booster

A booster is a compound machine consisting of a direct-current motor mechanically coupled to a direct-current generator on the same shaft. The motor draws power from the incoming supply line, and the generator delivers power to the load circuit. The voltage output of the generator can be adjusted independently of the supply voltage, making the booster useful for maintaining constant voltage to equipment despite fluctuations in the incoming line or changes in load current.

The motor and generator are usually wound as separate machines but share a common frame and rotating shaft. The motor typically runs at constant speed, often synchronized with AC line frequency through a synchronous or induction motor drive. The generator field winding is controlled by a voltage regulator that senses the load voltage and adjusts field current to compensate for voltage drop. This closed-loop correction keeps the output voltage within a narrow band, typically within 2 to 5 percent of the setpoint.

Application and Variants

Boosters are commonly found in DC power distribution systems where voltage regulation over distance is critical: electroplating plants, DC motor drives, telecommunications switching equipment, and railway traction supplies. Sizes range from fractional horsepower units handling a few amperes to large industrial machines rated at hundreds of kilowatts. Some designs use a permanent-magnet generator field for faster response, while others employ electronic voltage sensing and automatic field rheostats for fine control.

The booster has become less common since the mid-twentieth century, displaced by solid-state voltage regulators and thyristor-based power supplies that require no moving parts. However, boosters remain in service in older installations and in applications requiring high current capacity, ruggedness, and freedom from electronic failures. The term is sometimes loosely applied to any voltage-regulating motor-generator set, and occasionally to solid-state DC regulators, though that usage is technically imprecise.

Common failure modes include bearing wear from continuous rotation, commutator pitting from load transients, and loss of voltage regulation if the generator field winding develops short circuits. Maintenance requires periodic brush replacement and commutator cleaning. The efficiency of a booster is typically 75 to 85 percent, with losses shared between motor friction, copper resistance, and core heating.