Power Quality and Drives LLC

A variety of definitions of terms that are used in the motor industry are given below.
Copyright © 2007 Francis J. Martino

Motor Starters are used for reduced voltage starting with motors that can be connected for both wye and delta winding configurations. Star-Delta is also known as Wye-Delta.
There are two types available:
Open Transition operation. The wye contactor is opened before the motor reaches full speed, the motor is disconnected from the power, the motor looses speed, the delta contactor is closed, the motor draws an inrush current as it now continues to full speed. The inrush current will be less than the inrush with a full voltage start. The advantage of open transition is lower cost of the starter.
Closed Transition operation. The contactors change from wye to delta and momentarily maintain power to the motor through a resistor bank as the contactors switch. The motor does not loose speed and no inrush current will flow during the transition. The advantage is less mechanical stress on the motor, pump and pump systems.

Enclosure types that are available:
NEMA 1 and UL Type 1 are for use in a clean environment.
NEMA 12 and UL Type 12 have a gasket for protection against severe dust and oil.
NEMA 3R and UL Type 3R are for use in the rain.
NEMA 4 and UL Type 4 will protect against pressurized water from any direction.
NEMA 4X and UL Type 4X will protect against pressurized water from any direction and are corrosion resistant.
Chassis or Open Type is intended for mounting in a customer supplied enclosure. With smaller controls, a NEMA 1 enclosure may often be supplied and the customer must then remove the cover to use the control for panel mounting.

Programming of Variable Frequency Drives:
PID Control will cause the controller output speed to follow an input signal at an inversely proportional manner. Thus, a 0 to 10 VDC signal will cause the motor speed to increase from minimum to maximum as the voltage signal changes from 10 to 0. A 4 to 20 mA signal will cause the motor speed to increase from minimum to maximum as the current signal changes from 20 to 4 mA.
The "P" allows the proportional gain to be programmed. The change in output speed will be proportional to the magnitude of the input signal. "I" allows the value of integral gain to be programmed. The change in output speed will be proportional to the integral of the magnitude of the input signal. The integral is therefore an average value over a period of time.
"D" allows a differential gain to be programmed. The change in output speed will be proportional to the rate of change of the magnitude of the input signal.
Due to the difficulty of programming the differential in many applications and due to the limited advantage of having a differential gain, many manufacturers are manufacturing their drives with PI control and not PID.

V/Hz is the ratio of Volts to Hertz that is generated by the variable frequency drive. The standard configuration is to have a constant V/Hz ratio as the output frequency increases from 0 to the base frequency of either 50 or 60 Hz. When operating above the base frequency of 50 or 60 Hz, the output will normally have a varying V/Hz ratio, with the volts remaining constant as the frequency increases. The torque capability of the motor will then drop off at a rate inversely proportional to the square of the frequency. In the range above base frequency the motor will then exhibit a constant horsepower capability rather than a constant torque. The base frequency is normally the frequency at which the motor is rated, thus, a two-pole motor is rated at 3600 RPM at 60 Hz, or 3000 RPM at 50 Hz.

Vector, Flux Vector or Field Oriented describes a drive that will operate similarly to the V/Hz control except that the V/Hz ratio output of the drive will be determined by the position of the motor shaft as calculated by the drive. Thus, the V/Hz ratio will not be constant and will be altered as the drive attempts to maintain the desired motor speed when the motor experiences an increasing or decreasing speed that is due to load changes. A vector drive will allow better control at low speed and higher efficiency at all speeds.

Variable Torque Load: fan, blower, centrifugal pump. The torque requirement diminishes with the cube of the speed. Constant Torque Load: positive displacement pump (a pump that uses pistons) and most other machinery. The torque requirement remains constant as speed varies.
Constant Horsepower or Constant kW load: a drill press. The torque requirement increases with the square of the reduction in speed.

Amperage Interrupting Capacity (AIC) in RMS symmetrical amperes: The amperage which the device will safely pass and/or interrupt under a fault or short-circuit condition. If the interrupting capacity is exceeded, the device will burn or fragment.

Power Quality and Drives LLC
P.O. Box 83
Middlebury, CT  06762
Phone: (203) 217-2353
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