Motor Soft Starter

From Wikipedia

A motor soft starter is a device used with AC electric motors to temporarily reduce the load and torque in thepowertrain of the motor during startup. This reduces the mechanical stress on the motor and shaft, as well as theelectrodynamic stresses on the attached power cables and electrical distribution network, extending the lifespan of the system.^[1]

Motor soft starters can consist of mechanical or electrical devices, or a combination of both. Mechanical soft starters include clutches and several types of couplings using a fluid, magnetic forces, or steel shot to transmit torque, similar to other forms of torque limiter. Electrical soft starters can be any control system that reduces the torque by temporarily reducing the voltage or current input, or a device that temporarily alters how the motor is connected in theelectric circuit.

Electrical soft starters can use solid state devices to control the current flow and therefore the voltage applied to the motor. They can be connected in series with the line voltage applied to the motor, or can be connected inside the delta (Δ) loop of a delta-connected motor, controlling the voltage applied to each winding. Solid state soft starters can control one or more phases of the voltage applied to the induction motor with the best results achieved by three-phase control. Typically, the voltage is controlled by reverse-parallel-connectedsilicon-controlled rectifiers (thyristors), but in some circumstances with three-phase control, the control elements can be a reverse-parallel-connected SCR anddiode.

Another way to limit motor starting current is a series reactor. If an air core is used for the series reactor then a very efficient and reliable soft starter can be designed which is suitable for all type of 3 phase induction motor [ synchronous / asynchronous ] ranging from 25 KW 415 V to 30 MW 11 KV. Using an air core series reactor soft starter is very common practice for applications like pump, compressor, fan etc. Usually high starting torque applications do not use this method.

Contents   [hide]  1 Applications 2 Motor and machine 3 Motor start-up 3.1 Direct on-line starting 3.2 Star-delta start-up 3.3 Soft start-up 4 See also 5 References

[edit]Applications

Soft starters can be set up to the requirements of the individual application. In pump applications, a soft start can avoid pressure surges. Conveyor belt systems can be smoothly started, avoiding jerk and stress on drive components. Fans or other systems with belt drives can be started slowly to avoid belt slipping. In all systems, a soft start limits the inrush current and so improves stability of the power supply and reduces transient voltage drops that may affect other loads. ^[2][3][4]

[edit]Motor and machine

Across-the line starting of induction motors is accompanied by inrush currents up to 7 times higher than running current, and starting torque up to 3 times higher than running torque. The increased torque results in sudden mechanical stress on the machine which leads to a reduced service life. Moreover, the high inrush current stresses the power supply, which may lead to voltage dips. As a result, the operability of sensitive consumers may be impaired.^[1]

[edit]Motor start-up

A soft start-up eliminates the undesired side effects. Several types based on control of the supply voltage or mechanical devices such as slip clutches were developed. The list provides an overview of the various electric start-up types. The current and torque characteristic curves show the behavior of the respective starter solution.

[edit]Direct on-line starting

• Three-phase motor with low to medium power rating
• 3 conductors to the motor
• High starting torque
• High current peak
• Voltage dip
• One simple switching device

[edit]Star-delta start-up

• Three-phase motor with low to high power rating
• Six conductors to the motor
• Reduced starting torque, 1/3 of the nominal torque
• High mains load due to current peak during switchover from Y to D
• High mechanical stress due to torque surge during switchover from Y to D
• Two or three switching devices, more maintenance
• Very efficient one

[edit]Soft start-up

• Three-phase motor with low to high power rating
• 3 conductors to the motor
• Variable starting torque
• No current peak
• No torque peaks
• Negligible voltage dip
• One simple switching device
• Optional: Guided soft stop, protective functions, etc.
• Zero maintenance
• Compared to contactor solutions, soft starters, sometimes also referred to as soft starting devices, offer considerable advantages.

Torque surges entail high mechanical stress on the machine, which results in higher service costs and increased wear. High currents and current peaks lead to high fixed costs charged by the power supply companies (peak current calculation) and to increased mains and generator loads.

A soft starter continuously controls the three-phase motor’s voltage supply during the start-up phase. This way, the motor is adjusted to the machine’s load behavior. Mechanical operating equipment is accelerated in a gentle manner. Service life, operating behavior and work flows are positively influenced.

