Welcome to this installment of MC² covering ac motors and drives.


A common way to classify ac motors is based on the magnetic principle that produces rotation. So there are two fundamental types of ac motors; induction motors and synchronous motors. Induction motors are more common in motion control applications, but synchronous motors do find use in applications requiring precise and constant speed.


Industrial applications such as conveyors and packaging lines require motor and drive systems that are reliable with long service life. And ac motors and drives are well suited to these and other industrial uses with high output power.


Drives for ac motors interface between a controller that sends control signals to the drive which converts those signals to currents that drive the motor. An ac motor’s speed is determined by the number of poles and the frequency. So as frequency is adjusted the motor’s speed can be controlled as well. One of the most common types of drives is a variable frequency drive (VFD), which are covered in this Motion Control Classroom.


In addition to some basics of ac motors including motors for harsh environments, cogging, and the use of soft starters, this installment of Motion Control Classroom covers the basics of VFDs as well as various motor control methods including V/Hz control as well as some different motor braking methods.


You’ll find these and many more resources on a wide array of motion control components and systems, including other MC Classroom installments, at www.designworldonline.com/mc2/.


Miles Budimir


AC Motors & Drives

Senior Editor, Motion Control

Design World Magazine

AC Motors & Drives  Classroom Sponsored by:

Basics of AC Motors

What are AC motors? Technical Summary for Motion Engineers

An AC motor, like any electric motor, converts electrical energy to mechanical energy. AC motors take as their input an ac current, but differ from dc motors in that there is no commutation involved, and can be single or multi-phase.

When do you need a soft starter for an AC motor?

A soft starter can eliminate problems by gradually increasing voltage to the motor terminals during startup, providing a controlled ramp-up to full speed. 

What is direct torque control (DTC) for AC induction motors?

Direct torque control (DTC) is similar to field oriented control, in that it decouples torque and flux and controls them independently. 

FAQ: How do drives affect cogging in AC motors?

If you put two magnets in close proximity to each other, you know that they’ll experience an attractive force when opposite poles are near each other and a repulsive force when like poles are near. 

Industrial Automation Motors

Reliability and fantastic service are critical in industrial applications where production and processing facilities can often run 24/7. Quality comes as standard in every Parvalux product and for more than 70 years we’ve manufactured durable industrial DC and AC motors that work effectively, sometimes for decades.

AC induction motors are used to drive loads such as fans, pumps, lifts, and conveyors. But in some applications, the motor is also used as a brake to stop the load, reverse its direction, or hold the load and prevent it from moving.

What is flux braking and how does it differ from regenerative braking

Basics of AC Drives 

What is a VFD (Variable Frequency Drive)? 

Variable-frequency drives, or VFDs, are a means of controlling the speed of an ac motor, typically an ac induction motor. It does so by varying the frequency of the power source.

What are leading methods for VFD control of AC motors?

AC motors are often used in equipment that runs at constant speed regardless of load, such as fans, pumps, and conveyors.

What is droop control in the context of AC motors and drives?

One of the fundamental characteristics of an AC induction motor is that the rotor spins at a slower rate than that of the stator’s rotating magnetic field. 

FAQ: What is V/Hz control mode for AC drives?

AC motors are commonly paired with variable frequency drives (VFDs), which control motor speed by regulating the frequency of the supplied voltage. 

Electric Motor Catalogues 

Information on the different product types that Parvalux manufacture and produce, as well as the different services and industries that Parvalux work in.

The Leading Global Electric Motor Manufacturer 


Over 70 years' experience creating the perfect combination of motor and gearbox to match our customers’ applications and we offer thousands of standard AC, PMDC and BLDC electric motor options. We can easily customise any motor or gearbox to meet your exact requirements and we can do this at no or minimal extra cost. Our design team can help you choose, customise or design from scratch, geared motor solutions, ensuring you gain a competitive advantage in your market.