Speed-torque

In addition to secondarv resistance control, other devices such as reactors and thyristors (solid-state controllable rectifiers) are used to control wound-rotor motors. Fixed secondary reactors combined with resistors can provide veiy constant accelerating torque with a minimum number of accelerating steps. The change in slip frequency with speed continually changes the effective reac tance and hence the value of resistance associated with the reactor. The secondaiy reactors, resistors, and contacts can be varied in design to provide the proper accelerating speed-torque curve for the protection of belt conveyors and similar loads. 

A dc motors inherent speed-torque curve can be varied widely by adjusting the relative amounts of shunt and series fields. The series field may also be connected to aid or buck the shunt field. The usual practice is to connect the series field so that it adds to the shunt field (cumulative compound), which gives a stable, drooping speed with increasing load. 

The speed-torque characteristics of a motor will largely depend upon its rotor parameters such as ( 2 The higher the rotor resistance / 2. the higher will be the torque. From equation (1.3) we cun draw a speed-torque curve of a motor as shown in Figures 1.5(a) and (b). 

Figure 1.5 Speed-torque and speed-current curves at the rated stator voltage...

During a run, if the supply voltage to a motor terminal drops to 85% of its rated value, then the full load torque of the motor will decrease to 72.25%. Since the load and its torque requirement will remain the same, the motor will star to drop speed until the torque available on its speed-torque curve has a value as high as 100/0.7225 or 138.4% of T to sustain this situation. The motor will now operate at a higher slip, increasing the rotor slip losses also in the same proportion. See equation (1.9) and Figure 1.7. 

The maximum value of the output and torque of the motor can be obtained by dropping perpendiculars CC and CC3 on the output and torque lines respectively from the centre C.CjCi and C C4 indicate the magnitude of the maximum output and torque, respectively, that the motor can develop. This torque is the pull-out torque Tpp. In slip-ring motors it can be obtained at any speed on the normal speed-torque curve by inserting a suitable resistance into the rotor circuit to vary the slip. 

Figure 2.2 Tj, too high to have on the speed-torque curve...

Flowever, motor manufacturers may adopt more flexible designs with more reserve capacity and better speed-torque characteristics to suit the requirements of a particular sector. These are particularly for installations where the distribution system may have wider voltage fluctuations or the load itself may have varying load demands. It is possible that the same motor may have to drive more than one type of loads at different times. An agricultural pump motor may be one such application where it may also have to drive a thrasher or a winnower at different times. A motor with higher flexibility would be more desirable for such applications. 

Figure 2.3 Speed-torque characteristics of motors as per NEMA standard...

Figure 2.5 Speed-torque characteristics of a double squirrel cage motor...

The recommended practice would require that at each point on the motor speed-torque curve there should be a minimum 15-20% suiplus torque available, over and above the load torque, for a safe start (Figure 2.14). The torque thus available is known as the accelerating torque. 

To achieve the required performance, it is essential that at every point on the motor speed-torque curve the minimum available accelerating torque is 15-20% of its rated torque. In addition, the starting time must also be less than the thermal withstand time of the motor. For more details see Section 2.8. 

Here we analyse the effect of variation in the incoming supply parameters (voltage and frequency) on the characteristics and performance of an induction motor (such as its flux density, speed, torque, h.p., etc). We also assess the effect of variation of one parameter on the other, and then choose the most appropriate solid-state scheme to achieve a required performance. We generally discuss the following schemes ... 

Figure 6.2 Actual speed-torque characteristics by a convehlional frequency control (I///control)...

Figure 6,8 Speed-torque characteristics by flux (/ ,) control (single phasor control)...

Figure 6.10 Speed-torque characteristics by field-oriented control (FOG) (flux and torque control) (Source Allen Bradley)...

As discussed earlier, the motor speed-torque characteristics depend largely upon the rotor current... 

Beyond /V. the h.p can be kept constant by keeping the voltage fixed and raising/(h.p. T.N). The speed-torque characteristic is similar to a d.c. machine as shown in Figure 6.51... 

The speed-torque characteristics are almost linear, with torque falling with speed (Figure 7.1 I). 

They are suitable for stepless speed variation and can be controlled through speed, torque, temperature or flow of a process. They can also be programmed for any sequence of operation. 

Test for speed-torque and speed-current curves 11/257... 

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