Unit 31 - DC Motors
DC Servomotors
- Small PM Motors - Small, lightweight armatures that contain little inertia -Can be operated at high speed and then stopped or reversed very quickly - Have 2-6 poles - Used to power spindles on numerically controlled (NC) machines
PM motors of comparable horsepower ratings are _____ and ______ than conventional field-wound motors
- Smaller -Lighter in Weight
Motors powered by the trapezoidal waveform produce about _______% more torque than than those powered by sine waves, but motors powered by sine waves operate more _____ and with less ____
- 10% - Smoothly - Less torque ripple at low speeds
Shunt Motor
- An external power source supplies current to the shunt field and maintains a constant magnetic field - Shunt motor has very good speed characteristics - Full-load speed generally remains within 10% of the no-load speed - Often referred to as "constant-speed motors" - Maintains the most constant speed as load is added and armature current increases
Characteristics of DC ServoDisc Motors
- Armature contains no iron which permits very smooth operation at low speeds - Extremely fast acceleration - Thin, low-inertia disc armature permits exceptional torque-inertia ratio - 0-3000 rpm in 60 degrees (1/6 revolution) -Low-inertia armature alos permits rapid stops and reversals - Operate at speeds over 4000 RPM
How are the permanent magnet pairs arranged in a DC ServoDisc Motor?
- Arranged around the circumference of the motor housing in such a manner that they provide alternate magnetic fields
Horsepower Conversions (Do practice examples in notes)
1 W = 3.412 BTUs per hour 1055W = 1 BTU per second 4.18 W = 1 calorie per second 1.36 W = 1 ft-lb per second GET ALL EQUATIONS FROM NOTES
1. What is a motor? 2. What principle do motors operate on? 3. Two factors that determine torque produced are?
1. Machine that converts electrical energy into mechanical energy 2. Principle of attraction and repulsion of magnetism 3. Strength of magnetic field of pole pieces and strength of magnetic field of armature
What are the 3 classes of DC motors?
1. Shunt Motors 2. Series Motors 3. Compound Motors
What 2 factors is torque determined by?
1. Strength of the magnetic field of the pole pieces 2. Strength of the magnetic field of the armature
21. Brushless motors generally require less ______ and have less ______ 22. PM Motors use permanent magnets as the ______ and do not require _____ 23. Operating characteristics of a PM motor are similar to______
21. Less Maintenance and have less downtime than conventional DC motors 22. Pole Pieces; do not require series or shunt field windings 23. Similar to those of a shunt motor with external excitation
24 V supplied... on time is 10 microseconds and off time is 40 microseconds
24V is now supplied 20% of the time therefore average voltage is 24 x 0.2 = 4.8 V`
4. What is torque proportional to? 5. What is current flow through the armature limited by? 6. Three factors that determine amount of CEMF produced by a motor are?
4. Armature current if field excitation current remains constant 5. CEMF and armature resistance 6. - Strength of magnetic field of pole pieces - Number of turns of wire in armature - Speed of armature
What can the torque-to-weight ratios of PM motors equipped with rare-earth magnets exceed conventional by?
40%-90%
How much can the power-to-weight ratio exceed conventional?
50%-200%
7. What are the 3 types of DC motors? 8. What are shunt motors otherwise known as? 9. Series motors must never be_______
7. Series, shunt, compound 8. Constant-speed Motors 9. Operated under a no-load condition
What is a motor?
A device used to convert electrical energy into mechanical energy - construction is similar to DC generator (use same magnetic induction)
What do most large compound DC motors have as a protective device?
A field-loss relay which is connected in series with the shunt field
What do most large DC motors use for speed control?
An electronic controller to supply variable voltage to the armature circuit separately from the field which permits continuous adjustment of the speed from zero to full RPM
When are brushless motors used?
Applications where high torque and low speed are required - have brushless motors that have a very high number of stator poles
How many poles do some of these motors have?
As many as 64 stator poles per phase - at 60 Hz, this would produce a speed of 112.5 rpm.
