harmonic
A multiple of a base frequency. For example, the fifth harmonic of a 50 Hz signal is 250 Hz. The distortion of a periodic wave can be described in terms of the base frequency and a set of harmonics with varying amplitudes. The common harmonics produced by a three phase adjustable frequency drive are odd harmonics that are not multiples of 3 (5th, 7th, 11th, 13th, etc.)

hertz
Hz. Measurement of frequency. It means "cycles per second".

head
A measure of pressure, usually expressed in inches or feet of water.

head, dynamic
A measure of the kinetic energy of a moving fluid, usually expressed as the height of the fluid equivalent to its velocity pressure.

head, static
The static pressure of a fluid, usually expressed in terms of the height of a column of fluid that it would support.

head, total
In a flowing fluid, the sum of the static and dynamic heads at the point of measurement.

heater coil
(thermal overload relay) A heater coil is a part of a thermal overload relay (motor overload) that is intended to produce heat when conducting current. Excessive heat generation indicates that the motor is overloaded. This causes the overload to open a contact which in turn is used to open the motor starterís contactor or to stop the drive. Heater coils are sometimes referred to as heaters, thermal units, current elements or heater elements. While they can provide overload protection for a motor, they do not provide short circuit protection since they may burn out under short circuit conditions. see motor overload

high line
This refers to the voltage level in the input power line being above the acceptable range. An electronic drive may protect itself from operating under this condition by shutting itself off.

horsepower
HP A measure of power that is commonly used to measure the capacity of a motor. 1 HP = 0.746 kW, or 33,000 ft-lb per minute.

hunting
Undesirable fluctuations in motor speed.

HVAC
Heating, ventilating and air-conditioning. Adjustable frequency drives are commonly applied to such systems to control the speed of fans and centrifugal pumps. The torque required by HVAC loads tends to decrease as the motorís speed is reduced.