Rotor Speeds. Standards list dc generator speeds as high as are reasonable to reduce their size and cost. The speeds may be limited by commutation, maximum volts per bar, or the peripheral speeds of the rotor or commutator.

Generator commutators seldom exceed 5000 ft/min, although motor commutators may exceed 7500 ft/min at high speeds. Generator rotors seldom exceed 9500 ft/min. If the prime mover requires lower speeds than these, generators can be designed for them but larger machines result.

Rotor Diameters.
Difficult commutating generators benefit from the use of large rotor diameters, but diameters are limited by the same factors as rotor speeds. The resultant armature length should be not less than 60% of the pole pitch, because such a small portion of the armature coil would be used to generate voltage.

Direct-current motor speeds must suit the application, and often the rotor diameter is selected to meet the inertia requirements of the application. Core lengths
may be as long as the diameter. Such motors are usually

Number of Poles and Other Rotor Design Factors.
The rotor diameter usually fixes the number of main poles. Typical pole pitches range from 17.5 to 20.5 in on medium and large machines. When a choice is possible, high-voltage generators use fewer poles to allow more voltage space on the commutator between the brush arms.

However, high-current generators need many poles to permit more current carrying brush arms and shorter commutators. Commutators for 1000 to 1250 A/(brush arm) (polarity) are costly, and lower values should be used where existing dies will permit.

The main-pole air-gap flux density Bg is limited by the density at the bottom of the rotor teeth. The reduced taper in the teeth of large rotors permits the higher gap densities.

Ampere conductors per inch of rotor circumference (q) is limited by rotor heating, commutation, and, at times, saturation of commutating poles. The commutator diameter is usually about 55% to 85% of the rotor diameter, depending on the sizes available to the designer, the peripheral speed, and the resulting single clearances.

Heating may also limit the choice. Brushes and brush holders are chosen from designs available to limit the brush current density to 60 to 70 A/in2 at full load, to obtain the needed single clearance, and to obtain acceptable commutator heating.

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