UNIT 3 TEST ALTERNATORS/ AC 3-PHASE MOTORS

Type A: Disadvantages:

High resistance produces poor running characteristics. Higher resistance results in greater speed reduction when load is added. High percent slip.

Type A: Highest value of resistance. Advantages:-

Highest starting torque per line amp due to rotor currents being in-phase with stator currents. -High resistance limits starting current.

Type B-E: Moderate value of resistance. Key Characteristics:-

Fair starting torque. -Low starting current.-Fair speed regulation.

Because the rotor is stationary at the time power is applied, the following is true

The rate (or speed) at which the magnetic field cuts the rotor bars is at its greatest. -The voltage induced into the rotor bars is at a maximum. -The induced voltage in the rotor will cause current to flow through the rotor, which develops a magnetic field. -This magnetic field is attracted to the magnetic field of the stator, and begins to rotate.

Counter-Clockwise: Marked as ____ ,indicates the incoming generator is operating at a slower frequency.

Slow

Alternators do not feature a ____________, their output is not __________, an thus alternates polarity every half-cycle.

commutator, rectified

alternators convert the mechanical energy produced by the prime mover, into ?

electrical energy

AC alternators function on the very same principles of magnetic induction as DC ______________ ?

generators

Outer Bars: Features

high resistance (due to small cross-sectional area), and low reactance (placed further from rotor core.)

Inner Bars: Features

low resistance (due to large cross-sectional area), and high reactance (placed closer to rotor core.)

When the voltages applied to the coils are the same (_____________, ______________, ____________________), the needle will be stationary.

magnitude, frequency and sequence

If the lights blink __ and ___ at the same time (in unison), than the phase rotation of each machine is matched.

on and off

If the _____ sequence of the two machines is matched, the lights will be off.

phase

Each phase winding is connected to a ________?

slipring

Because of this change, ___________________ is used to minimize (by matching the power system frequency/sequence/voltage) the sudden shock associated with the frequency change of the generators rotating magnetic field.

synchronizing

One method for paralleling alternators is the use of ____________.

synchronizing lights.

an AC induction motor can never reach _______________________!

synchronous speed

In the utility industry, the use of a ___________________ is a common method for confirming that two portions of the electrical system are in-phase(sequence/phase angle, frequency) before connecting them.

synchroscope

The armature is wound with one winding per phase. These windings are connected in?

wye and delta

Failure to Synchronize results in:

-Excessive mechanical shock as the generator transitions to the power systems frequency (transient torque.) -Damaging current in the event of severely faulty synchronization (in excess of generator's three-phase bolted fault duty). -Interference with power system. -Potential for generator to appear as, and operate as, a motor load (more likely when transitioned at a lower frequency than system) .-Operation of generator protection (such as reverse power protection), causing generator to be tripped off-line.

As load is added to the motor, the following occurs:

-Increase Load = Reduced Speed. -Reduced Speed = Increased Cutting Action. -Increased Cutting Action = Increased Induced Voltage .-Increased Voltage = Increased Current flow in the rotor = Increased Magnetic Field in Rotor. -The increased current flow in the rotor causes increased current flow in the stator---- and increased magnetic field. -The increased current flow of the stator and rotor results in increased torque due to increase magnetic fields.

Type F-V: Low value of resistance. Key Characteristics:

-Poor starting torque .-High starting current. -Good speed regulation.

synchronous speed, is based on two factors:

-The number of stator poles (per phase)-More poles per phase results in slower the synchronous speed. -The frequency of the applied voltage. -The higher the frequency, the faster the synchronous speed.

As the rotor continues to accelerate:

-The rate (or speed) at which the magnetic field cuts the rotor bars will decrease. -As the cutting action decreases, so does the induced voltage. -As the induced voltage decreases, so does both rotor current and stator current. -In an unloaded state, the motor will continue to accelerate to just below the motors synchronous speed.

The magnitude of induced voltage is based on the following three factors:

1) The strength of the magnetic field of the stator. 2) The number of turns of wire (or rotor bars) cut by the magnetic field. 3) The speed of the cutting action.

The needle of the synchroscope is used to determine the whether the frequency and phase angle of the two connected signals is safe for connection. IEEE StandardAngle: +/- __degrees Slip (Frequency): +/- _.___Hz

10 and 0.067Hz

Two available methods to safely discharge the field include:

1: Field Discharge Resistor 2: Kickback Diode

In order to parallel two alternators, the following conditions must be met:

1: The phases must be connected in such a manner that the phase rotation of the machines is the same. 2: Each phase must be in sequence with each phase of the other machine. (Phase A of each machine peaks at the same time.) 3: The output voltage of each machine must be the same. 4: The output frequency of each machine must be the same.

The torque of an AC induction motor is based on the following three factors:

1: The strength of the magnetic field of the stator. 2: The strength of the magnetic field of the rotor. 3: The phase angle difference between the rotor and stator fields.

Assume that the percent slip of a four pole, 60Hz, 1800 rpm motor is 5%.This means that the speed of the cutting action is equal to 90 rpm (5% of1800 rpm) when the motor has reached full speed. Therefor the frequency of the induced voltage and current in the rotor has dropped to _Hz. (f = (P x rpm)/120)

3Hz. (f = (P x rpm)/120)

Frequency of Induced Voltage Before connection to power system:

Determined by the rotational speed of the rotor, which is driven by the prime mover. After connection to power system: Determined by the power system frequency.

Clockwise: Marked as ____, indicates that the incoming generator is operating at a higher frequency.

Fast

________________ Source: Voltage signal from the generator to be paralleled to the power system or running generator.

Incoming

_____________ diodes have been discussed previously. They are installed in parallel to the inductor (the rotor winding) so that they are normally reverse biased with respect to the power supply.

Kickback

There are two main types of AC alternators:

Revolving Armature-Type and Revolving Field-Type

________________ Source: Voltage signal from the power system or running generator.

Running

Output voltage of an alternator is based on three factors:-

The length of the armature or stator conductors (number of turns). -Strength of the magnetic field of the rotor. -Speed of rotation of the rotor.

magnetic field is based on three factors:-

The voltages of a three-phase system are 120degrees out of phase with each other. -The voltages of a three-phase system reverse polarity at regular intervals. -The stator windings are arranged in a specific alternating sequence.

Three-phase generators, referred to as?

alternators

The ________________________ will indicate whether the incoming signal, which is from the generator to be paralleled, has a higher or lower frequency than the system or running generator.

direction of rotation