Wednesday, January 7, 2009

Test & Measuring Equipments

Test & Measuring Equipments For enameled wires Tan Delta Bending Point Test:
An Important Parameter

By Pankaj Gaikwad, Sivananda Electronics


Tan Delta Bending Point Test

Tan Delta Bending point test is very important test to know the curing quality of enameled wires. It is useful test for manufacturers of enameled wires as a guideline for process control at the same time it is useful for the user Industries at Incoming quality control as it gives nearest simulation of actual application.



Manufacturers can set the process

parameters so as to produce properly cured enameled wire. In this test the small AC sinusoidal current is passed through enamel film covering & heat is applied to the specimen which may occur during actual application when this wire is used in motors, transformers and coils. From this test the user can get an idea about the temperature up-to which the dielectric losses in enamel wire will remain steady. It can be related to actual application operating temperature.



Basics about Tan Delta (The Dielectric Loss Factor)

In case of ideal capacitor when an AC sinusoidal signal is passed through it current leads voltage by 90 degrees. In practical conditions some power is lost in the dielectric material / Insulation due to which slight shift in this angle occurs.

This small angle is called delta & denoted as the loss angle.

Tan of this angle represents the loss factor in reactive material.

Tan Delta=R/Xc =CR as (Xc=1/C),=2 F

R = equivalent series resistor Xc= Capacitive reactance. F= Test Frequency



How the Capacitance is formed in enameled wires?

As shown below the while performing Tan Delta Test on enameled wire Sample is prepared on sample holder. The inner conductor acts as one plate of capacitor where as molten wood's alloy in which the sample is dipped acts as another plate of capacitor. Enamel between these two plates acts as the dielectric material in capacitor. i.e. measurement of tan delta of this capacitor is actually the measurement of Dielectric Loss Factor in Enamel Film.

The Capacitance of the sample prepared from the wires having SWG 18 to SWG 26 is generally in the range 160pF to 450 pF.

Above SWG 26 the single turn on sample holder results in capacitance less than 160pF. Hence in order to increase the sample capacitance 2/3 turns of sample are wound on the sample holder for SWG 26 & above.

The Tan Delta Test can be carried out on round enameled copper wires as well as enameled strips. The difference is only in type of sample holder & sample capacitance generally strips sample have capacitance above 640pF. The strip sample holder construction is slightly different than the wire sample holder.

Enameled wire Tan Delta depends on following factors

1. Type of Enamel used

2. Curing quality of enameled wire



Curing quality of enameled wire depends upon the speed of wire & Temperature of enameling section.



In Tan Delta measuring equipment the enameled wire Tan Delta is measured with respect to the Temperature & the graph of Tan Delta Vs Temperature is plotted , a typical Enameled wire shows following characteristic where the tan delta remain steady up-to certain temperature & after this temperature it increases rapidly .i.e .the graph actually bends at this temperature. This point of graph is called Bending Point & Temperature of this point is called Bending Point Temperature.

Enamel manufacturers generally provide the table for the bending point temperature Data for different types of enamels. Minimum & Maximum Limits. If the Bending point Result is below the Minimum limit the wire is considered as under cured wire where as above the maximum limit the wire is considered as over cured wire.



Interpretation of Results & Process Quality Control

Under cured wire: If this result is indicated it shows either the speed of wire to be reduced or enameling temperature to be increased.

Over cured wire: If this result is indicated it shows that the speed of wire to be increased or enameling temperature to be decreased.



Bending Point Limits for Different Types of Enamels

Bending Point Temperature Limits for different types of enamels with various combinations are generally specified by enamel manufacturers as follows. Minimum/ Maximum limits depends upon the various combinations.The Table provided by the enamel manufacturer is the best reference to maintain the process parameters within the limits in order to get properly cured enameled wire.

Single Point Test

If more samples to be tested in less time instead of plotting complete graph a single X,Y reference point is checked. In this method the temperature of Bath is kept constant & many samples can be checked at the set temperature for Tan Delta results. This method to be used as indicative only wherever possible the wires to be tested for Bending point.



Single Point Test & Bending Point Test

In Bending Point Test the Result is related to the Temperature where as in single point test the result is related to Tan Delta.



In other words Bending point is related to X axis where as Single Point Test is related to Y Axis.



Hence in Single point Test Higher value results indicate under-cured sample.

& in Bending point test higher value results indicate over-cured sample.



Equipment used for Enameled wire Tan Delta Test

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