Half Wave Rectification: In the rectifying process, half wave rectification passes only one-half of each incoming sinusoid, and does not pass the opposite half cycle. The output contains a single half sine pulse for each input cycle. A single rectifier provides half wave rectification.
Head: A measurement of pressure, usually in feet of water. A 30 foot head is the pressure equivalent to the pressure found at the base of the column of water 30 feet high.
Heater Coil (Thermal Overload Relay): A heater coil is a part of a thermal overload relay that is intended to produce heat when conducting current. Heater coils are sometimes referred to as heaters, thermal units, current elements or heater elements.
Heat Loss: Losses due to resistance take the form of heat which has to be dissipated into the air or surrounding cooling medium. Heat loss is also referred to as I2R loss because the current squared, multiplied by the resistance will yield the heat loss value (in watts).
Heat Loss at Full Load: The total heat loss at full load of a motor usually given in Kilowatts. This value is needed to determine total power requirements when a number of motors are supplied by a power source, or more often when the motor or motors are in an enclosed area, to fine the cooling needed.
Heat Loss From Starting: A necessary figure to answer two questions: (1) If the motor is started often will the accumulated heat damage the winding, and (2) Does additional cooling have to be added to overcome this accumulated heat from starting.
High Potential Test: A test which consists of the application of a voltage higher than rated between the winding and the frame or between two or more windings for the purpose of determining the adequacy against breakdown of insulating materials and spacings under normal conditions. It is not a test of the conductor insulation in any one winding.
Horsepower: 33,000 foot pounds of work per minute or 550 foot pounds per second.
Hot Spot Allowance: An insulation system is only as good as its weakest link. Thus it is necessary to locate the hottest part of the machine as this limiting temperature determines the motor life. The difference in degrees of temperature between the readily accessible points and the true "hot-spot" is called the hot spot allowance.
Hunting: Undesirable fluctuations in motor speed that can occur after a step change in speed reference (either acceleration or deceleration) or load.
Hysteresis Loss: The resistance offered by materials to becoming magnetized results in energy being expended and corresponding loss. Hysteresis loss in a magnetic circuit is the energy expended to magnetize and demagnetize the core.
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