Since the introduction of solid state control of ac motors, a number of power semiconductors have been developed, each with a unique set of characteristics. These semiconductors include the silicon controlled rectifier (SCR), gate turn-off device (GTO), power (giant) transistors (GTR), and insulated gate bipolar transistors (IGBT).

Under normal operating conditions, power semiconductors function as oneway switches; current flows in one direction, from the highest voltage potential to a lower voltage potential. Their three basic connections include terminals for input power, output power and a control terminal. A current or voltage signal applied to the control terminal closes (turns on) the semiconductor switch.

The most appropriate power semiconductor for an application depends on its speed (response to a control signal), gain or amplification, efficiency, and control method (voltage or current). These four basic characteristics are compared in the Table.

With its introduction in the 1950s as a variable voltage-controlling device in acto- dc conversion applications, SCRs are the oldest controllable power semiconductor. GTOs became available for widespread usage in the late 1970s, then GTRs in the early 1980s and, most recently, IGBTs.

As speed of switching increases from SCRs through IGBTs, waveforms are produced that contain increasingly higherfrequency components. This produces correspondingly higher leakage currents, which must be considered in the the circuit design.

Silicon controlled rectifier (SCR)

To easily understand how an SCR works, consider its function in a typical dc drive, which was the first general industrial application. In these drives, SCRs control dc motor speed and torque by converting ac line voltage to adjustable dc voltage. To do this, the controller determines when in the ac half cycle to turn the SCR on so it conducts to the end of that half cycle. The earlier in the half cycle the SCR turns on, the higher the average output voltage is to the motor. To turn the SCR on, the controller applies a current to the SCR gate, Figure 1.

If the load (such as a capacitor) can store energy in the form of a voltage, the SCR can turn on only when the instantaneous value of ac line voltage exceeds the value of stored voltage.

SCRs are capable of handling large amounts of current — 5,000 A to 10,000 A. But large devices are needed to handle high currents; and the larger the device, the slower its switching characteristics.

Typically, the time it takes an SCR to turn fully on after receiving a gate pulse is in the order of 10s of μsec. To turn on, SCRs require a heavy pulse of gate current (about 50 A/μsec ) for a short period of time and then lower-current sustaining pulses to keep it on. If the device is starved (insufficient carriers or electrons to start conduction), high-resistance areas can be developed leading to hot spots and failure. Such failures can also occur if, once turned on, sufficient current through the device is not maintained.

Several approaches are available involving external circuit components (such as saturable reactors) to ensure that the device will safely turn on. Some schemes for turning SCRs on involve continuous pulse firing; others provide a heavy turn-on pulse followed by a sustained pulse to maintain conduction of the SCR.

Turn-on characteristics are the most significant limitation of an SCR in a power-control application. The circuit must be capable of handling the heavy turn-on current, if even for a short time.

Since SCRs are rather slow to turn on and require line commutation to turn off, other devices such as SCR-rated, fastblow fuses are necessary to protect the SCR and turn it off if some event occurs, such as a short or a fast impedance change, while the device is conducting.

In the early 1960s, SCRs were used to control the frequency of ac drives. Now, GTOs, GTRs and IGBTs have replaced SCRs for this function, particularly in low-voltage ac drives, because of their greater efficiency and controllability.

Efficiency of SCRs is fairly low because the voltage drop across the device (typically 2 to 2.5 V) produces high losses when conducting heavy current.

In PWM ac drives and in low-voltage ac motor-control applications, SCRs function as rectifiers to convert ac line voltage to a dc voltage, which, in turn, is connected to the ac motor by other types of semiconductor switches.

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