AC motors Flashcards
What are the benefits of using an AC motor over a DC motor?
AC motors cost less than DC motors.
Some types of AC motors do not use brushes and commutators. This eliminates many problems of maintenance and wear. It also eliminates the problem of dangerous sparking.
An AC motor is particularly well suited for constant-speed applications. This is because its speed is determined by the frequency of the AC voltage applied to the motor terminals.
What is a synchronous motor?
Synchronous motors are machines whose rotor speeds and the speeds of the stator magnetic field are equal.
What is an asynchronous motor?
Asynchronous motors are machines where rotors rotate at the speeds less than the synchronous speed.
Where is the current applied in a synchronous motor?
In AC synchronous motors, three-phase current is applied to the stator windings.
What is a salient pole rotor?
poles of a salient type rotor are projected outward
What does load angle mean?
Equates to slip
What is slip?
It is the difference between the speed of the stator magnetic field and the rotor speed.
What is a salient pole machine?
On a salient pole machine, the rotor is a magnetic wheel with radially arranged pole shanks. Each pole shank consists of a pole core and a pole shoe and is attached to the excitation winding. This machine is especially suitable for low rotational speeds (e.g., water turbines).
What is a non salient pole machine?
Non-salient pole machines (shown in the graphic below) use a cylindrical rotor with the excitation winding placed inside its slots. These machines are especially suitable for high rotational speeds (e.g., steam or gas turbines). They are also referred to as cylindrical-rotor (round-rotor) machines or drum-type machines.
How does a synchronous motor start?
When starting from a standstill, the rotor cannot immediately follow the rotating field, due to inertia. Even if the excitation is switched on, the synchronous motor does not start by itself; the rotor needs to be spun up until its speed is near the rotating field speed (synchronous speed).
What are damper windings?
Damper windings are most widely used method of starting a synchronous motor. The damper winding is made up of heavy copper bars inserted in slots cut into the pole faces of the rotor. These copper bars are then short-circuited by end rings at both ends of the rotor. This arrangement of copper bars and end rings form a squirrel cage winding
What would happen if a synchronous motor runs out of step?
The motor would also be thermally overloaded at standstill. A motor must only be put on load between no-load operation and breakdown torque. Depending on the load, the rotor lags the stator’s rotating field by the load angle. Exceeding the load angle will cause the motor to reach the breakdown torque and stop.
