VFDs: The Next Best Thing to Motion Control

For applications with budget constraints, VFDs can provide surprisingly good performance at an affordable price.

There was a time when permanent magnet motors and servo drives were the only game in town for precision motion. Today, variable speed drives (VFDs) are being used with AC induction motors and, yes, PM motors, to achieve high-accuracy motion. Properly specified and configured, motion axes based on VFDs can be very effective solutions for cost-sensitive applications with more forgiving performance specifications.

VFD Basics  tr0
VFDs, sometimes called variable speed drives or AC inverter drives, generate output of varying frequencies. 

The relationship between drive frequency f and motor velocity ω in an AC induction motor is given by

where ω is angular velocity in RPM, f is frequency in hertz, n is the number of poles, and 120 is a unit conversion factor. The higher the frequency, the higher the RPM value. 

Standard drives generate a signal at the frequency of AC wall plug power – 60 Hz, in the US. When a standard drive is used to run an AC induction motor, the motor operates at top speed. To run the motor at a lower speed, the drive needs to supply a lower-frequency signal. VFDs are able to modify the frequency of the signal driving the motor. Thus, they provide a means for adjusting the output speed, torque, and horsepower of the motor.

Variable-frequency drives use a variety of techniques to change frequency. Probably the most effective and certainly the most common method is pulse-width modulation (PWM).

A PWM VFD consists of three basic components: the AC to DC conversion block (the rectifier), the filtering block (the DC bus), and the DC to AC conversion block where the frequency modulation takes place (the inverter, see figure 1). The rectifier generates a DC pseudo-with ripple, which is removed by the filtering block. In the inverter, an array of insulated-gate bipolar transistors (IGBTs) chops the signal into voltage square waves that are combined to synthesize an AC signal of specified frequency.

The VFD includes an onboard microprocessor run by specialty firmware. VFDs for motion control also incorporate high-speed I/O ports for feedback, built-in connectivity such as MODBUS, CC link, and Ethernet/IP. The microprocessor gives the VFD the ability to do simple motion for a single axis or even a master-slave architecture. More sophisticated operations require a PLC or dedicated motion controller.

VFDs in Action
The initial use case for VFDs was to provide energy savings and increased lifetimes for fixed-speed AC induction motors used for rotating assets such as fans, pumps, and blowers. At the time, the approach to achieving variables speeds or volumes was to run the motor at top speed and use mechanical means such as chokes, valves, cams, and gears to adjust the end results. The approach squanders energy, increases wear on the motor, adds complexity, and increases maintenance and points of failure. VFDs provide an alternative.

VFDs enable users to adjust the speed of the motor directly, reducing power consumption, motor wear, maintenance, and downtime. VFDs are straightforward to deploy, reliable, and when properly sized and selected, deliver rapid ROI. 

VFD Control Schemes
VFDs control motors using one of four methods:

• Volts per hertz (V/f) open-loop
• V/f closed-loop
• Closed-loop vector control
• Open-loop vector control

Volts per Hertz (V/f)
Volts per frequency control, sometimes called volts per hertz control, is a scalar control method that commands the drive to deliver a specified frequency that corresponds to the desired speed. AC induction motors depend upon current in the stator windings to generate a magnetic field distribution. The windings act as an inductor. We can express the inductive reactance X_L part of the impedance, as

where L is coil inductance. Equation 2 means that increasing frequency also increases impedance in the coil and vice versa. Using Ohm’s Law, we can tie this back to current and voltage:

Reducing frequency while maintaining voltage boosts current draw. This could increase torque but also has the potential to burn out the windings. V/f VFDs incorporate voltage limiters that maintain a set ratio of voltage to frequency.

V/f control can only be used to control speed, which it does to within a few percent. 

V/f VFDs are commonly used for variable speed operation of pumps, fans, blowers, and conveyors. In these applications, and air of a few RPM will not make a difference. The drives are very inexpensive and don’t require feedback. They typically do not require tuning. They should not be used at low speeds or in applications requiring full torque at zero speed. 

V/f with Encoder
Note that in basic V/f control, the system operates without feedback. The drive hands down the commands but has no means of confirming that the motor is operating as commanded, or whether the motor shaft is even turning at all. More demanding applications can benefit from the addition of an external encoder to monitor the motor shaft. 

Closed-loop feedback improves performance significantly: V/f VFDs can achieve speed regulation on the order of a few hundredths of a percent of maximum frequency. Like the open-loop version, they cannot be used to control torque or position. Common applications include machine tools, spindles, roller beds and other traversing carriages.

Closed-loop Vector Control
Vector control, or field-oriented control, is a control scheme capable of delivering highly accurate positioning performance. Vector control is based on the fact that induction motors operate based on two currents: flux-producing current in the stator windings that induce magnetic fields in the rotor, and torque-producing current in the rotor windings induced by the stator. A vector control VFD acts to maintain those two currents 90° out of phase to maximize torque. The motor operates analogously to a DC motor, in which the brushes act mechanically to maximize torque.

Vector control delivers tighter speed control and positioning, along with higher starting torque and higher low-speed torque.

AC induction motors don’t include magnets. Instead, the magnetic field of the stator induces a magnetic field in the rotor. The rotor field always chases the stator field, which moves from coil to coil in sequence. This lag is known as slip, and it leads to asynchronous operation. As a result of slip, the actual speed of an AC induction motor is lower than its rated speed. A motor rated at 1800 RPM, for example, might run at 1750 RPM. Slip increases with torque load. As a result of slip, AC induction motors operated open-loop, even used with a VFD, can’t deliver precision motion. Compensation schemes have been developed that significantly improve matters, but the issue remains.

