Download here

Activity: Read about the control of the BLDC motors and note the diagrammatic drawing of
the synchronous machines with the main directions and coil markings; the purpose of the fits;
the reasons for pulsating moment formation; the differences between BLDC and synchronous
motors; and in connection with magnet arrangements, the surface-mounted bread loaf shaped,
ring-shaped and buried arrangements, as well as types with external rotors.
1. The principle, regulation and features of permanent magnet synchronous motors
controlled by a current vector in vehicle drives.
1.1 Essential properties of permanent magnet synchronous motors
Essential properties of classical synchronous motors, as have learnt from the previous
chapters, are as follows:
-inflexible, maintaining speed;
-not self-starting, therefore completely unsuitable for vehicle drives (but);
-this electrical machine has the highest power density, with its cos φ=1.
Making synchronous machines and synchronous motors depicted in previous chapters capable
of vehicle driving has been an attractive challenge for a long time. This is due to the best
indicators of the synchronous machines that are related to volume utilisation.
The rigid assignment to the frequency was the most important hindrance, but this hindrance
could be eliminated after the appearance of the inverters.
The ability to construct it with permanent magnets can provide the same torque in a decreased
machine size and also the pole wheel. In addition the DC excitation system can be omitted.
The special change in the principle enables synchronous motors, known as electric motors
that have the most inflexible nature, to operate dynamically with rapidly changing velocity
and with a positive sign thus, having the ability to be built into servo drives or in vehicle
drives. High speed operation of the latter one might lead to unwanted excess voltages due to
the permanent magnets, which is an additional challenge for this construction.
The self-controlled and computer aided synchronous motor controlled with a current vector is
a special self-guiding system (see later), with properties free from the above-mentioned
disadvantages:
• appeared in electric locomotives (P>2000kW): 1974
• in robot and industrial servo drives (P<1 kW): 1986
• in cars (P>30kW) : 1997
1.1 Construction. Decreasing pulsating moment Material and arrangement of the
magnets
Vehicles with electric drives and servo drives run with synchronous motors excited by
permanent magnets and the general arrangement of such motors is shown in figure 1. Stator
coils a, b and c are shown with simplified graphical symbols, which occupy the stator bands,
with a 1/6 width of phases and current directions. The symmetrical axis of the stator coil "a"
is, as usual, the vertical centre-line of the stator.
The synchronous motor in the figure has 2 poles and the rotor made with permanent magnets
indicates a North-South direction as well as an axis d of the rotor and this is also the direction
of Φ main flux.
Figure 1. A simplified sketch of the synchronous motor.
The motor torque is produced by the magnetic interaction between the induction field created
by the permanent magnet rotor and the current flow in the stator coil. The circumferential
value of the torque depends on the product of the actual values of the two aforementioned
factors, that is, assuming a quasi-stationery state, the torque will have a constant value in the
function of the angular displacement if these factors fit to each other in such a way that their
product can be constant. On the basis of this, supply provided with normally sinusoidal
voltage will require such an induction distribution and fit along the circumference.
If this condition fails to occur pulsating torques are generated and the rate of such torques
may show considerable differences. Appearance of such torques lead to disturbances in the
position control of the servo drives, therefore the circumferential torque curve of the motors
used there can only include alternating constituents below 1%. In vehicle drives the
appearance of pulsation is disturbing, especially during start-up, and pulsation is not desired
for higher speeds due to its effect of generating vibration in the masses, which may cause a
surplus load and fatigue in the structure.
The non-sinusoidal and square-wave supply can be realised at the lowest cost. It needs a
matching and simple implementable magnet arrangement that results in an induction
distribution with a similar shape and this can mitigate the torque pulsation. These motors are
the so-called "brushless direct current" (BLDC) electrical machines, which are, from the
aspect of their properties, designed mainly according to the synchronous motors. Their
control and supply are simpler and cheaper therefore a lot of motors like them are used in
motorized bicycles, scooters and models.
