basics of 3-phase induction motor (part 2) _ eep

7/30/2019 Basics of 3-Phase Induction Motor (Part 2) _ EEP

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Basics of 3-phase Induction Motor (part 2)

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Basics of 3-phase Induction Motor (part 2)

Continued from first part Basics of 3-phase Induction Motor (part 1)For the purpose of standardization, the preferred rated voltages of three phase inductionmotor shall be in accordance with IS 12360: 1988.

The voltages for three phases, 50 Hz motors are: 415 V, 3.3 kV, 6.6 kV and 11 kV.

As per Motor voltage ratings are defined by NEMA MG 1, Motors and Generators (Ref.1), and ANSI C50.41, Polyphase Induction Motors for Power Generating Stations(Ref.2). According to ANSI C50.41, Section 6.3 , preferred voltage ratings are asfollows:

460V; 2300V; 6600V575V; 4000V; 13,200V

Note that these ratings do not correspond exactly to the standard ANSI C84.1 (Ref. 16)


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nominal system voltages of 480, 600, 2400, 4160, 6900, and 13,800V. Instead, each of these ratings is roughly 4 percent lower than the nominal system voltage. The reason for this is to provide some allowance for voltage drop.

For Coordination of Voltages and Output of three-phase induction motors it isrecommended that the minimum rated output should be greater than the limits givenbelow in terms of the rated voltage:

2 < Voltage < 3.3 for and up to 100kW

3.3 < Voltage < 6.6 for and up to 200kW

6.6 < Voltage < 11 for and up to 1000kW.

This is the voltage rating given by IS-325 Three phase induction motor Specification butthis cannot be followed in all cases as there have been cases where motors of 150kWbeing supplied by 415V and 160kW to 4500kW being supplied by 11kV.Now herecomes the application of motor in picture & the load torque requirement of the concerned

motor. Torque developed in Induction motor is directly proportional to the square of EMFinduced in rotor. At standstill the EMF induced in rotor is almost equal to applied voltageto stator and in running condition the EMF induced in rotor is slip times the appliedvoltage to stator.

If concerned Motor is used as compressors for chillers or any application which does notrequire very high starting torque and is of say 400kW then whether we have more voltageor low voltage that does not matter. So we can use either 11kV for this as given by IS or even contradict it and use 6.6kV.For 11kV supply voltage will be less compared to 6.6kVsupply but in both cases the load torque requirement will be met.

But if the same motor is used as a grinder motor in a refinery or any high starting torqueapplication then you cannot feed it with 0.415kV supply as load torque requirement willnot be met. We will find that load torque requirement is met at 11kV.

Because of more voltage requirement load torque requirement was not met at 0.415kVso next higher voltage level was selected even 6.6kV would have done if available.

Now question is how to find load torque requirement?

If electric motor is driving any pump/compressors then the load torque characteristics of pump/compressor is prepared by mechanical engineering department and electricmotor vendor gives his load torque characteristics for required kW at a particular voltagelevel.

Electrical department matches the two characteristics if found not matching then other voltage level is selected whose load torque characteristics matches.

So after making a detailed study of:- 1 - Manufacturing feasibility with respect to costing of motor. (For low kW HT machineinsulation cost will be more. So more money is required.) Transformers with large motorsconnected, 25% 100% of the ONAN rating , need special evaluation.


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The voltage drop on the system during starting must be calculated balancing theimpedance of the transformer and inrush of the motor to best match the voltage drop andshort circuit needs of the system. In addition the transformer manufacturer may need toenhance the internal bracing of the transformer to accommodate the shock loading as aresult of the motor starting current.

- 2 - Load torque requirement. HT motor has high Starting torque because of morevoltage. So load torque requirement or application has to be studied.

- 3 - So depending upon application we deviate from IS codes in this regard.

- 4 - So we define motor utilization voltage in our DBR at the starting of project keeping inmind the above mentioned points.

As a general standard engineering practice the following utilization voltage is adoptedfor 3 Phase motors especially in power plants:

Motor type & rating Voltage No of Phases &Frequency

Grounding

AC Motor above180kW

6.6kV 3Ph, 50Hz Non effectivelyearthed

AC Motor upto180kW

415V 3Ph, 50Hz Effectivelyearthed

DC Motors 220V or 110V

2 Wire DC Unearthed

Why anti condensation heating or space heaters are employed when a three phaseinduction motor is not under operation?

Often the motors are kept in a store for some time or they are transported under verydamp conditions and in such cases, the insulation resistance generally becomes low andit is dangerous for the motor to he connected up before the condition has been rectified.

IEEE 43 places special emphasis on determining the insulation condition of suchmachines before energizing and even before conducting a high-voltage test. This can bedetermined by the insulation test as noted below.

Insulation resistance of the windings is a measure to assess the condition of insulationand its suitability for conducting a high-voltage test or for energizing the machine. A lowreading may suggest damage to the insulation, faulty drying or impregnation or absorption of moisture. The insulation resistance may be measured according to theprocedure laid down in IEEE 43 between the open windings and between windings andthe frame by employing a direct-reading ohm meter (megger) .

The recommended minimum insulation resistance of the machine is obtained by thefollowing empirical formula:


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Rm=kV+1

Where:R = recommended minimum insulation resistance in MR (mega ohms) of the entiremachine windings, at 40C or 1 MR per 1000 V plus 1 M , andkV = rated machine voltage in kV

At the site, when commissioning a new or an existing motor after a long shutdown, itmust have a minimum insulation level according to the above equation. An 11 kV motor,for instance, must have a minimum insulation of 12 M. In normal practice, it is observedthat when first measured the resistance reading may show more than the minimum valueand may mislead the operator, while the winding condition may not be adequate for ahigh voltage test or an actual operation.

One must therefore ensure that the winding condition is suitable before the machine isput into operation. For this purpose, the polarization index (PI), which is determined fromthe insulation test data only as noted below, is a useful pointer. It must be evaluated at

site while conducting the insulation test then compared with the manufacturers referencedata for the machine to assess the condition of insulation at site and its suitability for operation.

This is usually a site test, but to establish a reference record of the machine, it is alsocarried out at the works on the completed machine and test records furnished to theuser.

Drying OutIf the measured insulation resistance of the motor is less than 1 M/kV with a minimumof 1 M when the machine is cold, it should first be dried out by anti condensation heater before full voltage is applied to the terminals of the motors.

References:

1. NEMA MG-1.2. Industrial Power Engineering and Application Hand Book by K C Agarwaal.3. Industrial Power System Hand Book by Shoaib Khan.4. Theory and Calculation of Alternating Current Phenomena by Charles ProteusSteinmetz5. Motor protection relay (MM30) manual from L&T

basics of 3-phase induction motor (part 2) _ eep

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