Open-loop vector

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

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.

Common applications

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.

Open up

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 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.

Closing in

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.