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ABB - General about motors

ABB - General about motors

Modern electrical motors are available in many different forms, such as single phase motors, three-phase motors, brake motors, synchronous motors, asynchronous motors, special customised motors, two speed motors, three speed motors, and so on, all with their own performance and characteristics.
For each type of motor there are many different mounting arrangements, for example foot mounting, flange mounting or combined foot and flange mounting. The cooling method can also differ very much, from the simplest motor with free self-circulation of air to a more complex motor with totally enclosed air-water cooling with an interchangeable cassette type of cooler.

To ensure a long lifetime for the motor it is important to keep it with the correct degree of protection when under heavy-duty conditions in a servere environment. The two letters IP (International Protection) state the degree of protection followed by two digits, the first of which indicates the degree of protection against contact and penetration of solid objects, whereas the second states the motor’s degree of protection against water.
The end of the motor is defined in the IEC-standard as follows:

  • The D-end is normally the drive end of the motor.
  • The N-end is normally the non-drive end of the motor.

Note that in this handbook we will focus on asynchronous motors only.

Squirrel cage motors

In this chapter the focus has been placed on the squirrel cage motor, the most common type of motor on the market. It is relatively cheap and the maintenance cost is normally low.

There are many different manufacturers represented on the market, selling at various prices. Not all motors have the same performance and quality as for example motors from ABB. High efficiency enables significant savings in energy costs during the motor’s normal endurance. The low level of noise is something else that is of interest today, as is the ability to withstand severe environments.

Current diagram for typical squirell cageThere are also other parameters that differ. The design of the rotor affects the starting current and torque and the variation can be really large between different manufacturers for the same power rating. When using a softstarter it is good if the motor has a high starting torque at Direct-on-line (D.O.L) start. When these motors are used together with a softstarter it is possible to reduce the starting current further when compared to motors with low starting torque. The number of poles also affects the technical data. A motor with two poles often has a lower starting torque than motors with four or more poles.


Three-phase single speed motors can normally be connected for two different voltage levels. The three stator windings are connected in star (Y) or delta (D). The windings can also be connected in series or parallel, Y or YY for instance. If the rating plate on a squirrel cage motor indicates voltages for both the star and delta connection, it is possible to use the motor for both 230 V, and 400 V as an example.

The winding is delta connected at 230 V and if the main voltage is 400 V, the Y-connection is used. When changing the main voltage it is important to remember that for the same power rating the rated motor current will change depending on the voltage level. The method for connecting the motor to the terminal blocks for star or delta connection is shown in the picture below.

Wiring diagram for Y- and Delta connection

Power factor

A motor always consumes active power, which it converts into mechanical action. Reactive power is also required for the magnetisation of the motor but it doesn’t perform any action. In the diagram below the active and reactive power is represented by P and Q, which together give the power S.

Diagram indicating P, Q, S and Cos φThe ratio between the active power (kW) and the reactive power (kVA) is known as the power factor, and is often designated as the cos φ. A normal value is between 0.7 and 0.9, when running where the lower value is for small motors and the higher for large ones.


The speed of an AC motor depends on two things: the number of poles of the stator winding and the main frequency. At 50 Hz, a motor will run at a speed related to a constant of 6000 divided by the number of poles and for a 60 Hz motor the constant is 7200 rpm.

To calculate the speed of a motor, the following formula can be used:

n = speed
f = net frequency
p = number of poles

4-pole motor running at 50 Hz

This speed is the synchronous speed and a squirrel-cage or a slip-ring motor can never reach it. At unloaded condition the speed will be very close to synchronous speed and will then drop when the motor is loaded.

Diagram showing syncronous speed vs.rated speedThe difference between the synchronous and asynchronous speed also named rated speed is ”the slip” and it is possible to calculate this by using the following formula:

s = slip (a normal value is between 1 and 3 %)
n1 = synchronous speed
n = asynchronous speed (rated speed)

Table for synchronous speed at different number of poles and frequency:

Table for synchronous speed at different number of poles and frequency


The starting torque for a motor differs significantly depending on the size of the motor. A small motor, e.g. ≤ 30 kW, normally has a value of between 2.5 and 3 times the rated torque, and for a medium size motor, say up to 250 kW, a typical value is between 2 to 2.5 times the rated torque. Really big motors have a tendency to have a very low starting torque, sometimes even lower than the rated torque. It is not possible to start such a motor fully loaded not even at D.O.L start.

