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.

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What is an AC drive?

2. What is an AC drive and how does it work?
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.
The technology
Rectifier unit Inverter unit
The AC drive is supplied with power from the The inverter unit takes the electrical energy
electrical network via a rectifier. The rectifier unit from the DC circuit and supplies it to the motor.
can be uni- or bidirectional. When unidirectional, The inverter uses modulation techniques to
the AC drive can accelerate and run the motor by create the needed 3-phase AC voltage output
taking energy from the network. If bidirectional, for the motor. The frequency can be adjusted to
the AC drive can also take the mechanical match the need of the process. The higher the
rotation energy from the motor and process and frequency of the output voltage is, the higher the
feed it back to the electrical network. speed of the motor and, therefore, the output
from the process.
DC circuit
The DC circuit stores the electrical energy from
the rectifier for the inverter to use. In most
cases, the energy is stored in high-power
capacitors.
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3. Benefits of an AC drive
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.
Process control
> Controlling the process output to match the > Reducing the mechanical shock in start and
need stop situations
> Synchronizing the different parts of the main Energy savings
process to secure a smooth flow between
subprocesses > Saving electrical energy compared to
conventional methods of process control. For
> Easily changing the setup when the process instance in pump and fan applications, energy
requirements change savings are typically 20-50%.
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
Figure 1: AC drive main components; rectifier, DC circuit and inverter.
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4. 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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5. Fans and pumps
Using an AC drive to control the fan or pump > Tuning the HVAC system during and after
output, rather than using dampers, vanes, commissioning is easier with the flexibility of
valves or on/off control, brings substantial an AC drive
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 air
ducts and water piping
> Slowing down the speed rather than damping Graph 1: The electrical power consumed by a pump at partial
the output results in lower noise levels loads is significantly less than with valve or on/off control.
Compressors
Compressors in HVAC systems are often used water temperature and condenser water
in chillers for cooling water, which again is temperature is based on the outdoor and
used for cooling the air. Utilising AC drives indoor temperatures and the humidity. The
in compressor applications potentially gives most effective way to obtain energy savings is
energy savings compared to on/off control. to tune the overall system rather than optimize
individual functions. The AC drive gives the
Energy savings are achieved by optimising the flexibility to tune the set up of the system to
system setup of the compressor, chilled water operate at the most energy efficient operating
circulation and condenser water circulation. point.
The optimum set point for the chilled
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6. 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
Summary
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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