MOTORS AND DRIVES 61
What exactly is an ac drive and just how does it work?
Olli Tevä outlines ac drive technology and, in particular, its uses in HVAC applications.
A
n ac drive is a device that is 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 three-phase voltage in the national electrical grid
connected to a motor creates a rotating magnetic field in it. The rotor of the electrical motor will follow this rotating magnetic field.
Speed of motor
An ac drive converts the frequency of the network to anything between 0 to 300Hz or even higher, and thus controls the speed of motor proportionally to the frequency. The technology consists of:
n Rectifier unit: The ac drive is supplied by 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.
n Dc circuit: The dc circuit will store the electrical energy from the rectifier for the inverter to use. In most cases, the energy is stored in high-power capacitors.
n 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 three-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 thus, the output of the process.
The benefits of an ac drive
The types of motors that ac drives control are normally operating at constant speed. Enabling the user to control the speed of motor potentially gives him various benefits in terms of process control, system stress and energy savings. n Process control: controlling the process output to match the need; synchronising different parts of the main process to secure smooth flow between subprocesses; easily changing the setup when the process requirements change.
n In system stress: Reducing the start-up current, which allows use of smaller fuses and supply connections and reduces peak loads on the electrical network; Reducing the mechanical shock in start and stop situations.
n Energy: Saving electrical energy compared to traditional methods of process control. For instance in pump and fan applications, energy savings are typically 20–50 per cent.
HVAC applications
In heating, ventilation and air conditioning (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 the building or providing additional heat energy to the building. The majority of HVAC applications where ac drives are used are fans, pumps and compressors. 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.
Fig. 1. Ac drive main components; rectifier, dc circuit and inverter.
www.engineerlive.com The ac drive
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