Sensorless Vector control; Sensorless Vector control Vs V/f control

Variable Frequency Drive (VFD) Sensorless Control: Working, Features & Comparison

Variable Frequency Drives (VFDs) are widely used to control the speed and torque of three-phase induction motors. In addition to the basic V/f (voltage-to-frequency) characteristic, modern drives offer sensorless control features that optimize motor performance without external feedback devices like encoders.

🔹 Key Features of Sensorless Control in VFDs

1. Slip Compensation

• What is Slip?
  Slip is the load-dependent difference between the motor’s reference speed and actual speed.

• How it Works:
  The VFD measures output current and adjusts the stator frequency to compensate for slip.

• Activation:

  • Automatically enabled during guided commissioning.

  • Can be manually activated by entering the stator resistance value (from the motor datasheet).

• Important Parameters:

  • Amplification → Defines how much speed correction is applied.

  • Max. Slip Ramp → Limits frequency change per second to avoid motor overload.

  • Frequency Lower Limit → Defines the minimum frequency at which slip compensation becomes active.

2. Current Limit Value Controller

The current limit controller ensures the VFD and motor are not overloaded during operation.

• How it Works:

  • Monitors motor current.

  • Reduces or increases output frequency to maintain current within the limit.

• Adjustable Parameters:

  • Current Limit → Maximum allowable current.

  • Amplification & Integral Time → Control responsiveness (higher amplification = faster response).

➤ Behavior in Motor Operation

• If motor current exceeds the set limit, the VFD reduces output frequency until current stabilizes.

• Once current falls back below the limit, frequency increases again.

➤ Behavior in Generator Operation

• If current exceeds the limit, VFD increases frequency (instead of decreasing) to reduce current.

• Frequency is raised up to the Maximum Frequency parameter.

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🔹 Control Methods: Scalar vs. Vector vs. Sensorless Vector

1. Scalar Control (V/Hz Control)

• Simple: Output voltage & frequency follow the reference setting.

• Limitation: VFD doesn’t “know” if the motor achieves the desired speed/torque.

• Result: More slip, less precise performance.

2. Vector Control (with Feedback – FOC)

• Uses encoder feedback to measure motor speed & position.

• Processor adjusts voltage and frequency in real-time.

• Provides high torque at any speed, including zero RPM.

• Ideal for: Lifts, hoists, and applications needing full torque at start.

3. Sensorless Vector Control (SVC / Open Loop Vector)

• Uses internal current sensors + motor model instead of external encoder.

• More accurate than scalar control, suitable for most industrial applications.

• Limitation: Cannot control motor at zero speed, since no feedback is available when the motor is stationary.

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🔹 When to Use What?

• Scalar Control → For simple applications (fans, pumps).

• SVC (Sensorless Vector Control) → For general-purpose industrial loads requiring stable torque and better accuracy.

• FOC (Closed Loop Vector with Encoder) → For high-performance drives where starting torque at zero speed is critical.

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✅ Key Takeaways

• Sensorless control improves VFD performance without external sensors.

• Slip compensation and current limit control protect motor and drive.

• SVC is a cost-effective solution for most applications, but FOC is necessary for zero-speed torque requirements.

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