Explaining AC drives
By Bryan Sisler
When 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 variable 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 produce magnetic flux within the motor. The motor revolutions per minute (rpm) can be defined as:
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)
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.
The drive produces a set of pulse-width modulated 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 approximately 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 deadtime 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 frequency at which the waveform is created, providing 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.
Figure 1. AC inverters, also called AC drives or variable frequency
drives, provide the frequency and voltage needed to drive an AC motor.
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 examples 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 cookies through a baking oven.
From the simple air fan in the warehouse to the complexity of energy storage systems, water supply, or wastewater management systems-AC motors running at variable speeds play a key role. Because AC motors consume much of the electricity generated worldwide, using AC drives to limit energy use can produce significant savings.
Best practices for AC drive integration
Best practices for AC drives include a thorough review of the application. A properly selected motor type, with the right-sized motor and drive, is the starting point for any application, and this requires a detailed review of the load characteristics. Also consider the nature of the energy supply to the drive and the environment in which the system will operate. In some instances, an AC transformer or supply reactor may be needed to provide a clean source of power.
The need for dynamic braking units, by energy injection or by removal of energy with choppers, and braking resistors should be defined. Regenerative energy-handling requirements should also be defined. Stopping a variable load, especially quickly and often, will stress the drive and motor and should be considered.
An installation review, including protection required from other energy consumers on the AC drive supply side, is necessary in many cases, and proper grounding systems are also needed. Finally, there must be adequate room for the AC drive and its protective devices. For example, the enclosure type and size must be sufficient to provide cooling and protection from the environment (figure 2).
Create a plan for programming the AC drive to provide the connected motor with the best operational methodology and energy savings possible. Fortunately, many of the required capabilities are built into AC drives for this purpose. Some common capabilities include:
- preprogrammed control for V/Hz applications
- quick-start menus
- safe torque off for the protection of operators and users
- autotuning for motors when in sensorless vector mode
- efficient handling of acceleration and deceleration ramps
- current, frequency, and voltage limits
- methods for handling excess system energy
- options for controlling emergency stops
All of these capabilities and more can be used to optimize operation of an AC drive/motor system and to ensure safe operation over the life of the system.
Figure 2. Unless specifically rated for field mounting, AC drives,
such as these AutomationDirect GS2 drive units, should be installed
in a control cabinet to protect them from the environment, with proper spacing for cooling.