Nonexcited Electric Motors are made in reluctance and hysteresis designs. These electric motors employ a self-starting circuit and require no external excitation supply.
Dc-excited Electric Motors come in sizes larger than 1 hp, and require direct current supplied through slip rings for excitation. Direct current may be supplied from a separate source or from a dc generator directly connected to the ac motor shaft.
Single-phase or polyphase synchronous electric motors can't start without being driven, or having their rotor connected in the form of a self-starting circuit. Since the electric motor field is rotating at a synchronous speed, the electric motor must be accelerated before it can pull into synchronism. Accelerating from zero speed requires slip until synchronism is reached. Therefore, separate starting means must be employed.
In self-starting electric motor designs, fhp sizes use starting methods common to induction electric motors (split-phase, capacitor-start, repulsion-start, and shaded-pole). The electrical characteristics of these electric motors cause them to automatically switch to synchronous operation.
Although the dc-excited electric motor has a squirrel cage for starting, called an amortisseur or damper winding, the inherent low starting torque and the need for a dc power source requires a starting system that provides full electric motor protection while starting, applies dc field excitation at the proper time, removes field excitation at rotor pull out (maximum torque), and protects the electric motor's squirrel-cage winding against thermal damage under out-of-step conditions.
The electric motor's pull-up torque is the minimum torque developed from standstill to the pull-in point. This torque must exceed load torque by a sufficient margin so that a satisfactory rate of acceleration is maintained under normal voltage conditions.
Electric Motor: Basics of Electric Motor Design Engineering