PECM Issue 53 2021 | Page 72

Tailoring the motor to the application through appropriate coil selection

GEARS , DRIVES & MOTORS THE MOST EFFICIENT SOLUTION

PORTESCAP
Tailoring the motor to the application through appropriate coil selection
During the selection of a miniature DC motor , the design engineer will of course consider the dimensions needed to fit the desired space as well as the working point ( the torque and speed at which the motor is required to operate ). This information will be readily available in most motor product catalogues . Alongside this information there will also often be a choice of coils .
Valentin Raschke , application engineer at Portescap , examines how choice of coil impacts on the motor specification , making it possible to adapt the motor to the available power source to create the most efficient solution for a given application .
Regardless of its construction , a DC motor is always performing the same job – converting electrical energy into mechanical energy . For any given application , there might be a whole range of motors available that could potentially do the job , with the designer making the eventual selection based on parameters such as size , weight , efficiency , torque & speed requirements , lifespan and cost .
Taking those factors into consideration might lead the designer to a single , best choice of motor . But what is not always appreciated is that there might be the opportunity to tweak motor design – and therefore boost efficiency – by selection of the most appropriate coil .
So how does the coil impact on our design considerations ? The torque produced by the motor will be determined by the current consumed and the torque constant . The torque constant , in turn , is defined by the radius of the coil , the magnetic flux density , the length of the motor and the number of turns of the coil . The first three of these parameters are fixed by the chosen motor and its diameter . Adjusting the number of turns of the coil , though , can impact the torque constant for a given size of motor .
This is important because , for a given motor and neglecting friction , the no-load speed of the motor is defined by the available supply voltage and the torque constant of the coil . If we look at a motor manufacturer ’ s catalogue for a suitable product to meet a specific operating point and power , we might find a choice of several coils that meet our needs , but with different supply voltages and different current requirements . The coil is therefore chosen to adapt the motor to the available power supply .
Choice of coil also impacts on the motor efficiency in the application . Look for example at ironless DC motors : these are renowned , among other things , for their high efficiency . Where there are losses , aside from friction , it tends to be heat generated by the current running through the copper wile of the coil . These losses are proportional to the coil resistance multiplied by the square of the current . Motor efficiency is higher at lower torque due to the lower motor current , so here again choice of coil can have a big impact .
MAKING THE CHOICE
If we were to consider an application requiring continuous operation at a fixed speed and torque – perhaps something like a medical infusion pump – then we might find a number of coils in the motor manufacturer ’ s catalogue that would seem to meet our needs at the required working point and power .
If the available power supply for our application is a voltage source , supplying a fixed voltage , that will narrow down the choice of coils further , and the designer can select the one that is the best fit to optimise the application . With a voltage source , it is typical for a reduced number of options of coils , and indeed , there may only be one option for a given application .
The number of options tends to be greater if a current source – a given maximum current across a range of supply voltages – is available instead of a voltage source to achieve the same working point . Now we might have a higher number of possible coils to meet the application requirement , and the design engineer can really begin to tailor the application .
If total efficiency is the prime consideration , that might lead us in one direction . In other applications , the coil with the lowest current consumption might provide the best choice , as it will result in a longer lifetime of the commutation system and increase the number of cycles with a single battery charge for battery-powered applications .
CUSTOM MANUFACTURED COILS
The availability of a number of standard coils can take us a long way towards tailoring the motor to the application , but even so there may be instances where the design engineer needs something even more optimised . In a critical application where cost pressures are less of a consideration than ultimate performance , and where the design engineer doesn ’ t have the freedom to adapt the power supply for a standard coil , then a custom designed coil could provide the best solution .
We can see , then , that the choice of coil based on the required motor working point and the power supply has a big impact on the optimisation of the application . For a given task , there may be different standard coils which can achieve the same working point with different voltage and current requirements , with each having different applications for efficiency , current consumption and lifespan . Or it may be desirable to ask the motor supplier to manufacture a custom coil if the application warrants it .
But the coil considerations also emphasise the advantages of engaging with a with knowledgeable motor manufacturer such as Portescap at the earliest stages of a design , before critical parameters are set in stone , and when there is the greatest opportunity to truly tailor the motor to the needs of the application .
For more information , visit www . portescap . com
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