The rated torque of a motor can be calculated using the following formula:

Mr = Rated torque (Nm)
Pr = Rated motor power (kW)
nr = Rated motor speed (rpm)

Torque diagram for a typical squirrel cage motorSlip-ring motors

In some cases when a D.O.L start is not permitted due to the high starting current, or when starting with a star-delta starter will give too low starting torque, a slip-ring motor is used. The motor is started by changing the rotor resistance and when speeding up the resistance is gradually removed until the rated speed is achieved and the motor is working at the equivalent rate of a standard squirrel-cage motor.

Torque diagram for a slip-ring motor | Current diagram for a slip-ring motor

In general, if a softstarter is going to be used for this application you also need to replace the motor.

The advantage of a slip-ring motor is that the starting current will be lower and it is possible to adjust the starting torque up to the maximum torque.



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Cost benefits of AC drives

Cost benefits of AC drives

In addition to their technical advantages, AC drives also provide many cost benefits. In this chapter, these benefits are reviewed, with the costs divided into investment, installation and opera- tional costs.

At the moment there are still plenty of motors sold without variable speed AC drives. This pie chart shows how many motors below 2.2 kW are sold with frequency converters, and how many without. Only 3% of motors in this power range are sold each year with a frequency converter; 97% are sold without an AC drive.
This is astonishing considering what we have seen so far in this guide. Even more so after closer study of the costs of an AC drive compared to conventional control methods. But first let’s review AC drive technology compared to other control methods.

How many motors below 2.2 kW are sold with frequency converters, and how many without

How many motors below 2.2 kW are sold with and without frequency converters


Technical differences between other systems and AC drives

AC drive technology is completely different from other, simpler control methods. It can be compared, for example, to the dif- ference between a zeppelin and a modern airplane.

We could also compare AC drive technology to the develop- ment from a floppy disk to a CD-ROM. Although it is a simpler information storage method, a floppy disk can only handle a small fraction of the information that a CD-ROM can.

The benefits of both these innovations are generally well known. Similarly, AC drive technology is based on a totally different technology to earlier control methods. In this guide, we have presented the benefits of the AC drive compared to simpler control methods.

Technical differences between other systems and AC drives

Technical differences between other systems and AC drives


No mechanical control parts needed

To make a proper cost comparison, we need to study the configurations of different control methods. Here we have used pumping as an example. In traditional methods, there is always a mechanical part and an electrical part.

In throttling you need fuses, contactors and reactors on the electrical side and valves on the mechanical side. In On/Off control, the same electrical components are needed, as well as a pressure tank on the mechanical side. The AC drive provides a new solution. No mechanics are needed, because all control is already on the electrical side.

Another benefit, when thinking about cost, is that with an AC drive we can use a regular 3-phase motor, which is much cheaper than the single phase motors used in other control methods. We can still use 220 V single phase supply, when speaking of power below 2.2 kW.

Conventional methods:AC drive:
• Both electrical and mechanical parts• All in one
• Many electrical parts• Only one electrical component
• Mechanical parts need regular maintenance• No mechanical parts, no wear and tear
• Mechanical control is energy consuming• Saves energy


Factors affecting cost

This list compares the features of conventional control methods with those of the AC drive, as well as their effect on costs. In conventional methods there are both electrical and mechanical components, which usually have to be purchased separately. The costs are usually higher than if everything could be pur- chased at once. Furthermore, mechanical parts wear out quickly. This directly affects maintenance costs and in the long run, maintenance is a very important cost item. In conventional methods there are also many electrical components. The installation cost is at least doubled when there are several different types of components rather than only one.

And last but not least, mechanical control is very energy con- suming, while AC drives practically save energy. This not only helps reduce costs, but also helps minimise environmental impact by reducing emissions from power plants.

