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.

There 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.

Voltage

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.

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.

The 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.

Speed

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

Example:
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.

The 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:

Torque

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:

M_r = Rated torque (Nm)
P_r = Rated motor power (kW)
n_r = Rated motor speed (rpm)

Slip-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.

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.

SOURCE: ABB – SOFTSTARTER HANDBOOK

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Simulacija rada dvostrano napajanog asinhronog motora

Automatizacija proizvodnih procesa uslovila je potrebu za stalnim usavršavanjem regulisanih elektromotornih pogona od kojih se zahtevaju pogodne regulacione karakteristike, smanjenje utroška električne energije, povećana pouzdanost, smanjenje tekućeg održavanja i dr.

Neke statistike ukazuju na to da se u razvijenim zemljama preko polovine proizvedene električne energije pretvara u mehaničku energiju u elektromotornim pogonima. Poslednjih godina se, dugo nezamenljivi koncept regulisanih pogona baziran na mašinama jednosmerne struje (DC) , zamenjuje regulisanim mašinama naizmenične struje (AC). Široka rasprostranjenost pogona sa mašinama jednosmerne struje uslovljena je mogućnošću raspregnutog upravljanja fluksom i momentom mašine uz relativno jednostavan elektronski izvor napajanja. Razvojem statičkih pretvarača i teorije vektorskog upravljanja mašine naizmenične struje sve više istiskuju jednosmerne zahvaljujući prednostima koje do sada nisu mogle da dođu do izražaja kao što su : nepostojanje komutatora, jednostavnost, robustnost i to što, gotovo da, im nije potrebno održavanje.

Jedna od mogućnosti regulisanja brzine asinhronog motora sa namotanim rotorom je pomoću dvostranog napajanja. Sa jedne strane, asinhroni motor, napajamo iz mreže. Mrežna učestanost i amplituda napona su konstantni, sa druge strane motor napajamo iz regulisanog izvora čiju je učestanost i amplitudu moguće menjati. Asinhroni motor sa dvostranim napajanjem u sinhronom režimu rada radi kao sinhrona mašina kojoj se može regulisati brzina. Brzina obrtanja se zadaje jednom učestanošću napajanja i nezavisna je od opterećenja. Pri tome, brzina može da se reguliše i u jednom i u drugom smeru, iznad i ispod sinhrone brzine a mašina može da radi ili kao motor ili kao generator.

Mogućnost regulisanja brzine obrtanja asinhronog motora dvostranim napajanjem uočena je još početkom ovog veka [ 1 ]. Dugo vremena ovaj način regulacije brzine nije našao širu primenu zbog problema vezanih za izvor promenljive učestanosti. Razvojem poluprovodničkih prtevarača omogućeno je da ovaj način regulisanja brzine dobije širu primenu, s’ obzirom na ostale dobre osobine koje poseduje.
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Kratak sadržaj rada

Tema ovog rada je “Simulacija rada dvostrano napajanog asinhronog motora”. Simulacija je realizovana uz pomoć simulacionog programskog paketa VisSim. VisSim je program, baziran na Windows okruženju, za modelovanje i simuliranje složenih dinamičkih sistema. On je spoj jednostavnog vizelnog rešavanja problema potpomognutog moćnim simulatorom. Vizuelni blok dijagrami omogućavaju pojednostavljeno rešavanje i prepravljanje složenih problema, dok moćni simulator omogućava brzo i precizno rešavanje linearnih, nelinearnih, kontinualnih i diskretnih problema. VisSim je simulacioni program koji ne zahteva linijsko pisanje programa, pregledan je, ima moćan matematički aparat.

Sve su ovo razlozi koji olakšavaju njegovu upotrebu i štede vreme onome ko ih koristi. Nešto više možete saznati o ovom simulacionom programu na web sajtu čija je adresa: www.vissim.com . Izrada simulacionog programa zahtevala je modelovanje konfiguracije prikazane uprošćenom zamenskom šemom na sledećoj slici:

Slika 1. Uprošćena zamenska šema celokupne konfiguracije

Zbog pojednostavljenja simulacije, modelovan je samo trofazni asinhroni motor u uslovima dvostranog napajanja odnosno sama dinamika prelaznih procesa koji se odigravaju pri tome. U okviru simulacionog programa zadaje se željena brzina obrtanja dok se veličine učestanosti i amplitude napona napajanja, sa strane regulisanog izvora napajanja, proračunavaju na osnovu zadate brzine.

