What is the rotor current usually induced by in a-c motors?

TEXT 1  MOTOR STARTING

In contrast to a-c motors where the rotor current is usually induced by transformer action from the stator winding, current is introduced into the rotor or armature windings of d-c motors by means of brushes and a commutator. The winding coils are progressively energized as they are oriented with respect to the field poles to produce the desired rotating torque. As with any switch, there is a limit to the amount of current that can be interrupted as each commutator segment moves away from the brush. Motors are available that can commutate as high as 500% of the full-load motor current. In most cases, however, 250 to 275 % is a safer value. As shown in Fig.2, the commutating ability is also a function of motor speed and applied voltage. These capabilities vary for different motor designs. Switches to connect the motor to the power supply can be a set of manually operated contacts, such as on a drum switch or a set of magnetically operated contacts. Addition of an overload relay to give motor running-overload protection is all that is needed to produce a d-c line stater. Manually operated  switches are available to handle fairly high motor currents. NEMA lists general-purpose across the-line manually operated starters up to 2 hp, and drum switches up to 300 amp and 100 hp for intermittent duty. General-purpose protective panels provide overload and low-voltage release for manual starters, controllers, and drum controllers with a reset contact in the off position. When a reset contact is not provided in the manual control, the controller must be in the off position before the protective panel is reset.

An inverse-time-limit overload permits momentary overloads, such as encountered in starting, but disconnects the motor from the line on maintained overloads. Protective panels are available in ratings from 1

to 150 hp at 115, 230 and 550 v. Multimore panels with a separate

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13.03.18= 4 сем. 2018 (Э-325 СН) З /О АВИЭТ ВАРИАНТ 1

overload for each motor and one power circuit contactor are often use for applications, where hoist, trolley and bridge drives are involved.

Magnetically operated contactors usually provide a safer and more convenient means for completing or interrupting electric power circuits, particularly in large-horsepower ratings. A magnetic contactor utilizes an electromagnet to actuate its contacts. Coils for the electromagnet are designed to meet a range of operating voltages. According to NEMA Standards, contactor coils must withstand 110% rated voltage continuously without injury to the coil. They must also operate the contactor successfully at 80% voltage.

What is the rotor current usually induced by in a-c motors?

Why are the winding coils energized?

What are manually operated switches used for?

What must be added for larger motors?

What happens with the resistance in a manual face-plate starter?

TEXT 1 SYNCRONOUS MOTORS

Syncronous  motors characteristically operate at constant speed, independent of load.Speed is directly proportional to the line frequency of the power supply and inversely proportional to the number of poles in the stator or rotor.

The motors are made in all sizes – from subfractional-horse power, non-excited units to large-horsepower, direct-current-excited motors for industrial drives. Their electrical input may vary from a few watts, as in the case of electric clocks and timing motors, to thousands of kilowatts

for large industrial motors driving loads such as compressors or pumps.

MOTOR TYPES. Commonly used synchronous motors include hysteresis, inductor, permanent magnet, reluctance, and direct-current-excited types. All but the last type are generally in the small-horsepower category, with the hysteresis ad inductor types having subfractional-horse power ratings. Permanent magnet and reluctance types extend into the integral-horsepower range. Direct-current-excited machines can range from small integral-horsepower sizes to 25, 000hp or more. Following a brief review of each of the four smaller types, the remainder of this chapter dwells on the d-c excited machines – particularly the salient pole type.

Subfractional-horse power hysteresis motors have a smooth, cylindrical, cast magnetic-alloy rotor, which starts by virtue of hysteresis losses induced in it by the rotating field of the stator. The rotor locks into step due to the retentivity of the rotor material. These motors are used for timing and as drives for recording instrument where precise, constant speed is required.

Inductor motors have a slotted or toothed rotor. The stationary member

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13.03.18= 4 сем. 2018 ( Э -325 СН ) З / О АВИЭТ ВАРИАНТ 2

is equipped with a main winding and either permanent magnets or an

exciting winding to establish magnetic poles. Forces between the stationary poles and the rotor teeth produce torque. These motors are used on timer applications where load inertia is low.

Permanent-magnet motors have permanent magnets embedded in a squirrel –cage type rotor to produce fixed poles which lock into step with the armature field. Because of its relatively high efficiency and power factor, this motor is becoming increasingly popular in the fractional and lower integral- horsepower range.

Reluctance motors are the most common of the non-excited synchronous types, found in sizes from sub-fractional values up to about 100 hp. The rotor has salient poles (without permanent magnets or exciting windings) which lock into step with the rotating field due to reluctance torque. Some reluctance motors have smooth, cylindrical rotors with cast aluminium filling the space between the magnetic salient poles and also forming the rotor-cage winding for starting.Efficiency and power factor are lower than for wound, salient- pole, direct-current-excited synchronous motors. However initial cost is also lower.

TEXT 1  MOTOR STARTING

In contrast to a-c motors where the rotor current is usually induced by transformer action from the stator winding, current is introduced into the rotor or armature windings of d-c motors by means of brushes and a commutator. The winding coils are progressively energized as they are oriented with respect to the field poles to produce the desired rotating torque. As with any switch, there is a limit to the amount of current that can be interrupted as each commutator segment moves away from the brush. Motors are available that can commutate as high as 500% of the full-load motor current. In most cases, however, 250 to 275 % is a safer value. As shown in Fig.2, the commutating ability is also a function of motor speed and applied voltage. These capabilities vary for different motor designs. Switches to connect the motor to the power supply can be a set of manually operated contacts, such as on a drum switch or a set of magnetically operated contacts. Addition of an overload relay to give motor running-overload protection is all that is needed to produce a d-c line stater. Manually operated  switches are available to handle fairly high motor currents. NEMA lists general-purpose across the-line manually operated starters up to 2 hp, and drum switches up to 300 amp and 100 hp for intermittent duty. General-purpose protective panels provide overload and low-voltage release for manual starters, controllers, and drum controllers with a reset contact in the off position. When a reset contact is not provided in the manual control, the controller must be in the off position before the protective panel is reset.

An inverse-time-limit overload permits momentary overloads, such as encountered in starting, but disconnects the motor from the line on maintained overloads. Protective panels are available in ratings from 1

to 150 hp at 115, 230 and 550 v. Multimore panels with a separate

                                            -5-

13.03.18= 4 сем. 2018 (Э-325 СН) З /О АВИЭТ ВАРИАНТ 1

overload for each motor and one power circuit contactor are often use for applications, where hoist, trolley and bridge drives are involved.

Magnetically operated contactors usually provide a safer and more convenient means for completing or interrupting electric power circuits, particularly in large-horsepower ratings. A magnetic contactor utilizes an electromagnet to actuate its contacts. Coils for the electromagnet are designed to meet a range of operating voltages. According to NEMA Standards, contactor coils must withstand 110% rated voltage continuously without injury to the coil. They must also operate the contactor successfully at 80% voltage.

What is the rotor current usually induced by in a-c motors?

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