Torque accelerating

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. 

When external secondaiy resistance is used for improved starting charac teristics, short-time-rated resistors are employed. As the motor accelerates, steps of resistance are cut out on a time or current basis to give the desired accelerating torque and current characteristics. 

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. 

A rotor fails during start-up, possibly due to a lower supply voltage than desired or a smaller accelerating torque than required or reasons leading to similar conditions. In such cases the rotor tails first, due to higher rotor currents and a prolonged acceleration time or a locked rotor. At this instant, unless the motor control gear trips, the stator may also fail due to excessive heat. Instances can be cited where even... 

This expression, except for the mechanical design, is totally independent of the type of start and the electrical design of the motor. Electrically also, this is demonstrated in I he subsequent example. The expression, however, does not hold good for an ON-LOAD start. On load, the accelerating torque diminishes substantially with the type of load and the method of start, as can be seen from Figure 2.14, and so diminishes the denominator of equation (2.5), raising the time of start. 

Against an opposing torque, the accelerating torque of the motor, which hitherto had varied in proportion to the type of switching, will now diminish disproportionately... 

Maintaining a minimum accelerating torque at each point, during the pick-up may also not be adequate sometimes, when the starting time exceeds the locked rotor or thermal withstand time of the motor, as discussed below. 

To determine the exact accelerating torque, measure the ordinates of torque as shown at different speeds and calculate the average torque as follows ... 

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. 

Figure 5.7(a) Determining the number of steps and accelerating torque between each step... 

These are required to make threads, i.e. the final drawing, twisting and winding of cotton. Such motors must possess very smooth acceleration to eliminate breakage of threads. They are recommended to have a starting torque of 150-200% of T, and a pull-out torque of 200-275% of with a mean acceleration torque ol 150-175%. A normal acceleration time of 5-10 seconds is recommended. Faster acceleration may cause more breakages, while a slower acceleration may result in snarls and knots in the yarn as a result of insufficient tension. 

Assume that the motor is designed for an average speedup torque of 135% and TpQ of 220% (Figure 7.21). If the average load torque is assumed as 68%, the average accelerating torque, 7, available will be 67% on DOL starting, i.e. 

Let us use a fluid coupling and start the motor lightly, similar to Figure 8.3. The revised accelerating torque and approximate clutching sequence of the coupling is illustrated in Figure 7.22. 

With the s inie pull-out torque as above, if we select a slip-ring motor and arrange the starter to develop an average pick-up torque of 200%. the avaihible accelerating torque will be (200-68) = 142%... 

AT = average accelerating torque over the speed interval (difference between motor and load torque) g = gravitational constant WR -- torsional moment of inertia... 

The damper and field windings each produce an effect on accelerating torque. The resistance of the damper winding is usually selected to produce its maximum torque during the first part of the starting period and a field-discharge resistor is selected so that the field-circuit torque peaks near pull-in speed. 

Constant-speed application High starting torque, high acceleration torque, high speed with light load... 

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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