Speed Control Flow Diagram
As resistance is added to the shunt field, field current decreases which causes a decrease in the flux density of the pole pieces -> this decrease in flux density produces less CEMF in the armature, which permits more armature current to flow -> the increased armature current causes an increase in the magnetic field strength of the armature -> this increased magnetic field strength of the armature produces a net gain in torque, which causes the motor speed to increase
What are the disadvantages of adding resistance in the armature circuit ?
Even though it permits the motor to be under sped, there are disadvantages: 1. As current flows through the resistors, they waste power in the form of heat 2. Also, the speed of the motor can only be controlled in steps. There is no smooth increase or decrease in speed
What do the conductors in the armature have in a DC ServoDisc Motor? What does this do?
Have a current flow which is perpendicular to the magnetic field (radial to the shaft) - This produces a torque perpendicular to both the magnetic field and the current and this force rotates the shaft
What increases torque?
Increasing the number of loops and turns
What is the use of this many poles referred to as? Why?
Magnetic Gearing - because it eliminates the need for mechanical gears and other speed-reducing equipment
What must happen in order to produce more torque?
More current must flow through the armature, which increases the magnetic field strength of the armature
*Note
Most large DC motors have voltage applied to the shunt field at all times, even when the motor is not in operation The resistance of the winding produces heat, which is used to prevent any formation of moisture inside the motor
In a DC ServoDisc motor, the armature is very thin and therefore, permits the permanent magnets to be _______
Mounted on either side of the disc and parallel to the shaft
What can electronic power supplies also do?
Sense the speed of the motor and maintain a constant speed as load is changed - most of these power supplies are current-limitied and the max. current output can be set to a value that does not permit the motor to be harmed if it stalls or if the load becomes too great
Which type of motor produces the most torque of the three motors?
Series Motor
What allows the flow of current through the armature when the motor is in operation?
The CEMF
What happens if the field excitation current is constant?
The amount of CEMF is proportional to the speed of the armature - the faster the armature turns, the higher the CEMF (and when speed of armature decreases, the DEMF also decreases)
What happens if the shunt field current stops completely?
The compound motor becomes a series motor and increases rapidly in speed which can cause damage to both the motor and the load
What is speed determined by?
The number os stator poles per phase and the frequency of the AC voltage
Why are shunt field leads often changed on small motors?
This is because the amount of current flow through the field is much less than the current flow through the armature which permits a small double-pole double-throw (DPDT) switch to be used as a control for reversing the direction of rotation
*Armature current is controlled by a _________
Variable-voltage power supply
What is the turning force (torque) of a motor caused by?
- Caused by the interaction of the magnetic fields surrounding the wire loops and the pole pieces
Compound Motors can also be __________ or ___________
- Cumulative-compound motors - Differential-compound motors
Series Motor Speed Characteristics
- Have no natural speed limit and should therefore never be operated in a no-load condition - Large series motors that suddenly lose their load race to speeds that destroy the motor - For this reason, series motors should be coupled directly to a load - Have the ability to develop extremely high starting torques (average of 450% of full torque is common) - Generally used for application the require a high starting torque, such as the starter motor on an automobile, cranes, and electric buses
How high do the ratings for PM motors go?
Over 15 horsepower
What is the armature?
The rotating part of a DC motor
Counter-Electromotive Force
- When the windings of the armature spin through the magnetic field produced by the pole pieces, a voltage is induced into the armature - This induced voltage is opposite in polarity to the applied voltage and is known as CEMF or back-EMF
What do large compound motors often use to reverse direction of rotation?
Often use a control circuit that uses magnetic contractors to reverse the flow of current through the armature - only the current through the armature changes direction. The current flow through the shunt and series fields remain the same
What happens when load is added to the motor?
The torque is not sufficient to support the load at the speed at which the armature is turning and therefore the armature slows down - When this happens, CEMF is reduced and more current flows through the windings which produces an increase in magnetic field strength and an increase in torque - This is the reason that armature current increases when load is added to the motor
What is the function of the field-loss relay?
To disconnect power to the armature if current flow through the shunt field decreases below a certain level
Motors that use a high pole count for low-speed, high-torque applications are often called _____ or _____
- ring motors - ring torquers
What are some of the losses that enough torque can overcome?
