Lowell Christensen, Consultant • Lowell Christensen LLC
We have heard of the desire to make home deliveries using commercial drones. There are several logistic issues with drone deliveries. These issues will soon be overcome and commercial delivery drones will be a fast growing application. This will require changes in motor design to meet the requirement for high torque in a low weight motor.
There are motor designs for the hobby drones but these have a very limited load weight capacity. These motors have a very high resistance and are limited to applications for cameras and other low weight loads. The motor required for heavier loads will need a flatter speed torque curve to keep the speed fairly constant as the load varies from a full load for delivery and no load on the return trip. A very low winding resistance is needed to achieve this flat speed torque curve. A second issue will be motor weight. The motor weight needs to be small to improve the weight capacity of the drone. This is a difficult requirement to meet. Lowering winding resistance is usually done by methods that increase the volume of the motor. This would increase motor weight.
The main issue of motor design for these new commercial drone applications will be the ratio of the torque produced divided by the motor weight. This torque to weight ratio will need to be maximized for these applications. Ways to do this will be discussed in this article. The motor could be either an outer rotation or inner rotation motor. The motor discussed here is an inner rotation motor
The present hobby drones have low cost motors produced in China. These designs have a high resistance and a low voltage constant making their speed torque curve very steep. This makes the motor operate at about one half of its no load speed capability when operating on a drone. Figure 1 shows typical speed torque curves for a motor with a high resistance and a motor with low resistance. The slope of the speed torque curve is the negative result of the resistance divided by the motor voltage multiplied by the torque constant. If the motor torque is increased by a heavier load, the speed will drop. The drone with the higher resistance may not be able to rise off the ground. Lowering the resistance will decrease the slope and flatten out the speed torque curve. The speed will not have a large change as the load changes from the full load on the trip to the consumer to the no load on the return trip. The small change in speed will make the speed a lot easier to control for lift capability of the drone. The flatter speed torque curve will also lower the no load speed requirement. This will allow a higher voltage constant and higher torque constant which will also flatten the speed torque curve. However the effect of increasing the motor voltage constant is that the number of turns will need to increase or the length of the motor will need to increase. Both of these effects will increase the weight of the motor and the resistance of the motor. This design tradeoff of resistance and weight is a real problem and there is no correct answer that will fit all applications.
The resistance is going to be a factor of the volume of copper in the motor. The easiest way to lower resistance is to increase the cross section of the copper but this increases the volume of copper and the weight of the copper. Another possibility would be to increase the area that the winding can occupy. This would mean that the slot would need to be deeper increasing the size of the stator. This would increase the amount of iron needed and increase the weight of the motor. Another factor of the resistance is the total length of the copper needed to provide the turns per coil needed. The number of turns is a function of the speed needed divided by the voltage available. The speed needed is an application parameter so it will be fixed by the application. Lowering the voltage will lower the turns per coil and the resistance. The downfall of this is that the current will need to increase to achieve the torque required since the torque constant will also drop. Another factor that effects the turns per coil needed is the airgap flux density. Increasing the magnet strength will decrease the number of turns needed and lower the copper weight. This higher flux density from the magnet will require more iron volume to keep the flux density at a level to limit the iron from saturating.
The total motor weight will come from five major components. These will be the motor frame, rotor iron, magnet, lamination and the copper used in the winding. The frame is a mechanical assembly and does not affect the motor performance except in weight. The frame weight will need to be minimized by reducing the thickness of the components while still giving enough strength to enclose the motor in a sturdy frame. Figure 2 shows the effects of the component weights as the ID to OD ratio of the stator is increased. This curve is for an internal rotation motor. The magnets will need to be a Rare Earth material to get the high flux density in a small volume. The material grades will have the same density so weight will not be a factor in picking the magnet grade. This leaves the two components that can vary the weight as the lamination and the copper. The lamination weight will depend on the flux density allowed in the iron. Once the flux density is established, the volume of iron is determined from the stator diameters and length. As the rotor diameter increases, the magnet width increases. This will make the teeth wider so the lamination weight does not change a lot. The copper volume is the most flexible variable. Decreasing the copper volume will increase the resistance of the motor and have a negative effect on the motor performance. Increasing the copper volume will decrease the resistance and improve the motor performance but will increase the motor weight which hurts the drone performance. Copper weight is made up of two components. One is the conductors in the slot which produce the motor torque and the other is the part of the copper in the end turns that interconnect these conductors. These end turns do not produce torque and only add heat to the winding. The winding design to minimize the end turn length is the only feasible way to lower the motor weight. The other factors such as iron flux density, magnet strength, voltage, and rotor speed are variables that affect each other and are tradeoffs in design. The mechanical parameters such as the OD, ID and Length are also variables that need to be investigated in the tradeoffs above because these are the parameters that will determine the torque to weight ratio. The OD and length will probably be set by the application so the ID of the magnetic circuit is the parameter the motor designer will need to investigate in designing a motor with a high torque to weight ratio.
We have seen that the low weight requirement of the drone motor and the low resistance requirement of the application act against each other so the motor design will be a trade off of a bunch of design options. Figure 3 shows that as the ratio of the ID to OD changes, the weight will change linearly but the resistance will be an exponential curve. This curve will change for different designs but will be similar. The resistance curve has a knee and above the knee the resistance will increase rapidly. The position of the knee will change its position depending on the motor design. There are several other design parameters that affect this knee position. There is no correct single motor design for all commercial drones and each application will have different variables to use in the design process. A lot of creative effort will be needed in checking the tradeoffs of the copper and iron volumes to minimize weight and resistance. The voltage from the batteries will be an important input parameter since that along with the rotor speed will determine the motor voltage constant and torque constant. The slope of the speed torque curve will be determined by the ratio of the resistance to the product of the voltage and torque constants. The total drone weight and load weights will determine the two operating points on the speed torque curve. All of the tradeoffs in designing these motors make this a very intriguing design project for the design engineer.
This is a session to be presented at MAGNETICS 2017.
