Woodward 82510 Magnetic Pickups and Proximity Switches

The magnetic speed pickup (MPU) is used to detect the speed of the prime mover. An MPU is necessary

when the prime mover drives something other than an alternator. It is also often used where an alternator

is driven directly by the prime mover and when a control signal is necessary before the alternator comes

up to its proper output voltage. A speed sensor circuit, either a section on the governor amplifier chassis

or a separate unit, is needed to convert the MPU’s output signal to one usable by the governor amplifier.

The magnetic pickup produces a voltage output when any magnetic material moves through the magnetic

field at the end of the pickup. Since most engines and turbines have flywheels or other large gears made

of magnetic material (usually iron or steel), magnetic pickups can usually be installed without adding

attachments to a gear or shaft. Nonmagnetic materials, such as aluminum, brass, and some stainless

steels, will not excite the magnetic pickup.

Woodward 82510

The MPU makes use of a “stray magnetic field” and no provision for return magnetic circuits or paths is

necessary. Any device which produces a dynamic discontinuity of magnetic material in the field of the

pickup will produce an electrical voltage. Although gears are the normal devices measured by an MPU,

other devices such as a vibrating surface, moving bar, crank, wheel spokes, or a steel screw head

mounted on some moving surface will work equally well if surface speed and other factors are taken into

account. The MPU may be excited by a keyway or slot in a wheel, but there is likely to be an unwanted

background signal due to varying density or eccentricity of the material. It is better to excite the MPU from

a protrusion on the surface. This places the pickup at a relatively great distance from the materials

between excitation periods and it is less likely to pick up stray signals.