Renewable Energy Systems CH-02 · 5-4 DC Motors Three basic types of wound stator motors are the...

Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd

Copyright © 2015 by Pearson Education, Inc.All Rights Reserved

Buchla, Kissell, Floyd

Renewable Energy

Systems5



5Solar Tracking

5-1 MOVEMENT OF THE SUN

5-2 COSTS AND BENEFITS OF TRACKING

5-3 SINGLE-AXIS AND DUAL-AXIS SOLAR TRACKERS

5-4 DC MOTORS

5-5 STEPPER MOTORS

Renewable Energy SystemsBuchla, Kissell, Floyd

Chapter Outline



The earth is divided by reference lines called latitudeand longitude. Latitude lines are imaginary lines that are parallel to the equator. Longitude lines are imaginary lines that form great circles that converge at the poles.

5-1 Movement of the Sun

The equator is at 0o

latitude and the North Pole is at 90o N latitude.

The prime meridian is the longitude that runs through Greenwich, England and defines 0o.



Someone at the North Pole has the earth’s axis going through their body and pointing to an imaginary pole in space (at their zenith). Likewise, someone on the equator has the projection of the earth’s equator (called the celestial equator) going directly overhead and the pole on the horizon.


(a) First day of summer at the North Pole (b) At the equator on the day of the equinox



The sun appears to move in the sky along a path known as the ecliptic, which is an imaginary circle inclined to the equator by 23.4o.


The sun appears to travel around the ecliptic over the course of a year. On the day of an equinox, the ecliptic crosses the celestial equator and there are approximately 12 hours from sunrise to sunset.



In the northern hemisphere the sun is north of the ecliptic in summer; in the southern hemisphere the sun is south of the ecliptic during summer.





The analemma is the figure 8 pattern the sun makes in its annual motion if the position is observed regularly at the same location and time of day.

David

Buchla



5-2 Costs and Benefits of Tracking

Tracking devices are used with solar concentration systems because they work best with direct radiation. Tracking systems can use one or two-axis tracking.

Dual-axis tracking is useful

for higher concentrations

Single-axis tracking is

useful for trough collectors

© 2015 by Pearson Higher Education, Inc.

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A Heliostat is a device that uses a movable mirror mounted in a manner so as to reflect the sunlight to a fixed target (such as a tower collector).

In a large tower array, each mirror needs to move independently to aim the reflection to the top of the tower.

For utility scale operations, a type of concentrating collector is the tower that is surrounded by many heliostats.


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Trackers require more space to allow for motion and avoid shading problems; this is a consideration when space is at a premium.




The decision to track or not track a flat panel depends on the result of cost/benefit analysis. Tracking with a flat panel will generally produce 20 to 40% more power than not tracking.




5-3 Single-axis and Dual-axis Solar Trackers

Solar trackers can be categorized as either active or passive.

An active tracker usually

moves by a motor. During

the day, the sun moves 15o

per hour from east to west.

At sunset, the motors can

turn the collectors back to

the east.

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Passive trackers do not need external power to move.

A passive tracker can

move by shifting weight of

a volatile liquid between

two canisters. The liquid

vaporizes easily and will

shift to the other canister if

one canister receives

more sun. The shadow

plates shade the canisters

evenly when collector is

perpendicular to the sun.




Equatorial trackers align the main driving axis along the north-south line of the earth’s axis. Panels need to be mounted in a manner to prevent interference with the daily motion.

In the northern hemisphere, the rotating axis points to the north star, which is in line with the earth’s pole. This arrangement is good for single axis trackers because the north-south motion of the sun is small compared to the east-west motion.

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Dual-axis trackers can follow the sun precisely. Most are designed as alt-azimuth mountings, whereby the axes move vertically (the altitude) and horizontally (azimuth). The system shown is a two-axis concentrating mount that was built for the Arizona Public service and uses concentratinglenses.

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5-4 DC Motors

DC motors are well suited for tracking motors, as power from the array can be tapped for power and because they develop high torque at low speeds and can start heavy loads.

All motors have two basic

parts: the rotating part that is

called the rotor, and the

stationary part called the

stator. The stator of a dc motor

on the bench for repair is

shown in the photo showing

the stationary field coils.

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5-4 DC Motors

The rotating magnet (the rotor) on a dc motor is the armature because power is taken from the rotor. Current is supplied to the rotor coils through the brushesand commutator.

The rotor is removed from the dc motor shown in the last slide. A bearing and the commutator is in the foreground. This motor shows obvious signs of wear on the commutator from the brushes making contact.

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5-4 DC Motors

A simplified view of a dc motor is shown (showing only one rotor winding). Motors typically have a number of low resistance windings, which allow high current (and hence high torque) at startup.

Current in the rotor (armature

current) creates a magnetic

field which interacts with the

field from the permanent

magnets (in this case).



5-4 DC Motors

A general curve of torque and speed is shown. In general, for a Permanent Magnet DC (PMDC) motor, the torque and speed are related with a negative linear correlation as illustrated.

The power delivered by the

motor depends on the

speed and the torque.

