educed voltage starting conditions to realize the majorbenefits of this type of starting arrangement.1. Part Winding StartThis is not a true reduced voltage start, but it accomplishesthe same job - limiting inrush current - by utilizing onlypart of the motor windings. Since it uses both the start<strong>and</strong> run contactors to carry the motor current duringoperation, it costs less than the other types.To utilize part winding start, the motor must have a dualwinding. Copel<strong>and</strong>® br<strong>and</strong> 208/220/440 volt, 3 phasemotors are wound with two identical stator windingswhich are connected in parallel on 208 or 220 voltoperation. For part winding start, the first step utilizesonly one winding or 1/2 of the stator, <strong>and</strong> these motorsmay be used whenever part winding start is requiredon 208 or 220 volt power. Part winding start cannot beused on these motors when used on 440 volts, sincethe entire winding must be connected in series for 440volt power .Copelametic® model 4R <strong>and</strong> 6R compressors arecurrently available with dual wound motors, <strong>and</strong> some4R <strong>and</strong> 6R models are available with specially woundmotors for part winding start on 550 volts.Basically all that is required for part winding start aretwo contactors, each capable of carrying the winding’sfull load <strong>and</strong> locked rotor current requirement, with atime delay between the contactors. When the starteris energized, the first magnetic contactor closes <strong>and</strong>puts half of the motor winding across the line. A presettime delay relay is energized at the same time, <strong>and</strong> atthe completion of the timing cycle, the second magneticcontactor closes <strong>and</strong> puts the second half of the motorwinding in parallel with the first.The normal Copel<strong>and</strong>® br<strong>and</strong> compressor motorprotectors must be used. Where current sensingprotectors are required, they must be installed in atleast two phases of each contactor. Motors equippedwith a Thermotector need no other external protection.To prevent tripping of the protectors during starting, thetime delay between the first <strong>and</strong> second contactor mustbe well within the protector’s tolerance for locked rotorconditions, <strong>and</strong> a time delay device having a time cyclesetting of one second ± 10% is required.The exact current <strong>and</strong> torque characteristics of a motorwill vary with design. For Copel<strong>and</strong>® br<strong>and</strong> motorcompressorsstarting on one winding, the motor willdraw approximately 65% of the normal across-the-linestarting current, <strong>and</strong> produce approximately 45% of thenormal starting torque. Under heavily loaded conditions,it is possible that the motor may not start until thesecond winding is energized, or if it does start, it maynot accelerate. An unloaded start may be desirableunder extreme conditions.On part winding start applications, occasionally anelectrical starting noise or “growl” of short durationmay be noticed. This occurs when the first half windingstarts the motor, but is unable to accelerate it beyonda few hundred rpm. As soon as the second winding isenergized, the motor instantly accelerates, <strong>and</strong> the noisedisappears. Since the time delay between windings isno greater than one second, the noise duration is veryshort.The noise will vary with voltage, speed, pressuredifferential, motor horsepower, <strong>and</strong> will vary slightlyfrom compressor to compressor. In addition, motorssupplied by different sources may have slight differencesin motor characteristics, <strong>and</strong> the resulting sound maybe slightly different.Occasionally service personnel mistake the starting noisefor bearing drag. The starting noise is quite normal, willbe more pronounced on larger motors, <strong>and</strong> does notharm the compressor In any way.2. Star-Delta StartingFor star-delta starting (also referred to as wye-delta)a specially wound motor is required with both ends ofeach phase winding brought out to terminals. By meansof contactors a motor designed for normal operation indelta is first connected in star, <strong>and</strong> after a predeterminedtime delay, the star connection is changed to delta. Thisstarting arrangement is relatively simple <strong>and</strong> inexpensive,<strong>and</strong> is widely used in Europe.Recently three phase, 50 cycle motors have beendeveloped for most Copel<strong>and</strong>® br<strong>and</strong> compressors 7½HP <strong>and</strong> larger specially wound for star-delta starting.Leads were brought out from both ends of each phaseso that the motors could be connected in either star ordelta. Motors are available for star-delta start connectionson either 380 volt, 50 cycle, 3 phase or 220 volt, 50cycle, 3 phase circuits.When a motor designed for delta operation is connectedin star, the voltage across each phase is reduced to 58%of normal, <strong>and</strong> the motor develops 1/3 of the normalstarting torque. The inrush current in star is 1/3 of normalinrush current in delta.Star-delta starting is suitable for low torque starting dutyonly. To insure starting on the star connection, somemeans of pressure equalization across the compressor(continued on p. 10-7)© 1967 Emerson Climate Technologies, Inc.All rights reserved.10-5
10-6© 1967 Emerson Climate Technologies, Inc.All rights reserved.
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