week 10

Conductive Rotor

he gaps between the bars are filled with conductive plates because it is more permeable to magnetic fields than air.

Wound-Rotor Resistance

high resistance at low speeds and low resistance at high speeds

The relative motion

between the rotor and the rotating magnetic field is called slip and is usually expressed as a percentage of the speed of the rotating magnetic field.

The higher the rotor resistance the greater the

starting torque

Reversing direction When using a poly-phase induction motor

switch two ofthe phases. Thus switching two of the three wires in a three-phase motor will reverse its direction.

how a squirrel cage rotor can be turned into an electromagnet by induction

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hree basic requirements for an induction motor to produce torque

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Relative Motion

which means the rotor will always spin slower than the rotating magnetic field.

single phase motors are not self-starting becuase

cannot start on their own is because with only one phase the AC current doesn't create a rotating magnetic field but a pulsing magnetic field. Because all the stator coils are powered with the same phase, thus they all experience the same peak amplitude of voltage at the same time.

Actual Speed

depends upon the amount of load that is being applied to the motor. As the torque, or load, increases then the actual speed of the motor must decrease

downside of the synchronous motor

the motor is not strong enough to start itself

Slip

the percent difference between a rotor's actual speed and the synchronous speed

Wound-Rotor Resistance cons

the speed regulation at the rated load and at low slip percentages is poor

plus to having higher starting torque

with higher rotor resistance is that the starting current (also called locked-rotor current)decreases with increases in rotor resistance. This is quite unusual considering that usually you need more current to provide more current, but in this case you get more starting torque and less starting current with increases in rotor resistance.

positive feature of the synchronous motor

without slip, the speed is equal to the synchronous speed and is only a function of the input frequenc

Rotating Magnetic Field

Using single or poly-phase AC power, a rotating magnetic field is generated naturally because of the alternating behavior of AC voltage.

Shaded-Pole Motor applications

These motors produce a very low starting torque and are thus best suited for low-power household applications like fans, can openers, blowers, and electric razors.

Synchronous Speed

An induction motor's synchronous is dependent upon the frequency of the AC input and the number of poles per phase.

Universal Motor

Universal motors are the only single-phase AC motor we will discuss that has a wound rotor. Because it is not an induction motor, it does not require a rotating magnetic field, nor relative motion reverse the polarity of the input voltage to either the rotor or stator windings, but not both.

Wound-Rotor Resistance pros

As resistance goes up, the starting torque goes up and the starting current (locked-rotor current) goes down,

Universal Motor applications

Because they are capable of higher speeds, they are useful in household applications that require high speeds, like blenders, vacuum cleaners, and hair dryers.

with the increase in starting torque

Type C and D motors have worse speed regulation at the normal operating torque (100% torque), which means for a small change in the load the speed of the motor changes more with type C and D motors.

induction motors can overheat when stalled

When running an AC motor at a frequency below the rated frequency (60 Hz), then the current into the motor will increase and overheat the motor.

Split-Phase Capacitor Motor cons

motors are higher than regular split-phase motors, but they are used to create more starting torque with less starting current and to operate at higher efficiencies.

Shaded-Pole Motor

motors overcome the problem of starting the single-phase AC motor by adding a small copper loop, shaded-pole shifts the magnetic flux just enough to create a rotating field to get the motor spinning.

The synchronous speed

of a rotor is the maximum possible speed that the motor could theoretically obtain

Split-Phase Motor

order to start a split-phase induction motor, the single-phase AC current is split in parallel into two different stator windings (start and run) strategically placed around the stator such that they simulate two different phases of AC current about 45 degrees apart.This creates a rotating magnetic field which will get the rotor to start turning

Split-Phase Capacitor Motor

similar to split-phase motors but they incorporate a capacitor into the circuitry that simulate two different phases of AC current about 80 deg t, which is greater than 45 deg that is typical in a split-phase motor, provide more than double the starting torque with one-third less starting current than the split-phase motor.

limitation of a single phase induction motor

single phase motors are not self-starting single-phase induction motors use a squirrel cage rotor motors require single phase motor is initially stationary, there is no relative motion,