Squirrel cage rotor is one of important components of AC asynchronous motor. A pure aluminum guide bar is cast in each slot of rotor. Guide bars are short-circuited with two end rings at both ends of iron core to form a short-circuit winding, which is used to generate induced electric potential and thereby electromagnetic torque. Because squirrel cage rotor has no windings, rotor requires no maintenance, has low noise and high reliability. If rotor is seriously broken during die-casting operation, motor will have insufficient output, current will increase, periodic swings will occur, and speed will decrease. In severe cases, stator winding will be burned out. It can be seen that improving mold structure is the key to obtaining excellent quality cast aluminum rotors.
Graphical results

Cast aluminum rotor is shown in Figure 1. Material of end ring and guide bar is cast pure aluminum (99.8% Al). Rotor core is made of laminated rotor punched sheets (100 pieces), and its outer edge helix angle is 4.9° ( skewed by one tooth pitch). There is a marking groove with a depth of 2mm and a width of 4mm at the edge of inner hole of rotor punching piece, and head has a sharp angle of 45°. Purpose is to ensure that front and back directions are consistent when sorting pieces, see Figure 2. So-called film sorting is to use weighing method to weigh out 100 punched films, pass one end of a thin lead wire of appropriate length through marking groove, the other end through guide groove closest to marking groove. Twist the two lead wire heads twice with needle-nose pliers so that core pieces are relatively fixed and lead wire is hidden in 45° sharp angle of marking groove, will not interfere with skewed spiral key.

 

Figure 1 Simplified diagram of cast aluminum rotor

 

Figure 2 Rotor punching

 

Figure 3 Calculation of skew bond helix angle

 

Due to uneven air gap of open slot rotor, air gap reluctance is uneven throughout circumferential range (reluctance of slot is larger than that of teeth), causing back electromotive force to contain cogging harmonics. When rotor adopts inclined grooves, additional torque generated by cogging harmonic magnetic field can be effectively suppressed, so that electromagnetic vibration and noise can be suppressed. Whether rotor core skew angle is in place will directly affect effect of overcoming cogging harmonics in back electromotive force. Therefore, a fast and efficient core skew method must be found before rotor die-casting. Insert mandrel (see Figure 4) that fits inner hole of punched piece into prepared inner hole of punched piece set of rotor core (align notch with mark groove of inner hole of punched piece), until shafts at both ends are stretched. Then insert skew key along mandrel slot (see Figure 5) and 2mm wide skew convex key of skew key at the same time (or gently tap it in with a small hand hammer) ) in marked groove of iron core, so that skew work is completed; cut and pull out tightening lead wire used when arranging slices, and obtain rotor core assembly as shown in Figure 6.

 

Figure 4 Mandrel

 

Figure 5 Skew key

 

Figure 6 Twist mandrel assembly

 

Figure 7 Rotor die-casting mold
1. Fastening screw 2. Unloading tie rod 3/13. Fastening screw 4. Washer 5. Unloading sleeve 6. Pad 7. Upper mold pad 8. Upper mold cavity 9. Guide sleeve 10. Mandrel 11 .Flat-type middle ring 12. Skew key 14. Unloading plate 15. Guide pillar 16. Base plate 17. Rotor punching group 18. Unloading plate insert 19. Runner insert 20. Split cone 21. Upper mold cavity Fixed plate 22. Positioning key

(1) Analysis of initial positioning and pouring system of rotor core. Four rotor punching groups 17 are placed in corresponding positioning holes of flap-type middle ring 11. After tightening, screw it into positioning stop of unloading plate insert 18 with inner sprue taper hole (make sure to insert positioning key into corresponding slot); sprue is a φ60mm hole connected to cold press chamber of die-casting machine. Lateral runner uses a modified butterfly-shaped sheet structure to enhance strength of lateral runner and will not deform when it withstands tensile force of point gate. Each rotor adopts 6 uniformly distributed inner gates to feed materials to minimize pressure loss and greatly improve yield. 6 conical inner gate scraps attached to lower end ring of rotor casting can be removed by lathe .
(2) Analysis of demoulding mechanism. Success of demolding is related to success or failure of mold design. There are 4 rows of 6 φ2mm ingates in mold. Rotor casting can only be taken out after ingates and butterfly runner are broken. Therefore, a discharge plate discharge mechanism driven by return force of upper mold is adopted.

During rotor die-casting process, if rotor core is scrapped due to insufficient pouring or broken bars, etc., rotor core will also have to be scrapped. This is also one of important reasons for increased cost in motor manufacturing. The key to success of rotor die-casting mold is to select a fully vertical die-casting machine suitable for placement of iron core inserts, a tapered expansion point gate with minimal filling pressure loss, a flap-type middle ring suitable for precise positioning of four parts in one mold, and a unloading plate point gate driven by a rigid telescopic tie rod. Compared with 1-cavity fully vertical point gate mold, mold greatly improves economy and production efficiency; compared with traditional multi-cavity horizontal side gate rotor die-casting mold, it has advantages of convenient clamping of iron core inserts, good mold stability, and low maintenance costs.