Recent events have made energy production and the efficiency of energy utilization the subject of much discussion, especially during the past few months. This interest will probably continue and will likely have widespread effects on the appliance industry. Specifically, motor efficiency, especially small motor performance, is being re-examined and regulated through overall appliance energy usage standards.

The desire for a more energy efficient and cost-effective electric motor is widespread in the appliance industry. However, while the motor industry has achieved more efficient integral horsepower, large diameter motors, it is much harder to make small diameter, fractional horsepower motors highly energy efficient. This means that for millions of applications in the appliance industry, the motors used are often not very efficient and many perform poorly.

Fig. 1. Fractional horsepower motor efficiency.

We broadly define a very high efficiency motor as one with an efficiency of 90 percent or greater. Currently, most small induction motors have operating efficiencies between 50 and 70 percent and sometimes even lower. Most small permanent magnet brushless DC motors are in the 70 to 80 percent range. Fig. 1 summarizes these general efficiency ranges and illustrates current motor performance ranges.

While there are many motor choices currently being offered, the problem is that the motor solutions offered today usually yield small incremental efficiency gains coupled with one or more disadvantages, such as higher cost, larger size and limited operating conditions. The reason for this is that these motor solutions are typically the result of small incremental changes in conventional motor designs. NovaTorque motors utilize a substantially different design and offer greatly improved efficiency.

Fig. 2. Field pole core and coil structure for NovaTorque motor.

The major power loss mechanism in small electric motors is copper loss, which is calculated as motor current squared times winding resistance. The primary factor influencing copper loss in a conventional motor is the amount of copper winding that can be installed in the slots of the motor. This is set by the diameter of the motor and the required rotor geometry.

The NovaTorque motor design uses a different geometry so that the coils are wound parallel to the motor’s axis of rotation, similar to the coil layout in an axial gap motor. (See Fig. 2). Having a coil with axial orientation creates a number of flexible design options for the motor. The most important one is that the copper loss can be reduced by changing the length of the coil.

Varying coil length

This axial coil geometry of the NovaTorque motor design means that the overall copper loss and motor efficiency can be varied by changing only the coil and overall motor length without modifying any of the other motor components or dimensions. To better understand this feature, consider the following motor example as shown in Fig. 3. If we design a motor to have a coil length of L and a winding wall thickness W, this coil when wound with a specific magnet wire will have N turns with resistance of R. When operated at I amperes of current, the motor has NI ampere turns and will produce a torque of T.

If we now double L so that the coil length is 2L and keep the coil thickness the same at W, the number of turns with the same size wire will be 2N with a resistance of 2R. Since there are now 2N turns, the current necessary to produce T torque will be one-half I or 0.5I, since this results in the same NI ampere turns for the motor.

The power in the first case will be I squared times R and the power in the second case is (0.5I) squared times 2R, or equivalently one-half I squared R. By doubling the coil length (without changing the coil or motor diameter), the power dissipated by the copper has been cut in half, yet the exact same amount of torque is produced. Since the copper loss is usually the predominant loss in a motor, reducing the copper loss by a factor of two can greatly increase the motor efficiency.

The flux path of this motor is not materially affected by this increase in field pole length because the permeability of the steel core is so much higher than any alternate magnetic path. No changes are necessary to the magnets or the motor end bells; only the coil and field pole core length needs to be altered. So to improve efficiency, the only costs are the additional copper and steel required to extend the coil length. The only change to the motor is a change in length.

By using this coil extension technique, small diameter motors can achieve efficiencies of 85 percent and higher. With some additional modifications, these motors can reach efficiencies of between 90 and 95 percent for a motor with a 55 mm outer diameter.

Table 1 presents design examples of a NovaTorque 55 mm diameter motor implemented with three different lengths of windings. This motor is designed to output 0.75 N-m (106 oz.-in.) of torque continuously at 3,600 RPM (0.38 HP). In these examples the price for steel is taken as $2.50 per Kg, the price for copper is assumed to be $5.50 per Kg and electricity is assumed to cost $0.10 per KW/H.

The increased cost of the copper is directly proportional to coil length. However, the steel costs only increase a modest amount, since only a portion of the total motor steel is in the core section under the winding. The three times multiplier on manufacturing cost is used to adjust raw manufacturing costs to selling price. These cost assumptions can be modified as desired to actual numbers reflecting different specific manufacturing situations. However, as can be seen, the payback for obtaining higher efficiency by using long coil lengths is quite attractive. This presents the appliance manufacturer an option to offer the consumer higher operating efficiency at a very attractive cost.

Additional benefits

While direct energy savings may be the primary factor in specifying a highly efficient motor, considerations related to waste heat may also be important. In a number of applications, the waste heat generated by the motor presents severe design constraints and operating disadvantages. This is especially true in applications such as refrigeration. In these applications the reduction of waste heat in the energy efficient motor can be of significant importance, especially in applications where this waste heat adds to the refrigeration load.

For a motor running a refrigeration compressor with an operating output load of 187 W at 3,600 RPM (1/4 HP), if the efficiency of the motor currently being used is 80 percent, then 233 W of power is consumed in the motor. This means that 46 W of heat is generated during operation, and that heat must be conducted out of the compressor package. This heat is generated in the location where one is trying to compress and then cool the working fluid. In most applications, this waste heat is dumped to the environment around the refrigerator, thereby increasing the ambient operating temperature and further increasing losses.

NovaTorque 55 mm diameter motor.

If that 80 percent efficient motor is replaced with a motor that is 90 percent efficient, then only 208 W are needed to drive the motor and only 21 W of waste heat is generated. This is 2.2 times less heat, a reduction of 55 percent. This is a highly significant benefit of more efficient motors. Of course, if the motor being replaced is less than 80 percent efficient, then the savings are proportionally greater. Less motor heat generation also can result in lower operating temperatures for the motor, which increases motor life and reliability.

This reduced heat generation also facilitates packaging in other applications. In some applications, the lower operating temperature of the motor may make it possible to completely eliminate cooling holes in the external case, providing benefits of greater case strength and eliminating the concern of foreign matter and fluids entering the motor of the appliance.

Fig. 3. Coil length change to reduce copper loss.

The NovaTorque permanent magnetic motor design can be scaled in size from sub-fractional (less than 1 in. in diameter) to small integral horsepower motors (up to 10 HP). It offers a number of other advantages including high power density, excellent thermal performance, high peak torque capability, very high-speed operation and smooth torque production with low ripple and low audible noise. In volume the manufacturing costs for the NovaTorque motor are predicted to be very competitive with conventional motor designs.

NovaTorque has been developing this new motor technology over the last three years and has been granted a U.S. patent on the basic motor structure. In addition, multiple U.S. and international patents have been filed on various aspects of this motor’s basic structure, design methods and manufacturing techniques. NovaTorque currently has advanced pre-production motors undergoing evaluation and testing and is in licensing discussions with several major motor manufacturers. Research and development is continuing to further optimize the motor performance and reduce production costs.