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Fan drives for building facility and industrial systems
Why power electronics for fan drives
Fans and blowers are used for the widest range of applications when moving
and blowing air. In addition to pure ventilation, important applications
also include the cooling of equipment and systems and the climate control
both of technical or biological systems as well as warehouses, residential
and recreational areas.
Generally, fans and blowers operate for many hours of a year - even continually.
This results in appropriately high operating costs with a relatively low
capital investment costs. This means that the lifecycle costs comprise,
to a large extent, the energy costs. Further, for most applications, three-phase
induction motors are used for these types of drives. The speed of these
drives can be adjusted in a highly efficient way - for example by using
frequency inverters with the appropriate control characteristics.
Depending on the particular application, it is not sufficient to just connect
the fan drive directly to the line supply through a breaker without using
any closed-loop control. Technical requirements demand specific solutions
which, today are increasingly being viewed from the energy cost perspective.
The starting characteristics of large fans, generally with high associated
moments of inertia, require other solutions where it is important to control
the starting operation without stressing the line supply as well as the
mechanical system - electrically, thermally or as far as the force is concerned.
Other perspectives and recommended solutions are restricted to the large
number of drives with three-phase induction motors.
Problems from the field – starting
Every drive must be initially accelerated from either standstill or a low
speed. While for small fans this can be simply done using a breaker, for
larger fans with higher power ratings, other solutions must be used:
Mechanical jerk: Significant stressing of the couplings, slipping pulley
belts, load on the fan mounting system
Pressure surges: Filter overload, noise as a result of pressure surges
in the ventilation duct
Starting current: The line current is significantly increased (by a factor
of between 5 and 8 of the rated current); power supply companies often
specify maximum values, high motor temperature rise in the acceleration
phase, high switch-on losses, extremely high inductive voltage drop as
a result of the reactive current component
Starting noise: Noisy drive belts, pressure surges, significantly increased
motor noise, especially when changing poles
Previous starting solutions
Up until now, both mechanical starting as well as electrical starting equipment
Hydro-coupling start: Additional equipment using an additional oil-filled
hydraulic coupling, in order to reliably handle even heavy-duty starting
- the problem of high motor inrush currents on starting remains.
Reactor starting: This is an extremely effective method but which only
becomes effective above the center of the speed range above which the fan
represents a significant load - the electrical problems when starting the
Star/delta changeover: Widely established, the power supply companies frequently
specify this technique above a certain kW motor power, more control equipment
is required in the form of contactor combinations, significantly reduced
torque while the motor is accelerating
Pole changeover: Requires pole-changing motors (2nd operating speed), good
acceleration, high starting currents remain, voltage spikes when changing
over, noise and an additional jerk when changing over
Wound-rotor motors with resistor starting: Extremely good starting characteristics,
not well established in the field due to the fact that special motors are
required with sliprings and wound rotor
Resistors in the stator circuit: This solution can practically only be
used for low power levels, the starting jerk is reduced and the starting
current is conditionally reduced (for 3-phase circuits)
Voltage reduction using a transformer: By reducing the voltage, the motor
starts with a lower magnetization. This is the reason that this principle
functions the same as the star/delta changeover but only if the fan drive
can accelerate with a significantly reduced torque
Electronic solution using soft-starting
For those applications, where the load on the fan drive is significantly
reduced at lower speeds, an induction motor can be very effectively and
softly accelerated by reducing the voltage with soft starting equipment:
Starting currents in the line supply are only slightly increased (approximately
200% rated current): Can also be used for weak networks
Star/delta changeover is not required
Electronic starting equipment is compact
High efficiency of the voltage adjustment
Can be bypassed after run-up: No continuous losses. The small compact devices
have an integrated bypass contactor.
Continuous mode of operation
Cost-effective series solution up to an operating voltage of 690 V even
for extremely high fan powers of several 100 kW up to 1 MW
Soft stopping possible: Soft, controlled down ramp
For those applications where the voltage can be reduced in operation (three-phase
controller), permanent closed-loop control and speed reduction are possible
For specialists: Some physical basics
of an induction motor
Most of the motors used worldwide are rugged induction motors which are
available at a relatively favorable price. These types of motors have a
characteristic relationship between their electrical and mechanical behavior
even in spite of the different types of constructions, framesizes and various
manufacturers. This is the reason that these types of motors can be easily
used together with KIMO electronic devices without having to make extensive
adjustments to the particular motor.
