Quiescent point of a transistor

Methods of speed control of an induction motor

The speed of the induction motor can be controlled by any of the following methods:

1.) Cascade control method

• When no signal is applied, the transistor voltage and current conditions are as indicated at the quiescent point (Q point) on the dc load line.
• When an AC signal is applied, the transistor voltage and current vary above and below point Q.
• The load line is drawn between collector current l_C and collector voltage V_CE for different values of base current l_g.
• When a value for the maximum possible collector current is considered, that point will be present on the Y-axis, which is nothing but the saturation point.
• When a value for the maximum possible collector-emitter voltage is considered, that point will be present on the X-axis, which is the cutoff point.
• When a line is drawn joining these two points, such a line can be called a Load line. This line, when drawn over the output characteristic curve, makes contact at a point called an Operating point or quiescent point, or Q-point.
  ​

Operating modes of BJT

The transistor operates in four operating modes. These are as follows: