What Is Torque in an Electric Motor?

In simple terms, torque represents a motor's ability to generate rotational force to move a load.

In industrial applications, torque enables a motor to set a machine in motion, keep a load rotating, or overcome resistance during operation.

Therefore, evaluating only a motor's power or speed is not enough. Depending on the application, torque characteristics must also be considered to ensure that the drive system can meet the load requirements.

What Is the Role of a Variable Frequency Drive?

A variable frequency drive (VFD) allows operators to control important aspects of motor operation, particularly its operating frequency and, consequently, its speed.

However, its role goes beyond simply making the motor run faster or slower.

Depending on the control method used, the drive can adjust the power supplied to the motor according to the application's requirements, providing greater control over the torque produced.

This capability makes VFDs an important solution for applications that require different levels of force throughout operation.

Frequency and Speed: What Is the Relationship?

In AC motors, the supply frequency is directly related to the speed of the rotating magnetic field.

By varying the frequency supplied by the drive, it is possible to control the motor's rotational speed.

However, reducing frequency does not simply mean reducing speed while keeping all other operating characteristics unchanged. The drive must maintain appropriate voltage and current conditions so that the motor can produce the torque required by the load.

For this reason, controlling the voltage-to-frequency relationship is fundamental to strategies such as V/f control.

What About Torque?

The torque produced by a motor depends on its electrical operating conditions and the characteristics of the load.

In certain applications, particularly those requiring greater force at low speeds or during startup, a more advanced control method may be necessary.

This is where features such as torque boost and vector control come into play.

Torque boost compensates for certain operating conditions to improve the motor's torque capability, while vector control provides more precise control over the variables involved in motor operation.

For example, Provolt's PI100 series supports sensorless vector control and V/f control, as well as automatic or manual torque boost.

V/f Control vs. Vector Control

Both strategies can be used to meet different application requirements.

With V/f control, the drive operates according to an established relationship between voltage and frequency. This strategy is widely used in applications that do not require highly precise torque and speed control.

Vector control, on the other hand, allows more advanced control of motor performance. It is particularly useful in applications that require greater precision, faster dynamic response, and reliable performance under varying load conditions.

For example, Provolt's PI550 series offers V/f control, sensorless vector control, and vector control with a pulse generator (PG), along with torque-related features such as automatic torque boost, torque control, and torque limiting.

Why Is Torque Important at Low Speeds?

Some applications require a motor to operate at low speeds without losing its ability to move the load.

This can occur in equipment such as:

• Conveyor systems;
• Winches;
• Elevators;
• Pumps;
• Centrifuges;
• Compressors;
• Process machinery.

In these situations, simply reducing the frequency is not enough. The system must maintain appropriate operating conditions so that the motor continues to produce the required torque.

Therefore, selecting the right drive control method requires considering the load characteristics and the expected operating conditions throughout the process.

Starting Torque Matters Too

Motor startup is one of the operating conditions that can place the greatest demands on the motor.

A load with high inertia or one that requires significant force to begin moving may demand high torque from the very start.

A properly configured VFD can help ensure a controlled startup, reducing mechanical stress and providing the motor with the conditions needed to accelerate the load.

For example, the Provolt PI550 specifies starting torque of up to 150% at 0.5 Hz under certain sensorless vector control conditions.

Constant Torque and Variable Torque

Another important consideration is understanding how the load behaves.

Some applications involve constant torque, where the torque demand remains relatively stable as speed changes. Conveyor systems and certain material-handling equipment are examples of this type of load.

Other applications, such as fans and some pumps, exhibit variable torque characteristics, where torque demand changes with speed.

This distinction directly influences drive selection and parameter configuration.

For example, Provolt specifies an overload capacity for the PI100 of 150% for one minute under constant-torque conditions and 120% for one minute under variable-torque conditions.

The Importance of Parameter Configuration

Even a VFD equipped with advanced features must be correctly sized and configured.

Parameters such as motor power, rated current, frequency, speed, load type, acceleration and deceleration ramps, and control method must be considered during setup.

Proper configuration allows the drive to respond according to the actual characteristics of the application, helping prevent unsuitable operating conditions and improving efficiency.

How to Choose the Right Control Method

The choice mainly depends on the application's requirements.

For simpler systems, V/f control may be sufficient. Applications requiring greater speed precision, high torque at low speeds, or faster dynamic response may benefit from vector control.

Before selecting a VFD, it is important to evaluate:

• Load type;
• Required torque;
• Starting torque;
• Speed range;
• Required precision;
• Acceleration and deceleration behavior;
• Overload conditions;
• Motor characteristics.

Conclusion

The relationship between variable frequency drives and torque is directly linked to the ability to adapt motor performance to the needs of an application.

Beyond speed control, a VFD offers different control strategies and features that help ensure the motor delivers the required performance under different operating conditions.

In practice, understanding the torque demanded by the load is essential for selecting the right drive, defining the appropriate control method, and configuring the system correctly.

When speed and force need to work together, proper torque control is an important part of drive efficiency and reliability.