AC Drives: An Introduction

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The term "AC drive" is a bit vague. Commonly, it refers to a variable-frequency drive (VFD) -- also known as an inverter -- that's used to adjust the speed (among other parameters) of an AC motor. This is definition used in all topical discussion on this web site.

Variable-Frequency Drives (also known as AC Drives and Inverters)

Variable-frequency drives (VFD) for AC motors have been the innovation that has brought the use of AC motors back into prominence. The AC-induction motor can have its speed changed by changing the frequency of the voltage used to power it. This means that if the voltage applied to an AC motor is 60 Hz (used in countries like US), the motor will run at its rated speed. If the frequency is increased above 60 Hz, the motor will run faster than its rated speed, and if the frequency of the supply voltage is less than 60 Hz, the motor will run slower than its rated speed. The variable-frequency drive is the electronic controller specifically designed to change the frequency of voltage supplied to the motor.

In the 1960s, the frequency drives had rather small solid-state components that limited the amount of current the drive could supply to the motor. This usually limited the size of the motor that could be controlled by a frequency and they were not commonly used. When larger transistors became available in the 1980s, variable-frequency drives allowed the largest motors to have their speed controlled. The earliest drives used linear amplifiers to control all aspects of the drive. Jumpers and dip switches were used provide ramp-up (acceleration) and ramp-down (deceleration) features by switching larger or smaller resistors into circuits with capacitors to create different slopes.

The arrival of advanced microprocessors has allowed the variable-frequency drive to become an extremely versatile device that not only controls the speed of the motor, but protects against overcurrent during ramp-up and ramp-down conditions. Newer drives also provide methods of braking, power boost during ramp-up, and a variety of controls during ramp-down. The biggest savings that the variable-frequency drive provides is that it can ensure that the motor doesn't pull excessive current when it starts, so the overall demand factor for the entire factory can be controlled to keep the utility bill as low as possible. This feature alone can provide payback in excess of the price of the drive in less than one year after purchase. It is important to remember that with a traditional motor starter, they will draw locked-rotor amperage (LRA) when they are starting. When the locked-rotor amperage occurs across many motors in a manufacturing plant, it pushes the electrical demand too high which often results in the plant paying a penalty for all of the electricity consumed during the billing period. Since the penalty may be as much as 15% to 25%, the savings on a $30,000/month electric bill can be used to justify the purchase AC drives for virtually every motor in the plant even if the application may not require variable speed.

Allen-Bradley PowerFlex 700 AC drive

above: An example of a modern variable-frequency drive. The Allen-Bradley PowerFlex 700 AC drive offers a flexible package of power, control and operator interface designed to meet the demands for space, flexibility and reliability while providing a broad spectrum of features, allowing the user to easily configure the drive for most application needs.

Today the variable-frequency drive is perhaps the most common type of output or load for a control system. As applications become more complex the frequency drive has the ability to control the speed of the motor, the direction the motor shaft is turning, the torque the motor provides to a load and any other motor parameter that can be sensed. Newer drives have a variety of parameters that can be controlled by numbers programmed into it or downloaded from another microprocessor-controlled system such as a programmable controller (PLC). These drives are also available in smaller sizes that are cost-efficient and take up less space.


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