1. I^2R loss in the armature windings 2. Windage Loss 3. Bearing Friction 4. Brush Friction
10. What can series motors develop? 11. Compound motors can contain ____ and _____
10. Extremely high starting torque 11. Both series and shunt field windings
12. When full voltage is applied to BOTH the armature and shunt field, what happens? 13. When full voltage is applied to the field and reduced voltage is applied to the armature, what happens? 14. What happens when full voltage is applied to armature and reduced voltage is applied to the shunt field?
12. Motor operates at base speed 13. Motors operates below base speed 14. Motor operates above base speed
How can the speed be controlled in a PM motor?
By variable voltage applied to the armature
What are two-phase stator windings commonly used for?
Motors intended to operate small fans
What is speed regulation?
The amount that speed decreases as mechanical load is increased and is proportional to the resistance of the armature - Lower armature resistance means better speed regulation - this is because armature current determines the torque produced by the motor if the field excitation current is held constant
When is torque produced in a DC ServoDisc motor?
When current flowing through the copper conductors of the disc produces a magnetic field that reacts with the magnetic field of the permanent magnets
What are the difference between brush-type and brushless motors?
- Brushless motors do not have a commutator or brushes, they are generally smaller and cost less - Added expense of the converter, however, makes the cost about the same as the brush-type - Brushless motors dissipate heat more quickly because the stator windings can dissipate heat faster than a wound armature - They are more efficient than brush-type motors, require less maintenance, and as a general rule have less downtime
Because the fields are permanent magnets, the field flux of PM motors remains ______. What does this do?
- Constant at all times - Gives the motor operating characteristics very similar to those of conventional separately excited shunt motors
PM (permanent magnet) Motors
- Contain wound armature and brushes - Pole pieces are permanent magnets - Eliminates need for shunt or series field windings - Have higher efficiency because power is supplied to armature circuit only - Popular where batteries are used
Brushless DC Motor
- Does not contain a wound armature, commutator or brushes - Armature or rotor (rotating member) contains permanent magnets - The rotor is surrounded by fixed stator windings - Stationary armature or stator winding is generally three phase, but some motors are designed to operate on a four-phase or two-phase power - Phases are provided by a converter that changes the DC into AC (used to create a rotating magnetic field inside the stator)
How is the first shunt field connected? How is the second shunt field connected?
- One shunt field is connected to a fixed voltage and maintains a constant field to provide an upper limit to motor speed and is connected to the field-loss relay - The second shunt field is connected to a source of variable voltage and is used to increase speed above the base speed
18. What doesn't a brushless DC motor contain? 19. The stator windings for brushless motors are generally ______ 20. What do brushless DC motors require? Why?
18. Wound armature, commutator or brushes 19. 3- phase but can be 4-phase or 2-phase 20. A converter to change the DC into multiphase AC
What does the rotating magnetic field attract?
Attracts the permanent magnets of the rotor and causes the rotor to turn in the same direction as the rotating field
How is base speed achieved?
By applying full voltage to the armature and both shunt fields
How can the direction of rotation of a DC motor be reversed?
By changing the connections of the armature leads or the field leads (common practice to change the connection of the armature leads - to prevent changing a cumulative-compound motor into a differential-compound motor) - It is also not uncommon to reverse the rotation of small shunt motors by changing the connection of the field leads (if a motor only contains a shunt field, there is no danger of changing the motor from cumulative to differential-compound motor)
What is one method of reducing armature current?
Connect resistance in series with the armature
What do many large DC compound motors intended to operate in an overspeed condition contain?
Contain two separate shunt fields - one shunt field is used to provide a stable speed and the other shunt field provides overspeed control
Who discovered Horsepower and what is it?
James Watt found that the average horse could do work at a rate of 550 ft-lbs per second (this is the horsepower measurement) and can also be expressed in the basic electric unit for power which is the Watt 1 hp = 746 W
How do you determine which way to control the speed of DC motors?
