Locked rotor torque

No-load running current Checking of current balance Pull out torque Locking rotor current Starting torque... 

Power factor.Torques Locked-rotor.% Pull-in. . . ... 

Power factor. Torques Locked-rotor. % Pull-in. 

Power factor.Torques Locked-rotor.% Pull-in.. 

Motors are classified by size, apphcation, electrical type, NEMA design letter, and environmental protection and cooling methods. They are rated for special standard environmental and operating service conditions by performance and mechanical configuration voltage and frequency, locked-rotor kVA, service factor, horsepower, speed, torque, locked-rotor current, performance, temperature rise, duty cycle, and frame size. 

BB will determine the locked rotor torque and power loss while the rotor is locked. 

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

With a view to achieve yet more standardization in motor design, NEMA Standard MG-1 has also recommended the maximum locked rotor current of single-speed three-phase motors for the various rotor designs A, B, C, and D, for various recommended torque values. These have been derived for a 415 V a.c. system and are shown in Table 2.2. 

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... 

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. 

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)... 

Maximum torque occurs at the locked rotor condition, i.e. when S = I (Figure 7.11). 

The motor should be mounted with a suitable loading arrangement and the rotor fully locked. The rated voltage at the rated frequency will then be applied to the motor terminals in the locked rotor condition. The loading on the motor will then be reduced slowly so that the motor can start and pick up speed. The value of pull-up torque at which the rotor picks up speed and attains speed corresponding to pull-out torque condition must be noted. 

When the locked rotor torque cannot be measured by the Table 11.7 Protection against contact with live or moving parts... 

When the torque is determined by the above method, the voltage during the test should be so adjusted that the locked rotor current is approximately equal to the full load current. After the locked rotor test, the resistance of the stator windings should be measured and may also be considered for calculating the I R losses. 

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). 

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

The voltage drop during starting should be calculated using the speed current data, locked rotor power factor, and the distribution system con stants. Speed-torque values at this reduced voltage can be calculated, assuming that the torque varies as the voltage squared. These values are compared to driven equipment curves. 

Molecular weight, effect on centrifugal sizing, 159 Mollier charts, 27 Monitoring system, 356 Motor, 146 enclosure, 260 equations, 267 insulation, 257 locked rotor torque, 270 selection, 270 service factor, 262 starting characteristics, 270 starting time, 273, 274 synchronous vs induction, 265 variable frequency drives, 27/, 280 voltage, 258 Motors... 

For horizontal shaft pumps and vertical shaft pumps having no thrust bearing (entire thrust load carried by the motor), the locked-rotor torque required is usually between 5 and 20%, and for vertical shaft machines having their own thrust bearing a locked-rotor torque as high as 40% is sometimes required. 

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. 

Locked-rotor (static) torque, starting, or breakaway The minimum torque that a motor will develop at rest for all angular positions of the rotor, with rated voltage applied at rated frequency. 

D. Performance characteristics. (Usually includes following electrical characteristics efficiency, power-factor, locked rotor KVA, fullload speed, temperature rise and starting, break-down, and full-load torques.) Be sure to advise whether special test certification is to be furnished this always extends delivery time and increases cost. 

Design D. A Design D motor is a squirrel-cage motor designed to withstand full-voltage starting and to develop high locked-rotor torque as shown in MG 1-12.37 with... 

All four designs can withstand full-voltage starting directly across the power lines, that is, the motors are strong enough mechanically to withstand magnetic stresses and the locked-rotor torques developed at the time the switch is closed. 

Tables 3-5 and 3-6 give the typical locked-rotor torque developed by Designs A, B, and C motors [8].

Locked-Rotor Torque of Design A and B Motors with Continuous Ratings [8]... 

---