Vector control (also called field-oriented control) is a powerful motor control method used in variable frequency drives (VFDs) to deliver precise torque and speed regulation. It comes in two main types: open-loop and closed-loop. Choosing the right one depends on your application’s demands for accuracy, torque, and feedback.
What Is Vector Control?
Vector control regulates motor torque and magnetic flux independently—like a DC motor—giving you faster response, better speed stability, and more control than traditional V/Hz drives.
Open-Loop Vector Control (Sensorless Vector Control)
How It Works:
- Uses software and motor models to estimate rotor position and speed.
- No encoder or feedback device required.
Advantages:
- Lower cost and simpler wiring
- Good dynamic torque and speed control
- Compact and easier to maintain
- Excellent for general-purpose applications
Limitations:
- Less precise than closed-loop, especially at low speed
- Torque may degrade under high load or sudden changes
- Not suitable for applications with frequent direction changes or zero-speed holding
Best For:
- Fans and pumps
- Conveyors
- Light-duty hoists
- General machinery without high precision needs
Closed-Loop Vector Control (Encoder Feedback Vector Control)
How It Works:
- Uses an external encoder or resolver to continuously monitor motor shaft position and speed.
- Allows real-time correction and precise torque control.
Advantages:
- High torque at zero or low speed
- Precise speed regulation (up to ±0.01% accuracy)
- Stable control during rapid load changes
- Enables true positioning and regenerative braking
Limitations:
- Higher cost (requires encoder + setup)
- More complex installation and tuning
- Requires maintenance of feedback device
Best For:
- Cranes and hoists
- Elevators
- Servo-like positioning systems
- Extruders, winders, high-inertia loads
Quick Comparison Table
|
Feature |
Open-Loop Vector |
Closed-Loop Vector |
|
Feedback Device Required |
No |
Yes (Encoder or Resolver) |
|
Torque at Low Speed |
Moderate |
High (including 0 RPM) |
|
Speed Accuracy |
±0.5–1% |
±0.01–0.05% |
|
Zero Speed Holding |
Not reliable |
Very stable |
|
Cost and Complexity |
Lower |
Higher |
|
Best For |
General loads |
Precision, heavy-duty control |
Which Should You Choose?
|
Application Type |
Recommended Control Type |
|
HVAC, Fans, Pumps |
Open-Loop Vector |
|
Hoists, Cranes, Lifts |
Closed-Loop Vector |
|
High-Torque Machining |
Closed-Loop Vector |
|
Basic Conveyors |
Open-Loop Vector |
|
Positioning Systems |
Closed-Loop (or servo drive) |