[edit]See also

• Braking chopper
• Space Vector Modulation
• Variable speed air compressor
• Vector control (motor)
• Korndorfer starter
• Motor controller
• Adjustable-speed drive
• Electronic speed control
• Variable-frequency drive
• Thyristor drive
• DC motor starter section of Electric motor

Adjustable Speed Drive

From Wikipedia

Adjustable speed drive (ASD) or variable-speed drive (VSD) describes equipment used to control the speed of machinery. Many industrial processes such as assembly lines must operate at different speeds for different products. Where process conditions demand adjustment of flow from a pump or fan, varying the speed of the drive may save energy compared with other techniques for flow control.

Where speeds may be selected from several different pre-set ranges, usually the drive is said to be adjustable speed. If the output speed can be changed without steps over a range, the drive is usually referred to as variable speed.

Adjustable and variable speed drives may be purely mechanical (termed variators), electromechanical, hydraulic, or electronic.

Contents   [hide]  1 Fixed speeds of electric motors 2 Reasons for using adjustable speed drives 2.1 Adjusting speed as a means of controlling a process 2.2 Example 2.3 Saving energy by using adjustable speed drives 3 Types of adjustable speed drives 3.1 Mechanical adjustable speed drives 3.2 Hydraulic adjustable speed drives 3.3 Continuously variable transmission (CVT) 3.4 Electric adjustable speed drives 3.4.1 Types of control 3.4.2 Types of drives 3.4.3 DC drives 3.4.4 Eddy current drives 3.4.5 AC drives 4 See also 5 References 6 External links

[edit]Fixed speeds of electric motors

Alternating-current electric motors run at speeds closely determined by the number of poles in the motor and the frequency of the alternating current supply. This is unlike the steam engine, which can be made to run over a range of speeds by adjusting the timing and duration of valves admitting steam to the cylinder.

AC motors can be made with several sets of poles, which can be chosen to give one of several different speeds (say, 720 rpm/12 Hz or 1,800 rpm/30 Hz mechanical for a 60 Hz electrical motor). The number of different speeds available is limited by the expense of providing multiple sets of windings. If many different speeds or continuously variable speeds are required, other methods are required.

Direct-current motors allow for changes of speed by adjusting the shunt field current. Another way of changing speed of a direct current motor is to change the voltage applied to the armature.

An adjustable speed drive might consist of an electric motor and controller that is used to adjust the motor's operating speed. The combination of a constant-speed motor and a continuously adjustable mechanical speed-changing device might also be called an adjustable speed drive. Power electronics based variable frequency drives are rapidly making older technology redundant.

[edit]Reasons for using adjustable speed drives

Process control and energy conservation are the two primary reasons for using an adjustable speed drive. Historically, adjustable speed drives were developed for process control, but energy conservation has emerged as an equally important objective.

[edit]Adjusting speed as a means of controlling a process

The following are process control benefits that might be provided by an adjustable speed drive:

• Smoother operation
• Acceleration control
• Different operating speed for each process recipe
• Compensate for changing process variables
• Allow slow operation for setup purposes
• Adjust the rate of production
• Allow accurate positioning
• Control torque or tension
• Allow catching of spinning load (e.g., column of water) after outage.

[edit]Example

An adjustable speed drive can often provide smoother operation compared to an alternative fixed speed mode of operation. For example, in a sewage lift station sewage usually flows through sewer pipes under the force of gravity to a wet well location. From there it is pumped up to a treatment process. When fixed speed pumps are used, the pumps are set to start when the level of the liquid in the wet well reaches some high point and stop when the level has been reduced to a low point. Cycling the pumps on and off results in frequent high surges of electric current to start the motors resulting in electromagnetic and thermal stresses in the motors and power control equipment, the pumps and pipes are subjected to mechanical and hydraulic stresses, and the sewage treatment process is forced to accommodate surges in the flow of sewage through the process. When adjustable speed drives are used, the pumps operate continuously at a speed that increases as the wet well level increases. This matches the outflow to the average inflow and provides a much smoother operation of the process.

[edit]Saving energy by using adjustable speed drives

An adjustable speed drive often uses less energy than an alternative fixed speed mode of operation. Fans and pumps are the most common energy saving applications. When a fan is driven by a fixed speed motor, the airflow may sometimes be higher than it needs to be. Airflow can be regulated by using a damper to restrict the flow, but it is more efficient to regulate the airflow by regulating the speed of the motor. It follows from the affinity laws that reducing fan speed to 50% results in a power consumption drop to 12.5%.^[2]

[edit]Types of adjustable speed drives

Speed adjustment techniques have been used in transmitting mechanical power to machinery since the earliest use of powered machinery. Before electric motors were invented, mechanical speed changers were used to control the mechanical power provided by water wheels and steam engines. When electric motors came into use, means of controlling their speed were developed almost immediately. Today, various types of mechanical drives, hydraulic drives and electric drives compete with one another in the industrial drives market.