The solution is to use external feedback (see figure 2). The sensor input enables the control system to position accurately even with slip. “It’s not that the slip is not there, but you’re counting pulses,” says Craig Dahlquist, Application Engineer, Lenze Americas (Uxbridge, Massachusetts). “If you’re going one inch, the system says, I need to go one inch, and I have so many pulses per inch. It’s not worried about how many sinusoidal waves it puts to the motor and how much gets slip.  It doesn’t care.  It just goes until it gets to the right amount of counts.”

VFDs running under closed-loop vector control are capable of very high accuracy and repeatability. Applications include automated guided forklifts, storage and retrieval units, hoists, smart conveyors, palletizers, freight elevators, and similar (see Figure 3).

When to Choose a VFD
VFDs are effective for budget-sensitive applications requiring only moderate positioning accuracies and resolutions. “When you need accuracy between 0.5 mm to 2 mm, in general, VFDs will work,” says Dahlquist. “When you get into accuracies around 0.1 mm or 0.01 mm, that’s when we’ll go to a servo.” 

“A servo drive is just a much precise type of control,” says Ortiz. “Accuracy for a VFD is roughly 1.5° compared to 0.05° for a servo. If you want really high precision type of control, a servo is still the best solution to go with but if the customer is just looking for basic type of movement, then a VFD could do the job.”

Sensor-less Vector Control
The term sensor-less vector control is something of a misnomer. The scheme gets its name from the fact that the drive is operated without external feedback. Instead, it receives input from current sensors inside the motor. The data is combined with a mathematical model to create an adaptive magnetic flux observer to track the motion of the motor.

Although VFDs operated with sensor-less vector control can be used with AC induction motors, in practice, they are used with permanent magnet DC motors. The synchronous behavior of the PM motor allows the combination to achieve good positioning accuracy and repeatability, as well as tight speed control.

Although it might seem odd to run a PM DC motor open-loop with a VFD rather than closed-loop with a servo drive, there can be several very good reasons for doing so. Assuming the VFD can deliver the appropriate performance for the application, the design eliminates the cost and complexity of an encoder/resolver and the cables that go with it. The system has fewer points of failure and reduced maintenance. Because the PM DC motor is synchronous, there are no concerns about slip and thus, no need for feedback.

Applications include many of the same items listed under closed loop vector control.

When to Choose a VFD
VFDs are effective for budget-sensitive applications requiring only moderate positioning accuracies and resolutions. “When you need accuracy between 0.5 mm to 2 mm, in general, VFDs will work,” says Dahlquist. “When you get into accuracies around 0.1 mm or 0.01 mm, that’s when we’ll go to a servo.” 

“A servo drive is just a much precise type of control,” says Ortiz. “Accuracy for a VFD is roughly 1.5° compared to 0.05° for a servo. If you want really high precision type of control, a servo is still the best solution to go with but if the customer is just looking for basic type of movement, then a VFD could do the job.”

Trade-offs of VFDs
As always, there are trade-offs. AC induction motors are bigger for the same amount of horsepower. They have more copper, for heavier weight. Similarly, VFDs are bigger than servo drives. The result is a system with much higher inertia, which limits the accelerations and decelerations that the system can achieve.

It’s also important to bear in mind that while VFDs don’t involve the same levels of programming as servo drives, they do require quite a bit of detailed configuration. In general, the types of applications described above will call for an engineer with significant experience in working with VFDs. “There are a lot of parameters, especially when you’re doing precision control,” says Ortiz. “Installation and programming is probably going to require a mid-to high-level person who knows VFDs. That said, there is a lot more involved when it comes to a programming a servo drive versus programming a VFD. With a VFD, you could probably do everything on the keypad.”

Success requires the correct choice of both motor and VFD. VFDs are designed and configured for specific control schemes. Work with your vendor to ensure that the VFD chosen matches the motor and can execute the control schema for the application.

Particularly at low speeds, it is important to choose inverter-duty motors. Induction motors are generally cooled by fans coupled to the motor shaft. At low speeds, cooling is insufficient. Inverter-duty motors are rated for higher temperatures. They include better insulation and active thermal management such as external blowers or even water cooling. In inverter-duty motors, the rotor is optimized for the internal sensors. This enables the sensors to work more effectively under sensor-less vector control.

The enhanced speed and position capabilities of VFDs at a certain amount of complexity. As a result, users sometimes underestimate what is required to install and commission fees axes. “It’s not as easy as connecting PM motor to the three phase output it in VFD, giving it a start command and a speed, and it’s good to go,” says Ortiz. “You have to have the right encoder. You have to have the right option card on the VFD to talk to the encoder, make sure the wiring is correct.”

VFDs continue to advance, given more powerful microprocessors and sophisticated algorithms. Safety features such as Safe Torque Off and Safe Stop have become increasingly common in the past several years.

“The technology has come such a long way,” says Jeff Payne, product manager for the drives and motors group with AutomationDirect (Cumming, Georgia). “It really is amazing how much you can do, especially if you have a system where the drive and the motor are a matched pair. If they can talk and you can get that feedback, then you can really increase your performance.  Not only in the torque, but also in positioning.”  

The end result is that VFDs may be worth considering for a general automation project. “There are a lot of applications that have traditionally used servo systems that can now be done by VFDs with AC induction motors,” says Dahlquist. “If people are willing to consider a new way, they can reduce the cost of their machine and become more competitive or they can make more money. It’s all about doing things more cost-effectively.”