The first motors produced by MAVILOR were no-cogging motors. Because of this feature they were commercially called "ironless motors". This term was not exact however, what it was meant was "slotless motors". Nevertheless, the absence of cogging - an obvious characteristic that can be easily noted when you turn the shaft of the motor with two fingers- is not the main feature. The main characteristics of these motors were probably the commutation. It gave the brushes a longer life than that of the mechanical parts.

After commutator motors, MAVILOR produced brushless motors, with slots in the stator, which were deeply studied to achieve a positioning between the slots and the magnets that gives lower cogging. However, the most important feature in case of an absence of slots, is high speed and near-to-zero ripple torque.

Other features: High speed

In the modern use of the motor there is a tendency to achieve high operating speed in order to increase power or to reduce weight. Values of 20,000 rpm were unthinkable only a few years ago. Now customers request them more and more often. The relative position of the slots respect to the magnets, when the motor runs, produces a modulation of the magnetic flux. Higher is the speed, higher is this modulation, which makes the eddy currents worse. These eddy currents can be improved using a higher iron lamination quality or reducing this thickness. In magnets, however, as they are conductive, the higher the speed is, the greater the losses are. The temperature inside the motor is drastically limited by the characteristics of the magnets. In high-speed motors, which at equal power are smaller than those that work at low speed, the surface for irradiating heat is smaller and they are therefore more sensitive to internal losses that unfortunately increase with speed.

In order to solve these problems MAVILOR is beginning to manufacture electric slotless servomotors. Like the brushless motors, there are two types: long motors with axial development (MLL) and flat motors with radial development (ML). The former is obviously more suitable for high speed and the latter for those with high torque.

Other features: minimum ripple torque

Working without slots implies not only no-cogging, which is not the most important quality, but also it implies a reduction of the ripple torque to its minimum value.

Let's remember the definition of Kt. Kt is the relationship between the torque and the current which is needed for obtaining this. This relationship, which is defined as constant, is not actually so. It varies at each point with the position of the rotor with respect to the stator. Higher and lower levels are found around the theoretical value (the Kt given in catalogues). This variation is called ripple torque. What is it caused by? It is caused by a non-appropriate position of the iron, magnets and wires in a turn of the rotor. If the slots are eliminated from a motor, the irregularity in the iron disappears. Moreover, the magnets are regularly placed as it is no longer needed to move them from their theoretical position to reduce cogging (fig. A) and the wires are no longer constricted in the slots and can give a more regular response to the stator magnetic flux.

Therefore the ripple torque is notably reduced in a motor more suitable for servomechanism.

Slotless/Classical brushless comparison.

Elimination of the slots leads to placing the conductors in the air gap (picture enclosed). This reduces the ripple torque and eddy current losses, however, it increases the air gap itself and therefore also the height of the magnets. This is apparently negative but in fact the consequences are favorable: we are referring to an MLL motor, see the figures enclosed where there are two motors with the same external diameter in A) the motor with slots and in B) the slotless motors.

 

The difference between the Ra and Rb radius is obvious.

Since the torque depends on the radius it is clear that with motor B) there is a great increase in the torque. Not only this, but also heat evacuation is better in B) because the wires are closer to the armature.

The reasons why MAVILOR is producing these motors are evident.

For more information visit: www.automotioninc.com