How Much for That Motor?

Most engineers only have experience with mass-produced electric motors and very few have an understanding of the design and tooling required to make custom motors. The initial attempt to obtain a custom motor typically results in sticker shock.

When a few custom motors are necessary to meet specific application requirements and these requirements are not met by simple modifications of mass-produced products, the design process begins. The first step is the definition of the problem. To the inexperienced, a statement that says the load has to move at a certain speed appears to be a definition of the problem. To the motor designer, such a statement is akin to saying you want a green car. A real design specification includes mechanical envelope, power requirements, thermal specification, drive interface requirements or even terms like torque constant, cogging torque, back EMF and more. Typically, it requires 15 to 30 man hours by an experienced engineer working with the customer to define the problem and communicate the proposal.

Design tasks include magnetic design, mechanical design, thermal design, material selections, heat treatments, lamination insulation, stacking and bonding process specification, coil and insulation specification, testing and QC. A recent project — the magnetic design for a scaled version of a motor where we already had the concepts in place — required 75 hours of Ph.D. time to run the calculations to estimate the motor performance in advance.

Motor design is followed by a long list of manufacturing items that need to be considered including punching dies, stacking machines, alignment fixtures, Stator (lamination bonding, OD/ID grinding) and rotor (stacking, bonding, grinding). Let us not forget grinding magnets, after we have bonded them to a suitable shaft.

With each part to be manufactured there is a cost to obtain material and set up the manufacturing process. For a typical motor shaft: Minimum material order, cutting bar stock to length, installing centers, turning the output shafts, turning the bearing journals, cutting bearing locators and knurling the mid section to allow pressing of the rotor stacks. A machine shop will have a set-up charge for each different machine operation.

Now, presume each machine operation met the specified tolerances but that each tolerance was on the high side, so the total accumulation of error in the parts is such that the rotor will not turn in the stator. All the suppliers say, “We met the specification.” The customer is screaming for delivery, QC checked all the incoming parts and they were all OK. But the assembled motor doesn't perform as it should. How do you put a value on taking responsibility for giving the customer what he expects, when he expects it? What price do you put on the risk?

But wait, how did we know the motor wasn't performing as required? This would require testing, which requires test equipment and test procedures. A good motor test dynamometer with appropriate fixtures can cost upward of $50K. And they have limited ranges, so it might be necessary to buy one to support testing of your custom unit.

Perhaps this will help the reader understand why he or she is likely to see price quotes ranging from $80,000 to $600,000 to design, tool, fabricate and support a custom motor product.