0195136047.pdf

554 ROTATING MACHINES

e

T

Te, ωm

(a)

i

+

−

v

Field

Armature

e

Te

T, ωm

(c)

i

+

−

v

Field

Armature

e

Te

T, ωm

(b)

i

+

−

v

Field

Armature

Figure 13.1.1Three modes of operation of

a rotating electric machine.(a)Motoring

mode.(b)Generating mode.(c)Braking

mode.

2. Thegenerating modehas mechanical power input and electric power output. The torque
   Tapplied externally to the shaft drives the machine against the electrically developed torqueTe.
   The generated emfedrives current out of the winding against the terminal voltagev.


3. Thebraking modehas both mechanical and electric energy input. The total input is dissi-
   pated as heat. The machine is driven by the externally applied torqueT, while the electromagnetic
   torqueTeis opposingT, thereby braking the machine. The electric braking of motor drives
   is achieved by causing the motor to act as a generator, receiving mechanical energy from the
   moving parts and converting it to electric energy, which is dissipated in a resistor or pumped
   back into the power line. Note that the applied voltagevand the generated emfedo not oppose
   each other.
   As mentioned in Section 12.3, polyphase windings can be arranged to yield sinusoidally
   distributed current sheets and rotating mmfs. A number of possible doubly excited combinations,