Stalled conditions

When the motor overheats, the thermostat opens, interrupting motorline current. Pilot thermostats mounted on the windings of larger motors trip the motor starter rather than interrupt line current. This method gives good protection for sustained overloads, but because of the thermal time lag between the copper winding and the thermostat it may not provide adequate protec tion for stalled conditions or severe overloads. 

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. 

Heating phenomenon in a motor during a stalled condition 2/45... 

For safe stall conditions t, f should be less than the thermal withstand time of the motor under locked or short-circuit condition. 

To ascertain whether the stator or the rotor would fail first during a stalled condition, the thermal withstand time of the rotor should also be determined separately for the rotor bars and the end rings. The lowest values for the stator or the rotor will be the safe stall time for the entire motor. The limiting temperatures in rotor components may be considered as follows ... 

Therefore the permissible rise in temperature in a stalled condition will be as follows ... 

The temperature of the windings and other parts must not exceed the limiting temperature specified in Table 7.5, even if the motor, after a prolonged operating period, remains energized in a stalled condition, for a specified time of fg seconds while /g will not be less than 5 seconds. 

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

Vane wear must be monitored in order to schedule replacement before the vanes become too short and wear the rotor slots. If the vanes are permitted to become too worn on the sides or too short, the vane may break and wedge between the rotor and the cylinder wall at the point of eccentricity, possibly breaking the cylinder. Shear pin couplings or equivalent torque limiting couplings are sometimes used to prevent damage from a broken vane under sudden stall conditions. 

At no-flow (stall) conditions, these fans will not generate any... 

While all these factors affect the good running of positive-displacement compressors, the effect is far worse with centrifugal machines, which can approach stall condition and so give a much reduced cooling duty... 

Figure 12.18 Motor run-up, thermal and instantaneous relay curves. The cold and hot thermal images of the relay are presented. The stalling conditions are indicated. The increase in the motor run-up time due to a...

Low voltage motors used in the oil industry usually have modest starting times, since the majority of their driven machines are pumps. Reciprocating compressors and ventilation fans can have reasonably long starting times. It is therefore not normally necessary to provide special relays to detect the stalling condition. 

Motor stall torque Induction motors can tolerate 100% stall torque indefinitely because current is shared equally by all three phases at slip frequency. A synchronous motor will require a DC current at stall, and therefore thermal capacity is reduced in both motor and inverter at stall conditions. 

Other rapidly varying forces arise from the wind turbulence, which increases as the stall conditions are approached. 

For example, the normal discharge rates for a power tool during a motor start-up or stall condition can appear to be the same as the short-circuit current conditions for a notebook computer even if both used a 4-series cell pack configuration. Therefore, the... 

Ability to deliver rated torque at stall conditions... 

Stall Condition in which flow separates from most of a lifting surface, sharply lowering lift and raising drag. 

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