Motion engineers generally fall into two camps when it comes to ac drive technology: Those already taking advantage of its benefits — low price, compact design, simplicity — and those who will soon start. Both groups, however, face a similar challenge in that they must understand the difference between ac drive types in order to select the right one for a particular job.
In general, ac drives work (controlling ac motor speed) by varying the frequency of the current supplying the motor. Although frequency can be varied many ways, and in relation to other variables such as voltage, the most common methods in use today are “volts per hertz,” open-loop vector, and closed-loop vector. How these techniques differ determines where each drive type works best.
Volts per hertz
Volts per hertz (V/Hz) technology is the most economical and easiest to apply of the three speed-control methods. Here, the drive controls shaft speed by varying the voltage and frequency of the signal powering the motor.
Now, the rotor of an ac induction motor is magnetically coupled to the stator through an induced magnetic field. The speed at which the magnetic field rotates around the stator is known as synchronous speed and is determined by:
n = 120 ƒ/N
where n is synchronous motor speed, 120 is an electrical constant, ƒ is the applied frequency, and N is the number of motor poles.
The equation illustrates one of the basic principles of speed control: Reducing applied frequency to an ac induction motor causes the magnetic field to turn at a proportionally slower rate, thereby reducing rotor speed.
This is only part of the story, however. Induction motors are designed to operate from line voltage at line frequency. But the whole purpose of V/Hz drives is that they don't hold systems to power line shapes. What they do instead is maintain an optimal voltage-to-frequency ratio, so that the motors they power will produce their rated torque over the widest possible speed range.
Consider a 460-Vac motor designed for 60-Hz operation. If applied frequency is reduced to 30 Hz, the shaft will slow to half its original speed. In this situation, a V/Hz drive also halves the voltage (here, to 230 Vac) in order to maintain the 7.67 V/Hz ratio, which allows the motor to continue producing its rated torque.
The design architecture of an open-loop vector drive is similar to that of a V/Hz drive. From a hardware standpoint, the only change is the addition of current sensors. The real difference is in firmware.
Open-loop vector drives use sophisticated motor-control algorithms that independently control both magnetizing and torque-producing current. The algorithms incorporate a detailed motor model that accounts for stator resistance and inductance as well as rated voltage, current, and speed. Using this information, the drive maintains a 90° angle between the magnetizing and torque-producing current vectors.
By independently optimizing magnetizing and torque-producing current, open-loop vector drives significantly raise the level of ac motor performance. Even without a sensor, vector-controlled ac motors will respond quickly to changing load conditions. They also generate more torque and more precisely regulate speed.
Closed-loop vector drives typically incorporate more sophisticated firmware (including the microprocessor) than other drive types. They also require a feedback device (usually an encoder) that's located on the motor.
By tracking speed and position, closed-loop vector drives are able to accurately control motor torque, speed, and position. Benefits include better speed regulation, full torque production at zero speed, basic positioning, and software-based electronic gearing.
Many motion applications — particularly pumps, fans, conveyors, and mixers — require nothing more than an inexpensive drive with simple speed control. Here, a V/Hz drive is usually the best bet. It's the easiest to install and has the lowest price point of any drive type. In fact, for these reasons, V/Hz drives are increasingly replacing older forms of motor control, including mechanical variable-speed drives, solid-state starters, and conventional motor starters.
With centrifugal loads, variable-frequency drives also save energy. To illustrate, consider the “affinity laws” that govern centrifugal loads. If Q is flow, n is speed, and hp is horsepower:
Q is proportional to n
P is proportional to n2
hp is proportional to n3
These relationships highlight the benefit of using V/Hz drives to control flow, for example, instead of dampers, inlet vanes, or throttling valves. Unlike on-off mechanisms, V/Hz drives allow power consumption to fall with flow — and a small drop in flow results in a large drop in power consumption. For example, a fan operating at 80% consumes only 51% of the energy required at 100% flow.
Not all motion applications are satisfied this easily, however. Some require more than simple speed control. In applications demanding tighter speed regulation and high starting and accelerating torque, an open-loop vector drive will usually work better.
The graph on Page 23 shows how a high-performance open-loop vector drive responds to a 100% step change in load. Here, the motor is operating at 50 Hz with no load applied. It is then slammed with an instantaneous 100% shock load to show how quickly the drive can return the motor to a stable 50-Hz operating frequency. It only takes 0.15 sec to fully recover, which is particularly remarkable, considering that this is accomplished open loop, without the benefit of a feedback device.
Just as V/Hz drives are replacing some mechanical controls, increasingly simple open-loop vector drives today are replacing older dc drives without a drop-off in performance. Common applications include extruders, filling machines, forming machines, and presses.
For applications that are even more demanding — lifts, hoists, incline or decline conveyors, and extruders of fragile material — it may be necessary to step up to a closed-loop vector drive. Closed-loop vector drives can control motor speed down to 0 Hz, while producing controlled holding torque. They also respond faster and more effectively to load changes, and are migrating into areas once reserved for high-end servo technology — where ac motors offer a cost advantage.
Comparing speed-control methods
Drive controls voltage and frequency
Speed range: 20:1 (typical)
Speed regulation: 1% to 3% (typical)
Starting torque: 150% (typical)
Easiest method to apply; offers multiple motor control
Open-loop vector (control torque)
Drive controls voltage and torque at speeds above 0 rpm
Speed range: 60:1 (typical)
Speed regulation: 1% (typical)
Starting torque: 200% (typical)
Good dynamic response; does not generate holding torque
Closed-loop vector (control the motor)
Drive controls voltage, freq-uency, torque, and position
Speed range: 1,000:1 (typical)
Speed regulation: 0.01% (typical)
Starting torque: 200% (typical)
Achieves holding torque at 0 rpm; offers position control