Fundamentals of DC Electric Machinery

1. MECH1200 Fundamentals of DC Electric Machinery • History • Introduction • Principle of Operation of Electric Machinery • Principle of Operation of AC Synchronous Machines • Principle of Operation of DC Machines • Types of Electric Machines • Construction of DC Machines • DC Machine Torque Characteristics • DC Machine Speed Control • Exercise Questions

2. MECH1200 History • Very advanced mechanisms are developed in the middle ages. • Development of the steam engine leads to the industrial revolution in the mid 1700’s. • Michael Faraday runs the first electric motor experiment in 1821. Faraday and Daniell in the lab A mechanical clock made in 1510 James Watt Images are in the public domain. Source: Wikimedia Commons, Wikimedia Foundation

3. MECH1200 Introduction • Electric Machinery: - Electric Motors: convert electrical energy into mechanical energy. - Electric Generators: convert mechanical energy to electric energy. • The Electric machine is composed of: Stator Rotor Image is in the public domain. Source: Wikimedia Commons, Wikimedia Foundation

4. MECH1200 Permanent Magnet and Electro Magnet N S Follows the right hand rule S N

5. MECH1200 Principles of Operation of Electric Motors • Faraday’s Law is the principle of operation of electric machines. • There are three methods by which electric motors work: • Having a stationary magnetic field in the stator and an alternating magnetic field in the rotor. - This method is employed in all DC motors except for the Brushless DC • Having a rotating magnetic field in the stator and a stationary magnetic field in the rotor. - This method is employed in synchronous motors, Stepper and Brushless DC Motors • Having a rotating magnetic field in the stator and an induced magnetic field in the rotor. - This method is employed in induction motors.

6. MECH1200 Types of Electric Machines Electric Machines DC Machines AC Machines Special Motors Permanent Magnet DC (PMDC) Motor Stepper motor Induction Machine Synchronous Machine Shunt Motor 3Ph Wound Rotor Brushless DC motor Separately Excited motor 3Ph Squirrel Cage Hysteresis motor Series Motor Reluctance motor Capacitor Start Compounded Motor Universal motor Split Phase Capacitor Run –Capacitor Start

7. Principle of Operation of AC Synchronous Machines MECH1200 N S N Alternators such as the one shown above are three phase synchronous machines. In synchronous motors, the magnetic field in the stator rotates, and the rotor tries to follow the stator’s magnetic field, i.e. it tries to synchronize itself with it, and thence the name synchronous motor. N S S N S S N

8. Principle of Operation of DC Machines When the rotor is energized, it rotates to the most stable position: Every half cycle the current flowing in the rotor switches direction due to commutation, watch: A circuit that uses a commutation technique is used to supply an alternating current into the rotor. But how can we make the magnetic field alternate in the rotor? A Stationary magnetic field in the stator An alternating magnetic field in the rotor Once the rotor rotates half a cycle, it switches polarity, so that the magnets are again repelling MECH1200 N Commutators N N S S S Brushes (To maintain electrical contact) Commutation Circuit

9. MECH1200 Construction of DC Machines • Field Windings: In DC machines, this is the winding in the stator. Note: Permanent magnet dc motors do not have field winding but rather a permanent magnet. • Armature Windings: The windings in the rotor . • Commutator segments: the method to convert the dc current to an alternating current in the rotor of the dc motor is called commutation. They are connected to the rotor windings, and stay in contact with the brushes. • Brushes: pieces made of carbon or graphite and pushed against a spring to maintain electrical contact with the commutator segments. • Interpole windings: A third set of windings are mounted on the stator, and connected in series with the rotor to reduce the sparking between brushes and the commutator.

10. MECH1200 • If the armature and field windings are connected in series, then the machine that results is called the DC series machine. • If the armature and field windings are connected in parallel, then the machine that results is called the DC shunt machine. • If the armature is connected with a field winding in series and another field winding in parallel, then the resulting machine is called the compounded machine. • Note that the interpole windings are always connected in series with the rotor.

11. MECH1200 • Name the type of each of the motors A, B, and C shown in the figure below: A: Shunt motor B: Series motor C: Compound motor

12. MECH1200 Shunt DC Motor Terminal Characteristics Question: At which speed should we operate the motor to achieve maximum power? Hint: The power corresponds to the area of the rectangle under the operating point on the speed-torque line. Which of the three rectangles has the largest area? Speed (RPM) No load speed 3000 Maximum power is at 50% of no load speed 2400 1800 Stall point 1200 600 Torque (ft-lb) 10 5

13. MECH1200 Shunt DC Motor Speed Control • There are three methods to control the speed of a DC motor: 1. Changing the armature voltage. 2. Changing armature resistance (by adding a resistor in series with armature) 3. Change the field (stator) magnetic field Speed (ω) Speed (ω) As Va increases As Ra increases Torque Torque

14. MECH1200 Series DC Motor Terminal Characteristics Speed (RPM) If the series motor is unloaded, speed increases significantly (this condition is called Runaway, and it must be avoided) Shunt motor Very high starting torque Series motor Torque (ft-lb)

15. MECH1200 Exercise Questions • How does the rotor of a dc motor maintain electrical contact with its commutation circuit? By using brushes that are pushed against a spring. • Name two motors that do not need brushes for their rotor windings. Squirrel cage induction motor and the brushless DC motor, and some types of stepper motors • Which motor supplies the highest output torque to weight ratio? The series DC motor.

16. MECH1200 Exercise Questions • Why is it not recommended to use brushed dc motors in spacecraft and some aircraft applications? Because in the space environment there is no air to carry away the heat generated by the brushes of the dc motor, causing them to wear very quickly. The same thing applies in the low pressure environment of aircraft. Thus a brushless dc motor is used. • In an environment that contains explosive gases, such as in mines, which motor do you recommend using: a) series dc motor b) shunt dc motor c) induction motor d) universal motor

17. MECH1200 • Which motor is recommended for artificial heart applications a) Series dc motor b) Shunt dc motor c) Induction motor e) Brushless dc motor • What type of machines is used as an electric generator in vehicles to recharge the battery? The synchronous generator • What is added to the dc motor to reduce sparking in dc motor brushes? Interpole windings are added.

18. MECH1200 • Which machines have a more complex construction, DC or AC machines? DC machines are much more complex, due to existence of interpoles and commutation circuits. • Which motor is potentially dangerous to operate with no load on it? The series dc motor • Why series dc motors should not be used in belt drives? Because if the belt breaks, the motor will be unloaded. Series dc motors are dangerous to operate if unloaded, as their speed will increase significantly until the motor is damaged. • Which type of motors can operate on both ac and dc power supplies? The universal motor