An induction motor is also known as asynchronous motor. However, most motors
run, in operation, at a constant speed which does not significantly deviate
from the synchronous speed of the line supply: For example, 1435 RPM instead
of a synchronous 1500 RPM
When loaded, the speed only reduces slightly and when the load is removed,
the motor speed increases again towards the synchronous speed
Rugged drive: Most motors can be overloaded briefly, e.g. 200% - for many
applications, this load is not even considered and the drive is simply
used in this way
High starting current: The starting current is generally between 500 and
800% of the rated current
Versions with several changeable pole numbers for different speeds – widely
established Dahlander circuit with a 2:1 pole ratio
The typical load characteristic of an induction motor is clearly defined
in the starting characteristic: This characteristic indicates the possible
torque as a function of the speed
When starting at speed zero, the motor has approximately 100 to 200% rated
torque (and an inrush current of between 500 and 800% the rated current!).
The motor only reaches the typical steep characteristic with a high maximum
torque (stall torque) close to the rated speed: The motor only has its
intrinsic high efficiency while it is operated in this steep part of the
characteristic. The current drawn is then proportional to the load. Further,
in this area, the speed is only slightly depending on the load.
The following well-known formula applies with a good approximation in the
steep part of the characteristic:
Md ~ Ø x I
Ø is the flux (magnetization) of the motor
I is the current in the motor feeder cables
Md is the torque at the shaft
Further, the following relationship applies:
Ø ~ U
This also clearly indicates how the motor behaves when the operating voltage
Initially, with the load remaining the same, the current increases inversely
proportionally to the voltage and the motor speed remains approximately
the same as the operating voltage is proportional to the flux.
Due to the additional relationship
Mstall ~ Ø2
This also clearly indicates how the speed can be adjusted by reducing the
If the stall torque, which has now been reduced, is indirectly exceeded
as a result of the decreasing flux, then the speed of the motor under load
decreases. However, this control principle is only stable if the motor
load torque decreases over-proportionally with decreasing speed, which
is the case for a fan drive. This means that a stable operating point can
again be obtained along the motor characteristic. However, this operating
point with reduced operating voltage is associated with relatively high
losses and with significantly increased currents when compared to frequency
inverter operation. Further, the motor has a poor efficiency.
A load characteristic is now assumed, as example, which corresponds to
a directly driven fan:
As the speed decreases, the required torque decreases according to a square
law. In the reduced speed range, the fan is permanently operated in the
starting range of the induction motor characteristic. At 50% rated voltage,
only one quarter of the previous stall torque is available: The characteristic
of the induction motor is correspondingly reduced and shifted downwards.
The motor is continually operated in the starting range; unfortunately
the current which is drawn does not decrease in proportional to the lower
Even more physics: An induction motor
with frequency inverter
The optimum arrangement would be if an induction motor could be continually
operated in its steep section of its characteristic. And it is precisely
this which is possible if the motor is not connected to a rigid line supply
with constant frequency where the only variable control parameter is its
operating voltage, but the frequency can also be decreased (or, for higher
speeds, also increased). A frequency inverter provides this type of supply.
The frequency inverter changes both the operating voltage and also the
If an induction motor is operated so that with reduced operating voltage
V, the operating frequency f is reduced to the same degree, then the flux
Ø in the motor remains constant (constant main flux).
The following relationship applies:
Ø ~ U/ f
The motor then has a characteristic which corresponds to a continuous shifting
of the steep range of the characteristic with a high associated efficiency.
With the appropriate control, the motor no longer leaves this favorable
operating range, and by shifting the sections of the characteristic, it
is permanently operated in this range at each load speed. In order to further
reduce avoidable losses as a result of the motor magnetization, for fan
drives, the flux can be reduced with decreasing speed, because the torque
which is demanded, also decreases: This means that the frequency inverter
is no longer operated with a linear characteristic between the frequency
and voltage (this results in a constant motor flux), but with a square-law
characteristic, which is reduced to the same degree. This provides enough
flux as is required for permanent operation.
