Nowadays, most countries are making efforts to increase the energy efficiency of rotating electric machines such as induction motor and permanent magnet motor since normally these machines consume substantial amount of electrical energy [1-4]. The foregoing researches include shape optimal design [1, 2] and employment of better quality of electrical steel sheet (ESS) with more advantageous magnetic properties . Recent researches practically reveal that the manufacturing process of stator core is one of the major factors which deteriorate the magnetic performance of ESS via mechanical stress; in addition, iron loss mostly increases during the laminating process [5, 6]. Nevertheless, until now, we can hardly find any research on the measurement and quantitative comparison of iron losses according to the ways of lamination such as welding, embossing, and bonding.
Fig. 1 shows the structure of bonded- and embossedtype laminated cores, where the embossed-type lamination looks as if it imposes more mechanical stress on the ESS and cause more deterioration of the magnetic properties than the bonded-type one . For this reason, it is believed, in general, that the welded- and embossed-type laminated stator cores have more iron loss than the bonded-type one.
Fig. 1.Configuration of laminated core.
Iron loss inside the ESS, conventionally, is evaluated based on the measurement of magnetic flux density and field intensity under the assumption that they are uniform along the magnetic flux path as in the Epstein frame, the Single Sheet Tester (SST), and the Ring-type specimen method [8, 9]. The stator core of a motor of which segment is shown in Fig. 2. However, it has non-uniform magnetic flux density and field intensity along its magnetic flux path.
Fig. 2.A stator core segment.
Based on the above background, in this paper, a novel iron loss measuring system is developed for a stator core segment with the help of auxiliary yokes to form a magnetic flux path. The developed system evaluates the iron loss based on the measurement of the induced voltage of a B-search coil and exciting current, and is applied to a bonded- and embossed-type laminated segments of stator core, shown in Fig. 2, for IPMSM.
2. Iron Loss Measurement System
2.1 Measurement system
Fig. 3 shows the configuration of the developed measurement system. It consists of a PC, DAQ board (NI PCI 6110), low pass filter (NF P-82), power amplifier (NF 4520) which is connected to the left and right legs of the auxiliary yoke, respectively, isolation amplifier (NF P-62A), and shunt resistance (0.05 Ω) for measuring the exciting current.
Fig. 3.Measurement system.
For the measurement of stator core segment, the iron loss is measured with the help of auxiliary yoke, as shown in Fig. 4, in two cases by controlling the exciting voltage waveforms: case 1 is when the magnetic flux passes the tooth part, and case 2 is when there is only return yoke part. In the design of the auxiliary yoke, the width of the central leg (case 1 in Fig. 4) is set equal to the width of tooth and those of the left and right ones that of return yoke of the stator core segment. Therefore, it guarantees almost same magnetic flux density is obtained along magnetic flux path.
Fig. 4.Measurement cases.
On the other hand, for the measurement of 24-segment assembled stator core (case 3 as shown in Fig. 4), iron loss is measured by using the Ring-type method , where the exciting coil and B-search coil are located at the return yoke.
During the measurement, exciting voltage waveform of exciting coil is controlled using digital feedback method so that the sinusoidal B-waveform is obtained at the B-search coil. The B-search coil is placed between the auxiliary yoke and exciting coil . The measuring frequencies are set to 50 Hz and 400 Hz considering the rotating speed of the IPMSM.
According to measuring frequency, number of turns of exciting coil is set, as shown in Table I, to utilize maximum available power of the power amplifier . Other specifications of different cases are also shown in Table 1.
Table 1.Specifications of measuring system
2.2 Iron loss calculation
2.2.1 Conventional method 
Conventionally the specific iron loss PL is calculated, as in the Epstein frame and the SST, as follows:
where B and H are magnetic flux density and magnetic field intensity , respectively; ρ is mass density and T is a time period. In general, the magnetic flux density is measured by using B-search coil as follows:
where e is the induced voltage of B-search coil of which the area turn is (NA)B, and the magnetic field intensity can be measured by using H-coil method or Ampere’s circuital law by measuring the exciting current [8-10].
It should be noticed that this method is effective only when distributions of B and H are uniform along flux path.
2.2.2 Proposed method
Fig. 5 shows an equivalent electric circuit of the measuring system, where the iron loss along the magnetic flux path (including the auxiliary yoke) corresponds to the power dissipated at the resistance Rc, and can be calculated as follows:
Fig. 5.Equivalent electric circuit of the measuring system, where Re, RB are resistances of exciting and B-search coils, respectively; Rs is shunt resistance, Lm and LL are magnetizing and leakage inductances; Rc represents the iron loss resistance; NeL and NeR are the winding turns of exciting coils on left and right legs, respectively; NBL and NBR are winding turns of B-search coils on left and right legs, respectively.
where the exciting current, i(t), is measured, as shown in Fig. 5, using a shunt resistor Rs. The voltage e1(t) is indirectly measured via eB(t) with the assumption that the leakage inductance is ignorable. The iron loss can be measured as follows:
where eBL(t) and eBR(t) are induced voltages of the B-search coils on left and right legs, respectively.
