MAY-JUN 2018

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42 INTECH MAY/JUNE 2018 WWW.ISA.ORG By Bryan Sisler To produce this desired frequency waveform, an AC drive is provided with an AC voltage supply and rectifies it to DC voltage, normally through a diode bridge, or if it is a regenerative four-quadrant drive, then through insulated-gate bipolar transistors. This rectified power is stored in a capacitor bank as part of a DC bus. This is known as the converting section of the drive. The DC power stored in the capacitor bank is then supplied to switching devices to create the required frequency. This AC power is supplied to the motor and enables it to spin at a desired speed, normally measured in revolutions per minute. W hen powering AC motors, the electrical supply can be at full power to run at full speed or at variable power to run at variable speed. Applications with varying loads benefit from the vari- able speed provided by an AC drive, because less power is used at lower loads and speeds. This article explains how an AC drive accomplishes this. Simply put, an AC drive, also referred to as a variable frequency drive (VFD), is a power supply control and conditioning device for AC motors (figure 1). Because AC motors require frequency to spin, the drive must provide the required energy waveform with enough voltage to deliver the needed current to pro- duce magnetic flux within the motor. The motor revolutions per minute (rpm) can be defined as: Explaining AC drives Motor rpm = Frequency (in Hz) • 120 (a constant) / # motor poles i.e., 60 Hz • 120/4 poles = 1800 rpm (minus slip = 1750 rpm for a four-pole motor) The drive produces a set of pulse-width modulat- ed signals that are positively or negatively oriented to create the desired waveform. The height of the pulse, typically 162.5 or 325 VDC, is a result of the stored energy level, with 230 VAC rectified to ap- proximately 325 VDC or ±162.5 VDC, and a 460 VAC supply rectified to 650 VDC or ±325 VDC. The width of the pulse, the modulation, is regulated by the length of time the switch is on, and the dead- time is the time between pulses. This switching of the power devices enables control of the amount or level of voltage that passes through the switches, as well as the fre - quency at which the waveform is created, provid- ing the required energy to control motor speed. The AC drive enables control of the operating characteristics of an AC motor, which can reduce energy used by the motor. This energy savings comes through adjusting or limiting the applied voltage and current during controlled acceleration/ deceleration and during normal operation. AC drive application basics Pumps, fans, and conveyor applications alone can generate hundreds of applications. Even the terms within applications can be synonymous. Bulk conveying can be carried out with pumps, fans, or conveyors, for instance. Application ex - amples include tons of flour blown through pipes for a bagging operation, pumps handling tens of thousands of gallons of soda through a bottling line, conveyors moving coal or rock over miles of a quarry, or a conveyor carrying thousands of cook - ies through a baking oven. Figure 1. AC inverters, also called AC drives or variable frequency drives, provide the frequency and voltage needed to drive an AC motor.

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