Locked rotors

Synchronous-motor rotor frequency can be detected because the rotor field circuit is available. Special control schemes have been devised which take into account both speed and induced rotor current in providing locked-rotor and accelerating protection. 

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. 

Table 2.2 Recommended maximum locked rotor currents for various rotor designs...

HP Approx, maximum Rotor design locked rotor current ... 

Note For motors beyond 200 h.p., NEMA has not covered these data. It is, however, recommended that larger motors may be designed to have even lower locked rotor currents than the above to reduce the starling transient effects on the distribution system as well as on the motor windings. ... 

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. 

P40 - resistivity of copper at 40°C h is known as the middle temperature during the entire temperature variation in the locked rotor condition. 

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 locked rotor thermal withstand time is much higher than for a standard motor. 

For motors with a starting time of up to 20 seeonds, with the minimum permissible applied voltage (80%), the locked rotor withstand time under hot conditions at rated voltage must be at least 2.5 seconds more than the starting time. 

Since the themial wilhsttind lime refers to locked rotor conditions and normally corresponds to 600 to 700%... 

By circulating roughly 50-60% of the rated current under a locked rotor condition, at a reduced voltage. To facilitate easy escape of the moisture to the atmosphere, the bearing covers may be removed. 

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. 

The motor should not be kept in the locked rotor condition for more than a few seconds to avoid damage to the windings. 

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. 

Locked rotor current of squirrel cage motors with. short- +20 7c of the guaranteed ciirrem... 

Operating conditions that may overload a machine and raise its temperature beyond permissible limits may be called unfavourable. This overheating, however, will be gradual (exponential), unlike rapid (adiabatic) heating as caused during a locked rotor condition. The machine now follows its own thermal curve and therefore a conventional thermal protection device can be used to protect it from such conditions. These conditions may arise due to one or more of the following ... 

In certain cases, for example in large motors where Tpf, is normally not very high, during a severe drop in voltage Tpo may fall to a value less than the load requirement and result in a stalling or even a locked rotor condition (see Figure 12.7). 

If the motor stalls during pick-up, it will come to a standstill as a result of a locked rotor. The motor will... 

Stalling or locked rotor protection. This is also detected by the prolonged starting time as well as overheating of the machine. It is possible that the machine was already under operation and hot when it had stalled. Under such a condition, the rotor operates at a high freqitency and is more vulnerable to damage. Since it is not possible to create a replica of the rotor, separate... 

Locked rotor current and corresponding hot thermal withstand time... 

The maximum it can operate is under a locked rotor condition of a motor, in which case the maximum current may not exceed six to seven times its rated current, which is moderate and far less than a short-circuit condition. Hence, it is not considered in Table 13.1.3. 

Interrupting when the motor is in a locked rotor condition (Section 1,2) or is still accelerating and carrying a highly inductive current of the order of six to seven times the rated current at a p.f. of about 0.2-0,3,... 

Interrupting an induction motor under a stalled condition which is almost a locked rotor condition. 

On the load side, the interrupter is connected through a cable to the equivalent circuit, with the required quantities of lumped resistance and reactance, to represent the motor to be tested, under a locked rotor condition. The circuit would also represent an interruption immediately after a start, to check for the most onerous operating condition for the interrupter to generate the highest surges as discussed earlier. 

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