Electrical Sciences AC Motor
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The maximum value of torque. If load on the motor is increased beyond this point, the motor will stall and come to a rapid stop
A motor where the AC currents are induced into the rotor by a rotating magnetic field
The torque required to pull the motor into synchronism
The maximum value of torque that a motor can develop without losing synchronism
Two stator windings of unequal impedance spaced 90 electrical degrees apart and connected in parallel to a single-phase source
Single-Phase AC Induction Motor
The percentage difference between the speed of the rotor and the speed of the rotating magnetic field
The synchronous motor is excited by an external DC source and, therefore, requires slip rings and brushes to provide current to the rotor
Describe how a rotating magnetic field is produced in an AC motor
A magnetic field is produced in an AC motor through the action of the three-phase voltage that is applied. Each of the three phases is 120° from the other phases. From one instant to the next, the magnetic fields combine to produce a magnetic field whose position shifts through a certain angle. At the end of one cycle of alternating current, the magnetic field will have shifted through 360°, or one revolution
Describe how torque is produced in an AC motor
Torque in an AC motor is developed through interactions with the rotor and the rotating magnetic field. The rotating magnetic field cuts the bars of the rotor and induces a current in them due to generator action. This induced current will produce a magnetic field around the conductors of the rotor, which will try to line up with the magnetic field of the stator
Given field speed and rotor speed, calculate percent slip in an AC motor
Slip is the percentage difference between the speed of the rotor and the speed of the rotating magnetic field
Explain the relationship between speed and torque in an AC induction motor
In an AC induction motor, as slip increases from zero to ~10%, the torque increases linearly. As the load and slip are increased beyond full-load torque, the torque will reach a maximum value at about 25% slip. If load is increased beyond this point, the motor will stall and come to a rapid stop. The typical induction motor breakdown torque varies from 200 to 300% of full-load torque. Starting torque is the value of torque at 100% slip and is normally 150 to 200% of full-load torque.
Describe how starting torque is produced in a single-phase AC motor
Starting torque is the value of torque at 100% slip and is normally 150 to 200% of full-load torque
Explain why an AC synchronous motor is not self-started
A synchronous motor is not a self-starting motor because torque is only developed when running at synchronous speed; therefore, the motor needs some type of device to bring the rotor to synchronous speed
Describe how an AC synchronous motor is started
A synchronous motor may be started by a DC motor on a common shaft. When the motor is brought to synchronous speed, AC current is applied to the stator windings. The DC motor now acts as a DC generator and supplies DC field excitation to the rotor of the synchronous motor. The load may now be placed on the synchronous motor
Describe the effects of over- and under-exciting an AC synchronous motor
Keeping the same load, when the field excitation is increased on a synchronous motor, the motor operates at a leading power factor. If we reduce field excitation, the motor will operate at a lagging power factor
State the applications of the following types of AC motors
The induction motor is the most commonly used AC motor in industrial applications because of its simplicity, rugged construction, and relatively low manufacturing costs
Single-phase motors are used for very small commercial applications such as household appliances and buffers.
Synchronous motors are used to accommodate large loads and to improve the power factor of transformers in large industrial complexes.
The change in torque with respect to slip shows that, as slip ________ from zero to ~10%, the torque _______ linearly
As the load and slip are increased beyond full-load torque, the torque will reach a maximum value at about _______
The maximum value of torque is called the _________ torque of the motor
On an induction motor the _____ must rotate at some speed ______ than that of the stator if relative motion is to exist between the two.
If two stator windings of unequal impedance are spaced 90 electrical degrees apart and connected in parallel to a single-phase source, the field produced will appear to rotate
_____________ motors use a wound rotor. This type of rotor contains coils of wire placed in the rotor slots. Slip rings and brushes are used to supply current to the rotor
What are the effects of over and under exciting an AC synchronous motor?
Over exciting the motor results in operation with a leading power factor; while under exciting the motor results in operation with a lagging power factor
The use of two stator windings of unequal impedance, spaced 90 electrical degrees apart, creating a phase difference to provide a rotating magnetic field, describes how
torque is produced in a single-phase AC motor
In an AC induction motor, as __________ increases from zero to approximately 25%, the ____________ will increase to a maximum value, beyond which any increase in load will cause the motor to __________
slip, torque, stall
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