In this method, there exists leakage inductance LL which may significantly affect on the accuracy of the measurement unless it is ignorable. To resolve this issue, in this paper, B-search coils are located under the exciting coil . The measured iron loss in this method is from not only the stator core segment but also the auxiliary yoke . The iron loss from the stator core segment only cannot be separately measured.
3. Experimental Results and Discussions
The segmented stator core is one of twenty-four (24) parts of the whole stator core, which is quite small comparing with the whole stator core. Furthermore, the measurements for case 1 and case 2, shown in Fig. 4, consist of four parts, i.e., the segmented stator core and auxiliary yokes. The auxiliary yoke may cause small airgaps between the segmented stator core and the auxiliary yoke even though auxiliary yoke is tightly fixed to segmented stator core. As a result, the measurement result for the segmented stator core may be slightly different at each measurement. For this reason, for both the embossedand bonded-type cores, the iron loss is measured with ten samples and their average value is taken as the final measurement result.
When the laminated core segment is assembled into a stator core, it has different distributions of magnetic flux as shown in Fig. 6. The different flux distributions circularly occur under the rotating field condition. Therefore, the combination of iron losses measured from case 1 and case 2 in Fig. 4 can be used to evaluate the iron loss in a segment when it is assembled into an IPMSM operating with a specific rotating speed.
Fig. 6.Flux distribution assumption of real operation of segmented stator core.
On the other hand, the iron loss measured from the case 3 explains the iron loss from the return yoke of the stator core. This is especially important in the segment-assembled stator cores as the IPMSM in this paper and takes most of the iron loss of the stator core. It is because the tooth of the core segment is, in general, along the rolling direction of the ESS and has B-waveform very close to alternating one, thereby very small iron loss while the B-waveform at the return yoke is generally similar to rotating one.
Fig. 7 and Fig. 8 show measurement results at frequency of 50 Hz and 400 Hz, respectively. The results for the magnetic flux density of 0.1 T ∼ 0.3 T do not have enough physical meaning since the eB(t) is very small containing a lot of noises.
Fig. 7.Measurement results at 50 Hz.
Fig. 8.Measurement results at 400 Hz.
For the embossed- and bonded- type stator cores in the experiment, the design, thickness of lamination and number of used electrical steel sheets are exactly same. Therefore, to show the difference in the iron loss between the embossed- and bonded-type stator cores, the difference is defined as:
where PE and PB represent iron losses of embossed and bonded cores, respectively.
It is shown, in Figs. 7 and Fig. 8, that case 2 always gives bigger difference than case 1, for example 1.75% in case 2 and 1.26% in case 1 at the magnetic flux density of 1.2 T under 400 Hz. Considering case 2 is when the magnetic flux passes through embossments while case 1 is not, this reveals that the embossment significantly deteriorates the magnetic properties of the ESS to have more iron loss at the same level of magnetic flux density. It is also shown that the differences become bigger as the measuring frequency increases from 50 Hz to 400 Hz.
When an IPMSM operates, magnetic flux distribution at a core segment will change as shown in Fig. 6. The iron loss at a core segment while motor operation therefore, can be deduced from those measured for cases 1 and 2, i.e., iron loss per segment per period will be approximated to the summation of that from case 2 and twice of that from case 1.
The corresponding results are shown in Fig. 9, where the difference of iron loss between embossed- and bonded-type core segments is 1.2% at (1.6 T, 50 Hz), and 1.5% at (1.2 T, 400 Hz), respectively.
Fig. 9.Iron loss at the motor operation of assembled stator core.
For measurements of assembled stator core in case 3, the iron loss difference between the bonded- and embossed-type stator cores is about 12.1% at (1.0 T, 50 Hz), and about 10.83% at (1.0 T, 400 Hz). Compared with a segment, this bigger difference comes from the assembled twenty-four (24) cores. It means the bonded-type stator core has less iron loss than the embossed-type one when segments are assembled into an IPMSM.
With the measurement results and iron loss calculation for motor operation, it is obvious that the bonded-type core has less iron loss than the embossed-type core due to smaller mechanical stress during the lamination process.
A comparison of iron loss measurement in bonded- and embossed-type segmented stator cores is carried out in this paper. The core segment and segment-assembled stator core are measured to compare the iron losses for a core segment and assembled stator core when lamination is changed from embossment to bonding. From the measurement, it becomes clear that bonding lamination is better than embossment one from the viewpoint of iron loss.