Frequency inverter operation with reduced flux results in:
Lower noise (reduces the magneto-restriction in the motor iron)
Lower magnetizing losses
Higher motor currents in relationship to the decrease (this is acceptable
as long as an overcurrent condition does not occur)
Low acceleration reserves for fast speed changes (for fans, this is essentially
undesirable due to the faster pressure fluctuations)
Why are frequency inverters used for
For many applications, the fan delivery must be continually adjusted or
adapted to a plant or system. The possibility of achieving the correct
average fan delivery by switching-in and switching-out, is seldom a realistic
possibility. Frequently, it is necessary to set or adjust the fan delivery.
This is the reason that for some time now, conventional mechanical and
electrical devices have been available which allow this adjustment to be
However, these techniques all have the disadvantage of significantly reducing
the efficiency of the particular fan or blower. In some instances, the
noise is increased to such an extent that this can only be implemented
on a case-for-case basis or with additional noise reducing measures.
KIMO has many years of experience in the area of variable-speed drives
so that together with you a changeover can be made to such drives, utilizing
the cost-saving and user-friendly frequency inverter operation.
closed-loop control techniques and their physical disadvantages
Today, in the field, there is still a high proportion of closed-loop throttle
and vane controls and, to a lesser extent, speed adjustment using voltage
reduction. Generally, transformers are used or electronically with three-phase
controllers (the same as soft starting devices but with a permanent voltage
These methods will now be described in more detail. Fan and pump technicians
are still being trained to apply and service them. In spite of their low
efficiency they are still widely established.
Closed-loop throttle control
Closed-loop throttle control is based on the principle of changing the
differential pressure of the impeller wheel in operation so that the fan
delivery is reduced. Generally, this is achieved by reducing the cross-section
in the air or gas duct, e.g. using adjusting flaps – the so-called throttle
vanes. Unfortunately this has a high negative impact on the efficiency.
A continually increasing pressure drop is then obtained across the throttling
element, which in turn reduces the volume flow of the gas or air because
the fan or blower must work against an artificially generated counter-pressure.
If the delivery pressure could be adjusted by reducing the speed, then
the square-law relationship between the pressure difference and the volume
flow would have a direct effect:
In order to reduce the volume flow by half, the speed would have to be
reduced so that the pressure difference at the fan would be reduced to
a fourth of the rated value. This is the case at 50% speed. Correspondingly,
at 50% speed, only 25% of the torque is required; the drive power is then
12.5% of the rated value.
When closed-loop throttle control is used, the fan rotates at approximately
a constant speed (the induction motor is operated, directly connected to
the line supply and runs in almost synchronism to the line frequency with
an extremely low slip). If the cross-section is reduced (throttled), initially,
the pressure at the fan, in front of the throttling element, appropriately
increases and the fan delivery is reduced. The pressure differential as
a function of the fan delivery, is shown in the diagram. For a typical
fan, for half of the volume flow, the pressure at the fan increases to
125% of its nominal value. The drive power required by the fan is only
reduced to 72% of the rated value – even though the volume flow has been
halved. If speed control were used, it would be physically possible to
reduce this to 12.5%. This means that it is therefore 5.8x of the ideal
case. However, the speed remains approximately constant so that the required
torque which the motor must provide is also 72% of its nominal value.
Closed-loop vane control
For closed-loop throttle and vane control, the speed hardly changes when
the delivery is changed (an induction motor rotates at an almost constant
speed). This means that its drive torque is also proportional to the required
Using as an example a fan, where the delivery is to be reduced to half
the nominal value, as a result of the vane control, there is still only
a differential pressure of 75% of the nominal value. The required drive
power is reduced to 51% and is therefore significantly more favorable than
a pure closed-loop throttle control – but is still 4x value of the ideal
case. Also in this case the speed remains approximately constant which
means that the required torque decreases to 51% of the nominal value.
Energy saving by electrically controlling
Using this control principle, the fan delivery is reduced by additionally
causing eddies in the air duct. In this case, not all of the rotating fan
surfaces are fully effective as a result of the eddies and, in some cases,
flow reversal takes place. This technique also has a poor efficiency but
is still better than pure closed-loop throttle control.
