Rating short-time motors

Rating of short-time motors 3/59 Equivalent output of short-time duties 3/63... 

When external secondaiy resistance is used for improved starting charac teristics, short-time-rated resistors are employed. As the motor accelerates, steps of resistance are cut out on a time or current basis to give the desired accelerating torque and current characteristics. 

If a short-time duty is performed on a Continuous Maximum Rating (CMR) motor with some no-load or idle running, the temperature rise 0 may not reach its maximum value, 0 , us shown in curve (c) of Figure... 

I. A CMR motor therefore can be operated at higher outputs on short-time duties as shown in curve (d). The extent to which a CMR motor cun be over-rated to perform a particular short-time or intermittent duty is considered in the follow ing example. While evaluating the rating for such duties, the heat during start-up and during braking and their frequency of occurrence should be considered. 

If the cycle has a short-time rating, with a period of energization and one of rest, the motor will obviously cool during the de-energizing period, and depending upon the peak load and tlie rest periods, a comparatively lower output motor can perform duties at higher loads. The equivalent output for the load cycle of Figure 3.16 is... 

Figure 3.17 Determining the motor rating (CMR) for a short-time duty...

Since the transformer will be in the circuit for only 15 to 20 seconds, the approximate short-time rating of the transformer can be considered to be 10-15% of its continuous rating. The manufacturer of the auto transformer would be a better judge to suggest the most appropriate rating of the transformer, based on the tapping and starting period of the motor. 

For motors with a short-time rating, the duration of the test should correspond to the specified short-time rating. At the end of the test the specified temperature rise limits should not exceed. At the beginning of the test, the temperature of the motor should be within 5°C of that of the cooling air. 

Duty cycles Continuous duty (CMR) (S ) Periodic duties Factor of inertia (FI) Pleating and cooling characteristic curves Drawing the thermal curves Rating of short motors Equivalent output of short time duties Shock loading and use of a flywheel... 

For loads requiring an occasional overload, such as startup of pumps with cold water followed by continuous operation with hot water at lower hp loads, using a motor with a short time overload rating will probably be appropriate. 

Bit Pressure Loss. It is necessary to choose the bit pressure loss such that the thrust load created in combination with the weight on bit will yield an on-bottom load on the motor thrust bearings, which is less than the maximum allowable load for the bearings. Since this is a deviation control run and, therefore, the motor will be drilling only a relatively short time and distance, the motor thrust bearings will be operated at their maximum rated load for on-bottom operation. Figure 4-206 shows that maximum allowable motor thrust bearing load is about... 

The required performance of the hoist drive and motor for single drum operation must be clearly stated. As a minimum, the drive and motor must be able to provide sufficient torque to lift a fully loaded conveyance in single drum at the bottom of the shaft. The chent should specify clearly all the operations expected in unclutched mode. The requirement for unclutched operation could vary from short time, slow speed operation to full speed hoisting at rated load. 

A universal motor has the highest horsepower per pound ratio of any ac motor because of its abihty to operate at much greater speeds than any other 60-Hz motor. It is ideally suited for operation at a rated output, where an occasional overload or intermittent heavy load occurs. Stall torque may be as much as 10 times the continuous rated torque. The motor may even be operated in a stalled condition for short periods of time. 

A nonproportional sampler is suitable for near-constant flow conditions. The sample is simply drawn from the waste stream at a constant flow rate. Sampling lines should be as short as possible and free from sharp bends, which can lead to particle deposition. Proportional samplers are designed to collect either definite volumes at irregular time intervals or variable volumes at equal time intervals. Both types depend on flow rate. Examples of some of these are the vacuum and chain-driven wastewater samplers. Other types, which have cups mounted on motor driven wheels, vacuum suction samplers, and peristaltic pump samplers, are also available (26,27). 

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. 

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