A variable speed drive can save 20% to 50% depending on the operating cycle and average flow. Applying variable speed drives to ventilation systems creates 20% to 70% in energy savings!

1. Efficient Motor Control with Power Drives System and Active
Energy Efficiency Speed Control
A variable speed drive can save 20% to 50% depending on the operating cycle and average flow. Applying
variable speed drives to ventilation systems creates 20% to 70% in energy savings!
Amit Kumar Senapati
Certified Energy Auditor: Reg. No. EA-13771

2. Efficient Motor Control with Power Drives System and Active Energy Efficiency Speed Control
Purpose of the document: - Understand Efficient Motor Control with Power Drives Systems and improving
know how on opportunities in energy saving by effective speed control.
Power Drive Systen(PDS)
Some of the major components of Power Drive System are as follows:-
a) First, it includes the motor and its sensors. The purpose of the motor is to transform energy into mechanical
power.
b) The gear, or transmission, conveys that power from the motor, to the machine that is performing the work.
c) The variable speed drive module controls the speed of the motor and supports smooth startup.
There is a huge potential for energy savings by looking at the following facts in motors:-
a) Motors Oversizing.
b) Low maintenance and no speed control.
c) Inefficient or poorly maintained power drive system.
An efficient system uses less than half the energy to do the same work

3. On average 97% of the life cycle cost of purchasing and operating a motor is energy related. Still the only factor
considered when selecting a motor is the initial purchase prise.
Reducing the consumption of a motor by 10% can save enough to buy three more motors.
So, the resons to have a power drve system is as follows:-
i) Motors consume a high proportion of total energy(60% - 70% of industrial use)
ii) A lot of motors are oversized and inefficiently controlled.
iii) 97% of the cost of ownership is energy, hence the focus should be more on operating cost rather can
initial purchase.
Less than 5% of the motors are DC motors.
Disadvantages of DC Motors:-
i) The manufacturing and maintenance cost is high.
ii) They require DC supply.
Advantages of DC motors: -
i) The motors are easy to control in speed and have very good performance in torque and speed precision.
Permanent Magnet Motors
A small subset of motors is permanent magnet synchronous motor. Here the rotor is fitted with rare earth magnet in
order to achieve increased field strength in a small volume. Due to their cost they are reserved for high performance
applications(like fast cycle machines). But as the energy consumption is less than indusction motors, they come up
more in applications like lifts.

4. Asynchronous Motors
Asynchronous motors are the most commonly used motor today.
Advantages of Asynchronous motors:-
i) They are very simple, robust and low cost.
Disadvantages of Asynchronous motors:-
i) They have high starting current(5 to 7 times the rated load)
ii) Uncontrolled acceleration.
iii) Their speed decreases slightly with load-This is called, “Slip”.
Basic Principle of Asynchronous Motor
The stator is equipped with three-phase windings, positioned at 120 degree intervals.

5. As the elctrical current rises and falls in each of the phases, a rotating magnetic field is created. This field causes
electric currents in the rotor. Electromagnetic interactions, between the rotor currents and the rotaing magnetic field
creates torque, causing the rotor to run. This then rotates the motor shaft.
The rotor follows the rotating field but rotates a little bit slower.This is referred to as “slip” between the stator rotating
field and the rotor. This slip generates the torque which moves the rotor. The slip also increases the torque. This
means that the speed of an asynchronous motor decreases slightly with load.
Asynchronous Motor : Squirrel Cage Type
The most common and most cost effective motor technology is the so called “Squirrel- cage motor”.
Depending on the supply voltage, the winding can be connected in different ways:-
i) Star coupling : One end of the stator coils are linked to a common node and the other ends to one of
the phases of the power supply.
ii) The windings are connected to form a triangle and each corner is connected to a phase of the
supply, there is no common node.
The squirrel cage rotor is comprised of metal bars short-circuited at their extremities by conducting rings.
Energy Efficiency in Motors
A motors maximum efficiency is obtained at 60%to 100% of full load.Most ofter electric motors are oversized.Motor
applications oversize the mtor because the motor because they are not very concerned about energy efficiency.They
want to ensure that there is no chance of the motor being overloaded. When energy is cheap, this approach is a low
cost trade off against overload risk.But as energy costs rise, the oversizing rapidly becomes wasteful and expensive
answer to this design issue.

6. Motors of a higher efficiency class are more complex to design and produce, and contain more metal, different
metals, or high grade metals to provide conductors with lower losses.The motor price will increase accordingly. In
relation to the motor lifetime the purchase proce is only a few percentage points and, due to the saved energy cost,
the pay back period is short.
Motor Efficiency
Electrical motor efficiency (η) is the ratio between the mechanical shaft output power and the electrical input power.
Internal losses include electrical, mechanical, and magnetic losses.
Electric input power can be determoined by input voltage, current and power factor of the motor.
Electric output power is equal to square root of three multiplied by the three phase volatge, current and power factor.
Rewinding motors generally cost about 60% of the cost to replace them. Efficiency drops when motors are rewound.
The best policy is to rewind highly efficient motors and to sell or salvage those that are not.
Gear Purpose
The output speed can be controlled very simply in the following manner. If the input gear with 20 teeth rotates at 10
revolutions per minute (rpm) and it’s connected to an output gear containing 100 teeth, that output gear will rotate at
2 revolutions per minute. The gear ratio in this example is 100:20 or 5:1.
Active Energy Efficiency Using Speed Control
An estimated 60% of motors are not speed controlled.
Many types of equipment can benefit from proper speed control – from pumps and fans to compressors and
machines.
Some of the Affinity rules are as follows: -
Flow is the output from a device such as a fan, pump or compressor, expressed in cubic meters per second or cubic
feet per minute. It is proportional to the shaft speed of the motor. This means that
i) With half the Shaft speed, we get half the flow.
ii) Pressure is proportional to the shaft speed, squared. So with half the shaft speed, you get a quarter of
the pressure.
iii) Power is proportional to the shaft speed, cubed.

