New Industrial Servo Drives Servopack YASKAWA Servo Drive 200-230V 400W SGDE-04VP

 

 

 

Before moving on to the matter of the air-gap, we should note that a question which is often asked is whether it is important for the coils to be wound tightly onto the magnetic circuit, and whether, if there is a
multi-layer winding, the outer turns are as eVective as the inner ones.
The answer, happily, is that the total MMF is determined solely by the number of turns and the current, and therefore every complete turn makes the same contribution to the total MMF, regardless of whether it happens to be tightly or loosely wound. Of course it does make sense for the coils to be wound as tightly as is practicable, since this not only minimises the resistance of the coil (and thereby reduces the heat loss)
but also makes it easier for the heat generated to be conducted away to the frame of the machine.

 

 

In motors, we intend to use the high Xux density to develop force on current-carrying conductors. We have now seen how to create a high Xux density inside the iron parts of a magnetic circuit, but, of course, it is
Iron Air-gap Coil
Leakage flux
Figure 1.7 Flux lines inside low-reluctance magnetic circuit with air-gap Electric Motors 11 physically impossible to put current-carrying conductors inside the iron.
We therefore arrange for an air-gap in the magnetic circuit, as shown in Figure 1.7. We will see shortly that the conductors on which the force is to be produced will be placed in this air-gap region.
If the air-gap is relatively small, as in motors, we Wnd that the Xux jumps across the air-gap as shown in Figure 1.7, with very little tendency to balloon out into the surrounding air. With most of the Xux lines going
straight across the air-gap, the Xux density in the gap region has the same high value as it does inside the iron.

In the majority of magnetic circuits consisting of iron parts and one or more air-gaps, the reluctance of the iron parts is very much less than the reluctance of the gaps. At Wrst sight this can seem surprising, since the distance across the gap is so much less than the rest of the path through the iron. The fact that the air-gap dominates the reluctance is simply a reXection of how poor air is as a magnetic medium, compared to iron.
To put the comparison in perspective, if we calculate the reluctances of two paths of equal length and cross-sectional area, one being in iron and the other in air, the reluctance of the air path will typically be 1000 times greater than the reluctance of the iron path. (The calculation of reluctance
will be discussed in Section 1.3.4.)
Returning to the analogy with the electric circuit, the role of the iron parts of the magnetic circuit can be likened to that of the copper wires in the electric circuit. Both oVer little opposition to Xow (so that a negligible fraction of the driving force (MMF or EMF) is wasted in conveying the Xow to where it is usefully exploited) and both can be shaped to guide the Xow to its destination. There is one important diVerence, however. In the electric circuit, no current will Xow until the circuit is completed, after which all the current is conWned inside the wires. With an iron magnetic circuit, some Xux can Xow (in the surrounding air) even before the iron is installed. And although most of the Xux will subsequently take the easy route through the iron, some will still leak into the air, as shown in Figure 1.7.
We will not pursue leakage Xux here, though it is sometimes important, as will be seen later.

 

 

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