When the speed is changed, the drive torque decreases to the square of
the speed. If a frequency inverter can be used to make this adjustment,
then the drive power almost corresponds to the ideal characteristic P~Q³.
When the speed is controlled by reducing the voltage, although the required
drive torque also decreases according to a square law, the motor efficiency
decreases drastically; the required power then has approximately the characteristic
as shown below.
Closed-loop speed control using frequency
When this technique is used, the motor is continually operated in its most
favorable operating range with the steep speed/torque characteristic:
Extremely high motor efficiency (in comparison to other control techniques),
practically the same as when directly connected to a fixed line supply
with rated speed
High frequency inverter efficiency
No high starting currents in the line supply
When the speed is reduced, significantly lower line currents
Can be controlled over the complete speed range
The flux can be reduced for a typical fan load characteristic (reduced
From the characteristic of generally used fans, it can be seen that as
the speed decreases, the required drive torque generally decreases as a
square law (with the exception of the frictional component caused by belts,
bearings etc.). The air delivery is also reduced, but favorably to the
same degree over the speed with an extremely high efficiency. Most ventilation
tasks have a load characteristic that principally has the same square-law
characteristic caused by flow losses in the filters, heat exchangers and
air ducts. This means that speed reduction is the ideal control technique
for these types of applications.
And it is precisely frequency inverters which are best suited to control
Additional advantages of a frequency
KIMO frequency inverters have an integrated control which can be used to
simply realize other functions and closed-loop controls – even up to small
automation tasks. Important cooling and ventilation functions can be simply
Restart-on-the-fly circuit: The frequency inverter is powered-up with the
fan still rotating (for example, the fan is slowly rotating due to the
natural air flow)
Closed-loop pressure control: The outlet pressure is kept constant and
the pressure sensor which is used for the closed-loop control is directly
connected to the frequency inverter and its auxiliary power supply is supplied
The noise of condensation fans used in refrigeration systems is reduced:
In the nighttime, the noise of the heat exchangers of industrial and commercial
refrigeration systems is frequently considered to be disturbing. The speed
of these systems can be reduced, depending on the load and the particular
time of day, using frequency inverters.
Extremely heavy duty starting of fans and blowers: Using KIMO frequency
inverters, fans with extremely high moments of inertia can be automatically
accelerated, with the highest possible physical acceleration to the operating
point along the current limit
Phase synchronization: Using the KIMO phase control card, frequency inverters
can be cost-effectively used to start large blowers, which, in rated operation,
can be simply changed-over to the public utility without resulting in system
Special applications from the field
using KIMO – power electronics for specialists
The tunnel ventilation system in Monte Carlo requires starting equipment:
KIMO LEKTROMIK soft starting equipment has been successfully used for this
high load application for 10 years now
The new tunnel under the river Elbe in Hamburg is also ventilated with
the help of KIMO LEKTROMIK soft starting equipment
Large blowers are used in the Australian diamond mines for ventilation.
However, the discharged air contains a significant amount of diamond dust
which is retrieved using extremely sensitive fine filters. If the motors
were to be normally started, these filters would be damaged by the pressure
surges when starting and would frequently rupture whereby the damage caused
by the dust is relatively high. LEKTROMIK soft starters from KIMO have
been used for years to achieve a significantly higher lifetime with the
shortest payback time.
A herb drying system with several drying lines must be operated with a
constant air velocity. The required air quantity is simply adapted using
a KIMO frequency inverter.
For over 5 years now, KIMO frequency inverters in a larger
automation group have been used in the climate and ventilation system of
the old Commerzbank high-rise building in Frankfurt.
In a large Nuremberg washing facility, the pressing line is continually
operated with a constant warm air flow using KIMO frequency inverters with
differential pressure control – thus saving significant quantities of heating
oil when compared to previous solutions.
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KIMO Industrie-Elektronik GmbH
Am Weichselgarten 19, D-91058 Erlangen, Germany
Tel.:+49 9131-6069-99 Fax.:+49 9131-6069-35
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Cluster Mechatronik & Automation
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