13.2 INDUCTION MACHINES 563
Field
Shunt field
Series field
Field
rheostat
Field rheostat
(a)
(c) (d)
(b)
Dc source
Armature
Armature
Armature
Shunt field
Series field
Field rheostat
Armature
Figure 13.1.8Field-circuit connections of dc machines.(a)Separately excited machine.(b)Shunt machine.
(c)Series machine.(d)Compound machine.
Vt=Ea+IaRa (13.1.12)
whereIais now the armature currentinput.Under steady-state conditions,volt–amperecharac-
teristic curves are of interest for dc generators, andspeed–torquecharacteristics are of interest
for dc motors. Depending on the method of excitation of the field windings, a wide variety of
operating characteristics can be obtained. These possibilities make the dc machine both versatile
and adaptable for control. More about dc machines is presented in Section 13.4.
13.2 Induction Machines
We shall first consider polyphase induction motors and then single-phase induction motors. The
polyphase induction motors used in industrial applications are almost without exception three-
phase. The stator winding is connected to the ac source, and the rotor winding is either short-
circuited, as in squirrel-cage machines, or closed through external resistances, as in wound-rotor
machines. The cage machines are also known asbrushlessmachines; the wound-rotor machines
are also calledslip-ringmachines.
A review of the subsection on elementary induction machines in Section 13.1 can be helpful
at this stage to recall the operating principle of polyphase induction machines.
Equivalent Circuit of a Polyphase Induction Machine
The induction machine may be regarded as a generalized transformer in which energy is converted
and electric power is transferred between stator and rotor, along with a change of frequency and a
