Start time

From this information it is possible to prepare bar charts by using a 4-week month and noting on the chart the interdependence of the various functions. The starting time for each class of work is fixed on the chart, and the duration is calciilated from the labor-hours allocated to that work from the control estimate. Work should progress smoothly as time elapses, but the operations must be linked by the order of necessaiy precedence. Starting times for the various items of work will be staggered as drawings are released and also to smooth out labor requirements. 

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. 

A 250 kW motor has a cold thermal withstand time of 30 seconds and a hot thermal withstand time of 25 seconds. If the starting time is 7 seconds, determine the consecutive cold or hot starts that the motor will be able to sustain safely. 

On an ON-LOAD start, the starting time increases disproportionately, depending upon the type of switching. This load therefore cannot be accelerated within a safe stall time... 

To achieve the required performance, it is essential that at every point on the motor speed-torque curve the minimum available accelerating torque is 15-20% of its rated torque. In addition, the starting time must also be less than the thermal withstand time of the motor. For more details see Section 2.8. 

Note The above example is only for a general reference. The CDF of the transformer, for the short-time rating, should be increased writh the starting time and the number of starts per hour. Refer to the transformer manufacturer for a more appropriate selection. 

We must also check whether the starting time of the motor with this profile of starting torque would be safe for the motor to pick up to the rated speed. Considering the same data as for Example 7.1, GD = 1866 kgm ... 

Starting time and looked rotor withstand time... 

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. 

The use of a squirrel cage motor is, however, inevitable as discussed earlier particularly for a process plant or a power house application, where downtime for maintenance of a slip-ring motor is unwelcome, or a chemical plant or contaminated locations, where the application of a slipring motor is prohibitive. To meet such load requirements with a squirrel cage motor, the use of fluid couplings to start the motor lightly and reduce starting time, is quite common and economical, as discussed in Chapter 8, and must be adopted in the above case. 

Assuming a safe start time of 8.26 x 1.15. i.e. 9.5. seconds, a motor with a safe hot stall time of 30 seconds will be adequate to withstand three consecutive hot starts, since 3 X 9.5 < 30 seconds. 

Note A similar situation may be found in an aulomalic Y/A starter when the setting of the timer, to changeover from star to della, is al a higher value lo allow a longer starling lime to pick up speed. Then also the motor wotild meet the same fate if the starting time exceeded the thermal withstand time of the motor. 

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... 

Repeat start protection (Figure 12.31). The relay now delects both the summated starting times and temperature rises 0(. and 6(, of the windings. It also detects... 

Likely. settings - summated starting time versus tripping time, or 6 versus tripping time. 

---