Investment costs: Mechanical and electrical components

Price Comparison For Pumps

Price Comparison For Pumps

In this graph, the investment structure as well as the total price of each pump control method is presented. Only the pump itself is not added to the costs because its price is the same regardless of whether it’s used with an AC drive or valves. In throttling, there are two possibilities depending on whether the pump is used in industrial or domestic use. In an industrial environment there are stricter requirements for valves and this increases costs.

The motor
As can be seen, the motor is much more expensive for traditional control methods than for the AC drive. This is due to the 3-phase motor used with the AC drive and the single phase motor used in other control methods.
The AC drive
The AC drive does not need any mechanical parts, which reduc- es costs dramatically. Mechanical parts themselves are almost always less costly than a frequency converter, but electrical parts also need to be added to the total investment cost.
After taking all costs into account, an AC drive is almost always the most economical investment, when compared to differ- ent control methods. Only throttling in domestic use is as low cost as the AC drive. These are not the total costs, however. Together with investment costs we need to look at installation and operational costs.

ThrottlingAC drive
Installation material20 USD10 USD
Installation work5h x 65 USD = 325 USD1h x 65 USD = 65 USD
Commissioning work1h x 65 USD = 65 USD1h x 65 USD = 65 USD
Savings in installation: 270 USD!


Installation costs: Throttling compared to AC drive

Because throttling is the second lowest investment after the AC drive, we will compare its installation and operating costs to the cost of the AC drive. As mentioned earlier, in throttling there are both electrical and mechanical components. This means twice the amount of installation material is needed.

Installation work is also at least doubled in throttling compared to the AC drive. To install a mechanical valve into a pipe is not that simple and this increases installation time. To have a mechanical valve ready for use usually requires five hours compared to one hour for the AC drive. Multiply this by the hourly rate charged by a skilled installer to get the total installation cost.

The commissioning of a throttling-based system does not usu- ally require more time than commissioning an AC drive based system. One hour is usually the time required in both cases. So now we can summarise the total installation costs. As you can see, the AC drive saves up to USD 270 per installation. So even if the throttling investment costs were lower than the price of a single phase motor (approximately USD 200), the AC drive would pay for itself before it has even worked a second.

ThrottlingAC drive
Power required0.75 kW0.37 kW
Annual energy 4000 hours/year 3000 kWh1500 kWh
Annual energy cost with 0.1 USD/kWh300 USD150 USD
Maintenance/year40 USD5 USD
Savings in installation: 185 USD!


Operational costs: Maintenance and drive energy

In many surveys and experiments it has been proved that a 50% energy saving is easily achieved with an AC drive. This means that where power requirements with throttling would be 0.75 kW, with the AC drive it would be 0.37 kW. If a pump is used 4000 hours per year, throttling would need 3000 kWh and the AC drive 1500 kWh of energy per year.
To calculate the savings, we need to multiply the energy con- sumption by the energy price, which varies depending on the country. Here USD 0.1 per kWh has been used.

As mentioned earlier, mechanical parts wear a lot and this is why they need regular maintenance. It has been estimated that whereas throttling requires USD 40 per year for service, maintenance costs for an AC drive would be USD 5. In many cases however, there is no maintenance required for a frequency converter.

Therefore, the total savings in operating costs would be USD 185, which is approximately half of the frequency convert- er’s price for this power range. This means that the payback time of the frequency converter is two years. So it is worth considering that instead of yearly service for an old valve it might be more profitable to change the whole system to an AC drive based control. To retrofit an existing throttling system the pay-back time is two years.

Total cost comparison

Total Savings Over 10 Year - USD 1562

Total Savings Over 10 Year - USD 1562

In the above figure, all the costs have been summarised. The usual time for an operational cost calculation for this kind of investment is 10 years. Here the operational costs are rated to the present value with a 10% interest rate.

In the long run, the conventional method will be more than twice as expensive as a frequency converter. Most of the savings with the AC drive come from the operational costs, and especially from the energy savings. It is in the installation that the high- est individual savings can be achieved, and these savings are realised as soon as the drive is installed.

Taking the total cost figure into account, it is very difficult to understand why only 3% of motors sold have a frequency con- verter. In this guide we have tried to present the benefits of the AC drive and why we at ABB think that it is absolutely the best possible way to control your process.



