Miniature Disc Magnet Motors for Textile Applications
In the 80’s, Portescap was mainly involved in the watch industry providing small stepper motors to be used in watches and clocks. Using rare earth magnets, we then designed the first micro motor with a multipolar structure. Such a design avoids additional pole shoes on the rotor to create fewer steps per revolution. Later, Portescap expanded its expertise to include bigger motors for industrial applications [< 100 W].
We were quick to realize the benefits of multipolar technology that included very low inertia, a short magnetic circuit that allows for low iron losses and saturation, and no magnetic coupling between phases. While this technology was initially applied only to stepper motors, the next step was to redesign this motor to be used not as a stepper, but as a servo motor. The number of pole pairs was optimized to deliver higher torque and acceleration, with the ability to reach higher speeds.
A multipolar design allows for a higher torque density. Similarly, low inertia combined with high torque density allows for a higher acceleration. Usually multipolar structure does not reach high speeds due to number of commutations per revolution and the electrical time constant, both slowing down rising current in the coil. Moreover, frequency of commutation also increases iron losses due to hysteresis and eddy current.
Disc magnet motors can easily be designed with different number of poles according to the application. For applications that require high torque and lower speed, a higher number of pole pairs are recommended; whereas for an application that requires medium torque and speed, the number of pole pairs will be optimized. In the 90’s, Portescap identified some applications that require high acceleration with lower mass or inertia, such as pick and place assembly, dye bonding, winding machines, motors for textile yarn guides, etc. The following section illustrates an example of pole pair optimization for a textile application.
Disc Magnet Motors for a Textile Yarn Guide
A big challenge in textile applications has been to replace the mechanical cam system with an electrical actuator. A mechanical cam system has greater bandwidth but poor flexibility, for instances such as adjusting stroke length. Textile application was a perfect one for our kind of technology since the load inertia stays very small.
A typical requirement of a mechanical thread guide is that usually the thread has to be guided by a slot in the barrel (i.e. cam). An option could be to drive the thread with a belt and a motor, or with a lead screw.
Thread Guide with Slot in Barrel Thread Guide with Belt and Motor
If the typical displacement of thread is 0.20 m, linear speed is 6 m/s, typical thread guide weighs 4g and the distance to decelerate and accelerate is as small as possible, let us assume 5 mm. One of the criteria for selecting the actuator is the required mechanical power. Let us assume that, xo is displacement, xa is distance to decelerate (or accelerate), áº‹ is linear speed, áº is linear acceleration, ta is time to accelerate and M is weight of thread.
Then, ẍ = (ẋmax/ta) = (ẋ/(2.da/ẋmax)) = (ẋmax^2)/(2.da) & force to move thread = M.ẍ
Maximum power needed will be = M.ẍ.ẋmax=M.ẋmax^2/ta. This power, divided by the time to reach it, is the power rate given as, dPmech/dt = M.ẋmax^2/ta^2 = M.ẍ^2
Thus in short, we need a motor that is able to deliver peak mechanical power in a very short time. Subsequently, the power needed to run the motor at constant speeds will be much lesser.
In the above example, after calculations we find Pmech = 86W, power rate = 50kW/s, time to accelerate = 1.7ms, time to make one stroke = 33 ms, time to decelerate = 1.7ms. The motor is thus “boosted” during 1/10th of the time. Assuming there is no saturation of magnetic circuit, the power delivered in boosted mode could be 10 times the rated power. Since this power is a function of square of current, and torque a function of current, this boosted torque could be around 3 times the rated torque. Thus, we know the level of performance expected from our motor. Now let us find out the best motor.
A motor is characterized by few parameters like boosted Torque (Tb) and Inertia (J). We should calculate the optimum radius of the pulley R. With a smaller pulley the motor has to deliver lower torque but at a higher speed and with a big pulley, the torque is bigger and the speed lower. The optimum pulley will be one that delivers for minimum required power. The calculation can show us that optimum pulley radius will be one where the load inertia matches the motor inertia. Intuitively, it’s easy to understand that we would want 50% of the full power delivered by the motor to be used in moving the load, and the other 50% to be used in moving the motor.
In our case we will match the inertia if M.R^2= J (neglecting pulley inertia). For our example, we then need a motor able to deliver 172 W with a power rate for the system of 103 Kw/s, which means that power rate for the motor alone should be 206 kW/s.
Consider a mechanical system that requires a peak speed between 3 and 4 Krpm. The motor torque should be 0.5 Nm up to 350 rad/sec and motor inertia should be around 13E-7 kg.m^2.
In short we are looking for a motor technology able to get a power rate > 206 kW/s and speed up to between 3 to 4 Krpm.
Let us now compare a few motor technologies:-
The Disc Magnet Technology is by far the best for such an application. As described above, for this specific application we decided to use the disc magnet motor technology which allows for higher torque density and acceleration. The number of pole pairs has been optimized; therefore this motor has 12 pole pairs. Thanks to this motor technology, users now have much more flexibility in replacing an existing mechanical system. For instance, our customer is now able to go from -3500 rpm to +3500 rpm in 5 ms.
Portescap has developed a few motor technologies including DC ironless motor, BLDC slotted and slotless motor, stepper type permanent magnet motors and hybrid, disc magnet motors. Our application engineers with our experts typically help customers in looking for the best system and motors for their needs. Textile is a good example of a design optimization for a specific application requirement. Portescap’s P532 EN and P760 EN motors are typically used in textile applications.