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AC Generators - Three Phase Motors

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Three Phase Alternators

An alternator is a generator of alternating current. So you ask, "Why don't they just call this a generator then?" There are different terminologies for machines, and this definitely is one of those examples. The minor difference being there is no commutator in the alternator.



Types of Alternators

#1) Revolving Armature Type
-least used because of limited output
-can be connected delta or wye
-connected to sliprings (loops of wire)
-power is carried to the outside circuit via brushes riding against the sliprings.


#2) Revolving Field Type
-permits higher output because it is connected directly to the stator and not routed through brushes/sliprings
-can be connected delta or wye
-connected to stator
-three sets of windings 120 degrees apart
-two types of rotor designs used are 1) Salient Pole and 2) Cylindrical



Design of Alternators

The 'Rotor' or the rotating part of an alternator, is an electromagnet that provides the magnetic field needed to induce a voltage into the stator windings. This rotor may also have a squirrel-cage winding which dampens rotor speed from sudden changes in the load. This is also called an armortisseur or damper winding. Rotors need DC excitation to establish a magnetic field.

1) Salient Pole
-used for alternators with speeds less than 1800 RPM (rotations per minute)
-typical prime movers (diesel engines, water turbines)
-centrifugal force prevents the use of salient poles on high speed machines
-normally of LARGE diameter to allow poles to be mounted


2) Cylindrical
-used for high speed machines
-typical prime movers (steam, natural gas turbines)
- SMALL diameter due to centrifugal force limitations, and the fact that at high speed few poles are required.
-the long rotor increases the conductor length for increased voltage.



Characteristics of Alternators

Alternator Hunting
Hunting is a term used when there are speed changes in an alternator. It's a pulsating or oscillating effect that causes the current to surge back and forth between alternators in parallel. This normally occurs when alternators are driven by a reciprocating engine. The surge currents can be large enough to trip protective devices.
** This can be corrected by use of a heavy 'flywheeel' and using a 'damper winding' in the rotating field.

Brushless Exciters
As the above heading indicates the alternator is brushless. In order to provide the alternator with the current needed, a separate small armature is added to the shaft of the rotor. This rotates within the wound electromagnet stator field. This current is then rectified and used an excitation current for the main alternator. To control the field excitation of the main alternator, the field (stator) current of the exciter alternator is controlled.


Frequency
The output frequency of an alternator is determined by two factors
1) the number of stator poles
2) the speed of the rotation of the rotor
This can be held constant with a 'governor'


formula


Output Voltage

Since the windings can't be changed once the machine has been built, and the speed can't be changed without effecting frequency, the only practical way to control the voltage is by changing the field strength by adjusting the excitation level. The factors that determine output voltage are
1) the conductor length (of the armature or stator winding)
2) the strength of the field
3) the speed of the rotor (held by a governor)
The output voltage of an alternator can be held constant with an automatic voltage regulator.


Voltage Regulation
With (NL) connected to the alternator, its terminal voltage (VT) is equal to generated voltage (EG). When a load is added, this results in current flow in the stator coils. This current then causes, within the alternator, a voltage drop which results in reduced terminal voltage. So the terminal voltage will change with a load unless a correction is made to keep it constant.

Field Discharge Protection
When the excitation current is interrupted, the collapsing magnetic field can cause the contacts to arc and create high voltage surges. This will damage the rotor winding insulation. So two devices are commonly used to prevent this damage
1) field discharge resistors
2) free-wheeling diodes



Alternator Impedance
Factors that determine the internal impedance of an alternator are
-stator conductor resistance
-stator coil inductive reactance
-armature reaction
**Their effect on the alternators terminal voltage is influenced by the power factor of the load.

Armature Reaction
At 'unity' power factor the armature reaction causes minimal distortion of the main field.
- a lagging power factor happens when a magnetomotive force (mmf) is created by the current of the stator conductors which opposes the magnetizing force of the main field resulting in reduced terminal voltage. As the voltage drop increases, the power factor lags even more.
-a leading power factor happens when a magnetomotive force (mmf) is created by the current of the stator conductors which aid the magnetizing force of the main field resulting in increased terminal voltage. As the voltage is increased the power factor leads more. Having said that, leading (capacitive) power factor loads are not as common as lagging (inductive) power factor loads.

Induced Rotor Voltage
It is the stators rotating magnetic field that cuts the rotor bars, that induce a voltage in the rotor. Therefore, the speed of the rotating field is relative to the winding speed in rpm.

Slip
Slip is the difference between synchronous speed and rotor speed.
synchronous speed - rotor speed = slip


Percent Slip




Rotor Frequency
f = rotor frequency
p = number of stator poles
sr = rotor slip in rpm



Flux
Flux becomes more in phase as the motor speed increases







The Connections/Terminations of Alternators

Paralleling Alternators
Before an alternator is connected in parallel, certain conditions need to be satisfied
1) voltages must be the same
2) they have to be in phase
3) they have to be in the same phase rotation
4) and they have to be the same frequency



Lights can be used to check the 'right' connections. There are two common ones used.
1) Three Dark Method - is used to determine phase rotation. A to A.
2) Two Bright, One Dark Method - is used after phase rotation has been established. This is used to check if they are in phase with each other. A to B, B to C, etc.



Terminal Labelling
This is something you're going to have to memorize. Typical trade knowledge for you to store any way you can. There are ways to make it simple for you remember, and we believe that the simplest way is shown below. If you remember this list, you will remember any rotation of 'Y' or 'Delta' windings in the field. Remember this is three phase connections.





Terminal Block Connections
The following has been uploaded for you to be able to utilize as a graphical representation. Feel free to copy and paste these graphics as needed. Apprentices using this graphic is much better than apprentices not.




















Torque




**rotor design influence starting current and starting torque.

A -
Starting Torque - the maximum torque the machine will develop at zero speed.

B-
Pull Up Torque - the minimum torque developed while accelerating from rest to breakdown.

C-
Breakdown Torque (Stalling) - the maximum torque the motor will produce. If it is increased beyond this point, the motor will stall.

D-
Full Load Torque - the torque produced when running at rated horsepower and rated full load speed.


Three factors that determine torque are
1) the strength of the stator magnetic fields
2) the strength of the rotor magnetic fields
3) the phase angle difference between rotor and stator fields (see `flux` above)

Formulas for Pound - Inch or Pound - Foot






Horsepower

1 Horsepower = 746 Watts


Horsepower ratings are marked on nameplate as well as speed.



Speed Regulation





Efficiency




Motor Start Up

The actual amount of starting current is determined by
-type of rotor bars
-horsepower rating
-applied voltage

Example - 0.5 (HP) 208(V) 3 Phase - Code J (This code is found on Nema standards)
Code J = 7.1-7.9 - locked rotor KVA per Horsepower






Sizing Fuses and Conductors for Motors

This is going to be a big one people. Get your code books out, and turn to page 465 of your code book. Or go to Appendix B Section 28.

You'll see something similar to the drawing you're looking at below, but with a little less markings. The following is an example of ONE motor sizing. It takes several different scenarios, and several examples to get used to these, but this one will get you started.