1. 1 Motors and Generators ©Dr. B. C. Paul 2001

2. 2 More Fun with Flux Mechanically Rotated Shaft Slip Rings Wires with brush contacts to slip rings Electromagnetic Flux The coil rotates through the flux enclosing a changing amount of magnetic flux This induces a changing voltage in the coil. Brushes contact the slip rings and put wires at the same potential as the coil Output Voltage is Sinusoidal AC

3. 3 This Machine Converts Mechanical Energy to Electrical This is a Generator Note why mechanical to electrical conversion tends to produce an alternating current

4. 4 More Practical Machine Stator - a ring shaped iron shell Rotor - a big rotating electromagnet Current transfers onto the rotor by means of slip joints The wire wraps around the rotor creating magnetic flux that carries through the rotor - across an air gap and through the stator By rotating the rotor mechanically we can create a rotating magnetic flux (Slip rings connect to a direct current that magnetizes the rotor)

5. 5 Now What Will You Do? #1 Phase #2 Phase #3 Phase We put three separate coils through the stator The rotating flux is seen as a changing flux in each coil generating 3 voltage phases This is the secret to 3 phase AC current production

6. 6 This is Called a Synchronous Motor Three Phase Electric Current is run through the stator The AC current produces a rotating magnetic flux We now Apply a DC current to the windings around the rotor (via slip rings and brushes) making it an electromagnet It follows the rotating magnetic field at synchronous speed.

7. 7 The Stator Current An AC voltage is applied to the Stator The Stator has an impedance Z s We also have an electromagnet sweeping around the stator and inducing a voltage opposing the applied voltage VtVt ZsZs Generated back voltage V g + I a Z s = V t

8. 8 How Synchronous Motors Know How Much Power to Draw The Rotor actually lags just behind the terminal voltage (due to friction and load resistance) This means the induce voltage is slightly out of phase with the terminal voltage The bigger the load the further the rotor is behind and the more the induced voltage drops back out of phase

9. 9 The Voltage Phase Effect When the induced voltage is way out of phase with the applied voltage there is little cancellation V g + I a Z s = V t Power is a product of voltage and current Voltage stayed the same Current went up - it draws more current under load Constant

10. 10 The AC Generator (Called an Alternator) Coal is burned in a boiler The boiler makes high pressure super-heated steam The high pressure super-heated steam is cooled and de- pressurized through a steam turbine A governor on the turbine regulates rotation speed The turbine shaft goes through a gear reducer to produce 3600 rpm rotation The Shaft connects to a rotor in an alternator that goes through 60 rotations per second The rate of rotation controls the current frequency

11. 11 The Alternator There are variations that have 4 poles on an X shaped rotor instead of 2 poles on a bar shaped rotor The advantage to 4 poles is that rotation need only be 1800 rpm Reduces wear on the brush contacts Can also build 6 poles and rotate at 1200 rpm Beyond this increasing poles makes a very big impractical rotor

12. 12 Alternators Can Work in Reverse Suppose a 3 phase current is passed through the stator The rotor will act as a compass and try to align with the magnetic field The rotor is now generating torque that can turn a load An alternator running in reverse is called a synchronous motor

13. 13 Inductors Inductors allow conversion of electrical energy back and forth from magnetic flux This ability allows us to build transformers that are essential to moving power over electric lines The magnetic flux to electricity correlation allows us to mechanically turn a magnetic flux and produce electricity This is the foundation of motors and generators

14. 14 Mining Applications In many mining applications the conversion of electricity to kinetic energy is the primary objective most of our equipment is built of inductors

15. 15 Comments on Generation and Transmission Voltage We have seen that motors and generators all involve electric wiring in close proximity. The types of insulators needed to control real high voltages in limited space are not always available This is the reason that power must be generated and used at limited voltage With long range transmission we can build big poles and space wires to use air insulating We need high voltage to control transmission loss and we can get it.