Narednim poglavljima obrađena je kako problematika regulisanja brzine obrtanja dvostrano napajanih asinhronih trofaznih mašina tako i analiza svih važnih detalja vezanih za modelovanje samog procesa. Sva razmatranja u ovom radu su sprovedena za slučaj da se napajanje sa strane statora vrši iz mreže a napajanje sa strane rotora iz regulisanog izvora promenljive učestanosti i amplitude napona. Svi izvedeni zaključci bi važili i za obrnut slučaj.
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2.1 Analitička teorija asinhronog motora sa dvostranim napajanjem

2.1.1 Regulisanje brzine asinhronog motora dvostranim napajanjem

Regulisanje brzine i smera obrtanja asinhronog motora sa namotanim rotorom pomoću dvostarnog napajanja zasniva se na promeni učestanosti i napona sa jedne strane, uz konstantnu učestanost i konstantan napon sa druge strane. Najčešće su napon i učestanost sa strane statora stalni i on se napaja iz mreže dok se napajanje sa strane rotora vrši preko regulisanog poluprovodničkog pretvarača, promenljive učestanosti i napona. Najčešća su dva rešenja.

Jedno rešenje tj. uprošćena blok šema konfiguracije sa jednosmernim međukolom prikazana je na slici ispod:

Slika 2. Konfiguracija sa jednosmernim međukolom

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dok je uprošćena blok šema drugog rešenja sa ciklokonvertorom prikazana na narednoj slici:

slika 3. Konfiguracija sa ciklokonvertorom

Kao izvor promenljivog napona i učestanosti moguće je upotrebiti i poseban sinhroni generator. Mada se ovo rešenje gotovo i ne koristi.

Brzina obrtanja asinhronog motora iznosi :

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odnosno, gde su:
n₁ – sinhrona brzina
n – brzina obrtanja
s – klizanje,
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a kako je : 

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to je : 

Pošto se, u opštem slučaju, obrtne magnetopobudne sile statora i rotora mogu obrtati u istom ili u različitim smerovima to je:

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Na sledećoj slici je prikazana zavisnost brzine obrtanja rotora od učestanosti rotorskih veličina, pri konstantnoj, mrežnoj učestanosti.

Slika 4. Zavisnost brzine obrtanja od rotorske učestanosti

Pri tome se deo dijagrama, sa slike, koji odgovara negativnoj učestanosti odnosi na slučaj kada se obrtne magnetopobudne sile statora i rotora obrću u suprotnim smerovima. Deo koji odgovara pozitivnoj učestanosti je slučaj kada se obrtne magnetopobudne sile obrću u istom smeru.

Odavde se vidi da je dvostranim napajanjem moguće regulisati brzinu obrtanja asinhronog motora u jednom i u drugom smeru, ispod i iznad sinhrone brzine. Pri f₁ = f₂ brzina obrtanja može biti jednaka nuli ili dvostrukoj sinhronoj brzini, u zavisnosti od međusobnih smerova obrtanja magnetopobudnih sila.
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2.1.2 Osnovne jednačine i ekvivalentna šema asinhronog motora sa dvostranim napajanjem

Osnovne jednačine asinhronog motora sa dvostranim napajanjem mogu se dobiti polazeći od osnovnih jednačina asinhronog motora za standradni režim rada, uzimajući u obzir i napajanje sa strane rotora:

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Ukoliko uzmemo u obzir da je:

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nakon deljenja jednačina ( 2.1 ) i ( 2.2 ) klizanjem dobijamo sledeće:

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Uzimajući u obzir da je:

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dobijamo sledeće :

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Zanemarujući gubitke u gvožđu (Z_m ≈ jX_m):

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Izrazi ( 2.7 ) i ( 2.8 ) se mogu predstaviti u jednom praktičnijem obliku ukoliko uzmemo u obzir da je I₀ = I₁ + I₂ imamo:

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gde je

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Na osnovu dobijenih izraza ekvivalentna šema dvostrano napajanog asinhronog motora ima sledeći izgled:

Slika 5. Ekvivalentna šema dvostrano napajanog asinhronog motora

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Uskoro u nastavku stručnog teksta: Simulacija rada dvostrano napajanog asinhronog motora (2)

• Simulacioni VisSim model dvostrano napajanog asinhronog motora
• Zaključak

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AUTOR STRUČNOG TEKSTA:

Jovica Vranjković
Diplomski rad: Simulacija rada dvostrano napajanog asinhronog motora
Univerzitet u Beogradu Elektrotehnički fakultet Katedra za energetske pretvarače i pogone

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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

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

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

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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:

A 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

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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