On the other hand, pulsating torque may be caused by other reasons, namely, when a magnet
arcs passes in front of the stator teeth, or more precisely, when such arcs osculate the edges of
the teeth, this causes rapid changes in the induction distribution. The rate of this depends on
the air-gap induction, the size of the air-gap, the shape of the teeth, and the magnet edge shape
at the pole changes, taking into consideration one edge-pair of each. If the ratio of the slot
number and the pole pairs is an integer, which is an advantageous construction from other
aspects, pulses will appear simultaneously and, considering nominal torque, the lump sum
may range between 1 and 20 %.
Reasons for appearance of pulsating torque may be eliminated or may be made quasi
undetectable by means of sinusoidal supply of synchronous motors, through the arrangement
induction distribution aimed at this, and through the avoidance of osculation of said edges by
using buried magnets. However, R&D and partly additional manufacturing activities and costs
would be needed for this.
In synchronous machines phase voltage and flow of the stator are sinusoidal in an ideal case.
The main flux is generated by the rotor permanent magnets, producing a sinusoidal
distribution for the air-gap induction in the air gap, in an ideal case, figure 2 b), or in the case
of BLDC type motors, this will be a square-wave type, figure 2 a).
Figure 2 Circumferential induction distribution in permanent magnet synchronous motors
The following figure (figure 3) shows a potential realisation of the sinusoidal induction
distribution:
Figure 3 Curves of induction distribution in permanent magnet synchronous motors
There are several construction modes for the arrangement and fitting of the magnets:
- fitted in arc-shaped arrangement onto the rotor surface and "bread loaf" shaped,
- buried arrangement, consisting of several lineal or arc-shaped sections (figure 4).
Figure 4 Different kinds of permanent magnet arrangements in permanent magnet
synchronous motors
The following figure, Figure 5 represents a potential realisation of the constructions shown in
the diagrammatic figures for poles consisting of two magnet parts. On the right, a magnet
arrangement entitled "buried" is shown:
Figure 5 Buried type magnet arrangement.
Versions with external rotors and reversed arrangement use a surface-mounted arc-shaped
arrangement, which are mainly used in motorized bicycles and scooters. Figure 6 shows 1/4
of the arc of a motor with an external rotor.
Figure 6 A version with and external rotor that is used mainly in scooters. The arrows sign the
direction of the winding by a specified layout.
1.3 About magnet materials used
Magnets are mainly fabricated from rare earth metals over the last decades. The application of
Nd-FB (neodymium-iron-boron) based magnets has gained ground over the last 10 years.
Their coercivity (H rc) is 3 or 4 times of that of the rare earth metal based magnets and their
remanent induction (Br) is higher by 20-30 %.
Today approx. 30 different versions (Table 1) are available for this magnet. The appropriate
type can be selected according to the Br value (from 1,05 to 1,35 Vs/m 2) and partly according
to the planned operating temperature. For this new type of magnet material it is an unusually
important aspect to choose and keep the appropriate operating temperature, since its remanent
induction shows a highly negative temperature-dependence. The value of such dependence
varies but it may exceed -1 %/ oC. A negative phenomenon of this nature in the case of an
operation at a temperature above the planned value is the reduction of the Br induction and
will entail the reduction of the flux value at the same rate, and due to this reason, the torque at
the same motor current will also decrease by similar rate.
Although a current increase applied to compensate this phenomenon resets the value of the
torque, the efficiency will decrease due to the I 2R ohm increase in the coil loss. Torque
resetting through increasing the current can result in only a few percentages of increase in the
current intensity, taking also into consideration the surplus load of the inverter any further
decrease in the induction above this value will entail torque loss, which can only be reinstated
after cooling-down.
The final reduction and loss of the remanent induction occurs when reaching the Curie-point,
260-300 oC, however, the original value can be reset after cooling down by magnetisation.
For the appropriate engineering the particular Nd-FB types of the magnet materials are
indicated by the manufacturers on the bases of the matching values of Br and the planned
operating temperature. Column 1 shows the magnet type:
Table 1 The properties of the magnet materials