thyristor Inverter (cont’d)
If thyristor firing angle  > 0, inverter behavior.

If thyristor firing angle =0, rectifier behavior.

Max. power transfer occurs when =.
Inverter Operation Modes
Two inverter operation modes are established depending on the thyristor firing angle:
1) Load-commutated inverter
Applies when /2<<.

2) Force-commutated inverter
Applies when <<3/2.

Load-Commutated Inverter Mode
Consider =3/4. In this case vca < 0 => thyristor Q5 is turned off by the load. This requires load to operate at leading power factor => motoring mode of a synchronous machine operating in over-excitation.
Vd=-Vd0cos



Force-Commutated Inverter Mode
Consider =5/4. In this case vca> 0 and so thyristor Q5 is not turned off by the load. Thus some type of forced commutation is required in this case. Lagging VAR is consumed by the load => motoring mode of an induction motor. Vd=-Vd0cos



Force-Commutated Inverters
For driving an induction machine, a force-commutated inverter is required because of the phase lag characteristic of the induction motor.

The topology of a 3 bridge inverter with an auto-sequential method of forced commutation is shown on the next slide.
Force-Commutated Inverters (cont’d)

Force-Commutated Inverters (cont’d)
The current is switched sequentially into one of the motor phases by the top half of the inverter and returns to the dc page link from another of the phases via the bottom half of the inverter. By switching every 2/3 radians, a 6-step current waveform can be applied to the motor.

The series diodes and delta-connected capacitors force the commutation of the thyristors. The capacitors store a charge with the correct polarity for commutation and the diodes isolate them from the load.


Force-Commutated Inverters (cont’d)
Since current is constant, voltage drop across stator windings = 0 and voltage drop across winding resistances = constant.

Thus the motor terminal voltage is set by the motor not by the inverter.

Since the motor is wound with sinusoidally distributed windings, the voltages at the motor terminals are nearly sinusoidal.


Force-Commutated Inverters (cont’d)

The current ideally follows a six-step waveform. However, current cannot change instantaneously through the winding inductances and so the current transitions have a finite slope.

During these transitions the current transfers from one thyristor to the next via one of the six commutating capacitors.
Force-Commutated Inverters (cont’d)
Example: Commutation from Q2 to Q4




Force-Commutated Inverters (cont’d)
When Q4 is fired, Q2 is impressed with a reverse voltage across the capacitor bank. => Q2 turns off almost instantaneously. Id flows through Q3 and D3, phases b and c, D2, the capacitor bank and Q4. The capacitor bank charges linearly with Id. During this time D4 is reverse-biased. When the capacitor bank voltage equals the line voltage, diode D4 turns on and the current Id flows through D4 and terminates the commutation process.
Force-Commutated Inverters (cont’d)

Force-Commutated Inverters (cont’d)
Note the large voltage spikes (Ldi/dt). These can be suppressed either by designing the motor with small leakage inductance or by using a diode bridge at the motor terminal with a zener diode load.


Force-Commutated Inverters (cont’d)
Two positive features of CSI inverters compared to VSI inverters:
1) CSI inverters are able to ride through a commutation failure and return naturally to normal operation; costly preventive measures used for VSI inverters.
2) CSI inverters can be switched to regenerative mode simply by reversing the polarity of the dc rectifier output voltage. This is automatically accomplished when an induction motor operates in a negative slip mode. In the VSI inverter, the current flow must be reversed - much harder.