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DRIVE ISOLATION TRANSFORMERS

GeneralMotor drives, such as variablefrequency and adjustable speed,are becoming more and morepopular every day. They are usedto adjust the speed of inductionmotors by varying the frequency.Motor drives are becoming more

popular because of their impact onenergy savings, and because theyare easy to control. Plant andfacility managers are becomingaware that these motor drivescause harmonic voltage distortion,electrical noise, and line voltage

notching, but what is lesser knownis that a drive isolation transformerplaced in front of these motordrives is a simple, cost-effectivesolution to the power qualityproblems created by the motordrives.

Motor Drives and Power Quality ProblemsThe basic motor drive uses arectifier to convert AC to DC.Rectifiers are made up of diodebridges or silicon controlledrectifiers (SCR) which creates anon-linear current waveform. Mostmotor drives are three phase (6pulse) devices. In three phasecircuits, there is a brief momentwhen more than one SCR is on atthe same time creating amomentary short. This shortimposes mechanical stress on theupstream transformer and causes

a distorted voltage waveform as anotch. The number of notches percycle is equal to the number ofpulses or SCRs in the circuit. Thisdistorted waveform and notchinghave serious effects on otherequipment connected to the samecircuit as the motor drive. Thedistorted waveform containsharmonics that can cause powerfactor correction capacitors toresonate. Notching causes CPUsto shut down, controllers tomalfunction, synchronization

problems, UPS output voltageoscillation, and some digital clocksto run fast.

Motor drives also introduce largeground currents due to the rapidcurrent changes in the SCRs thatare capacitively coupled throughground. High frequency groundcurrents (noise) cause datadisruption in digitalcommunications.

SolutionInserting a drive isolationtransformer in front of the motordrive reduces the harmonicdistortion, attenuates electricalnoise, and diminishes line voltagenotching caused by motor drives.The addition of the inductivereactance from a transformer to themotor drives power sourceeffectively reduces the notchingeffects it has on the power to otherloads. In three phase applications,

drive isolation transformers reduce

harmonic distortion by eliminatingthe triplen harmonics (when thetransformer is connected delta -wye). The harmonics produced bythe motor drive are trapped in thedelta primary of the transformer.The transformer must be K-rated tohandle the heat generated by thetrapped harmonics in the primary.A reduction in harmonic distortionimproves the power factor andreduces the harmonic effects on

other loads.

Attenuation of electrical noise dueto ground currents is achieved byisolation and shielding. Thesecondary of drive isolationtransformers is isolated from theprimary providing a good amountof noise attenuation. Multipleshielded drive isolationtransformers provide excellentnoise attenuation needed toprevent data disruption in digitalelectronic equipment.

ukdriva18June1998

APPLICATION NOTES Ultra-K - UK#22

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Drive Isolation Transformers vs. Standard Isolation TransformersSome isolation transformermanufacturers will take theirstandard transformers and re-labelthem as drive isolationtransformers. They are able to dothis because there are no

standards to differentiate the two.That is why it is important to makesure the isolation transformer isable to operate as a drive isolationtransformer when it comes time tomake a purchase.

In order for a transformer to qualifyas a drive isolation transformer,there are a few criteria it mustmeet. The transformer must: meetthe IEEE 597 standard on

overloads; be listed to UL 1561 fornon-sinusoidal current loads; havean inductive reactance level of noless than 3%; and be designed tohandle the thermal and mechanical

stresses of highly cyclic processcontrol applications.

Standard isolation transformers donot meet the criteria of a driveisolation transformer Some of

these transformers may meet oneor two of the drive isolationtransformer criteria, but can nottruly be used as a drive isolationtransformer.

Ultra-KThe Ultra-Kmanufactured byControlled Power Company meetsthe stringent requirements of adrive isolation transformer. It

conforms to IEEE 597 standard onoverloads; is offered in K-factors ofK-4, K-7, K-13, and K-20 to handle

the heat generated by harmonics;has a typical inductive reactance of3%; and is manufactured with extrabraced coils to handle the

mechanical and thermal stressesimposed by the short circuitconditions of the rectifier. The

Ultra-Khas double or tripleshielding for optimal electricalnoise attenuation, and comes withthe option of a high frequency filter

and TVSS for additional surgesuppression.

SummaryToday, motor drives are verypopular in industrial applications.Rectifiers found inside of motordrives create line voltage notching,electrical noise, and harmonicswreaking havoc on electronicdevices. Drive isolationtransformers are needed to correct

for these power problems. Thefollowing conditions must be met inorder for a transformer to be adrive isolation transformer: IEEE597 standard on overloads; listedto UL 1561 for non-sinusoidalcurrent loads; obtain no less than3% inductive reactance; and built

for high mechanical and thermalstress. Controlled PowerCompanys Ultra-K is the onlytransformer that offers all of theimportant features of a driveisolation transformer all in onepackage.