## Identification of Some Additional Loss Components in High-Power Low-Voltage Permanent Magnet Generators

##### Hämäläinen, Henry (2013-08-13)

Väitöskirja

Hämäläinen, Henry

13.08.2013

Lappeenranta University of Technology

Acta Universitatis Lappeenrantaensis

**Julkaisun pysyvä osoite on**

http://urn.fi/URN:ISBN:978-952-265-429-8

#### Tiivistelmä

Permanent magnet generators (PMG) represent the cutting edge technology in modern wind

mills. The efficiency remains high (over 90%) at partial loads. To improve the machine

efficiency even further, every aspect of machine losses has to be analyzed. Additional losses

are often given as a certain percentage without providing any detailed information about the

actual calculation process; meanwhile, there are many design-dependent losses that have an

effect on the total amount of additional losses and that have to be taken into consideration.

Additional losses are most often eddy current losses in different parts of the machine. These

losses are usually difficult to calculate in the design process. In this doctoral thesis, some

additional losses are identified and modeled. Further, suggestions on how to minimize the

losses are given.

Iron losses can differ significantly between the measured no-load values and the loss values

under load. In addition, with embedded magnet rotors, the quadrature-axis armature reaction

adds losses to the stator iron by manipulating the harmonic content of the flux. It was,

therefore, re-evaluated that in salient pole machines, to minimize the losses and the loss

difference between the no-load and load operation, the flux density has to be kept below 1.5

T in the stator yoke, which is the traditional guideline for machine designers.

Eddy current losses may occur in the end-winding area and in the support structure of the

machine, that is, in the finger plate and the clamping ring. With construction steel, these

losses account for 0.08% of the input power of the machine. These losses can be reduced

almost to zero by using nonmagnetic stainless steel. In addition, the machine housing may be

subjected to eddy current losses if the flux density exceeds 1.5 T in the stator yoke.

Winding losses can rise rapidly when high frequencies and 10–15 mm high conductors are

used. In general, minimizing the winding losses is simple. For example, it can be done by dividing the conductor into transposed subconductors. However, this comes with the expense

of an increase in the DC resistance. In the doctoral thesis, a new method is presented to

minimize the winding losses by applying a litz wire with noninsulated strands. The

construction is the same as in a normal litz wire but the insulation between the subconductors

has been left out. The idea is that the connection is kept weak to prevent harmful eddy

currents from flowing. Moreover, the analytical solution for calculating the AC resistance

factor of the litz-wire is supplemented by including an end-winding resistance in the

analytical solution. A simple measurement device is developed to measure the AC resistance

in the windings. In the case of a litz-wire with originally noninsulated strands, vacuum

pressure impregnation (VPI) is used to insulate the subconductors. In one of the two cases

studied, the VPI affected the AC resistance factor, but in the other case, it did not have any

effect. However, more research is needed to determine the effect of the VPI on litz-wire with

noninsulated strands.

An empirical model is developed to calculate the AC resistance factor of a single-layer formwound

winding. The model includes the end-winding length and the number of strands and

turns. The end winding includes the circulating current (eddy currents that are traveling

through the whole winding between parallel strands) and the main current. The end-winding

length also affects the total AC resistance factor.

mills. The efficiency remains high (over 90%) at partial loads. To improve the machine

efficiency even further, every aspect of machine losses has to be analyzed. Additional losses

are often given as a certain percentage without providing any detailed information about the

actual calculation process; meanwhile, there are many design-dependent losses that have an

effect on the total amount of additional losses and that have to be taken into consideration.

Additional losses are most often eddy current losses in different parts of the machine. These

losses are usually difficult to calculate in the design process. In this doctoral thesis, some

additional losses are identified and modeled. Further, suggestions on how to minimize the

losses are given.

Iron losses can differ significantly between the measured no-load values and the loss values

under load. In addition, with embedded magnet rotors, the quadrature-axis armature reaction

adds losses to the stator iron by manipulating the harmonic content of the flux. It was,

therefore, re-evaluated that in salient pole machines, to minimize the losses and the loss

difference between the no-load and load operation, the flux density has to be kept below 1.5

T in the stator yoke, which is the traditional guideline for machine designers.

Eddy current losses may occur in the end-winding area and in the support structure of the

machine, that is, in the finger plate and the clamping ring. With construction steel, these

losses account for 0.08% of the input power of the machine. These losses can be reduced

almost to zero by using nonmagnetic stainless steel. In addition, the machine housing may be

subjected to eddy current losses if the flux density exceeds 1.5 T in the stator yoke.

Winding losses can rise rapidly when high frequencies and 10–15 mm high conductors are

used. In general, minimizing the winding losses is simple. For example, it can be done by dividing the conductor into transposed subconductors. However, this comes with the expense

of an increase in the DC resistance. In the doctoral thesis, a new method is presented to

minimize the winding losses by applying a litz wire with noninsulated strands. The

construction is the same as in a normal litz wire but the insulation between the subconductors

has been left out. The idea is that the connection is kept weak to prevent harmful eddy

currents from flowing. Moreover, the analytical solution for calculating the AC resistance

factor of the litz-wire is supplemented by including an end-winding resistance in the

analytical solution. A simple measurement device is developed to measure the AC resistance

in the windings. In the case of a litz-wire with originally noninsulated strands, vacuum

pressure impregnation (VPI) is used to insulate the subconductors. In one of the two cases

studied, the VPI affected the AC resistance factor, but in the other case, it did not have any

effect. However, more research is needed to determine the effect of the VPI on litz-wire with

noninsulated strands.

An empirical model is developed to calculate the AC resistance factor of a single-layer formwound

winding. The model includes the end-winding length and the number of strands and

turns. The end winding includes the circulating current (eddy currents that are traveling

through the whole winding between parallel strands) and the main current. The end-winding

length also affects the total AC resistance factor.

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