Posted on 1st Aug 2023
Electrical supplies can either be at full power to run AC motors at full speed or at variable power to run them at variable speed. Since less power is needed at lower loads and speeds, applications with varied loads can benefit from an AC drive's variable speed. This article describes how an AC drive makes this happen.
A power supply control and conditioning tool for AC motors is known as an AC drive, also known as a variable frequency drive (VFD) such as Lenze Drive, Keb Drive, Siemens Drive, Darwin Motion Drive and more in simple terms (figure 1). The drive must give the necessary energy waveform with sufficient voltage to deliver the appropriate current to produce magnetic flux inside the motor since AC motors need frequency to spin. The definition of the motor's revolutions per minute (rpm) is
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)
An AC drive is given an AC voltage source in order to create the desired frequency waveform. The AC voltage supply is rectified to DC voltage, often using a diode bridge or, in the case of a regenerative four-quadrant drive, by insulated-gate bipolar transistors. As part of a DC bus, this rectified power is kept in a capacitor bank. This area of the drive is referred to as the converting portion.
The needed frequency is then produced by switching devices using the DC power that was previously stored in the capacitor bank. The motor receives this AC power, which enables it to spin at the specified speed, typically expressed in revolutions per minute.
To create the appropriate waveform, the drive generates a set of pulse-width modulated pulses that are either positively or negatively orientated. The stored energy level, with 230 VAC rectified to about 325 VDC or 162.5 VDC, and a 460 VAC supply rectified to 650 VDC or 325 VDC, determines the height of the pulse, which is typically 162.5 or 325 VDC. The switch's on time determines the pulse's width, or modulation, and the deadtime, or the interval between pulses, determines how wide the pulse will be.
To create the appropriate waveform, the drive generates a set of pulse-width modulated pulses that are either positively or negatively orientated. The stored energy level, with 230 VAC rectified to about 325 VDC or 162.5 VDC, and a 460 VAC supply rectified to 650 VDC or 325 VDC, determines the height of the pulse, which is typically 162.5 or 325 VDC. The switch's on time determines the pulse's width, or modulation, and the deadtime, or the interval between pulses, determines how wide the pulse will be.
An AC motor's operating parameters can be controlled by the AC drive, which can lower the motor's energy consumption. The applied voltage and current are adjusted or limited to achieve this energy savings during controlled acceleration and deceleration as well as during normal operation.
Applications for pumps, fans, and conveyors by themselves can result in hundreds. Even terminology used within applications can be interchangeable. For example, pumps, fans, or conveyors can be used for bulk transporting. For example, 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 an oven are all instances of applications.
Whether it's a straightforward warehouse air fan or a sophisticated energy storage system, water supply system, or wastewater management system, AC motors operating at varied speeds are essential. Since AC motors account for a large portion of the electricity produced globally, adopting AC drives to reduce energy use can result in significant savings.
The application should be thoroughly reviewed as one of the best practises for AC drives. Any application must begin with a properly chosen motor type, the appropriate motor and drive size, and this necessitates a thorough examination of the load characteristics. Also take into account the drive's energy source's makeup and the setting in which the system will be used. In rare cases, a clean power source may need the use of an AC transformer or supply reactor.
It is important to establish whether dynamic brake units, braking resistors, and energy injection or removal with choppers are necessary. It's important to identify the requirements for handling regenerative energy. It should be taken into account that stopping a fluctuating load abruptly and frequently will put the drive and motor under stress.
In many circumstances, an installation inspection is required, along with any necessary protection from other energy consumers on the supply side of the AC drive and appropriate grounding systems. Finally, there needs to be enough space for the AC drive and its safety features. For instance, the kind and size of the enclosure must be adequate to offer cooling and environmental protection (figure 2).
Make a plan for programming the AC drive to deliver the optimal operational approach and energy savings to the connected motor. Thankfully, many of the necessary characteristics are already present in AC drives for this use. Typical abilities include the following:
An AC drive/motor system can operate as efficiently as possible while maintaining safety over the system's lifetime by utilising all of these features and more.