[edit]Mechanical adjustable speed drives

There are two types of mechanical drives, variable pitch drives and traction drives.

Variable pitch drives are pulley and belt drives in which the pitch diameter of one or both pulleys can be adjusted.

Traction drives transmit power through metal rollers running against mating metal rollers. The input/output speed ratio is adjusted by moving the rollers to change the diameters of the contact path. Many different roller shapes and mechanical designs have been used..

[edit]Hydraulic adjustable speed drives

There are three types of hydraulic drives, those are : hydrostatic drives, hydrodynamic drives and hydroviscous drives.

A hydrostatic drive consists of a hydraulic pump and a hydraulic motor. Since positive displacement pumps and motors are used, one revolution of the pump or motor corresponds to a set volume of fluid flow that is determined by the displacement regardless of speed or torque. Speed is regulated by regulating the fluid flow with a valve or by changing the displacement of the pump or motor. Many different design variations have been used. A swash plate drive employs an axial piston pump and/or motor in which the swash plate angle can be changed to adjust the displacement and thus adjust the speed.

Hydrodynamic drives or fluid couplings use oil to transmit torque between an impeller on the constant-speed input shaft and a rotor on the adjustable-speed output shaft. The torque converter in the automatic transmission of a car is a hydrodynamic drive.

A hydroviscous drive consists of one or more discs or connected to the input shaft pressed against a similar disc or discs connected to the output shaft. Torque is transmitted from the input shaft to the output shaft through an oil film between the discs. The transmitted torque is proportional to the pressure exerted by a hydraulic cylinder that presses the discs together.

[edit]Continuously variable transmission (CVT)

Main article: Continuously variable transmission

Mechanical and hydraulic adjustable speed drives are usually called transmissions or continuously variable transmissions when they are used in vehicles, farm equipment and some other types of equipment.

[edit]Electric adjustable speed drives

[edit]Types of control

Control can mean either manually adjustable - by means of a potentiometer or linear hall effect device, (which is more resistant to dust and grease) or it can also be automatically controlled for example by using a rotational detector such as a Gray code optical encoder.

[edit]Types of drives

There are three general categories of electric drives: DC motor drives, eddy current drives and AC motor drives. Each of these general types can be further divided into numerous variations. Electric drives generally include both an electric motor and a speed control unit or system. The term drive is often applied to the controller without the motor. In the early days of electric drive technology, electromechanical control systems were used. Later, electronic controllers were designed using various types of vacuum tubes. As suitable solid state electronic components became available, new controller designs incorporated the latest electronic technology.

[edit]DC drives

DC drives are DC motor speed control systems. Since the speed of a DC motor is directly proportional to armature voltage and inversely proportional to motor flux (which is a function of field current), either armature voltage or field current can be used to control speed. Several types of DC motors are described in the electric motor article. The electric motor article also describes electronic speed controls used with various types of DC motors.

[edit]Eddy current drives

An eddy current drive consists of a fixed speed motor and an eddy current clutch. The clutch contains a fixed speed rotor and an adjustable speed rotor separated by a small air gap. A direct current in a field coil produces a magnetic field that determines the torque transmitted from the input rotor to the output rotor. The controller provides closed loop speed regulation by varying clutch current, only allowing the clutch to transmit enough torque to operate at the desired speed. Speed feedback is typically provided via an integral AC tachometer.

Eddy current drives are a type of slip controlled drive. Slip controlled drives are generally less efficient than other types of drives. The motor develops the torque required by the load and operates at full speed. The output shaft transmits the same torque to the load, but turns at a slower speed. Since power is proportional to torque multiplied by speed, the input power is proportional to motor speed times operating torque while the output power is output speed times operating torque. The difference between the motor speed and the output speed is called the slip speed. Power proportional to the slip speed times operating torque is dissipated as heat in the clutch.

[edit]AC drives

AC drives are AC motor speed control systems.