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The Best Applications For VFDs

The Best Applications For VFDs

The most commonly used motor in building HVAC applications is the three-phase, induction motor, although some smaller applications may use a single-phase induction motor. VFDs can be applied to both.

While VFD controllers can be used with a range of applications, the ones that will produce the most significant benefits are those that require variable speed operation. For example, the flow rate produced by pumps serving building HVAC systems can be matched to the building load by using a VFD to vary the flow rate. Similarly, in systems that require a constant pressure be maintained regardless of the flow rate, such as in domestic hot and cold water systems, a VFD controlled by a pressure setpoint can maintain the pressure over most demand levels.

The majority of commercial and institutional HVAC systems use variable volume fan systems to distribute conditioned air. Most are controlled by a system of variable inlet vanes in the fan system and variable air volume boxes. As the load on the system decreases, the variable air volume boxes close down, increasing the static pressure in the system. The fan’s controller senses this increase and closes down its inlet vanes. While using this type of control system will reduce system fan energy requirements, it is not as efficient or as accurate as a VFD-based system.

Another candidate for VFD use is a variable refrigerant flow systems. Variable refrigerant flow systems connect one or more compressors to a common refrigerant supply system that feeds multiple evaporators. By piping refrigerant instead of using air ducts, the distribution energy requirements are greatly reduced. Because the load on the compressor is constantly changing based on the demand from the evaporators, a VFD can be used to control the operating speed of the compressor to match the load, reducing energy requirements under part-load conditions.

Additional VFD Applications

While the primary benefit of both of these VFD applications is energy savings, VFDs are well suited for use in other applications where energy conservation is of secondary importance. For example, VFDs can provide precise speed or torque control in some commercial applications.

Some specialized applications use dual fans or pumps. VFDs, with their precise speed control, can ensure that the two units are operated at the desired speed and do not end up fighting each other or having one unit carry more than its design load level.

Advances in technology have increased the number of loads that can be driven by the units. Today, units are available with voltage and current ratings that can match the majority of three-phase induction motors found in buildings. With 500 horsepower units or higher available, facility executives have installed them on large capacity centrifugal chillers where very large energy savings can be achieved.

One of the most significant changes that has taken place recently is that with the widespread acceptance of the units and the recognition of the energy and maintenance benefits, manufacturers are including VFD controls as part of their system in a number of applications. For example, manufacturers of centrifugal chillers offer VFD controls as an option on a number of their units. Similarly, manufacturers of domestic water booster pump systems also offer the controls as part of their system, providing users with better control strategies while reducing energy and maintenance costs.

A Few Cautions

When evaluating the installation of a VFD, facility executives should take into consideration a number of factors related to the specifics of the application. For example, most VFDs emit a series of pulses that are rapidly switched. These pulses can be reflected back from the motor terminals into the cable that connects the VFD to the motor. In applications where there is a long run between the motor and the VFD, these reflected pulses can produce voltages that exceed the line voltage, causing stresses in the cable and motor windings that could lead to insulation failure. While this effect is not very significant in motors that operate at 230 volts or less, it is a concern for those that operate at 480 volts or higher. For those applications, minimize the distance between the VFD and the motor, use cabling specifically designed for use with VFDs, and consider installing a filter specifically designed to reduce the impact of the reflected pulses.

Another factor to consider is the impact the VFD may have on the motor’s bearings. The pulses produced by the VFD can generate a voltage differential between the motor shaft and its casing. If this voltage is high enough, it can generate sparks in the bearings that erode their surfaces. This condition can also be avoided by using a cable designed specifically for use with VFDs.


SOURCE: facilitiesnet


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The Benefits of VFDs In HVAC Systems

The Benefits of VFDs In HVAC Systems

One of the most successful energy management tools ever applied to building HVAC systems is the variable frequency drive (VFD). For more than 20 years, VFDs have successfully been installed on fan and pump motors in a range of variable load applications. Energy savings vary from 35 to 50 percent over conventional constant speed applications, resulting in a return on investment of six months to two years.

