Q.: Regarding ""cutting a mass concrete wall"" (Problem Clinic, May issue, pages 428 and 429), what's a 400-Hertz winch? One that goes around 400 times a second, or one rented from the Hertz Corporation or neither of the above?

A.: Neither of the above. It's a creature of sloppy writing and editing, and the "Hertz" should have been spelled with a small "h."

The machine was actually a winch powered by a 1/2-horsepower aircraft-type 400-hertz electric motor. The motor was connected to the winch through a gear reducer to make the winch operate at very slow speed.

Electric motors of 400-hertz frequency are commonly used in the aerospace industry and they are also the chief kind of electric motor used by sawing and drilling contractors. One reported advantage is that a 400-hertz motor has 7 times as great a power-to-weight ratio as a 60-hertz motor. This means a 400-hertz motor of a given horsepower is lighter and more easily portable than a 60-hertz motor of the same horsepower. It is also said that the 400-hertz motor is more efficient where electric power must be transmitted a long distance to the motor. The 400-hertz motors are available over a wide range of horsepower ratings.