Slip controlled drives control the speed of an induction motor by increasing a motor's slip, either by reducing the voltage applied to the motor, or increasing the resistance of the rotor windings. Because they are generally less efficient than other types of drives, slip controlled drives have lost popularity and have recently been used only in special situations. See eddy current drives above.

In larger ratings (more than a few kilowatts), a wound-rotor motor has its rotor connected to a converter that returns energy to the power system, converting it from low slip frequency to the line frequency. This reclaims the energy that would otherwise be wasted in rotor circuit resistors. These are called "slip energy recovery drives" and are used on such applications as forced-draft blowers for boilers. ^[3] An electromechanical version using a rectifier, DC motor and AC generator is called a Kramer drive.

Adjustable-frequency drives (AFD) control the speed of either an induction motor or a synchronous motor by adjusting the frequency of the power supplied to the motor. Adjustable frequency drives are also known as variable-frequency drives (VFD).

When changing the frequency of the power supplied to an AC motor, the ratio of the applied voltage to the applied frequency (V/Hz) is generally maintained at a constant value between the minimum and maximum operating frequencies. Operation at a constant voltage (reduced V/Hz) above a given frequency provides reduced torque capability and constant power capability above that frequency. The frequency or speed at which constant-voltage operation begins is called the base frequency or speed. Whether the applied voltage is regulated directly or indirectly, the V/Hz tends to follow the general pattern described for the performance described. The variable-frequency drive article provides additional information on electronic speed controls used with various types of AC motors.

Regenerative AC drives are a type of AC drive which have the capacity to recover the braking energy of a load moving faster than the motor speed (an overhauling load) and return it to the power system.

[edit]See also

• Continuously variable transmission
• Regenerative variable-frequency drives
• variable-frequency drive
• Dc injection braking

Variable Speed Drives

What are variable speed drives?

Variable speed drives (VSDs) allow loads driven by AC induction motors (such as fans and pumps) to operate in a wide range of speeds compared to the motor fixed speed. VSDs are also called variable-frequency drives, adjustable-speed drives, variable-frequency inverters, or frequency converters.

VSD installations can increase energy efficiency (in some cases energy savings can exceed 50%), improve power factor and process precision, and provide other performance benefits such as soft starting and over-speed capability. They also can eliminate the need for expensive and energy-wasting throttling mechanisms such as control valves and outlet dampers.

Typical applications of variable speed drives

VSDs can provide significant savings in applications such as:

• Variable air volume air conditioning systems
• Chilled water pumping
• Exhaust air systems, such as dust extraction, paint shop exhaust, and fume cupboards
• Refrigeration systems
• Some modern compressors (including air and refrigeration compressors) 

Variable speed drives case study

Hotel saves energy and improves comfort 
A major Perth hotel installed variable speed drives on its air conditioning fans, improving conditions, reducing noise and cutting fan-related energy costs by about 40%.

Project description

The hotel has a large number of units supplying air conditioning to the guest rooms. As more rooms are occupied, more air is needed to maintain a comfortable temperature in each room. As a result, the fans supplying the air need to vary their output, based on the number of occupied rooms at any given time.

The initial installation had throttling dampers installed in the supply air ductwork. As the requirement for airflow decreased, these dampers closed to reduce airflow to the rooms.
Not only was this method of flow control inefficient in terms of energy use, it was noisy and required regular maintenance.

The hotel determined that installing variable speed drives (VSDs) on the fans would save energy, improve system performance and lower maintenance costs.

After a trial run, the hotel installed VSDs on each of the guests' rooms supply air fans. The work included disconnecting the existing supply air dampers, upgrading the air conditioning control systems and installing the new VSDs.

Savings achieved were around 40% of fan energy costs. This equated to approximately $23,000 per annum. A typical result (for one fan) is shown in the figure below.

Figure 1: Before and after installation 

Based on the monitored results the initial payback period will be 36 months. This does not include the maintenance savings.

This case demonstrates that it is possible to improve energy performance and service delivery at the same time. The upgrade was very well received by both hotel management and their guests.

Summary

Efficiency measure(s)	Installation of VSDs on fans
Potential users	Offices, hotels, hospitals, schools
Site	Hotel in Perth.
Cost of implementation	$67,000
Expected payback period	36 months

Note: Whilst care has been taken in the preparation of this material, it is intended to provide a general idea about what can be achieved in a particular situation. Quantitative indicators may vary with time and changing circumstances. Each situation is different, requiring its own evaluation; and therefore the results given above should not be taken as being directly transferable to other circumstances. Names and addresses have been withheld for privacy reasons.

