Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Motors starting current

The major differences between ac and dc starters are necessitated by the commutation limitation of dc motors, which is the ability of the individual commutator segments to interrupt their share of armature current as each segment moves away from the brushes. Normally 250 to 275 percent of rated current can be commutated safely. Since motor-starting current is limited only by armature resistance, line starting can be used only for veiy small [approximately 1492-W (2-hp)] dc motors. Otheiwise, the commutator would flash over and destroy the motor. External resistance to limit the current must be used in starting to prevent this. [Pg.2491]

Step I Assume the motor starting current versus time characteristics to be shown in Figure 12,32, and divide the... [Pg.299]

Depending on the value of the resistor, the motor will, in general, draw a heavier current then when started using star-delta or auto-transformer methods. The resistors must be rated to carry the limited motor starting current for the time they are in circuit. [Pg.223]

FIGURE 2.1 Motor-starting current waveform. A 5-hp motor was started across the line. The motor full-load current was 60 A. The first half-cycle peak reached a value of 860 A. [Pg.36]

Motor starting current versus speed characteristic 107... [Pg.7]

Motor Starting Current versus Speed Characteristic... [Pg.107]

I-squared-t margin between the fuse and the cable, and that the comer point of the motor starting current is well avoided. [Pg.233]

The d.o.l. starter switches the main suppiy directiy onto the motor. Since motor starting currents can be seven or eight times greater than the running currents, the d.o.l. starter is only used for small motors of less than about 5 kW rating. [Pg.127]

Line Starters and Combination Starters A line starter consists of a contactor (motor-starting switch) and motor-overload relays, (Contactors are capable of canving and interrupting normal motor-starting and -running currents they are not, hovv ever, normally capable of interrupting short-circuit currents. They must be backed up by fuses or a circuit breaker for this function,... [Pg.2488]

If a greater reduction in line current is required for starting ac motors than is possible with reac tor starting, autotrausformers may be used. Because of transformer action, the reduction in motor-starting torque is directly proportional to the reduction in line current. Table 29-3 compares reactor and autotrausformer starting with respect to... [Pg.2491]

Secondary Control of Wound-Rotor Motors Wound-rotor motors may be effectively reduced-voltage-started or have their speed controlled by using external secondaiy resistance. The addition of resistance into the secondaiy circuit of a wound-rotor motor reduces the starting current and affects the speed under load conditions. [Pg.2491]

This is a vital relationship, which reveals that during start-up and until such speed, the reactance of the motor windings / 2> the rotor current will also remain almost the same as the starting current and will fall only at near the rated speed. (Refer to the current curves in Figures 1.5(a) and (b)). The initial inrush current in a squirrel cage induction motor is very high. In a slip-ring motor, however, it can be controlled to a desired level. (Refer to Section 5.2.1.)... [Pg.8]

Consider a 750 kW, 4-pole motor, having a rated starting torque as 125% of the rated and starting current k as 600% of the rated. At 80% of the rated voltage the starting torque Ts, will reduce to... [Pg.12]

A squirrel cage motor has its 4, on DOL as six times its rated current. Find the required tapping on an autotransformer to limit the starting current to 1.5 times. [Pg.74]

For a 3.3 kV 450 kW motor, with a full-load current of 100 A and starting current on DOL as six times the rated current, the kVA rating of the transformer for 50% tapping will be... [Pg.74]

Soft starting minimizes the starting mechanical and thermal stresses/shocks on the machine and the motor. It results in reduced maintenance cost, fewer breakdowns and hence longer operating life for both. Reduced starting current is an added advantage. [Pg.77]

The external resistance adds up to the total impedance of the motor windings and limits the starting current. It also improves the starting power factor. [Pg.83]

Restriction in starting current and a requirement for high starting torque to accelerate heavy rotating masses sometimes limit the use of a squirrel cage motor. For such applications a slip-ring motor provides a better alternative. [Pg.83]

This is the time taken by the stator or the rotor, whichever is less, to reach the limiting temperature rise, as specified in Table 7.5, when the starting current /s, is pas.sed through the stator windings after the motor has reached thermal equilibrium, underrated conditions. For increa.sed safety motors, this time should not be less than 5 seconds (preferably 10 seconds or more). [Pg.181]


See other pages where Motors starting current is mentioned: [Pg.108]    [Pg.2490]    [Pg.302]    [Pg.495]    [Pg.210]    [Pg.253]    [Pg.2245]    [Pg.2494]    [Pg.182]    [Pg.182]    [Pg.275]    [Pg.108]    [Pg.2490]    [Pg.302]    [Pg.495]    [Pg.210]    [Pg.253]    [Pg.2245]    [Pg.2494]    [Pg.182]    [Pg.182]    [Pg.275]    [Pg.108]    [Pg.781]    [Pg.2483]    [Pg.2490]    [Pg.2491]    [Pg.7]    [Pg.20]    [Pg.40]    [Pg.44]    [Pg.71]    [Pg.78]    [Pg.84]    [Pg.102]    [Pg.102]    [Pg.139]    [Pg.140]    [Pg.140]    [Pg.160]    [Pg.175]   
See also in sourсe #XX -- [ Pg.70 , Pg.153 ]




SEARCH



Starting current

© 2024 chempedia.info