V/f control in Induction Motors; Volts per Hertz control

V/f Control of Induction Motors – Working, Characteristics, and Applications

Introduction

V/f control, also known as Volts-per-Hertz (V/Hz) control, is the simplest and most widely used method of controlling the speed of induction motors. It is especially popular where precise tuning is not required and motors need to operate up to 1000 Hz.

This method is widely adopted in industrial applications because it allows multiple motors to be started on a single VFD (Variable Frequency Drive), which is not possible with encoder-based vector control systems.

---

Principle of V/f Control

The principle of V/f control is simple:

• To maintain constant flux in the motor, the ratio of applied voltage to supply frequency (V/f) must remain constant.
• At lower frequencies, the voltage is reduced to avoid magnetic saturation, while at higher frequencies, the voltage is increased proportionally.
• This ensures the motor operates efficiently across a wide range of speeds.

---

Torque Formula for Induction Motors

The electromagnetic torque developed by an induction motor is given by:

👉 This equation shows that torque is directly proportional to the square of the applied voltage (V²) and inversely proportional to slip and impedance.

---

Limitations of V/f Control

While easy to implement, V/f control does have some drawbacks:

• Weak starting torque compared to vector control.
• Speed regulation is typically around 2%–3%.
• Slower speed response (about 3 Hz).
• Limited precision in torque and speed control.

---

VFD Speed Control Range in V/f Method

A VFD using V/f control typically has a speed control range of 1:40.

👉 Example: If the rated frequency is 50 Hz, then:

Minimum Controllable Frequency=50/ 40=1.25 Hz

So, the motor can be effectively controlled down to 1.25 Hz.

---

Torque–Speed Characteristics of Induction Motors

The torque-speed curve of an induction motor can be divided into regions:

1. Starting Region
• Motor draws 6–7 times rated current.
• Starting torque ≈ 1.5 times rated torque.
2. Acceleration Region
• As speed rises, current reduces significantly.
3. Base Speed (Rated Frequency)
• Motor delivers rated torque at rated current.
4. Breakdown Torque
• At ~80% of synchronous speed, the motor can deliver up to 2.5 times rated torque (called breakdown torque).
• Beyond this, torque falls rapidly, and the motor stalls if overloaded.

---

Torque–Speed Curve under V/f Control

The shape of the torque–speed curve depends on the load type:

• Variable Torque Loads (Fans, Pumps):
  Voltage is reduced at low frequencies, reducing magnetizing current and preventing faults. This improves efficiency.
• Constant Torque Loads (Conveyors, Crushers):
  Full magnetizing current is required at all speeds, so a straight V/f line is maintained.

---

Advantages of V/f Control

1. Provides a wide range of speed control.
2. Delivers good running and transient performance.
3. Voltage and frequency reach rated values at base speed.
4. Simple, low-cost wiring.
5. Low starting current compared to DOL (Direct On-Line) starting.

---

Normal Duty VFD vs Heavy Duty VFD

Feature	Normal Duty VFD	Heavy Duty VFD
Typical Applications	Variable torque (Fans, Pumps)	Constant torque (Mixers, Conveyors)
Overload Capacity	110% for 60 sec	150% for 60 sec
Continuous Current Rating	Higher	Lower
Motor Rating Adjustment	Full rating usable	Requires derating (e.g., 20 kW → 15 kW)

👉 Thumb Rule:

• Use Normal Duty VFD for variable torque applications.
• Use Heavy Duty VFD for constant torque applications.

---

Conclusion

V/f control is a cost-effective and simple method to control induction motors where high precision is not critical. The torque equation highlights that motor torque depends strongly on applied voltage and slip. While this method has limitations in starting torque and speed response, it remains a reliable choice for HVAC, pumps, and multi-motor operations. For constant torque applications, selecting the right VFD rating (normal vs heavy duty) is crucial to ensure motor protection, efficiency, and longevity.

---