Variable speed drive tips

Full load operation

VSDs provide dramatic energy savings by optimising the system, not by improving the actual efficiency of the motor in isolation (as an energy efficient motor retrofit would). In fact, a VSD system is about 4% to 6% less efficient at full load than an induction motor alone. This is mainly due to the losses in the VSD itself. However, it doesn’t take much operation at reduced load to save more energy than is lost at full load. Average loading as high as 90% can justify a VSD retrofit for high-duty applications.

Low speed operation

Most induction motors can operate with modern VSDs through moderate speed ranges (around 30% to 100% speed). Sustained operation at low speeds and, in particular, high load at low speeds may require a special or larger drive and special measures to cool the motor.

AC induction motors operate hotter with a VSD because of harmonics, impurities in the electric power they provide to the motor and also the slower rotating speed of the motor's integral cooling fans. This is usually not a problem if speeds are continuously above 40% or where there are brief periods of slow-speed operation. However, prolonged operation at or below about 30% speed, especially when driving significant loads, can cause rapid and potentially damaging heat in some motors.

Starting Torque

In VSD/motor systems, starting torque is typically determined by the drive (not the motor). For conventional VSD applications, the VSD/motor system will have a peak starting torque of about 130% of rated full-load torque, which is significantly less than what the motor could develop by itself. This level of starting torque is acceptable for most variable speed loads, but some loads (especially constant-torque loads such as conveyers, escalators, augers, or reciprocating compressors) may require greater starting torque.

Harmonics and power factor

Although they can improve displacement power factor (DPF), modern VSDs also create harmonics, which reduce real power factor. (Real power factor includes harmonics and DPF.) For instance, while a VSD can improve DPF to close to 1.0, the harmonics generated by the VSD can cause the real power factor to decline to between 0.75 and 0.80. These harmonic currents (most often the fifth and seventh harmonics) tend to exacerbate resistance losses and can even negate the benefits of improved DPF.

To minimise this problem, more and more VSD manufacturers are packaging harmonics-mitigating equipment (such as line reactors or isolation transformers) with drives. This lets users enjoy the full benefits of power factor improvement. What’s more, this added equipment can significantly reduce the impact of VSD-generated harmonics on other electronic equipment.

VSDs located too far from motor

Pulse-width modulated drives can cause significant damage to motors if the length of cable between the VSD and the motor exceeds 15 to 30 metres. (The number seems to differ by manufacturer.) Using pulse-width modulated VSDs on older motors with long cable runs may shorten the life of the motor.

Mechanical resonance

It is important to determine any mechanical resonance frequencies and to program the VSD to avoid steady operation at those speeds. These resonance frequencies, common in large fans, gears, and belt-driven systems, can cause significant damage through vibration.

Motor compatibility

To ensure that your VSD and motor are compatible, either buy them from the same company, or ask the manufacturer to test the VSD to make sure it’s compatible with another company's line of motors.

Further information

Find out how to reduce your energy costs, by contacting us.