Method Generally Employed is dictated by the requirements of the load - When full voltage is connected to both the armature and the shunt field, the motor operations at its base speed - If the motor is to be operated below base speed or under speed, full voltage is maintained to the shunt field and the amount of armature current is reduced - The reduction of armature current causes the motor to produce less torque, and the speed decreases
Some converters that supply power to brushless motors produce ______, but most produce a ______
- Sine Waves - Trapezoidal AC Waveform
DC ServoDisc Motors
- Uses permanent magnets to provide a constant magnetic field, but the armature design in completely different - Armature does not contain any iron - It is made of 2-4 layers of copper conductors formed into a thin disc (youtube video) - Conductors are printed on a fibreglass material therefore a.k.a. "Printed-Circuit Motor"
Speed-Torque Characteristics
- When a DC motor is first started, the inrush of current can be high because no CEMF is being produced by the armature - When current flows through the armature, magnetic field is produced and it starts to turn - As windings cut through the magnetic field of the pole pieces, CEMF is induced in the armature. CEMF opposes the applied voltage causing current flow to decrease - If motor is not connected to a load, the armature continues to increase in speed until CEMF is almost the same value as the applied voltage, at this point, motor produces enough torque to overcome its own losses
What 3 things is the amount of CEMF proportional to?
1. The number of turns of wire in the armature 2. The strength of the magnetic field of the pole pieces 3. The speed of the armature
How is the direction of rotation of a DC motor determined?
By facing the commutator end of the motor (generally the back or rear of the motor)
Series Motors
- Only has a series field connected in series with the armature - Armature current therefore flows through the series field - Speed of the series motor is controlled by the amount of load connected to the motor - When load is increased, the speed of the motors decreases which causes a reduction in the amount of CEMF produced in the armature and an increase in armature and series field current - Because current increases in both the armature and series field, the torque increases by the square of the current - If current double, torque increases 4x
24. PM motors are more _____ than conventional DC motors 25. Servomotors have _____ with _____ 26. What is a disc servomotor armature made of? 27. Disc servomotors can be accelerated ____
24. Efficient 25. Lightweight armatures with low inertia 26. Made of several layers of copper conductors 27. Very Rapidly
Example: peak voltage of 24 V and are turned on for 40 microseconds and off for 10 microseconds during each pulse....
24V are supplied to motors for 80% of the time, producing an average voltage of 19.2 V (24 x 0.8 = 19.2V)
How can a DC motor be overspeeded?
By connecting a full voltage to the armature and reducing he current flow through the shunt field
How is the direction of rotation reversed in PM motors?
By reversing the polarity of voltage applied to the armature leads
Characteristics of DC ServoDisc Motors x2
- Speed can be varied by changing amount of voltage supplied to armature - Voltage is varied using pulsewidth modulation - Most amplifiers produce a DC pulsating voltage at approximately 20 kHz - Average voltage supplied is determined by length of time it is turned off (pulse width) - At 20 kHz, pulses have a width of 50 microseconds (pulse width = 1/freq)
What is the amount of current flowing through the armature determined by?
- The CEMF and the armature resistance
What happens when a DC machine is used as a generator?
- The commutator performs the function of a mechanical rectifier converting AC into DC before is exits the machine through brushes
What happens when a DC machine is used as a motor?
- The commutator performs the function of a rotary switch and maintains the correct direction of current flow through the armature windings
Terminal Identification for DC motors
- The terminal leads of DC machines are labelled so the can be identified when they are brought outside the motor housing to the terminal box - DC motors have the same terminal identification as that used for DC generators
Compound Motors
- Uses both a series field and a shunt field - Used to combine operating characteristics of both - Series field of the compound motor permits the motor to develop high torque, and the shunt field permits speed control and regulation - Used more than any other type of DC motor in industry - Does not develop as must torque as the series, but develops more than shunt - Speed regulation is not as good as a shunt motor, but is much better than a series motor - Can be connected as short shunt or long shunt **long shunt is more common because of superior speed regulation**
15. How can the direction of a DC motor be changed? 16. What is common practice? 17. What is the shunt field relay used for?
15. By reversing the connection of either the armature or the field leads 16. To reverse the connection of the armature leads to prevent changing a compound motor from a cumulative to a differential compound motor 17. Used to disconnect power to the armature if shunt field current drops below a certain level
