Mechatronic systems – El. machines – AC synchronous motors
SYNCHRONOUS MOTORS
Principle of motor
The stator is essentially the same as for the asynchronous motor. It is equipped with AC,
usually three-phase windings. The rotor assembly of poles, excited by direct current, with
smaller motors are used in permanent magnets.
If you are bringing into the three-phase stator windings, there is a rotating magnetic field (see
Sec. Of asynchronous motors), which is able to abduct him round magnet excited rotor. Thus
operating a synchronous motor, which, after spin rotates together with the rotating magnetic
field of the exact synchronous speed. It does not change even under load.
The rotor of the synchronous machine is thus rotated together with the rotating magnetic field
of the stator.
Because of this direct dependence of frequency and speed are used the name of a synchronous
machine.
Arrangement of synchronous machines
Depending on the design of the excitation circuit we distinguish synchronous machines
1. The first permanent magnet on the rotor
2. Second coil excitation circuit - excitation winding on the rotor (wound rotor)
3.
Reluctance Motors
According to the arrangement of the rotor windings there are 2 types of synchronous
machines with a wound rotor
a) with the salient (expressed) poles - a rotor consists of a rotor wheel, on which is mounted a
number of poles (two or more) and each pole has a field coil.
b) with a smooth rotor - rotor forms a solid cylinder having longitudinal grooves on the
surface and in these are located concentric exciter coil supplied with direct current so that the
rotor is magnetized.
Alternators and synchronous motors are usually three-phase. Single-phase motors are used
only rarely, in some special cases.
Fig. 1. Design of synchronous machinee with a wound rotor
a) with the salient poles b) with a smooth rotor
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Mechatronic systems – El. machines – AC synchronous motors
There are four poles on the magnet rotor wheel (Fig. 1. a), the field winding is powered
with direct current from a separate source, either dynamo, or more recently, from
semiconductor converters, which gives alternating north and south pole. At the stator slots similar to an induction machine - is stored three-phase winding. Excitation current to the
excitation coils is supplied through two rings and brushes that they abut.
Types of permanent magnet synchronnous machines
The ideal waveform of induced voltage is sinusoidal. In practice, achievable only quasisinusoidal. This is achieved by suitably distributed stator winding pole pitch and reduced PM.
This reduces torque ripple to a minimum in the sinusoidal phase currents. To start request by
direct connection to the network rotor can be equipped with a squirrel cage rotor, which also
serves as a damping winding.
a)
b)
Fig. 2. Two basic types of permanent magnets imposition
Motors with sinusoidally distributed PM on the surface of the rotor (fig. 2a)
This type of machine (SMPMSM), which is widely used, is shown in Fig. 2a. The stator has a
three-phase sinusoidally (ideally) distributed winding that generates a rotating magnetic field
in the air gap. PM are glued to the surface of the rotor and generates a radial air gap magnetic
flux. The rotor has an iron core, which may consist of one piece or is composed of metal
metal plates. When the rotor machine rotates, three-phase sinusoidal voltage is generated.
Since the relative permeability of PM is approximately equal to one (1.3 < µr <1.1), and the
PM is deposited on the rotor, the air gap is relatively large and the machine appears as virtual
machine unexpressed poles (Ld = Lq). There are constructions, which are partially recessed
into the PM rotor, thereby improving the mechanical robustness of the machine. Editing also
cause the machine exhibits certain geometrical asymmetry (Ld ≠ Lq).
Motors with sinusoidally distributed PM inside the rotor (fig. 2b)
Unlike the previous case, these machines PM deposited inside the rotor in fig. 2b. However,
there are a number of variants, such as the PM rotor in place. A typical configuration is
precisely in fig. 2b. These machines usually have three-phase sinusoidally distributed winding
on the stator side and storing method of the PM machine, gives the following features:
(a) machines are more robust, allowing you to achieve higher speeds
(b) air gap in the axis [d] is greater than the axis [q], which causes the machine expressed
poles, and thus Ld < Lq
(c) the air gap is much smaller, which increases the armature reaction
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Mechatronic systems – El. machines – AC synchronous motors
TORQUE EQUATION OF SYNCHRONOUS MOTOR
In synchronous motors it occurs to increasing of the so-called load angle θ L (between stator
and rotor) with increasing of load according to the relationship
Me =
U U if

U2
p 
sin θ L+
( X d -X q ) sin 2θ L 
2 Xd Xq
ω s  X d

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where
U = U m / 2 … RMS phase value of stator supply voltage
p … number of pole pairs
reactance in the longitudinal and transverse directions X d = ω s Ld
X q = ω s Lq
ωs = 2π fs … electric synchronous speed
(mechanical rotor speed ωm = ωs /p )
Uif ...RMS phase value of induced voltage, it is proportional to the electromotoric forse EMF
(excitation current if,, respectively magnetic flux Ψf at motor with permanent magnets)
and the speed U if = ω sΨ fef
The equation for the torque machines can be understood that with increasing of motor load
(load torque) the load angle θ L (between stator and rotor) is increasing too.
Torque of the synchronous machine is generally composed of two components:
• Synchronous - made up of excitation of the rotor (the first member of the equation)
• Reactive - formed by changing the reluctance (magnetic conductivity) in longitudinal and
transversal direction of the rotor. Also arises at motor without excitation (the second member
of the equation)
Reactive component is thus formed e.g. by motor with salient poles.
This component is also used for special motors which have no excitation to the rotor (or
permanent magnets). The rotor is then formed by the teeth of magnetic material. These
machines are called reluctance. Principle view is in Fig. 3a). In Fig. 3b), then the actual
implementation of the rotor so-called axially laminated synchronous reluctance motor.
This principle is also for stepping motors.
Summary:
• for wound motor with excitation coils or permanent magnets Uif > 0
• for motor with salient poles Xd ≠ Xq
• for motor with smooth rotor Xd = Xq
• for reluctance motor Uif = 0 a Xd ≠ Xq
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Mechatronic systems – El. machines – AC synchronous motors
a) stator + rotor
b) rotor
Fig. 3. Synchronous reluctance motor
MECHANICAL CHARACTERISTIC OF SYNCHRONOUS MOTOR
The motor rotates at synchronous speed (i.e. no load speed) independently of load torque
 f 
60 f1
n0 =
= n0 n  1 
where f1 je supply frequency and p number of pole pairs
p
 f1n 
n
n0
M
0
Mmax
Fig. 4. Mechanical characteristic of synchronous motor
From the above equation the possibilities follow for speed control of the synchronous motor the only by supply frequency f1 (for a motor having the number of poles 2p).
Until recently, this method but alluded to the absence of available frequency converters.
However, this - alike as induction motors - with the development of power and control
electronics in recent years has changed and today they are commercially relatively cheap
transistor frequency converter.
For speed control of synchronous motors, however, must be a power supply (inverter)
provided with the corresponding control loops, as a synchronous motor, unlike the induction
motor is unable to start itself after connection of constant frequency voltage.
From the mechanical characteristics follows that the during loading the motor speed does not
change. But it increases - as already mentioned - the load angle θ L (between stator and rotor),
and after the excess over a certain limit - this corresponds Mmax in Fig. 4 - the motor will stop,
at this motor draws high short circuit current (it is a fault state).
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