A synchro rotor that has two large poles connected by a crossbar in its core is called a _________ rotor.

The term used to describe a synchro rotor that features two large poles connected by a crossbar in its core is "salient-pole." This design is characterized by distinct and protruding poles that are physically noticeable, which allows for effective magnetization and magnetic field generation. Salient-pole rotors are typically used in applications where high torque and better control are needed, as the design aids in achieving greater magnetic reluctance and improving performance in synchronous machines.

The other options do not accurately describe this rotor configuration. A flat rotor generally has a uniform cross-section with no protruding parts, while a cylindrical rotor embodies a more continuous and even surface. The term "spin-pole" is not widely recognized in the field of motor design and does not correspond to any standard rotor structuring conventions. Thus, the salient-pole designation is the most appropriate for a rotor with the described features.