Soft magnetic composite is maybe the best-kept secret in powder metallurgy. We’ve exhaustively covered the benefits it can offer vs., say, a steel lamination. But that doesn’t mean soft magnetic composite (SMC) should be used for making all powder metal components.
Does your project or its goals fall under one of the categories below? If so, a different powder metal material process may be more beneficial than SMC.
(Related post: When SHOULD You Use SMCs?)
When Not to Use Soft Magnetic Composites for Powder Metal Components
1. Standard 50/60 Hz Induction Motors
The inherent performance of soft magnetic composites at low frequencies like 50-60 Hz is not adequate for induction motors (i.e. the little motor that drives your bathroom exhaust fan).
There’s a fundamental reason.
These motors don't have any windings in the rotor. They’re induced through their permeability aspect (the magnetic field from the motor’s stator). Because the permeability of soft magnetic composites is generally lower than that of a lamination, it’s a poor fit with these motors. You won’t get the starting torque and performance you’re hoping for.
2. Most 50-60 Hz Applications Where You Have No Desire to Design
You can overcome the limitations above, but you have to be willing to rethink your design. The point is to optimize the motor to fit soft magnetic composites’ unique properties. If you don’t want to redesign, then it's a waste of your time and the powder metal manufacturer’s time to bother with SMCs.
This applies to motor types similar to case #1 above.
Cost efficiency is another factor in choosing materials for 50-60 Hz projects. Large-scale manufacturers making motors have stamping presses that run tremendously fast. There are certain segments of the market -- commodity products -- where it makes no sense to choose SMCs
3. When Strength Is Critical
Stress during rotation can possibly exceed the strength of a component made from soft magnetic composite. So something that rotates at high speed -- like a rotor in a motor -- isn’t a good fit with soft magnetic composite.
You might get away with using SMC for a small rotor, but as you get into bigger ones, strength requirements can be high at the component edges. You’d need some careful calculation and design to avoid ending up with broken parts..
Unfortunately, powder metal parts are a bit brittle compared with laminations.
4. Devices Below 400 Hz (Usually)
This is not always true, but it’s a good general guideline for deciding whether to use soft magnetic composite.
Again, if you’re willing to rethink how the motor moves, you can probably eliminate this roadblock to using SMCs.
Perhaps you only need a simple design and aren’t worried about core los . You probably can’t improve much by a redesign. If your product isn’t going to improve with a material switch, why bother?
5. Very Large Motors
A gargantuan motor would require many soft magnetic composite parts to be both stacked and assembled. If so, there’d be potential for needing tolerances outside of your current component plans.
Does your motor component need to go under a compaction press? A motor that’s 2’ or even just 18” in diameter won’t fit in a powder metal press. So you’d have to make the components small and stack them together.
In other words, the assembly process is a pain. Standard large-motor designs don’t need that hassle -- unless it's such a unique design that SMC is the only way to achieve it.
More Questions About Your Project?
Hopefully you have a better idea of the ideal material process for your application. If you haven’t checked off any boxes on this list, check out this article on ideal uses for soft magnetic composites. You’ll be surprised what you can create with the flexibility and performance of SMCs!
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