ENERGY EFFICIENCY IN ASYNCHRONOUS MOTORS WITH AND WITHOUT FREQUENCY CONVERTERS

Frequency converter efficiency

Energy Efficiency of Asynchronous Motors with and without Frequency Converter

Due to regulatory requirements, the demands on the energy efficiency of three-phase motors have been continuously increasing in recent years.

Synchronous and asynchronous motors differ in how their rotors operate. In an asynchronous motor, also known as a three-phase motor, the rotor follows the stator's rotating magnetic field with a time delay, meaning it operates asynchronously. This results in a phenomenon known as slip, where the rotor speed is slightly lower than the synchronous speed calculated from the power supply frequency and the number of pole pairs. Asynchronous motors are used in various applications, such as machine tools, pumps, electric vehicles, and fans.

In contrast, a synchronous motor’s rotor follows the predetermined frequency and the magnetic field in the stator synchronously. This is achieved through the use of permanent magnets (self-excitation, such as a magnetized ferrite cylinder as the rotor) or electromagnetic excitation (with a field coil on the rotor and power supplied through slip rings). A key advantage of synchronous motors is their constant operating frequency, making them well-suited for applications that require a stable speed. Synchronous motors are used in driving vehicles, trains, ships, and in industrial applications.

A frequency converter is a device that can regulate the speed of a three-phase motor. It drives an electric motor by varying the frequency and voltage. The converter is placed in front of the motor to generate a variable AC voltage. When a motor is connected directly to the power grid, its speed is determined by the grid frequency. However, when the motor is connected via a frequency converter, the motor speed can be altered by changing the output frequency. Additionally, the converter can regulate the starting current and implement acceleration or deceleration ramps.

There are several reasons for adjusting the motor speed, such as energy savings and improving the energy efficiency of various systems. Speed control helps adapt to specific process requirements and reduces the mechanical load on a machine, thereby extending its lifespan.

One of the main advantages of frequency converters is that they can adjust the power supplied to the driven device according to its energy needs, thereby supporting energy savings and reducing energy consumption. Frequency converters are commonly used in controlling pumps, compressors, and fans.

Whether a motor is more efficient with or without a frequency converter depends entirely on the application. In operation, the speed of an asynchronous motor follows the supply frequency. However, under heavy load, the motor's speed decreases. Although the applied magnetic field remains constant, this leads to a reduction in efficiency. When starting a motor, the torque behavior is not optimal, resulting in extreme current spikes. The speed and torque cannot be continuously adjusted to fit the operating conditions.

A frequency converter first converts the applied alternating or three-phase voltage into a direct current and then converts it back into alternating or three-phase current at any desired frequency. This allows the user to control the drive from 0 to its rated speed without steps and adjust it to meet actual requirements. Significant cost savings can be achieved in the following application examples:

• Demand-controlled pumps
• Conveying systems with variable delivery rates
• Load-dependent compressors or fan controls that account for contamination levels

Depending on the application, energy costs for a motor can be halved by switching to a frequency converter. This depends entirely on the specific use case. For pumps and fans, the use of a frequency converter is often a better choice, as throttling control in some applications leads to significant energy losses. Constant-speed drives always provide the maximum required flow rate, with the actual demand regulated by valves, leading to wasted energy. With a frequency converter, the motor speed can be adjusted to match the actual flow rate needed. By operating with variable speed, energy savings of up to 50% can be achieved.