There is no power

developed if the motor is

not turning (stall torque) or

if there is no torque (no-

load speed). Between

these extremes, the motor

delivers power to the load.



5-4 DC Motors

Small geared motors can be useful in tracking drives because the output turns slowly. Gearing reduces the shaft speed and increases the torque, which is important for tracking motors. The motor shown is a PMDC that is smaller because of the strong ceramic permanent magnets.

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The relationship between torque

and power (in SI units) is:

q

PT

Tq =torque, in newton-meters (n-m)

P=power, in watts (W) =rotational speed, radians/second (rad/s)



5-4 DC Motors

Another type of PMDC motor uses a permanent magnet for the rotor and an electromagnet for the coils.

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Because the rotor is a

permanent magnet, there is

no need for brushes or a

commutator; hence it is

called a brushless PMDC

motor.



5-4 DC Motors

Three basic types of wound stator motors are the series-wound motor, the shunt-wound motor, and the compound motor. These designations describe the type of connection(s) between the field winding and the rotor winding.

In a series dc motor, the field

current and the armature

current are identical. The

starting torque is high, so the

motor is useful for starting heavy

loads.



5-4 DC Motors

In a shunt wound motor, the field winding is in parallel with the armature. In the shunt motor, the torque is proportional to the armature current. Because the voltage is directly across the field, the field current is constant and independent of the load.

A dc shunt motor has

increased current when

the motor is loaded. The

torque is low at startup

and increases with

increased loading. It

tends to have fairly

constant speed

throughout its load range.



5-4 DC Motors

In a compound dc motor, there are two field windings: a series winding and a shunt winding. The combination of series and shunt windings allows the motor to have the torque characteristics of the series motor and the regulated speed characteristics of the shunt motor.

There are different ways

to wire a compound

motor; a cumulative

compound short-shunt

motor is illustrated. Here

the magnetic fields of

the field coils aid each

other.



5-5 Stepper Motors

Stepper motors rotate in small steps in response to a pulse signal. They are widely used as drive motors in solar trackers because of their ability to move precisely. The diagrams show a simplified permanent magnet stepper motor, in which the rotor magnets line up with an active pair of electromagnets as they are pulsed.



5-5 Stepper Motors

Another type of stepper motor is formed if the magnets are replaced with a soft iron core. The motor moves to minimize the air gap when selected electromagnets are active. This action minimizes the reluctance of the path, so the stepper is known as a variable reluctance stepper motor.

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A hybrid stepper motor combines the permanent magnet motor and variable reluctance motor.Hybrid stepper motors have good torque and can be run at higher step than other types, so they are popular for various applications.



Selected Key Terms

Brushes

Celestial

equator

Commutator

Ecliptic

In motors or generators, the electrical conductors that provide a path for current from a stationary

part to a moving part.

The projection of the earth’s equator in the sky.

The celestial equator traces a line that is at an angle above the horizon that is 90o‒ L, where L is

the latitude.

The sun’s path in the sky with reference to an

observer on earth.

A ring structure that has independent conductors

that is designed to switch the current in a rotor coil as the structure rotates.



Selected Key Terms

Heliostat

Holding torque

Latitude

Longitude

A device that uses a movable mirror mounted in

a manner so as to reflect the sunlight to a fixed

target (such as a tower collector).

With reference to a stepper motor, the maximum

torque that can be externally applied to a

stopped motor without causing it to rotate to the

next step position.

The angle in degrees formed by a line that extends from the center of the earth to the

equator and from the center of the earth to a

given point on the globe.

Reference circles that are perpendicular to the

equator and converge at the poles. Longitude increases in the positive direction in the

eastwardly direction.



Selected Key Terms

Meridian

Prime

meridian

Rotor

Stator

Synodic day

Zenith

A line of longitude that passes through both poles

and a point straight overhead.

A great circle of longitude that passes through

both poles and Greenwich, England.

The rotating part of a motor or generator.

The stationary part of a motor or generator.

The time for the earth to make one revolution on

its axis. It is almost exactly 24 hours.

A point directly overhead of an observer.



true/false quiz

1. At the North Pole, the latitude is 0o.



true/false quiz

2. On the equinox, the ecliptic crosses

the celestial equator.



true/false quiz

3. In a large tower array, the heliostats

all point in the same direction.



true/false quiz

4. To follow the sun, all trackers require

electrical power.



true/false quiz

5. Concentrating collectors will have

the greatest benefit from tracking.



true/false quiz

6. A commutator uses coils to

generate a magnetic field.



true/false quiz

7. Some dc motors do not use brushes.



true/false quiz

8. The current in the rotor and stator is

the same in a series wound motor.



true/false quiz

9. A series wound motor has high

starting torque.



true/false quiz

10. A hybrid stepper motor combines

the permanent magnet motor and

variable reluctance motor.



true/false quiz

Answers:

1.F

2.T

3.F

4.F

5.T

6.F

7.T

8.T

9.T

10. T

Renewable Energy Systems CH-02 · 5-4 DC Motors Three basic types of wound stator motors are the...

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