VFD Motor Control Techniques

Posted on 10th Oct 2022

Changing Frequency

Inverting operation uses IGBT for switching the voltage at specific intervals to adjust the frequency, then controls the power to the motor, allowing us to change the speed and direction as well as the torque characteristics for more efficiency. Darwin Motion Drive is used to control induction motors. It does this by varying the supply frequency by converting the AC supply into DC, a process known as rectification, and then converting it back into AC, a process known as inverter operation.

Darwin Motion VFD may manage the voltage applied to the motor by varying the power supply's frequency and voltage while delivering power to the motor. We can specify parameters in VFD for how we apply voltage on the drive, and there are a few options.

Induction motors connected to a Variable Frequency Drive can be managed using one of four standard motor control techniques.

Volts-per-freq (V/f), V/f with encoder, Open Loop Vector, and Closed Loop Vector are the four.


Motor encoders, an electro-mechanical device installed on the back of a motor case and connected to the motor shaft, are used to accurately determine speed.

Each revolution of the motor shaft will result in a series of electrical pulses (PPR).

The VFD receives these pulses and uses them as speed feedback.


You don't need to utilise a feedback mechanism for speed detection using this method, which is the simplest and uses less configuration options in the VFD known as "Volts per Hertz." Normally, we use encoders for speed detection in motors.

When high frequency operation is needed, the V/f control approach is employed.

Another benefit is the ability to connect many motors to a single VFD. V/f is the only control technology that enables the operation of numerous motors from a single VFD.

When using several motors, each motor will start, stop, and follow the same speed reference at the same time.

One drawback is that the VFD doesn't provide feedback that the motor shaft is rotating at the specified speed.

The motor's starting torque is also restricted to 150%, however this will more than plenty for the majority of applications requiring variable torque.

Most applications of variable-torque fans and pumps employ the V/f control technique.

With Encoder, V/f

An encoder can be utilised with the V/f Control method if your application needs speed precision and must operate at a higher frequency reference. Encoder feedback improves the V/f control with speed accuracy at +/- 0.03% of maximum frequency.

The output voltage is managed by the V/f pattern.

Since the voltage and frequency are predetermined, this supports high speed control without requiring a large dynamic response.

This method is uncommon because investing in an encoder and PG card does not improve the outcomes.

The open loop V/f control method's starting torque, speed response, and speed control range are all equal.

The encoder's PPR restricts operation frequencies above a certain level.

Vector Open Loop

Open Loop Vector (OLV), often known as sensor-less vector, differs significantly from the V/f control approach.

There is no encoder being utilised, as indicated by the term "Open loop," hence no feedback is obtained.

Greater and more dynamic motor control is what the OLV control approach aims to give.

Similar to how a DC motor is regulated, vector control is utilised to achieve independent control of motor speed and motor torque.

OLV motor can provide 200% of rated torque at 0.3 Hz. For many applications, larger starting torque at slower speeds is necessary.

We may configure four-quadrant torque restrictions using this way.

To prevent damage to machinery, products, or equipment, torque restrictions are typically employed to limit motor torque.

The OLV control system provides a higher speed response of 10 Hz with the benefits of torque limitations, which can enable a more dynamic reaction to impact loads.

A rock crusher would be an illustration of an application that could generate impact loads.

Depending on the kind and number of rocks going through the crusher, the load is continually changing.

Vector Closed Loop

This is the most effective control strategy. Similar to the OLV approach, the Closed Loop Vector (CLV) motor control method determines output voltage using a vector algorithm.

The main modification is the interface of an encoder.

With vector control and encoder feedback, a motor can start with a 200% starting torque at zero revolutions per minute.

This feature works best in applications that need elevators, cranes, and hoists.

The maximum speed responsiveness, over 50 Hz, as well as the largest speed control ranges, 1:1500, are both possible with encoder feedback.

The CLV technique can operate the motor in torque control mode in addition to these high performance operating requirements.

The VFD may directly adjust motor torque instead of motor speed while operating in torque control mode.

Any application where torque takes precedence over speed calls for this.

Torque control is frequently used in web applications, winders, reminders, and capping.