1 

 Transformer Products 

 1900 East North Street 

 Waukesha, WI 53188 

 Phone: (262) 524-4355 

 Fax: (262) 524-4513

 October 31, 2011 

Mr. Richard Parker jerry.corkran@cooperindustries.com 

richardparkerdc@gmail.com 

 

Subject: Analysis of M3 Compared to Amorphous Core Designs 

Dear Richard: 

We have examined the 25 kVA single phase Design Line 2 Pole Type Transformer to understand 

fundamental relationships between M3 Core Material and that of Amorphous Core. We offer these 

comments for your consideration for the DOE Negotiations. 

I. Observations: 

a. Amorphous core transformers have lower core loss than M3 Silicon iron transformers but 

higher winding losses. At 50% load, they both have the same efficiency. At less than 

50% load, amorphous core transformers have lower total losses than M3 silicon iron but 

that is always at night when electric power cost is at a minimum. During the day when 

the load peaks and exceeds 50% of nameplate load, amorphous core transformers have 

higher total losses than M3 transformers designed for the same efficiency at 50% load. 

During the day when peaking generators are running the electric power costs are the 

highest, sometimes hitting 3 times the night time rate and yet at that point the amorphous 

core transformers have higher total losses than the M3 transformers. 

b. Amorphous core transformers are generally more costly to build than M3 silicon iron 

transformers and sell for higher prices.

c. Amorphous core transformers may not result in energy savings and total cost savings to 

the end users and in fact may result in higher energy usage and costs.

II. Assumptions 

a. Considerations are made at the present 2007 mandatory energy efficiency standards of 

transformers but the starting point does not really affect the conclusions in this memo.

b. DOE Design line 2 is the 25 kVA and is most representative of single phase poles.

c. The present 25 kVA mandatory efficiency for Design Line 2 is 98.91% at 50% load.

d. Two designs are considered, one amorphous and one M3 and both meet the mandatory 

98.91% efficiency at 50% load, and with a total allowable loss of 137.8 watts.

e. The peak efficiency for each core material occurs when the core loss = the total load loss.

f. For the 25 kVA, with M3, the core loss is assumed to be 1⁄2 of 137.8 watts or about 68.9 

watts. The load loss at 50% load is also 68.9 watts. Hence the load loss of the M3 

design at full load is 4* 68.9 or 275.6 watts. Total loss for the M3 design at full load 

would be 68.9 + 275.6 or 344.5 watts.

g. We assumed that amorphous cores have ~1/3 the core loss of M3. Hence the amorphous 

transformer would have approximately 26 watts of core loss. Hence the amorphous 

transformer would have 68.9-26 or 42.9 more watts of load loss than the M3 design at 

50% load. This means that the amorphous design would have 68.9+42.9 or 111.8 watts 

of load loss at 50% load or 447.2 watts of load loss at full load. Total loss for the 

amorphous design would be 26 + 447.2 or 473.2 watts. 

h. Hence the amorphous design has 68.9-26 or 42.9 watts less than the M3 transformer at no 

load but 473.2-344.5 or 128.7 watts higher loss than the M3 design at full load.
2 

i. Night time load cycles may dip to 10% of rated load at night but rise to a normal 80% of 

rated load several times during the day. 

j. During cold load pickup after an outage, total load often reaches 120% or higher of rated 

load and persists for periods of up to an hour or more. 

k. The IEEE Loading guide C57.91allows for up to 200% of rated load for periods of up to 

30 minutes. During such periods the amorphous core has 1814.8 watts of loss while the 

M3 silicon iron transformer has 1171.3 watts, resulting on an added 643.5 watts or 55% 

greater strain to the system that requires extra generation when it is least available. 

III. Graphical results of the analysis 

a. Table I shows the efficiency of both transformers as a function of percent Load: 

Per Unit

Load M3 Amorphous

0.1 97.21 98.79

0.2 98.43 99.13

0.3 98.77 99.12

0.4 98.88 99.03

0.5 98.91 98.91

0.6 98.89 98.77

0.7 98.85 98.62

0.8 98.79 98.46

0.9 98.72 98.31

1 98.64 98.15

1.1 98.56 97.98

1.2 98.47 97.82

Efficiency

b. Graphical results of Table I 

Night load = 10%

Day Load = 80%

RMS load = 50%

Cold Start = 120%

97.00

97.50

98.00

98.50

99.00

99.50

0 0.5 1 1.5

M3 Efficiency

Amorphous Efficiency

c. Table II shows watts loss versus percent load
3 

PU Load Amorphous M3

0.1 30.6 71.6

0.2 44.0 79.9

0.3 66.3 93.7

0.4 97.6 113.0

0.5 137.8 137.8

0.6 186.8 168.1

0.7 244.9 203.9

0.8 311.8 245.2

0.9 387.7 292.0

1 472.5 344.4

1.1 566.2 402.2

1.2 668.9 465.6

Total Watts Loss

d. Graphical results of Watts loss versus load

Night load = 10%

Day Load = 80%

RMS load = 50%

Cold Start = 120%

0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

800.0

0 0.5 1 1.5

M3 Total Watts

Amorphous Total Watts

IV. Recommendations 

Our recommendation continues to be to base the new mandatory efficiency standard on M3 silicon iron and 

not to base it on amorphous cores; for the many factors of transformer weight, size, reliability, load 

regulation, and even ferroresonant overvoltage problems with amorphous. Silicon iron core transformers 

continue to have better losses and efficiency and total economics for load growth, cold load pickup and 

peak demand conditions. 

Very truly yours, Sincerely, 

 

Philip J Hopkinson Jerry L Corkran

Philip J Hopkinson, PE Jerry L. Corkran 

President & CEO HVolt Inc Staff Engineer-Cooper Industries