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Where are Fans and Blowers found?
While the typical application is air handling, some of these applications
and energy saving opportunities may surprise you…
- Paint Booth - Fans are used to balance the air pressure inside
the booth, to provide proper ventilation and filtration. AC drives
can be used to replace inlet vanes.
- Cooling Tower - Multiple fans force air through water spray,
removing heat from the water. AC drives provide an alternative
to cycling the fans on and off.
- Induced Draft - Found in many plants using boilers, ovens, or
kilns, the induced draft fan provides forced air to a combustion
process. AC drives can be an energy saving alternative to inlet
vanes.
- Dehydrators - Used in the food industry among others, dehydrating
processes usually require large volumes of air to be moved, another
AC drive opportunity.
- Dust or Fume Extraction - Many industrial processes generate
dust, gases, or fumes, which cannot be allowed to remain in the
production area. AC drives can be used on the extraction fans
are used to replace dampers or vane control.
Where are Pumps found?
Centrifugal type pumps offer the largest energy saving opportunity
- Water Treatment - Intake, storage and distribution, aeration,
and lift stations.
- Chemical Processing
- Food Processing
- Petroleum Refining and Processing
Where are Compressors found?
Compressed air is found more than 70% of manufacturing facilities,
and in fact is referred to as the "fourth utility". Operated at
peak performance, a well managed and maintained system can save
20% to 50% in running cost. By applying an adjustable speed drive,
the compresor's output can be optimized. For every 2 PSIG redustion
in system pressure, a 1% reduction in energy comsumption is realized.
Introduction
The information contained on this page is only an introduction to
the use of inverters as a means of varying the speed of ac motors
to control flow via fans, pumps, and compressors, and to highlight
the significant advantages available. Our outside sales team can
provide very specific information, tailored to your applications.
Principal of Operation of the Inverter
Basic Operation
An inverter is an electronic power device used to control the
speed of a standard industrial three phase ac motor in proportion
to an input signal. The inverter offers many other function such
as overload and short circuit protection, soft start/stop and
pre-set speeds.
The speed of an ac motor is nominally proportional to the frequency
of the applied voltage, therefore to vary the speed, the frequency
must be controlled. This is achieved using two power stages -
the first rectifying the ac supply to dc, the second a transistor
bridge switching the dc to produce an ac output. The rate at which
the transistors are switched will vary the output frequency and
hence the motor speed.
The Torque equation of an ac motor:
T (Torque) a Applied Voltage (V)
Voltage Frequency (f)
If constant torque is required over the speed range the applied
voltage must be proportional to the frequency. However the torque
required to drive most fans drops as a square of the speed (and
frequency):
Applied Motor Voltage (V) a Speed2 (N)
SSD Drives Inverters can be programmed to give the above characteristic,
greatly reducing the voltage at lower speeds. This reduction reduces
losses, giving cooler motor running temperatures and further energy
saving.
Open and Closed Loop Control
In an Open Loop System the fan or blower runs at a speed nominally
proportional to the setpoint signal. There is no monitoring of
the actual speed so no compensation can be made.
In a Closed Loop System the process variable is monitored and
compared to the setpoint value. This difference between the actual
and required value (error) is amplified and used to control the
motor speed. The measured and controlled variable could be any
of a range of variables including air flow, pressure, temperature
, level etc.
Basic Methods of Flow Control
Before identifying the advantages of inverter control it is worth
considering some of the more common mechanical methods of varying
flow.
- Dampers- A mechanical vane is opened or closed
in the air path to vary the resistance to the air flow. The fan
power is effectively constant irrespective of flow and therefore
wasteful of energy. Such a system can also be noisy.
- Bypass- Similar to the above but a return path
allows a proportion of the air flow to be cycled back to the inlet.
The vane is adjusted to determine the amount of air recycled and
hence the net flow. Although potentially less noisy than a damper
alone, there is little improvement in energy saving.
The Advantages of Inverter Control
Energy Savings
Some of the basic equations for centrifugal fans and blowers:
Flow a Speed
Torque a (Speed)2
Power a (Speed)3
It can be seen from the above that the power required to drive
a fan or blower is proportional to the cube of the speed. In all
the systems above the AC motor is run "across the line"
at a fixed speed with flow varied by a mechanical constriction.
When an inverter is used to reduce the flow rate, the speed of
the fan or blower is reduced with a corresponding cube root reduction
in power consumed.
Control
Inverter based flow controls can be integrated very easily into
larger control systems and can interface to analog voltage demands
(typically 0-10V), current loops (typically 4-20mA) or serial
communication links. Fiber-optic based systems are popular due
to their electrical noise immunity.
Maintenance
In non inverter driven systems the fan or blower motor runs at
full speed and usually near to full power continuously. If an
inverter is used, the motor only has to run at the speed and power
required causing less wear on the motor and mechanical transmission.
Noise
Fundamental equation for a centrifugal fan:
Noise a (Blade Tip Speed)5
Where a fixed speed fan is employed, the fan size and speed must
be designed to move the maximum required amount of air. However,
under normal conditions lower than maximum flow rate is required.
If an inverter is used, the fan itself can be slowed - a 15% reduction
in speed gives a 55% reduction in noise. The requirement to limit
environmental noise can be critical in offices and hotels etc.
Application Notes
Although the details of any specific design will vary upon the
particular application, consideration should be given to the following
points:
Enclosure Protection
SSD Drives can be supplied to IP20 or IP54 levels of protection.
IP20 units are suitable for mounting in separate enclosures or
direct mounting in very clean environments.
Motor Derating
The name plate rating of an AC motor will assume full speed running.
When running at less than full speed, the cooling effect of the
shaft mounted fan will be reduced and therefore full torque will
not be available. This will likely not be a problem on fan and
pumps because the torque requirement reduces rapidly with speed,
but in constant torque applications, derating must be taken into
account.
Multi Motor Systems
Some air handling systems require a number of fans to be speed
controlled together rather than a single larger one. In such systems
it is acceptable to connect the fan motors in parallel and control
them from a single inverter. Select the correct inverter by adding
the motor currents (not the horsepowerpower). Each motor should
have an appropriately rated overload connected in series as the
inverter protection will be based on the total current of the
motors.
Resonance
An inverter driven system can run over a wide speed range and
it may not be possible to design the mechanical transmission and
ducting to avoid all potential resonance. This can be overcome
by programming ‘skip frequencies’ i.e. speeds at which
the motor will not dwell.
Stopping Modes
Some applications require very rapid stopping (positioning systems
for example) others will need a gentler slow down (e.g. large
fans). The SSD Drives range of inverters have selectable stopping
modes including ‘coast’ and ‘injection braking’
allowing the correct braking method most suited to the application
to be used.
Training
Regular inverter training courses are given by SSD Drives providing
opportunity for in depth study of the principals and applications
of inverter based control Systems.
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