Speed-torque curves

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). 

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...

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. 

Motor speed-torque curve NEMA rotor designs Special designs of rotors Effect of starting current on torque Load torque or opposing torque Selection of motors Time of start-up and its effect on motor performance Thermal withstand time... 

A motor speed-torque curve for a compressor (250 to 1,000 hp or more) does not look the same as a smaller machine (10 or 20 hp) (see Figure 7-3). NEMA Standard MG-1 gives minimum locked rotor-torque values as a function of motor size at 1,800 rpm (see Table 7-2). 

Figure 7-3. Typical speed-torque curves for large and small motors [7],...

Figure 7-4. Speed-torque curve for two compressor types. Each type of compressor has its own curve [7].

Figure 7-5. There is no standard speed-torque curve for accelerating compressors. Attempting to make a standard motor to meet all needs (heavy line) would impose unnecessary demands on motor design [7].

Unloaded speed-torque curve (zero flow through compressor)... 

Speed-torque curve, intermediate discharge pressure (refrigeration service for restart evaluation)... 

Speed-torque curves at 70, 80, 90, and 100 percent of rated voitage. 

FIGURE 3-14. Performance Characteristics of Motors. For typical squirrel-cage induction motors. Points (I) show the speed-torque limits during initial acceleration. Area is the maximum torque range recommended for short-term overloads. Point is the reverse-curvature point on the speed-torque curve and is the point of minimum accelerating torque. 

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