Chapter 12 : Variable Frequency Drivers
Overview
Chapter 12 covers variable frequency drivers (VFD) aka “motor drives”. A motor drive is an electronic unit designed to control the speed of a motor using solid-state components. Motor drives provide the same function as a motor starter but they can also control motor direction, speed, acceleration, and deceleration time. Chapter information link.
Key points to remember:
- Motor drives are replacing contactors and motor starters because of continuous improvements that are being made in the performance, operating speed, and applications of circuit controls.
- AC motor drives are also referred to as VFDs or variable frequency drives because they alter the voltage and frequency to control the speed of a motor.
- AC power sources are used to power most types of VFDs even DC motor drives because AC voltage is more accessible and readily available in most cases.
- AC motor drives contain a DC bus used to filter the converted voltage (DC) required by the solid-state components that control the frequency and thus the speed of the motor.
- Motor drives are used most commonly in HVAC installations.
Introduction to Motor Drives
Motor drives are electronic devices used to control the speed and direction of a motor by varying the input frequency and applied voltage to the motor, hence the name “variable frequency drive”. Motor starters can be easily replaced with motor drives and offer additional functionality including increased performance with the biggest disadvantage being the initial cost. Motor drives are replacing mechanical magnetic motor starters in industry in part due to the inherently simpler system with less moving parts that translates to longer life and increased reliability. This flexibility ultimately leads to cost savings even though the new more versatile solid-state based devices are initially more expensive.
AC motor drives contain three main sections including the converter, DC bus, and inverter. The converter rectifies the AC voltage into a DC signal. The DC bus then filters the DC signal there by protecting the associated diodes used to maintain the DC voltage level. The inverters responsibility is then to control the speed and torque of the motor by varying the “carrier” frequency and the voltage applied to the motor. DC motor drives are available, but AC motor drives are more common in typical commercial and industrial electrical systems due to the prevalence of AC distribution circuitry and the high cost of DC motor maintenance.
Image source: AC Drives Basics (Benefits, Principle and Theory)
Keep in mind:
Even AC motor drives contain a DC bus as motor drives rely on solid-state components to alter the carrier frequency and thus the speed and torque of any given motor.
Control Wiring Methods: Hard Wiring vs PLC Wiring
Hard wiring is the oldest and most straight forward type of wiring that is used in control systems. While hard wiring is a tried and true method, much is left to be desired when compared to the more versatile PLC controlled systems. Unlike PLC controlled systems, hard wired systems need to be torn down and rewired any time changes need to be made. This liability is most noticed in larger systems that would experience the frequent need for future changes and upgrades.
PLC wiring offers improvements in performance and function with the ability to not only precisely control the speed and direction of a motor but also monitor real time operating characteristics or potential problems caused by unknown faults. Motor drives can replace electromechanical motor starters altogether in most cases. This shift from mechanical to electronic control provides additional abilities including controlled acceleration and deceleration time, and different types of stopping methods that not only offer the ability to gradually start and softly stop or reverse a motors’ direction easily but also help improve operating life.
Various methods to more precisely control the speed at which a motor is stopped are typically needed in commercial and industrial motor control systems. If left to its own devices, a motor will coast to stop gradually based on the size and inertia of the connected load. Dynamic and regenerative braking methods are employed to increase the rate that a motor will come to rest. “Dynamic” braking is a motor braking method that uses resistors connected across the motor armature to dissipate the rotational energy of the motor to provide a controlled stop. “Regenerative” braking uses the regenerated energy of the motor to provide counter torque to help stop the motor rotation.
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Looking Deeper
Motor drives are used most commonly in HVAC installations that require the energy saving speed control of AC motors. PLCs are most commonly used in manufacturing processes due to the flexibility their programmability offers. PLCs in industrial applications are designed to withstand fluctuations in noise, vibration, temperature, and humidity. The input/output sections of a PLC function as the eyes, ears, and hands of the PLC. Additionally, PLC controllers also offer the seamless integration of multiple systems required for larger and more complex applications.
Variable frequency drive used to control a motor through the use of a DC bus that alters the frequency and voltage applied to a motor.
(Programmable logic controller) Solid-state device that takes the place of more rudimentary motor starters and relays allowing for more seamless automation of control circuits.
The property of matter by which a mass persists in its state of rest or motion until acted upon by an external force.