While the number of applications suitable for early generation drives was limited based on the horsepower of the motor, today’s drives can be installed in practically any HVAC application found in commercial and institutional buildings. Systems can be operated at higher voltages than those used by earlier generations, resulting in off the shelf systems for motors up to 500 horsepower.

Early generation systems also suffered from low power factor. Low power factor robs the facility of electrical distribution capacity and can result in cost penalties imposed by electrical utility companies. Today’s systems operate at a nearly constant power factor over the entire speed range of the motor.

Another problem that has been corrected by today’s systems is operational noise. As the output frequency of the drives decreased in response to the load, vibrations induced in the motor laminations generated noise that was easily transmitted through the motor mounts to the building interior. Today’s drives operate at higher frequencies, resulting in the associated noise being above the audible range.

And VFDs continue to evolve. From numerous system benefits to an increasing range of available applications, VFDs are proving to be ever more useful and powerful.

The Heart of VFDs

Most conventional building HVAC applications are designed to operate fans and pumps at a constant speed. Building loads, however, are anything but constant. In a conventional system, some form of mechanical throttling can be used to reduce water or air flow in the system. The drive motor, however, continues to operate at full speed, using nearly the same amount of energy regardless of the heating or cooling load on the system. While mechanical throttling can provide a good level of control, it is not very efficient. VFDs offer an effective and efficient alternative.

Three factors work together to improve operating efficiency with VFDs:

1. Operating at less than full load. Building systems are sized for peak load conditions. In typical applications, peak load conditions occur between 1 and 5 percent of the annual operating hours. This means that pump and fan motors are using more energy than necessary 95 to 99 percent of their operating hours.

2. Oversized system designs. Designing for peak load oversizes the system for most operating hours. This condition is further compounded by the practice of oversizing the system design to allow for underestimated and unexpected loads as well as future loads that might result from changes in how the building space is used.

3. Motor energy use is a function of speed. The most commonly used motor in building HVAC systems is the induction motor. With induction motors, the power drawn by the motor varies with the cube of the motor’s speed. This means that if the motor can be slowed by 25 percent of its normal operating speed, its energy use is reduced by nearly 60 percent. At a 50 percent reduction in speed, energy use is reduced by nearly 90 percent.

The installation of a VFD in an HVAC application addresses the inefficiencies introduced by the first two factors, while producing the energy savings made possible by the third. The VFD accomplishes this by converting 60 cycle line current to direct current, then to an output that varies in voltage and frequency based on the load placed on the system. As the system load decreases, the VFD’s controller reduces the motor’s operating speed so that the flow rate through the system meets but does not exceed the load requirements.

VFD Benefits

The most significant benefit to using a VFD is energy savings. By matching system capacity to the actual load throughout the entire year, major savings in system motor energy use are achieved.

Another benefit of the units is reduced wear and tear on the motors. When an induction motor is started, it draws a much higher current than during normal operation. This inrush current can be three to ten times the full-load operating current for the motor, generating both heat and stress in the motor’s windings and other components. In motors that start and stop frequently, this contributes to early motor failures.

In contrast, when a motor connected to a VFD is started, the VFD applies a very low frequency and low voltage to the motor. Both are gradually ramped up at a controlled rate to normal operating conditions, extending motor life.

VFDs also provide more precise levels of control of applications. For example, high-rise buildings use a booster pump system on the domestic water supply to maintain adequate water pressure at all levels within the building. Conventional pump controls in this type of application can maintain the pressure within a certain range, but a VFD-based system can maintain more precise control over a wider range of flow rates, while reducing energy requirements and pump wear.


SOURCE: facilitiesnet


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We have updated the electrical engineering software list on our webpage Stručni programi. Two new software are added to the list: Short-Circuit Current Calculator and Group Motor Protection Guide. These software programs are intended to clearly present product data and technical information that will help the end user with design applications. Both softwares belong to Copper Bussmann.

Short-Circuit Current Calculator

Click to enlarge

Click to enlarge

An easy way to calculate prospective short-circuit current levels

The Cooper Bussmann Point-to-Point Short-Circuit Calculator is a simple, easy-to-use program that allows you to calculate prospective short-circuit currents with a reasonable degree of accuracy. These values can be calculated on the load side of a transformer, at the end of a run of cable or at the end of a busway. Calculations can be made for single or three phase systems.

