What is an AC drive?148Ø9

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What is an AC drive and how does it work?

The technology

An AC drive is a device used to control the speed of an electrical motor. The speed is controlled by changing the frequency of the electrical supply to the motor. The 3-phase voltage in the national electrical grid connected to a motor creates a rotating magnetic field in the motor. The rotor of the electrical motor follows this rotating magnetic field. An AC drive converts the frequency of the network to anything from 0 up to 300 Hz or even higher, and thus controls the speed of the motor in proportion to the frequency.

Rectifier unit

The AC drive is supplied with power from the electrical network via a rectifier. The rectifier unit can be uni- or bidirectional. When unidirectional, the AC drive can accelerate and run the motor by taking energy from the network. If bidirectional, the AC drive can also take the mechanical rotation energy from the motor and process and feed it back to the electrical network.

DC circuit

The DC circuit stores the electrical energy from the rectifier for the inverter to use. In mostcases, the energy is stored in high-power capacitors.

Inverter unit

The inverter unit takes the electrical energy from the DC circuit and supplies it to the motor. The inverter uses modulation techniques to create the needed 3-phase AC voltage output for the motor. The frequency can be adjusted to match the need of the process. The higher the frequency of the output voltage is, the higher the speed of the motor and, therefore, the output from the process.

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Benefits of an AC drive

Process control

Controlling the process output to match the need

Synchronizing the different parts of the main process to secure a smooth flow between subprocesses

Easily changing the setup when the process requirements change

System stress

Reducing the start-up current, which allows the use of smaller fuses and supply connections and reduces peak loads on the electrical network

Reducing the mechanical shock in start and stop situations

Energy savings

Saving electrical energy compared to conventional methods of process control. For instance in pump and fan applications, energy savings are typically 20-50%.

The types of motors that AC drives control normally operate at a constant speed. Enabling the user to control the speed of the motor potentially gives various benefits in terms of process control, system stress and energy savings.

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Figure 1: AC drive main components; rectifier, DC circuit and inverter.

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How can AC drives be used?Heating, ventilation and air conditioning (HVAC)

AC drives are important in all sorts of industries, including marine, oil & gas, water & wastewater management and pulping, to name a few. In HVAC applications the main processes are related to heating, cooling, drying and circulating air. Supporting processes are mostly related to taking the extra heat out of a building or providing additional heat energy to a building. AC drives are mainly used in HVAC applications in fans, pumps and compressors.

Figure 2: The main components of an HVAC system; air circulation, water cooling circuits and water heating circuits.

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Fans and pumps

Using an AC drive to control the fan or pump output, rather than using dampers, vanes, valves or on/off control, brings substantial energy savings, if the required output is less than nominal most of the time. The AC drive controls the speed of the pump and fan by changing the electrical energy supplied rather than damping the air or water flow. It is similar to reducing the speed of a car by pressing less on the accelerator, instead of using the brake. The payback time for an AC drive is typically one year or less.

Other benefits of using AC drive to control the speed of fan or pump are:

Smooth ramp up and down causes less stress to the mechanics of fans and pumps and to airducts and water piping

Slowing down the speed rather than damping the output results in lower noise levels

Tuning the HVAC system during and after commissioning is easier with the flexibility of an AC drive

Graph 1: The electrical power consumed by a pump at partial loads is significantly less than with valve or on/off control.

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Compressors

Compressors in HVAC systems are often used in chillers for cooling water, which again is used for cooling the air. Utilising AC drives in compressor applications potentially gives energy savings compared to on/off control.

Energy savings are achieved by optimising the system setup of the compressor, chilled watercirculation and condenser water circulation. The optimum set point for the chilled

water temperature and condenser water temperature is based on the outdoor and indoor temperatures and the humidity. The most effective way to obtain energy savings is to tune the overall system rather than optimize individual functions. The AC drive gives the flexibility to tune the set up of the system to operate at the most energy efficient operating point.

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Other benefits

Fewer starts and stops reduce the wear on the compressor

The piping and mechanics are subject to less stress in ramp up or down situations

Reduced noise level in low load situations

Possibility to use high-speed compressors

40% of all the energy used in Europe and North America is consumed in buildings. The biggest share of this energy is consumed by HVAC applications. With rising energy costs and concerns about CO2 levels and global warming, it is crucial to use all means available to reduce the energy consumption by HVAC applications. The savings potential is large. The key issue is to start looking more at the lifetime costs of a HVAC system, where energy costs play a big role, rather than at the initial investment in a HVAC system. To give one example, energy accounts for 90% of the lifetime costs of a pump or fan.

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Summary