7. Variable Speed Drives
i) Variable speed drive is more efficient. With variable speed drives, starting and stopping operations are
made smooth and perfectly controlled.
ii) Slow starting also minimizes inrush currents that often accompany a motor sudden starting. Inrush
currents cause voltage sags that can have damaging effects on the motor and other affected equipment.
iii) Variable speed drives is sometimes called variable frequency drive. This is because the drive receives AC
power at a constant frequency, such as 50 or 60 Hz, and converts it into a variable frequency for supply to
the motor.
Three options associated with starting a motor are as follows:-
i) Direct On Line
Direct On Line starting connects the motor stator windings directly to the main supply. The motor will start
and accelerate according to its natural characteristics.
The starting method is suitable for stable supplies, mechanical still and well designed shaft system.
The starting equipment consists of a main contactor – effectively an on/off switch which is suitable for the
required current – and a thermal or electronic overload relay.
The disadvantage of a direct-on-line starter is the high inrush current that occurs when the motor starts
turning.It may draw a current that is 5 to 7 times the nominal current until it is up to full speed. The inrush
currents can generate mechanical shocks that warrant oversized wiring and additional surge protection.

8. ii) Star Delta
This method requires 3 contactors and a control part to manage the contractor sequence. Both ends of the
motor windings must be accessible. The motor windings are connected in star formation upon starting.
Once the motor torque matches the load torque, the winding connection is changed to delta connection.

9. Star/delta provides a softer start than DOL but acceleration is still not controlled. Star/delta is always used
for motors above 10kW to preserve and optimize the electric installation and mechanical parts.The motor
is started with the Y(Star) connection and accelerated as far as possible, then switched to D(Delta)
connection.Star/Delta connection causes the starting current of about one third the current associated with
DOL starting.The rupture in the star/delta starting current is the result of the starting current making the
switch between star and delta.
iii) Soft Starting
Soft starters increase voltage gradually to generate a smooth, steady acceleration and are perfectly
compatible with DOL and star/delta motors. Although a soft starter does smoothly and gently start a motor,
and bring it up to full speed in a controlled manner, it does not provide substantial energy savings or
variable speed control. During the starting process, the soft starter progressively increases the motor
voltage so that the motor becomes strong enough to accelerate the load to rated speed without causing
torque or current peaks. Soft Starters can also be used to control the stopping of a process and provide
thermal and short circuit protection for the motor.
Direct starting methods are inexpensive, but their performance is also not upto mark.
DOL and star/delta starting methodologies can be stressful on mechanical parts and gear trains. They also
draw high inrush currents.
The soft starter preserves the efficiency of gear trains and mechanical parts, but does nothing to master
speed or torque, which are the main factors to control in a process to make significant energy savings.
The variable speed drive is also known as a frequency converter and is known for having every accurate
speed and torque regulation. It has a rectifier bridge followed by the inverter. Both allow voltage amplitude
and frequency to be varied.

10. Variable speed drives allow electrical and mechanical equipment to be down sized by limiting the starting
current.

11. The same is true for ventilation systems. In fact, using speed drives in fan installations brings 20% to 70% in
energy savings. Notice the power consumption versus air flow curve in this fan installation. The blue line is
the power versus air flow curve when the air flow is controlled with an outlet damper. The red line is the
power versus air flow curve when the air flow is controlled with an inlet vane.
The green line shows the power drawn by removing or fully opening the damper and controlling the speed.
Using an output damper to reduce the flow to 80% uses 95% of the motor’s power. Using an inlet vane to
reduce the flow to 80% still uses 70% of the motor’s power. And using a speed drive uses only 50% of the

12. motor’s power.
One of the major disadvantage of Frequency Converter is that they generate harmonic distortion in the power
network.
Harmonic currents and voltages are generated by non-linear loads connected to the power distribution system.
C-less drive technology uses a frequency converter with very low capacitance value on the DC supply. This
isbecause capacitance is the main source of dictortion. However reducing the capacitance also reduces the
performance of the motor control, which makes it suitable for applications such as HVAC where high performance
speed drives are not required. THDI can be reduced to 30%.
A 12 pulses system is a type of drive design that uses 2 rectifier bridges to supply the DC bus. Each rectifier
is supplied by a secondary winding of a specific transformer; each secondary has a de-phasing of 30°. The
result on the other side is an attenuation of the 5th and 7th orders of harmonics.
An 18 pulses drives involves the same principal but with 3 rectifier bridges.
There are two main types of active filters. First is the Active Front End (AFE). Here the classical rectifier
bridge is replaced by an inverter controlled in order to draw a sinusoidal current onto the network.
A line active filter is a device installed on the power distribution network that senses harmonics and injects,
in opposite phase, a balancing harmonic to maintain a sinusoidal line current. They are very effective but
can be expensive. They are normally used above 100kW and can reduce THDi to under 5%.
A variable speed drive can save 20% to 50% depending on the operating cycle and average flow. Applying variable
speed drives to ventilation systems creates 20% to 70% in energy savings!
References:
1. How to make money by Energy Auditing. Author. Balasubhraminiam.
2. Schneider Energy University.