Advancements in Brushless Motor Technology

The second configuration is called a cup or inverted motor and is shown as a cutaway in figure number 1. In this type of brushless motor, the wound stator is located inside of the permanent magnets. The permanent magnets are mounted on the inside diameter of a cup that rotates around the armature. The heat dissipation of this type of design is similar to that of a standard dc brush motor, and normally is used in blower applications where high ambient temperatures are not significant. The advantages of this type of design is that the bulk of the rotating mass is located within the bearing centers. This permits much easier balancing and therefore higher speed capability. AMETEK manufacturer blowers of this configuration at power levels of up to 1200 watts. The first transportation motors developed were the 47 frame and 64 frame platforms that are still used in our air conditioning and water circulating pumps as well as other designs. Examples of these designs are shown in figure 2. All of the earlier designs used Hall effect electronics for commutating the motors. The Hall effect designs use a shaft encoder to time the point of powering each stator winding to rotate the motor at its operational speeds. It was also recognized at that time that additional products were needed to meet the lower power requirements of many of the vehicle applications such as defroster, heater and drivers booster blowers. In 1995, we introduced our 3-inch motor design.  

The early 3-inch designs shown in figure 3 use the same type of Hall effect electronics for commutation and incorporate all of the same internal mechanical components that are used in the 47 frame motor. One of the primary differences is the elimination of the finned motor housing. Although this reduces the heat dissipation capacity of the motor, it also reduces the cost of the design. The 3-inch motor has been successfully incorporated into many defroster, heater and booster blower applications. In recent years, we have been able to reduce customer product costs by superceding the 47 frame motor with 3 inch motor designs. Later designs of 3-inch motors use a newer electronic technology that does not require the use of Hall effect switches. This is called a sensorless design. The availability of newer electronic components made it possible to sense the back EMF in the coils of the stator and power the windings in the correct sequence to provide the proper shaft rotational output. The development of the sensorless motor driver permitted more compact designs as axial space was not required to accommodate the hall switches.

In recent years, we have added a smaller 22 frame (1.7 inch) motor shown in figure 3 to the offerings to round out the product line. The 22 frame motor does not have extensive use in the Heavy Duty Transportation Market.



NEW DEVELOPMENTS - DURATEK 3.0 MOTOR
Our most recent development for the transportation market is the new DurA-tek 3.0 three inch motor platform. This design is currently offered with a sensorless electronics design, and is suitable for heavy-duty transit applications where significant heat loads are not present. The design incorporates new electronics that will provide up to four- speed operation simply by providing signal voltage on specific wires. A variable speed option will be available for the DurA-tek 3.0 later this year. Variable speed control will be accomplished by varying voltage on a signal wire. The DurA-tek 3.0 motors are shown in figure 4.

The primary advantage of the DurA-tek 3,0 is that it is designed for higher production volumes and more automated assembly. This results in lower manufactured cost and  therefore lower production prices. The other advantage is that the modern sensorless electronics package developed for DurA-tek 3.0 can be incorporated into existing designs that operate in the same power range (standard 3 inch and 47 Frame). Usage of the DurA-tek 3.0 electronic drive will increase the availability of multiple and variable speed electronic motor drives in existing products. 

One of the first new product developments using the DurA-tek 3.0 motor is a lower flow brushless sealless water- circulating pump. This pump will complete our transportation water circulating pump offerings. With the availability of this new design, we now offer three pumps as shown in figure 5. This includes our industry standard 47 frame pump that delivers 18 gpm at free flow, our 64 frame pump that delivers 26 gpm at free flow, and the new DurA-tek 3.0 pump that delivers 10 gpm at free flow. Comparative performance curves of all three pumps are shown in figure 6. All three pumps are sealless designs and the pump portion is magnetically coupled to the motor. This provides the longest possible life between maintenance intervals. When motor maintenance is required, the motor can be removed and replaced without disturbing the pump or its piping.

DESIGN STANDARDS  The primary difference between all brush DC motors and brushless DC motors is the maintenance required. The most common reason for brush motor maintenance is brush replacement, which usually is required on an annual basis. The second maintenance item is commutator wear. Brush DC motors require periodic disassembly to machine the commutator to correct for the wear caused by the brushes. This is required every two to three years. Eventually the commutator is worn to the point that it is no longer repairable and the armature must be replaced. This normally is required after about 4 to 6 years of service. The third cause of mechanical failure of a brush motor is bearing failure. This is not considered an issue as the brush DC motor is disassembled several times during its life for more frequent failure modes and bearings can be inspected and replaced at that time.

MOTOR DESIGN LIFE 
In brushless DC motors, the only mechanical failure mode is the bearings. For this reason, bearing design is extremely important in the mechanical design of any brushless motor. Bearings normally fail for either mechanical repeated stresses (fatigue) or loss of lubrication (wear). In order to properly design a long life bearing, consideration most be given to the following mechanical issues which include radial, axial and overhung bearing loading, dynamic vehicle loading and equipment balance, rotational speed, operating temperature, and environmental conditions. In long life bearing design, when the body of the motor is a dissimilar metal such as aluminum, the bearing needs to be supported in a stainless steel sleeve. This greatly improves the mechanical life and offsets the difference in thermal expansion. 

The bearings used in fractional and low HP motors are either sealed or shielded and packed with grease for their life. There is no simple way to re-lubricate a bearing of this size so that the bearing as installed in the motor carries its lifetime supply of grease. Care is needed in selecting the best grease lubricant for the operating conditions including grease viscosity and ability to withstand the operating temperatures without breaking down and loosing its lubricating properties.
Our experience over the years in determining true motor life has shown that the critical life factor with a properly supported bearing is lubrication. Lubrication life is significantly less than the mechanical life and in order to bring the lubrication life above 40,000 hours it is necessary to significantly oversize the bearing. This provides the bearing with sufficient lubrication storage to allow for normal operation in excess of 40,000 hours. In many cases, this results in a L10 bearing mechanical life that can be measured in millions of hours. It often is not practical to run a motor under full operating conditions to determine its bearing life before introducing the product into production. As a result, bearing manufacturers have developed empirical formulas based on laboratory testing of bearings. The use of these formulas provides a reasonably accurate method of determining the true bearing and motor life. Our own internal life testing of various motor products has verified that our motors do meet the full intent of the APTA specifications. The empirical formula for mechanical or fatigue life can be found in most engineering handbooks. The empirical formulas used for lubrication life are complex, specific, and proprietary to each bearing manufacturer.