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6. PERFORMANCEThe Magnastart’s linear impedance characteristic means that the stator current drawn onstart remains almost constant during three-quarters of the starting cycle. This hasconsiderable advantages when supply restrictions are in force, as it enables the fastestpossible start to be obtained within a given current limit.Typical torque and current to speed characteristics are given in the charts below to illustratethe benefits of using a Magnastart Inducer as a rotor starter, compared to direct on linestarting. (The impedance values 1-6 are nominal, and only reflect the typical range ofimpedances achievable from a Magnastart, not any particular tapping). Fig. 4 shows thevariation in starting torque as a proportion of Full Load Torque (FLT) with impedance.47Direct on Line6Direct on Line3Lowest Impedance 1Impedance 25Lowest Impedance 1Impedance 2Torque / FLT2Impedance 3Impedance 4Current / FLC43Impedance 3Impedance 41Impedance 5Highest Impedance 621Impedance 5Highest Impedance 600 10 20 30 40 50 60 70 80 90 10000 10 20 30 40 50 60 70 80 90 100Speed %Fig. 4 Fig. 5As can be seen, direct on line starting will give the least starting torque, and there is anoptimum impedance for maximising starting and run-up torque, with higher impedancesgenerally depressing both values (though reducing starting current). On most motors thismaximum starting torque occurs between 3 and 4 times the full load rotor current.Magnastart is most efficient, starting fan loads and large flywheels, at 2 times full loadrotor current.Fig. 5 shows the effect of the various impedances on starting line current, as a proportion ofnormal full load current (FLC), and illustrates that the starting current can be maintained atlow levels if high starting torque is not a key requirement. (As is normal, current andtorque both reduce to zero at 100% synchronous speed, ie no slip).Speed %Section 9 gives details of tappings and relative impedances achieved.Page 8 V1.3
7. RATING AND SELECTION OF MODELThe size of a Magnastart required is decided solely by the motor size and starting dutyrequired. Factors to be used in determining ‘starting duty’ are:• The number of starts per hour.• The starting period.• The starting current as a factor of full load rotor current.The rating of the required Magnastart is determined by the motor’s full electrical powerrating in kW/HP. It is not normally determined by the running mechanical load, sinceduring start-up the motor will draw the maximum power possible (usually well in excess ofthe normal running power) to accelerate the machinery’s mass.The ratings have been calculated on a basis of 15 second starts, with near-full-speed beingreached in ~10 seconds and being shorted in 15 seconds, generating a maximum of 1.5times the full load torque at start. For other starting conditions, the units can be up-rated orde-rated in proportion.In addition the motor’s rotor voltage and current ratings should be considered to make surethese do not exceed the Magnastart’s rating. Only very rarely do motors exceed theMagnastart ratings for rotor voltage or current. However, if the maximum rotor startingcurrent or voltage is exceeded, a larger Magnastart is needed. Note that the Magnastartrotor ratings allow a wide range of voltage and current capacity, but these must not be usedin conjunction beyond the maximum power rating of the Magnastart.The chart in Section 9 provides a selection of front panel tappings and their relativeimpedances. This is used to select the right tapping. If the impedance needed is not known(which is quite often), the right tapping can be found by attempting a start with a highimpedance tapping, then progressively trying lower impedance tappings until the rightstarting characteristics (line current vs time) are obtained. During the setting up operation,the rotor current should also be measured on each attempted start. This ensures that theMagnastart and motor rating is not exceeded and gives a guide to the additional torque thatmight be obtained on lower impedance tappings.8. INSTALLATION8.1. Enclosure and MountingThe Magnastart should be installed in a suitable metallic enclosure, protected fromhuman access. This need not be ventilated but reasonable space should be left eitherside of the Magnastart to allow circulation of air around the coils. The heat energydissipated in the Magnastart during a start is approximately the same as the storedkinetic energy in the motor and load.If it is mounted in the base of a control cubicle, ensure that enough ventilation ornatural cooling through the body of the cubicle is provided to prevent the heatgenerated during starting cycle(s) adversely affecting other equipment (eg PLC). ExtraPage 9 V1.3
Page 2: US Group• Fermilab (FNAL)• M. D
Page 7: 5. ELECTRICAL CHARACTERISTICSEach c
Page 11 and 12: 8.4. Control and Power CircuitryThe
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Page 16 and 17: 10. CONNECTING MAGNASTARTS IN PARAL
Page 18 and 19: Fig. 8 Mechanical OutlinePage 18 V1

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