EC Fans vs Axial Fans

In HVAC systems such as chillers or air conditioning units, the compressors are by far the biggest consumers of the energy that is required to run the devices.  Next to compressors, in terms of energy usage, are the fans with motors which also consume a significant amount of input power.  There are a few technologies that are used for the fan motors, and these can have a major impact on the energy efficiency of the unit in which they are fitted.

Asynchronous Motors

Traditionally, and still today, asynchronous motors are frequently used for HVAC fans. The main components of an asynchronous motor, used to drive the fan mechanism, are the stator and an internal rotor separated from the stator by a small air gap. The stator of the asynchronous machine carries, for our applications, a three-phase winding, each offset by 120°. The rotor is usually a so-called squirrel cage rotor with a short-circuited rotor winding. The asynchronous motor is operated directly with three-phase current.

As the asynchronous motor is connected to a three-phase current, a rotating field is created in the stator as it is with all three-phase motors. This rotating field induces a voltage in the rotor windings, so that a second magnetic field is generated (induced). The stator field can now act on the rotor field and exert a torque on the rotor, which then begins to rotate. The special feature of the asynchronous machine is that the rotor does not run in step with the stator, but follows the rotating field of the stator. In other words, the asynchronous motor only has torque when the rotor and the rotating field are running asynchronously.

The difference in speed between the rotor and the stator is called ‘slip’. In order to achieve good efficiency, the slip should be as small as possible.  At high loads, however, the speed difference of three-phase motor increases, because the greater the torque required by the rotor, the more the speed drops and the slip increases. For this reason, asynchronous motors are often equipped with a frequency converter, which keeps the speed of the asynchronous motor constant under any load situation.

Nevertheless, asynchronous motors bear two major disadvantages: (1) the aforementioned slip causes losses, and (2) high starting currents requiring special measures for cables and fuses.

Electronically Commutated Motors

In order to reduce the energy consumption of fans in ventilation and air conditioning applications, there is currently nothing to surpass  modern EC technology. EC motors are more energy-efficient, durable and quiet, and can save up to 50% electricity usage.

EC stands for “electronically commutated” which means in power electronics, that a current flow passes from one branch to another in a well-defined manner.

Several power electronics components are required for an EC motor drive. Firstly, the alternating current must be converted into direct current by means of a rectifier.  A PFC (Power Factor Correction) circuit can be used to reduce the reactive power (impedance) entailing losses.  It also allows variable input voltages.  The inverter, as the next stage, generates the voltage for the windings required to operate the EC motor.  This develops magnetic poles in the stator, which interact with the permanent magnets in the rotor.  These types of motor are also call BrushLess Direct Current motors.

To rotate the BLDC motor, the stator windings need to be energized in a dedicated sequence.  It is important to know the rotor position in order to understand which winding will be energized following the energizing sequence.  Rotor position is sensed using Hall effect sensors embedded into the stator.

Most BLDC motors have three Hall sensors embedded into the stator on the non-driving end of the motor.  Whenever the rotor magnetic poles pass near the Hall sensors, they give a high or low signal, indicating the N or S pole is passing near the sensors.  Based on the combination of these three Hall sensor signals, the exact sequence of commutation can be determined.

Shown here is a simple schematic of a so called Hall sensor.  These consist of thin semiconductor layers, which usually have four electrodes on the sides.  A current I is fed in through the two electrodes opposite each other, the two electrodes orthogonal to them are used to take the Hall voltage UH.  If a magnetic field B running perpendicular to the layer flows through the Hall sensor, it delivers an output voltage which is proportional to the amount of the vector product of magnetic flux density and current.  The cause is the Lorentz Force on charge carriers in the layer.  It is proportional to the current, the charge carrier mobility and inversely proportional to the layer thickness.

When a BLDC motor rotates, each winding generates a voltage known as back Electromotive Force or back EMF, which opposes the main voltage supplied to the windings according to Lenz’s Law.  The polarity of this back EMF is in opposite direction of the energized voltage.

The permanent magnets in the rotor are then attracted, in sequence, to the magnetic poles in the stator. Simultaneously, the rest of the stator windings are charged with the reverse polarity.  These attraction and repulsion forces combine to achieve rotation and produce the optimal torque.

Below is an example of an electronically commutated three-phase BLDC motor, (but the full sequence can be found here):

By means of a suitable control circuit, a control behaviour is achieved which largely corresponds to the behaviour of a direct current machine.  For this purpose, a simplified frequency converter with block commutation is used, in which the intermediate circuit can be fed directly with a variable DC voltage and thus controls the speed of the motor linearly.

Again, an electronically commutated motor differs from a standard asynchronous motor by a much better efficiency.  This is particularly evident in partial load operation: with an AC motor, the efficiency reduces as the motor speed reduces, this is due in particular to increased slippage losses.

In contrast, the EC motor runs synchronously, so that no slip occurs and thus losses can be avoided.

 

Advantages

  • Variable speed motors

With the simplest of controls, the EC motor can be controlled precisely, stepless and down to 10% of its output without humming or stopping.

Conventional AC motors have to be controlled by an external frequency converter. This consumed a lot of power and also caused humming noises.

  • Energy saving

EC motors consume up to 70% less electricity in ventilation systems.  The rule of thumb is that the additional costs for an EC fan are paid off within 6 months through electricity savings.

  • Performance

EC fans build up a considerably higher pressure.  This allows more air to be transported at lower costs

 

  • Other Advantages
    • EC Fans are maintenance free and silent in part load
    • Stepless speed control allows pressure control, volume flow control and air quality control
    • There is no need for peripheral components like motor protection switch, thermal contact evaluation device, or phase control unit – all of these capabilites are managed inside the EC motor and automatically
Sample calculation of the energy saving potential of EC fans
Short-term - 100% capacity
Air Volume
8,000 m3/h
Static Pressure
500 Pa
Power EC (1)
1,940 W
Power AC (2)
2,650 W
Operation time per day
2 hours

ENERGY CONSUMPTION PER YEAR

Power consumption EC
1,416 kWh/a
Power consumption AC
1,935 kWh/a

SAVING

518 kWh/a

SAVING IN %

26.8%

Cost saving per year

Emission reduction per year

Day operation - 80% capacity
Air Volume
6,400 m3/h
Static Pressure
320 Pa
Power EC (1)
1,040 W
Power AC (2)
1,810 W
Operation time per day
10 hours

ENERGY CONSUMPTION PER YEAR

Power consumption EC
3,796 kWh/a
Power consumption AC
6,607 kWh/a

SAVING

2,811 kWh/a

SAVING IN %

42.5%

Cost saving per year

Emission reduction per year

Night operation - 60% capacity
Air Volume
4,800 m3/h
Static Pressure
180 Pa
Power EC (1)
483 W
Power AC (2)
1,080 W
Operation time per day
12 hours

ENERGY CONSUMPTION PER YEAR

Power consumption EC
2,116 kWh/a
Power consumption AC
4,730 kWh/a

SAVING

2,615 kWh/a

SAVING IN %

55.3%

Air Volume
Static Pressure
Power EC (1)
Power AC (2)
Operation time per day

TOTAL

Power consumption EC
7,328 kWh/a
Power consumption AC
13,271 kWh/a

SAVING

5,944 kWh/a

SAVING IN %

44.8%

1,189 Euro

3.60 t C02