4. DYNAMIC THERMAL ANALYSIS WITH COUPLED ELECTROMAGNETIC−
4.6 Induction servomotor loss distribution vs. thermal behaviour
With industrial induction motors, stator copper losses are typically the dominating loss mechanism, and with small-power machines, they can comprise over 50 % of the total loss. Figure 4.14 shows the typical distribution of the loss components of a four-pole industrial cage-induction motor as a function of power.
0 10 20 30 40 50 60
0 10 20 30 40 50 60 70 80 90
Motor power [kW]
Share of losses [%]
PCu,s
PCu,r PFe
PMech PAdd
Figure 4.14. Loss distribution of four-pole totally enclosed fan-cooled (TEFC) squirrel cage motors as a function of power (Auinger 1997; Haataja 2003 (modified)).
As the temperature rise of the stator winding insulation is critical from the thermal point of view, the temperature level of the (stator) winding insulation must be kept below value given by the manufacturer, or otherwise the insulation failure and consequently the turn or turn-to-ground short-circuit may occur. As almost all the electrical insulators are also thermal insulators, the heat dissipation is very poor from the stator winding to ambient, as the winding is entirely surrounded by several insulation layers. Typically the critical part of the winding is the end-section, as it is fully surrounded by the air and the resin material, and further, it is far away from any conducting section. The heat conductivity of the air is the worst of all the materials present in electrical machines, as can be seen from Table 4.3, which lists the thermal conductivities of the most common materials.
Table 4.3 Heat conductivities of the most common materials used in electrical machines (Kylander 1995).
Material Thermal conductivity λth [W/mK]
Copper (pure) 395
Aluminium (pure) 237
Construction steel 50
Electrical steel (M600-50A in plane) 38.7
Lamination material 3.7
Epoxy-resin heat conductivity 0.2
Slot insulation varnish 0.2
Air heat conductivity 0.026
As can be seen from Table 4.3, the thermal resistance from the end winding to the conducting parts of the machine – and finally into the ambient – inevitably becomes large as it is surrounded by the air. In the slots, the empty space between the conductors is very often filled with the resin material, which not only mechanically strengthens the winding, but also improves the heat dissipation, as the thermal conductivity of the resin is almost a decade higher than that of the air.
Between the winding and the stator iron, there is a slot liner that protects the winding from mechanical shocks, and due to the roughness of the interfacing surfaces, the contact between the slot liner and the stator iron contains lots of cavities. These things together cause that the heat dissipation from the stator winding, especially from the end-section, is very poor, and the loadability of the machine is thereby determined to a large degree by the stator current.
Stator iron losses are easier to dissipate, as the thermal conductivity of the electrical steel is relatively good and there is only one boundary section that the heat flux must pass to reach the frame, which is the section between the stator yoke and the frame. Depending on the manufacturing technique, materials, and the contact pressure, this boundary section also contains cavities that degrade the heat dissipation, but as this section is also included on the heat dissipation from the stator windings to the ambient, it can be said that the thermal resistance from the windings to ambient is remarkably higher than that from the stator iron to the frame. If the load distribution of the motor can be changed in such a way that the stator copper losses are decreased while increasing the stator iron losses (by increasing the flux), the heat dissipation, and thus the thermal loadability of the motor will improve. Increasing the flux level also decreases the rotor copper losses with induction motor, which must be to a large degree dissipated through the stator.
With PMSMs having thick magnets, this significantly improves the heat dissipation compared to induction machine, and enables the fully closed construction of PMSM servo. The problem with the induction machine is that although the increase in the flux level decreases the torque-producing stator current, it increases the magnetizing current, especially when the motor is in saturation, and thus the total effect on the stator current can be very small or even the opposite. In Chapter 3, only the behaviour of the total loss (efficiency) as a function of flux density was studied. Figure 4.15 presents the individual loss components of the prototype as a function of flux level (calculated with FEM).
0
Figure 4.15. Simulated (FEM) fundamental wave losses of the prototype motor as a function of flux level.
a) Stator copper losses, b) Rotor copper losses, c) Stator iron losses, and d) Sum of the loss components (total losses).
As can be seen in Fig. 4.15, the flux has a substantial effect on the different loss components, especially the stator copper losses can be reduced substantially by decreasing the flux level. By decreasing the flux at rated torque by approx. 20 %, the total losses can be decreased by approx.
30 %. With smaller loads, the effect is even stronger. Figure 4.16 shows the measured end winding temperature rises of the prototype motor at four different loading levels, and flux level as a parameter.
20
Figure 4.16. Heating of the machine end windings at different flux levels, at 50, 100, 150, and 200 % load torque. a) 50 % of the rated torque, b) rated torque, c) 150 % of the rated torque and d) 200 % of the rated torque. There is a significant difference in the heating at different flux levels, especially at partial loads, although the gap for instance in temperature of the end winding between 80 % and 100 % at rated load is still approx. 30 °C after 15 min loading.
Figure 4.16 shows that the flux level optimization discussed in Chapter 3 has a substantial effect on the heating of the machine, as it could be assumed. By properly controlling the flux level, the thermal loadability can be increased and the lifespan of the windings lengthened. As a drawback, the load cycle must be known to avoid abrupt torque variations. This is because the dynamic stability of the drive diminishes when decreased flux is applied.