On the surface many objects look the same but quite often the differences are discovered through costly experience.  The same goes for electrical motors.  The drive to reduce cost often translates into inferior end product that is less reliable and increases the longer-term cost of ownership. Very often users are not aware of these inferiorities.  Obviously the impact of such inferiorities is more severe when purchasing a high cost, custom built, medium voltage motor.

Willingness to not only analyse cause of failure but also to continuously improve design, results in improved quality and long term reliability. 

Rotor Design

The use of aluminium die cast rotor technology is less costly for the manufacturer, however consistency cannot be guaranteed.  Many parameters need to be controlled to ensure the perfect cast.  Few operators de-gas their aluminium prior to casting, resulting in porosity and unexplained poor rotor performance.  Oily deposits on rotor laminations result in defects; often rotor bars are not formed at all.  Once the rotor is cast it is virtually impossible to determine quality or consistency of cast.  Poor performance and destructive testing often reveals the cause of the problem.  Figure 1 and 2 illustrate casting defects which cause poor efficiency.

Figure 1: Aluminium die-cast rotor: blow holes in end ring.

Figure 2: Blow holes in end ring on an Aluminium die-cast rotor.

The effect of casting defects is less prevalent in smaller electric motors where reduced efficiency is less critical.  However, when die cast rotors are used on larger, more expensive medium voltage motors (3.3 kV to 11 kV) the effect of casting deficiencies can be severe.

In many instances, on larger motors the rotor is too large to be cast as a single unit and thus two halves are individually cast and welded to form a single large rotor.  Control of consistency of the welding process is even more difficult to control than that of casting aluminium, resulting in porosity and defects being manufactured into the rotor!

Through expensive research, a practical test have been developed that can in fact test the consistency of the aluminium casting with a high degree of certainty. This advanced test is however difficult to perform and time consuming, resulting in a relativly high cost of R 9800[2] for each rotor test. Manufacturers are reluctant to this test, not only because of the high cost, but also because if a rotor fails this test, the only remedy is to replace the rotor completely.

Copper barred rotors are both electrically and mechanically superior to aluminium die-cast rotors.

The resistance of aluminium is more than double that of copper resulting in dramatically reduced rotor efficiency and energy losses across entire motor life span. Manufacturers try to compensate for this increased resistance by increasing the rotor bar area, and thereby reducing the resistance. The problem with this is that the area of the lamination steel is then reduced, which results in higher core losses.

When a perfectly cast die-cast rotor is compared with a copper barred rotor, mechanical efficiency is reduced due to increased slip.  Typically a 2 pole copper bar rotor will run at 2986 r.p.m. compared to 2972 r.p.m. for a perfect aluminium die cast rotor.

The table hereunder compares the difference in the performance characteristics between a copper barred rotor and a die-cast aluminium rotor.

Table 1:  Comparison between Aluminium Die Cast Rotor and Copper Barred Rotor

When a copper bar rotor is manufactured, the copper bars are pushed through the rotor lamination stack. This process ensures that all rotor bars are in place as illustrated in Figure 3.

Figure 3: Copper rotor bars in electrical steel rotor pack

The end rings are brazed onto both ends of the rotor.  There are two methods used to secure the end ring to the rotor. 

The “rolled ring” or the ”under-bar” configuration. The under-bar method is a more economical method of rotor design and manufacture however, it does have some distinct disadvantages.  Rolled copper end rings need to be brazed to close the ends, this induces stresses, which can result in cracking of the end ring.  Starting torque exerted on the end ring causes deflection and stress which after repeated starting leads to cracked rotor bar brazing and ultimately complete motor failure.  A typical failure of the under-bar method is shown in Figure 4.

Figure 4: Typical failure on a “rolled ring / under-bar” end ring construction.

The preferred ‘side-bar’ end ring method of rotor construction is shown in Figure 5.  The copper or chrome-copper rings are forged to ensure superior composition.   After machining the ring is brazed onto the ends of the copper bars using superior quality silphos brazing bar which ensures improved reliability and surface contact.

Figure 5: “Side-bar” end ring configuration.

