Torque Starting

Drive motors should be of the high-starting-torque type and selected for 1.33 times maximum rotational speed. For two- or three-diameter Idlns, the brake horsepower for the several diameters should be calculated separately and summed. Auxiliaiy drives should be provided to maintain shell rotation in the event of power failure. These are usually gasohne or diesel engines. 

Reduced-voltage starting. A reactor, resistor, or transformer is temporarily connected ahead of the motor during start to reduce the current inrush and limit voltage dip. This is accompanied by reduced starting torque. For reactor or resistor start, the torque decreases as the square of current for transformer start, the torque decreases directly with line current. The reactor, resistor, or transformer can be adjusted to give a proper balance between torque and current. 

Starting torque Normal High High Normal... 

If a greater reduction in line current is required for starting ac motors than is possible with reac tor starting, autotrausformers may be used. Because of transformer action, the reduction in motor-starting torque is directly proportional to the reduction in line current. Table 29-3 compares reactor and autotrausformer starting with respect to... 

Type of Drive Speed Range Starting Torque Maximum Speed Dynamic Response... 

During start-up, at lower voltages, the starting torque is... 

Consider a 750 kW, 4-pole motor, having a rated starting torque as 125% of the rated and starting current k as 600% of the rated. At 80% of the rated voltage the starting torque Ts, will reduce to... 

When the running torque requirement is high but the starting torque requirement not as high then a... 

The current in the copper ring opposes the main flux in that area of the pole and behaves like an artificial second winding, and develops a rotating field. Although the torque so developed is extremely low, it is enough to rotate such small drives, requiring an extremely low starting torque, of the order of 40-50% of the full load torque. 

Refer to Figure 2.1 where r, = starting torque or breakaway torque. 

The movement of rotor teeth around the stator produces a clogging effect, resulting into vibrations and noise. To reduce this effect, the common practice is not to provide the rotor slots parallel to the shaft axis but at an angle. This practice is known as rotor bar skew . A proper skewing can also improve the starting torque and reduce the starting cunent, in addition to the effects of space... 

Figure 2.8 Starting (locked rotor) currents corresponding to different starting torques...

This depends upon the applied voltage, i.e. type of switching, starting torque of the motor, counter-torque... 

The starting of an induction motor does not relate to simple switching alone. It also involves its switchgears to control its starting inrush current, starting torque, or both, and its overload and short-circuit protection. 

Restriction in starting current and a requirement for high starting torque to accelerate heavy rotating masses sometimes limit the use of a squirrel cage motor. For such applications a slip-ring motor provides a better alternative. 

Then, for a stator current of corresponding to a starting torque of the required rotor current will be... 

The torque and current curves are as shown in Figure 5.4. Determine the external resistance required to achieve a starting torque of 200%. 

Motor output in kW for different starting torques Standard no. of starting steps... 

We must also check whether the starting time of the motor with this profile of starting torque would be safe for the motor to pick up to the rated speed. Considering the same data as for Example 7.1, GD = 1866 kgm ... 

Tex = braking torque of the external brakes, if provided otherwise it may be considered to be zero Tj, = locked rotor (starting) torque of the motor k2 = factor to account for the average braking torque. This may be considered to be 1.3-1.7 (consult the manufacturer for a more accurate value)... 

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