DC motors are built the same way as generators Armature of a motor connected to a dc power supply

1. Motor Operation • DC motors are built the same way as generators • Armature of a motor connected to a dc power supply • When switch is closed a large current flows through the armature winding due to its low resistance • Armature is within a magnetic field • A force is exerted on the windings • The force causes a torque on the shaft • The shaft rotates Electro Mechanical System

2. Counter EMF • Rotating armature cuts through the magnetic field • Voltage is induced in the armature windings E = B l v • This induced voltage is called counter-electromotive force (cemf), its polarity acts against source voltage ES • Power is taken from the electrical system Electro Mechanical System

3. Acceleration of the Motor • The net voltage acting on the armature circuit is: ES – EO • The resulting armature current I is limited only by the armature resistance: I = (ES – EO) / R • At rest, the induced voltage is zero: EO,rest = 0 V • Starting Current is 20 to 30 times greater • The large current produces a large torque • I = ES / R • As speed increases, the counter emf increases and the voltage difference diminishes • Resulting in a reduced current Electro Mechanical System

4. Example • Armature of a permanent magnet dc generator has a resistance of 1 ohm and generates 50V at a speed of 500 rpm. If the armature is connected to a 150V supply, find: • The starting current • The counter-emf when the motor runs at 1000 rpm. At 1460 rpm • The armature current at 1000 rpm. At 1460 rpm • At start-up EO,rest= 0 V so starting current is: • I = ES / R = 150V / 1Ω = 150A • Generator voltage at 500 rpm is 50 V, so counter-emf of the motor at 1000 rpm will be 100V and at 1460 rpm will be 146V • Armature current at 1000 rpm is • I = (ES – EO) / R = (150 – 100)/1 = 50A • Armature current at 1460 rpm is • I = (ES – EO) / R = (150 – 146)/1 = 4A Electro Mechanical System

5. Machine Power and Torque • Power and torque characteristics can be determined over various shaft speeds • Counter emf: EO = Zn/60 • Power supplied: Pin = Pa = ESI • Voltage drop (IR losses): ES = EO + IR • Separating power and losses • Pa = ESI = (EO + IR)I = EO I+ I2R • The mechanical power : Pm = P = EOI • The developed torque: Home Work Page 99 Example 5-2 Electro Mechanical System

6. Speed of Rotation • We know that • EO = Zn/60 • The voltage drop across the armature resistance is always small compared to the supply voltage • Even as the load varies from no-load to full-load • EO is approximately equal to ES • EO = Zn/60 • n = 60EO /Z ≈ 60ES /Z Electro Mechanical System

7. Armature speed control • ES can be varied by connecting motor armature to a separately excited variable voltage dc generator G • Field excitation of the motor is kept constant, but the generator excitation current IX varies from zero to maximum or reverse which in turn vary the ES and motor speed • This method is known as Ward-Leonard system and is found in steel mills, high rise elevators and paper mills etc. • ES is adjusted slightly higher than EO, the armature will absorb power because I flows into the positive terminal • Let us reduce ES, now EO current I reverses as a result motor torque reverses and dc motor suddenly becomes generator Electro Mechanical System

8. Armature Speed Control • Speed is controlled by varying the armature voltage ES • Motor speed changes proportionally to the armature voltage • The armature voltage is controlled by an external variable power supply • The field winding is separately excited by a constant voltage source Electro Mechanical System