Myth Busters

Permanent magnet (PM) machines are harmful to humans.                   

Even though permanent magnet machines are not a new technology, some people have concerns that the use of magnets makes them risky. Nothing can be further from the truth. While adequate precautions must be taken to prevent exposed PM rotors to adjacent steel or magnetically attractive structures, an assembled PM machine has a completely enclosed magnetic field and has no residual external magnetic field.

 

Induction machines are more robust than PM machines.                        

PM machines are inherently as robust or more robust than induction machines. From a structural integrity perspective, the composite retention system of Calnetix PM machines is capable of handling higher stresses than an induction machine of equivalent power density and geometry. High-speed induction machines are often beset with issues of rotor integrity because of the laminar construction required to keep rotor losses low. The Calnetix PM rotors are specially constructed for high stiffness contributions from the components, keeping wide rotordynamic margins for subcritical operation.

 

PM machines are more expensive than Induction machines.               

This was the case about 20 years ago when high strength magnetic material technology and manufacturing methods were nascent. Improvements in manufacturing methods, yields and the inherent magnetic properties have brought PM machines in line with any standard motor technology available today. Combined with higher efficiencies and the compactness of design, PM technology is the path of choice for most applications, from medical devices to industrial machines.

 

High-speed machines are more expensive than low-speed machines with gear boxes.             

The initial cost of a high-speed machine with a variable speed drive (VSD) is marginally higher than a low-speed machine with a gear box. However, when you add up the auxiliary systems (oil and cooling) that support large gear boxes, the loss in efficiency in the gear transmission, the maintenance blackouts and the lower availability of the geared systems, high-speed solutions beat their rival geared systems every time in ROI and total lifecycle costs.

 

PM machines degrade over time.    

The magnets used in PM machines retain their magnetic potential within 4% over the lifetime of the machine. As long as the machines are operated within their design limits, the magnets are “permanent.”