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Machine Riddle No.1 – Measuring of generator winding resistance
Recently I participated in a generator electrical test program. Measuring of Stator winding  resistance is also of the mentioned tests.
The recommended test procedure by manufacturer to calculate winding resistance is wattmeter method. In this method, one 50HZ power supply is applied to the three phases of stator winding which is separated from other sections. In this condition electrical current of each phase ( I ) and active power consumption of three phases ( W ) are measured by suitable measuring instruments. Then resistance of each phase ( R ) is calculated as following relation:
                              
  
  W=w1-w2 = RI2


As you know, there are many ways to measure active power in three phase symmetrical circuits.One, two and three-wattmeter methods may be used but in this case, generator manufacturer only recommended "two-wattmeter method" and high accuracy wattmeter for this measurement.

How can you explain the reasons of manufacturer choice? Why the two-wattmeter method is the best choice to reduce measurement errors in this case?


Author : Hamid - From: Iran
 
#1
Thu, September 13, 2007 - 10:31

Effective AC resistance of a particular winding is usually determined by using direct current and volt-amp meter method.
This resistance is usually that of two phases since the neutral point of a Wye winding is frequently not accessible. In the less common delta-wound alternator, the windings are permanently spliced together at the Delta points

The per phase resistance of one phase in a Wye configuration is

Rdc = V/(2A) 
Author : guest
 
#2
Thu, September 13, 2007 - 10:32

It is not Resistance you are testing; it is the "impedence" you get by this method; at 50Hz. By two watt method you get more accurate readings.

 
Author : guest
 
#3
Thu, September 13, 2007 - 10:33

Measuring by V-I method is the accurate method. In idle condition, inject about 6-12V battery supply and measure the voltage(V) drop across the winding and the current drawn(I) using accurate, calibrated meters. Resistance =V/I. The winding resistances usually will be very less. So to safe guard the battery from getting drained use about 10 ohm in series. All R&D units and labs follow only this method.
 
Author : guest
 
#4
Thu, September 13, 2007 - 10:35

There is some confusion in my mind as to the terms you have used in your question.

Resistance (US engineering) generally refers to DC ohmic values, and when I measured a generator winding resistance, I used a 4 terminal kelvin bridge. This measured the DC resistance of the copper only.

The test procedure's that you describe would be the proper procedures for measuring the combined I2 R winding and core losses which is a means of providing a value of merit of the machine.

This is not resistance alone. It includes the complex power loss of all the 3 phase windings plus the core losses. (hysteresis)

ALSO Note, this measurement does not include any friction or wind-age losses.

Does this help you any? 
Author : guest
 
#5
Thu, September 13, 2007 - 10:36

Yes, you are right.But please note that mentioned test didn't do in winding rated voltage,therefore the core loss can be neglect.Also this test is done on generator without rotor system and in DC test skeen effects were not considered.


 
Author : Hamid
 
#6
Thu, September 13, 2007 - 10:37

Your comment is noted and taken into consideration, perhaps they want to measure the positive sequence component of resistance.

Are you familiar with symmetrical component vector notation mathematics. The system is a method of describing any complex three phase circuit by 3 complex component vector values. i.e. a positive, and negative and a zero sequence component. It is similar to expressing a vector quantity like (R + Jx) except that where Jx is used, it is replaced with A0, A1 and A2 and A is 0. 120 and 240 degrees instead of R= 90 degrees. This wattmeter method would provide the positive sequence values.

This is just a comment, I am trying to justify a reason for their method.

 
Author : guest
 
#7
Thu, September 13, 2007 - 10:40

According to Blondel's theorem, in any system of n phases the power can be measured by n-1 wattmeters, hence two wattmeters is sufficient for a three phase system.

 
Author : guest
 
#8
Thu, September 13, 2007 - 10:41
Thank you very much for your attention.
I don't know anything about Blondel's theorem, I like to learn about it and appreciate you for this reminder.
According to electrical measuring principles the three wattmeter and two wattmeter methods will have equal result in measuring of power consumption in three wire and three phase systems in ideal conditions. But we have not measuring device without any inhere errors. As you know amount of errors can be related to circuit, accuracy of measurement equipments and special measuring conditions. For example in that mentioned test, the power factor of circuit is very low because the generator winding inductance property is very high in compare its resistant and so on.
I think we must focus on our special condition of circuit.  
Author : Hamid
 
#9
Sun, November 18th, 2012 - 17:09
Can I use multimeter for measuring winding resistance 
Author : Sandip - From: India
 
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