Torque load

Two-winding motors may be built for constant torque, variable torque, or constant horsepower. Constant-horsepower motors are capable of handling the same horsepower at both speeds (i.e., higher torque at the low speed). Constant-torque motors can handle the same load torque at either speed (e.g., conveyor drives). Variable-torque motors are designed for loads in which load torque varies as the square of speed and horsepower varies as the cube of speed. Typical applications are as follows ... 

Torque. lEC 60034-1 stipulates for a minimum inbuilt capacity of a machine (all ratings and voltages) to sustain an excessive torque of 60% for a minimum of 15 seconds, without stalling or an abrupt change in its speed. This stipulation is to meet the need for an excessive torque of a transitory nature due to the above parameters or for a momentary excessive load torque itself during operation. This stipulation, however, would not apply to motors designed and manufactured to specific requirements. 

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. 

Judicious electrical design will ensure a pull-out torque slip as close to the full-load slip as possible and minimize the additional slip losses in such a condition. See Figure 1.8. A motor with a pull-out torque as close to full load slip as possible would also be able to meet a momentarily enhanced load torque during a contingency without any injurious heat or a stalling condition. 

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. 

Motor Torque, Load Torque and Selection of Motors... 

Motor torque, load torque and selection of motors 2/37... 

Tj = rated or the full-load torque and should occur as near to the synchronous speed as possible to reduce slip losses. 

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. 

Approximate torque curve during a soft start Normal torque curve Load torque... 

Ki = mean load factor, i.e. the ratio of the average load torque to the motor torque which depends upon the loading on the motor during start-up. 

T = average load torque between running speed and the final speed... 

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. 

P = kW required T = load torque in nikg N speed of drive in r.p.m. q = unit efficiency of the drive... 

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... 

Minimum locked rotor torque percent of full load torque... 

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

Inertia load (torque). This is usually determined by the manufacturer of the driven equipment and is evaluated by determining the time for the machinery to drift to a stop, being retarded by its own friction. Peak, fluctuating, or shock loads and their cycles or repetition must be considered. 

Torques in Percent of Motor Full-Load Torque... 

Torque is the turning effort developed by the motor or the resistance to turning exerted by the load. Usually torque is expressed in ft-lb however, the usual expression is as a percentage of the full load torque. Synchronous motors usually offer several types of torque. Starting or breakaway (called locked rotor) torque is developed at the instant of starting, see Figure 14-12. 

Full-load torque The torque necessary to produce the rated horsepower at full load speed. In lb at 1 ft radius, it is equal to the horsepower X 5,250 divided by the full load speed. 

The pull-out or breakdown torque must be greater than the maximum torque required by the driven equipment to prevent stalling, usually 150% of full load torque for unity-power fector motors and 220-225% for 0.8 leading power factor motors. ... 

All of the torque developed by the motor must either accelerate inertia or overcome load torque. 

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