Frequency Inverters

INTRODUCTION

Speed Control Air capacity control of fans is most efficiently done by controlling the fan speed and the principle benefits to be gained are:- control of capacity control of noise control of power absorbed. There are a number of methods that can be used to do this, including:- auto-transformers (external rotor motors only) star/delta switches triacs (external rotor motors only) frequency inverters multi-speed motors There are many applications where the full exhaust capacity is required for only a short period of time and if no speed control system is available, there is an unnecessary waste of heated or conditioned air taking place. In addition, the substantial reductions in generated noise and power absorbed that can be obtained are also lost. Controlling the speed of a fan by means of changing the supply voltage is a simple and often-used method, but it does require certain conditions to be fulfilled:- the motor must have a high resistance rotor (single-phase) the motor must have above-average cooling If more than one fan is to be controlled by a single controller, the fans must be identical. Dissimilar fans should not be connected to the same controller. Speed Control Of External Rotor Motors The most commonly used devices to control the speed of external rotor motors are:- auto-transformers star/delta controllers triacs External rotor motors generally have a high resistance rotor as an inherent feature of their design, and the motors� torque curve is quite different from those of ordinary motors. In addition the cooling of these motors is very efficient, so these motors meet the basic requirements necessary for speed control by voltage control. With voltage control heat is generated in the rotor, particularly at around two thirds of full speed. This heat is then transmitted directly to the impeller mounted on the motor and hence into the surrounding air without adding additional thermal stresses on the stator. Unless otherwise stated, motors operating with a speed-controller should not be run in ambient temperatures exceeding 40篊. Auto Transformers The auto-transformer is the best known method of reducing the supply voltage to a fan motor and Fantech have a wide range of 5-step units for both single and three-phase fans. They are very reliable and minimise motor hum and electromagnetic emissions which would upset sensitive electronic equipment. If controlling more than one fan, the total amperage of all the connected fans should not exceed 75% of the maximum rating of the controller. A speed reduction to as low as 30% is achievable. Star/Delta Control The three-phase external rotor motors fitted to the backward curved centrifugal fans have star/delta motors fitted to them as standard (except Ex e motors which have single-speed motors). This facility enables two speeds to be obtained using either a star/delta switch or star/delta controller. In addition some axial fans, fitted with conventional motors, have the star/delta feature. The speed ratio obtained with the star/delta feature is approximately 1.3:1. Triacs The triac is a type of semi-conductor device and is widely used to speed control single-phase external rotor motors. It functions as a quick-acting switch which turns on the current to the motor during each period of alternating current when it receives an ignition pulse. The power to the motor can be regulated depending on the point in the period at which the ignition pulse is received. If the current is switched on early in the period, the whole period is included and the motor receives full voltage and thus full power. The later the current is switched on, the smaller the amount of voltage is released to the motor and thus the power to the motor is lower. As the voltage supplied to the fan is no longer sine shaped, magnetic oscillations occur in the fan motor. This is most noticeable at very low speeds. This noise, which is loudest at 100 Hz, may spread throughout the duct system if the motor is not properly vibration-suppressed. The triacs are provided with radio interference suppression chokes to reduce radio interference produced on medium and short-wave bands. The interference suppression is such that no special precautions have to be taken in normal installations. On premises where there is very sensitive electronic equipment, it may be advisable to use shielded cables and a separate supply or, alternatively, use an auto-transformer control. If more than one fan is connected to a triac-type controller, the amperage of all the fans should not exceed 85% of the maximum rating of the controller. A speed ratio as low as 10:1 is possible. Capacitance Control The SSC controller provides 2-speeds to a selected range of single phase motors. This controller utilises capacitors and provides a noiseless control. Speed Control Of Standard Foot And Flange-Mounted Motors. We do not recommend standard motors being controlled by voltage reduction unless they have specially designed high-resistance rotors and suitable cooling systems. Experience has shown that trying to control the speed of a conventional motor by voltage reduction is seldom satisfactory. On a standard motor, all changes in speed when using a voltage reducing device, occur over a very narrow band of voltage and it is therefore hard to achieve an acceptable accuracy of adjustment. In addition, due to the level of heat generated, there is a risk of damage to the windings and bearings. Motors with these facilities are available on request. Frequency Inverters Standard motors can be speed-controlled satisfactorily using frequency inverters which vary the frequency and voltage of the electricity supply to the motor. This system provides infinite variation of the fan speed but the motors can become noisy. See the special note on page L-11 relating to EMC requirements. Multi-Speed Motors These can be either tap (Dahlander) or dual-wound. Tap-wound motors use a single set of windings with a control 憈ap� in each coil. The speed ratio is 2:1 and the maximum speed can be any synchronous speed. i.e 48, 24, 16, 12, rev/sec etc. Dual-wound motors are effectively two motors in one as there are two separate sets of windings in the one motor frame. With dual-wound motors almost any combination of standard motor speeds is possible. Multi-speed motors provide excellent power saving and the noise reduction is the same as any other fan with the same speed difference. The capacities available are directly proportional to the speeds. Although by no means exclusive the above methods are more frequently applied to conventional motors. General If you have requirements not detailed here, contact our Sales Engineers for assistance, as not all control devices have been covered in this catalogue. Summary Of Air Capacity Control Options Control Type   Speed Ratio   Approx. Power Savings   Relative Cost   Auto-transformer   3:1   to 95%   low/moderate   Star/delta switch   1.3:1   50%   low   Star/delta controller   1.3:1   50%   moderate   Triac   10:1   to 70%   low   Capacitance   Down to 2:1   varies   low/moderate   Frequency inverter   5:1   to 90%   highest   Tap winding   2:1   87%   low/moderate   Dual-wound   3:2   70%   low/moderate