Group Motor Protection Guide

Click to enlarge screensot

Click to enlarge

A quick and easy-to-use program to help you meet group motor protection requirements

The NEC® section 430-53 allows two or more motors, and other loads, to be protected by the same overcurrent protective device when specific requirements are met. The Cooper Bussmann Group Motor Protection Guide program is a quick and easy-to-use program that will tell you if you meet the requirements of group motor protection by asking a series of questions. Once it is determined that you can use group motor protection, you must still meet the group switching requirements of NEC® section 430-112. The Cooper Bussmann Group Motor Protection Guide program will ask another series of questions to see if you meet these requirements.

Both software are available for  download from our webpage Stručni programi.


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Motors for dusty atmospheres – a potentially explosive development

Motors for dusty atmospheres – a potentially explosive development

Industries dealing with solids handling, like food, pharmaceuticals and chemicals, must now use hazardous area motors, like the oil and gas industries have for many years. Combustible dust can be just as explosive as gas and needs to be treated accordingly.

These days, dust is classed as a hazardous atmosphere, on par with hazardous areas with combustible gas. For instance, in the food industry, substances such as grain, cereal, sugar, flour and milk powder are classed as hazardous when they are in the form of dust. Essentially, any combustible material can be highly volatile when reduced to dust. As dust, materials have an extremely large surface area and can burn rapidly. Under some conditions, this can cause an explosion with very high energy.

Since 2006, hazardous areas with dust come under the ATEX regulations that control installations in hazardous areas. Areas with dust are classified the same way as hazardous areas with gas and equipment is selected on the same basis. But while users in the chemical, oil and gas sectors have been dealing with hazardous atmospheres for decades, this is a fairly new field for many other industry sectors.

There are two types of potentially explosive atmospheres under ATEX, Group 1 for underground mines and Group ll for surface industries

In Group ll, ATEX defines categories of equipment, specified by their protection characteristics. It also designates the hazardous zones they can be used in. Hazardous areas are divided into three zones.

Hazardous dust

Motors for areas with hazardous dust are known as Dust Ignition Proof or DIP motors, alternatively Ex tD motors.

These are used in atmospheres where explosive dust surrounds the motor, or where dust settles under its own weight on the motor. They are designed for Zones 21 and 22; no motors can be used in Zone 20 or Zone 0.

Dust is measured either as a cloud of dust or a layer of dust. The ignition temperatures for various types of dust can be obtained from commercially available reference tables. The ignition temperature for a cloud of dust must be at least 50% above the motor’s marking temperature. The ignition temperature of a 5mm layer of dust must be 75°C above the marking temperature of the motor. It is the responsibility of the user to stage maintenance periods so that the dust layer does not build up above 5mm.

To decide whether hazardous area motors are needed, the ATEX regulations requires users to draw up an Explosion Protection Document, assessing each area of the plant for hazardous gas or dust and dividing the plant into zones. An area can be declared safe only as the result of a risk assessment. Once the plant is correctly divided into zones, the appropriate equipment for each zone can be selected.

It may be tempting to try and simplify the process by using a blanket zone to cover the entire site but this could be a mistake. More expensive, over-protected equipment will have to be bought, installed and inspected. The use of over-specified equipment can have long-term financial implications, as the maintenance and repair obligations under ATEX depend on the category of equipment. Blanket zoning also raises a suspicion that the risk analysis may not have been carried out in sufficient detail.

Manufacturers’ and users’ responsibilities

ATEX 95, the product directive, and ATEX 137, the worker protection directive, cover any electrical or mechanical product or equipment that constitutes a potential source of ignition risk and which requires a special design or installation procedure to prevent an explosion.
The Product Directive, ATEX 95, concentrates on the responsibilities of the equipment manufacturer. The directive draws up the distinction between the duties of the end-user, which include the definition of the Zones, and those of the manufacturer, who will be concerned with meeting the category requirements rather than the zones.
The Worker Protection Directive, ATEX 137, concentrates on the duties of the end-user. The directive requires a consistent assessment of all measures to prevent risks of explosions and injury to people both inside and outside the plant.