We fit precision laser cut compression plates on all our Large Machines, both on the stator as well as the rotor cores after the cores have been compressed with 30 tons! (See pictures 7 and 8) Some manufacturers still use the practise of merely punching the rotor bars with a chisel (see picture 9) or merely spot weld some of the laminations at the core ends together. These practises lead to loose laminations after a few months of operation, which then creates vibration and noise problems. (See pictures 10 and 11)

Picture 7: Precision laser cut 10 mm compression plate securing stator laminations.

Picture 8: Rotor compression plate.

Picture 9: Attempt at securing the rotor core pack by punching the rotor bars.

Picture 10: Typical result of the lack of a compression plate: loose end laminations on a stator core.

Picture 11: Loose rotor end laminations causing vibrations and damaging rotor bars.

We implemented a system of a Ball and Roller bearing or White Metal Sleeve bearing configurations on our large motors, where others still use the Ball-Ball configuration, which has worse bearing life and re-greasing intervals and can also not accommodate as much radial load as the Ball-Roller configuration.

Frame Design

Several of the OEM’s still supply Cast Iron frames to the South African market. All our frames are fabricated from steel. (See picture 12). A Cast Iron frame has a worse heat transfer coefficient than the fabricated frames – resulting in a “oven-effect” inside the motor. The fabricated frame can also be customised to suite the customer’s application and requirements. The most significant advantage of the fabricated frame is still that it is repairable! The pictures shows attempts to repair a cast iron frame – but these are of course are only temporary. (See pictures 13 and 14)

Picture 12: Fabricated 400 mm centre height frame.

Picture 13: Attempt at welding back ribs on Cast Iron frame.

Picture 14: Another crack on the same Cast Iron frame.

Stator Core Design

Our stator cores are shrunk fit and dowelled into the stator frame. This allows for relatively easy removal and repair. (See picture 15). Some of the OEM’s supply what we affectionately call “The Throw Away Motor”. This motor’s construction makes repair almost impossible due to the fact that the core is welded into the frame! Removing such a core will most likely result in it’s destruction. (See picture 16)

Picture 15: Pressed (up to 30 ton!) stator core.

Picture 16: Typical frame welded to the stator core.

We have upgraded from the industry standard 630V65 and 530V50 laminations steel, to premium quality M400V50 laminations steel! These laminations deliver lower core losses and better power factors than inferior materials. We are also able to source top quality M230V35 laminations steel with exceptionally low core losses of less than 2.3W per kilogram at 1.5 Telsa flux density! Every set of lamination material are tested and certified to the specified quality.

Conclusion

This is but a very short list of our efforts to Engineer a Quality Motor in an industry that is known for excuses. Excuses for vibration problems, noise problems, and motors! We are in this business to solve customer’s problems, not to create additional long term problems!

You get what you pay for!   Are you asking for the correct configuration?

We are proud to engineer quality solutions for our valued customers.

About the Author:

Henk de Swardt has a B. Sc. in Electrical and Electronic Engineering. He has more than eleven years of electric motors experience, both in the electric motor repair industry, as well as the electric motor manufacturing industry. He was employed for several years by the Largest OEM in South Africa . He also received specialized training in France on the designing of Electrical Motors. He is currently serving the Electric Motor industry at the Largest repairer of MV and HV motors in Africa . For a full C.V. visit http://www.qtime.co.za/CV_Main.html

Other articles written by the Author:

·         Can a small Voltage increase be used to improve an electric motor’s efficiency?.

·         Centrifugal Fans: Direction of Rotation Explained.

·         Critical Speed on an electric motor explained.

·         Electric Motor Design Enhancements: Ensuring high quality and long term reliability.

·         Electric Motor Failure Prevention: Wedge Failures.

·         Electric motor Revitalisation Program: Case Studies 1 - 4.

·         High Efficiency Motors: Fact or Fallacy?

·         How does build-up of residue in water heat exchangers influence their cooling efficiency?

·         Star-Delta Starting and Dual Voltage Motors Explained.

·         The effects of an increased air gap of an electric motor.

·         The Locked Rotor Test Explained.

·         Torque and Starting of High Inertia Loads Explained.

·         Winch motor failure analysis.