Operation of Electric Motors | Devices | Electrical Engineering

In this article we will discuss about the single and four quadrant operation of electric motors.

Single Quadrant Operation of Electric Motors:

Single quadrant operation means that load and the motor shall be required to run only in one specified direction under the action of developed torque.

When an electric motor is switched to electric supply mains and drives mechanical devices such as line shaft and machine tools, the machine is said to be operating in forward motoring mode or simply motoring mode. In this mode of operation armature current flows in opposition to the emf induced in the armature. The direction of emf induced in the armature is in direct opposition to the applied voltage. Electromagnetic torque developed is in the direction of armature rotation.

Four Quadrant Operation of Electric Motors:

In view of the fact that both active and passive load torques can be present in general, in a drive system, the motor driv­ing the load may operate in different regimes—not only as a motor, but for specific periods, also as a generator and as a brake.

Further, in many applications, the motor may be required to run in both directions. Therefore, in sketching the speed-torque characteristics of either the load or the motor, it is preferable to make use of all four quadrants of the speed-torque plane for plotting, rather than to confine, the characteristics to the first quadrant alone.

The conventions used for positive and negative values of speed, motor torque and load torque in a diagram of this type are as follows:

The speed is assumed to have a positive sign, if the direction of rotation is counter-clockwise or is in such a way to cause an ‘upward’ or ‘forward’ motion of the drive. In case of reversible drives the positive sign for speed may have to be assigned arbitrarily either to counter-clockwise or clockwise direction of rotation. The motor torque is taken to be positive when it causes an increase in speed in the posi­tive sense. The load torque is assigned a positive sign when it acts against the motor torque.

Figure 1.2 illustrates the four different modes of operation of an electric machine. Although the diagram indicates that the field polarity is maintained and the armature current is reversed to obtain negative torque, the same effect is obtained by reversing the field polarity and maintaining the armature current direction. Field reversal is necessary with some forms of rectifier control. These four quadrants of operation are feasible for any dc or ac rotating machine but the dc machine is much freer to transfer its operation between quadrants and operates satisfactorily at any point within the envelope.

Figure 1.3 illustrates the four quadrant operation of a motor driving a hoist consisting of a cage with or without load, a rope wound onto a drum to hoist the cage and a balance weight magnitude larger than that of the empty cage but smaller than that of the loaded one. The arrows in this figure indicate the actual directions of motor (or electro­magnetic) torque, load torque and motion in the four quadrants. It can be easily seen that they correspond to the sign conventions mentioned earlier for speed, motor torque and load torque.

The load torque of the hoisting mechanism may be taken to be constant (i.e., independent of speed); forces due to friction and windage being negligible in case of low speed hoists and the torque being primarily due to the gravitational pull on the cage. This torque being an active load torque does not change its sign even when the direction of rotation of the driving motor is reversed. Therefore speed-torque curves of a hoist load can be represented by means of vertical lines passing through two quadrants.

The speed-torque character­istic of a loaded hoist is shown in Fig. 1.3 by means of the vertical line passing the first and fourth quadrants. Since the counterweight is assumed to be heavier than the empty cage, the inherent tendency of the load viz., the empty cage is to move in an opposite direction to that of load presented by the loaded cage and hence the speed-torque curve of the unloaded hoist is represented by the vertical line passing through second and third quadrants.

In the first quadrant the load torque acts in a direction opposite to that of rotation. Hence, to drive the loaded hoist up, the developed torque in the motor M must act in the same direction as the speed of rotation i.e., T_M should be of positive sign. Since the speed is also positive being an upward motion, the power will also have a positive sign, i.e., the drive is said to be motoring. Quadrant I is arbitrarily and conventionally, thus, designated as ‘forward motoring quadrant’.

Since speed and torque are both taken as positive when motoring forwards, upwards or counter-clockwise, the prod­uct of E and I is taken as positive under these conditions. It is convenient to assume that both E and I are positive under this condition, i.e., to assume the flux to be positive, so that positive E corresponds to positive speed and positive I to positive torque. If in all circuits the direction of the current in the field winding is assumed to be the same, the flux is automatically assumed to be positive.

Negative armature cur­rent then indicates negative torque, i.e., the torque in the reverse, downward or clockwise direction. Similarly negative emf indicates negative speed. All that is required to avoid difficulties is to maintain these conventions consistently in the circuit analysis and the signs will indicate the mode of opera­tion. The motor equation V = E + IR must, of course, be used consistently for all cases, and no attempt made to prejudge the behaviour by using the “generator equation” V = E – IR at any time.

The hoisting up of the unloaded cage is represented in the second quadrant. Since the counterweight is heavier than the empty cage, the speed at which the hoist is moved upwards may attain a dangerously high value. In order to avoid this, the motor torque must act in a direction opposite to that of rotation, i.e., the motor should switch over to a braking or generator regime. Note that T_M will have a negative sign and speed still has a positive sign, being forwards, upwards and counter-clockwise, giving power a negative sign, corresponding to the generating or braking operation.

The third quadrant represents the downward motion of the empty cage. The downward journey of the cage opposed by the torque due to the counterweight and friction at the transmitting parts. So for moving the cage downwards, the motor torque must act in the same direction as the motion of the cage. The electrical machine acts as a motor as in the first quadrant, but in the reverse direction. Thus quadrant-III becomes ‘reverse motoring’. The motor torque has a nega­tive sign as it causes an increase in speed in the negative sense and the speed also has a negative sign being a down­ward motion. Power, thus, has a positive sign.

The downward motion of the loaded cage is shown in the IVth quadrant. The motion can take place under the action of load itself, without the use of any motor. But in order to limit the speed of the downward motion of the hoist, the electrical machine must act as a brake. The motor torque has a positive sign as it causes a decrease in speed in the down­ward motion. The speed, of course, has a negative sign, being a downward journey. The power, thus, acquires a negative sign, corresponding to the braking operation of the motor.