Safe operation of the product or equipment is the result of cooperation between the manufacturer, the end-user and, if involved, the contractor. However, the responsibility for explosion protection of the product or equipment can never be contracted out to a third party. While the end-user is responsible for installation of products and equipment, the motor manufacturer is responsible for safety of the motors and for delivering maintenance and installation instructions.

With responsibility divided up this way, responsibility for explosion safety rests squarely with either the equipment manufacturer or the end user. Nobody else can be held responsible. The manufacturer is responsible for the equipment being safe when it leaves the factory. The end user is responsible for ensuring that it is installed, maintained and operated in such a way that it does not pose a danger of explosion.

Employers are responsible for the actions of employees and suppliers. ATEX does allow outsourcing, but the end user is responsible for the quality and the end result of such work, for instance maintenance work. When equipment is to be repaired, the end user is responsible for selecting an appropriate repair shop.
Ex motors can be repaired or rewound, but this should only be done at an approved workshop. Repairs can be carried out either to IEC guidelines or to the manufacturer’s guidelines. If it is carried out to the manufacturers’ guidelines, all warranties and original documents continue to be valid. If not, it is the end user’s responsibility to ensure that the repair job is satisfactory. At the moment, ABB is the only manufacturer to offer certified premises for hazardous area motor repairs.

Drives and hazardous area motors
Drives and hazardous area motors

Drives and hazardous area motors

Variable speed drives can be used with hazardous area motors but certain considerations need to be kept in mind. For example, a variable speed drive may create extra losses inside the motor, because of its voltage-pulse based waveform, which is different to the sinusoidal waveform produced by the 50 Hz network.

Also, the air cooling of the motor will be affected by the speed of its fan.
The drive can also be the source of other undesirable side-effects, which can include reduced motor insulation life, electromagnetic interference and bearing currents. These are effects that can be prevented and for hazardous area duty, such prevention is essential.
An ATEX compliant drive system – including motors, sensors, cabling, filters etc – should be treated as a unit. The drive affects the motor performance and the motor affects the choice of drive. Matching your own motor/drive combination can be both time-consuming and difficult. Some manufacturers can supply a ready-made solution, with combined ATEX-approved drives and motors.

Assessing the risk

The decision on whether you need to employ hazardous area motors for dust depends on the results of a risk assessment.
First of all, you will need to identify and assess fire and explosion risks of dangerous substances.
EN Standard for Group ll: Dust environments

EN 61241 -0 General requirements

EN 61241 -1 Protection by enclosures tD

EN 50281 -1-1 Dust ignition protection

Keeping working areas clean and dust free, particularly near potential ignition sources, will go a long way to reducing risks, but the best advice is to employ a professional consultant. With relevant assistance, you will be able to assess the different areas of the plant, work out the zones and draw up detailed design documentation and inspection schedules for the plant.

You will then need to eliminate or reduce the risks from the use of these substances as much as possible. This could help make the hazardous area smaller, reducing safety risks as well as the costs.

‘Equipment for use in the presence of combustible dust’. One deciding factor is the type of dust, but a host of other factors also play their roles, such as particle size, moisture content and how the dust is formed.

SOURCE: Hazardex


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Motor Operation Efficiency Under Abnormal Conditions

Motor Operation Efficiency Under Abnormal Conditions

Operation under unusual service conditions may result in efficiency losses and the consumption of additional energy. Both standard and energy-efficient motors can have their efficiency and useful life reduced by a poorly maintained electrical system. Monitoring voltage is important for maintaining high-efficiency operation and correcting potential problems before failures occur.

Preventative maintenance personnel should periodically measure and log the voltage at a motor’s terminals while the machine is fully loaded.

Motors must be properly selected according to known service conditions. Usual service conditions, defined in NEMA Standards Publication MG1-1987, Motors and Generators, include:

  • Exposure to an ambient temperature between 0°C and 40°C
  • Installation in areas or enclosures that do not seri- ously interfere with the ventilation of the machine
  • Operation within a tolerance of ± 10 percent of rated voltage
  • Operation from a sine wave voltage source (not to ex- ceed 10 percent deviation factor)
  • Operation within a tolerance of ± 5 percent of rated frequency
  • Operation with a voltage unbalance of 1 percent or less

Over Voltage

As the voltage is increased, the magnetizing current increases by an exponential function. At some point, depending upon design of the motor, saturation of the core iron will increase and overheating will occur. At about 10 to 15 percent over voltage both efficiency and power factor significantly decrease while the full-load slip decreases. The starting current, starting torque, and breakdown torque all significantly increase with over voltage conditions.
A voltage that is at the high end of tolerance limits frequently indicates that a transformer tap has been moved in the wrong direction. An overload relay will not recognize this overvoltage situation and, if the voltage is more than 10 percent high, the motor can over-heat. Over voltage operation with VAR currents above acceptable limits for extended periods of time may accelerate deterioration of a motor’s insulation.

Under Voltage

If a motor is operated at reduced voltage, even within the allowable 10 percent limit, the motor will draw in- creased current to produce the torque requirements imposed by the load. This causes an increase in both stator and rotor I²R losses. Low voltages can also prevent the motor from developing an adequate starting torque. The effects on motor efficiency, power factor, RPM, and current from operating outside nominal design voltage are indicated in the diagram below.

Voltage Variation Effects on Motor Performance

Voltage Variation Effects on Motor Performance


Reduced operating efficiency because of low voltages at the motor terminals is generally due to excessive voltage drops in the supply system. If the motor is at the end of a long feeder, reconfiguration may be necessary. The system voltage can also be modified by:

  • Adjusting the transformer tap settings
  • Installing automatic tap-changing equipment if sys- tem loads vary considerably over the course of a day
  • Installing power factor correction capacitors that raise the system voltage while correcting for power factor

Since motor efficiency and operating life are degraded by voltage variations, only motors with compatible voltage nameplate ratings should be specified for a system.

For example, three-phase motors are available with voltage ratings of 440, 460, 480, and 575 volts. The use of a motor designed for 460-volt service in a 480-volt system results in reduced efficiency, increased heating, and reduced motor life. A 440-volt motor would be even more seriously affected.

Phase Voltage Imbalance

A voltage imbalance occurs when there are unequal voltages on the lines to a polyphase induction motor. This imbalance in phase voltages also causes the line currents to be out of balance. The unbalanced currents cause torque pulsations, vibrations, increased mechanical stress on the motor, and overheating of one and possibly two phase windings. This results in a dramatic increase in motor losses and heat generation, which both decrease the efficiency of the motor and shorten its life.
Voltage imbalance is defined by NEMA as 100 times the maximum deviation of the line voltage from the average voltage on a three-phase system divided by the average voltage. For example, if the measured line voltages are 462, 463, and 455 volts, the average is 460 volts. The voltage imbalance is:

voltage imbalanceA voltage unbalance of only 3.5 percent can increase motor losses by approximately 20 percent. Imbalances over 5 percent indicate a serious problem. Imbalances over 1 percent require derating of the motor, and will void most manufacturers’ warranties. Per NEMA MG1-14.35, a voltage imbalance of 2.5 percent would require a derate factor of 0.925 to be applied to the motor rating. Derating factors due to unbalanced voltage for integral horsepower motors are given in the diagram below. The NEMA derating factors apply to all motors. There is no distinction between standard and energy-efficient motors when selecting a derate factor for operation under voltage unbalance conditions.

Motor Derating due to Voltage Unbalance

Motor Derating due to Voltage Unbalance

Common causes of voltage unbalance include:

  • Faulty operation of automatic power factor connection equipment
  • Unbalanced or unstable utility supply
  • Unbalanced transformer bank supplying a three-phase load that is too large for the bank
  • Unevenly distributed single-phase loads on the same power system
  • Unidentified single-phase to ground faults
  • An open circuit on the distribution system primary

The following steps will ensure proper system balancing:

  • Check your electrical system single-line diagram to verify that single-phase loads are uniformly distributed
  • Regularly monitor voltages on all phases to verify that a minimum variation exists
  • Install required ground fault indicators
  • Perform annual thermographic inspections


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