Brushless exciterless field system for AC synchronous machines

A three phase AC motor which rotates at the synchronous speed determined by the number of poles of its main stator winding and the frequency of the AC input to it, which does not need any exciter machine or brushes to provide its DC field current, but has an internal, but independently adjustable excitation system to supply its field current. The internal excitation system consists of an auxiliary winding on the stator which is additional to the main power winding, another auxiliary winding on the rotor besides the main DC field winding, a diode rectifier circuit on the rotor from which the DC field current is supplied by rectification of the induced currents in the rotor auxiliary field. The field current adjustment can be made by adjustment of the input to the stator auxiliary winding. This can be done independently by eliminating any magnetic coupling between the main and auxiliary circuits. The elimination of the magnetic coupling between the main and auxiliary circuits is achieved by winding the auxiliary circuits for a suitably chosen pole number which is different from that of the main circuits. An optional damper winding will assist in starting when the machine is operated from a fixed frequency supply and ensure stability during steady state rotation. The machine is suitable for inverter fed adjustable speed drives and for fixed frequency operation. It can function in the motoring and the generating modes.

BACKGROUND OF THE INVENTION 
This invention relates to AC electric motors and generators of the 
synchronous type, specifically to the incorporation of the excitation 
system internally in the machine itself, eliminating the need for brushes 
and a separate exciter. 
A synchronous type of AC machine, whether it be a motor or a generator, has 
fixed magnetic poles on its rotor. These poles are typically excited by 
providing a DC current, called the field current or the exciting current, 
through the field windings which are located on the rotor. There are also 
synchronous machines constructed with permanent magnets on the rotor. 
These machines do not need field current and the field flux has a fixed 
magnitude which is not easily adjustable. This invention does not include 
such machines, but is applicable only to machines which are excited by a 
DC field current. 
The arrangement for providing the DC exciting current is called the 
excitation system. Typically the source of DC is another machine called 
the exciter, which is coupled to the synchronous machine and rotates with 
it. Earlier exciters were DC generators. Such machine plus exciter systems 
needed several brushes because the DC exciter also had to have brushes on 
its commutator. Later, totally brushless excitation systems became 
popular, which used an AC generator as the exciter. In such exciters the 
windings where AC is generated are located on the rotor. The generated AC 
is converted to DC by a diode rectifier circuit mounted on the rotor and 
fed directly to the field of the synchronous machine, thereby totally 
eliminating the need for any brushes or sliding contacts. The adjustment 
of the field current is made by adjustment of the field current of the 
exciter machine, whether it is an AC exciter or a DC exciter. Since the 
field of the exciter machine is on its stationary side in both types of 
exciters, the field current adjustment can be done from the stationary 
frame in both cases. 
BRIEF SUMMARY OF THE INVENTION 
In contrast to the above described schemes, the present invention, besides 
being a brushless scheme, totally eliminates the need for a separate 
exciter machine. The new scheme also enables the adjustment of the field 
current, and therefore the power factor of the synchronous machine, from 
the stationary frame. Basically it is an incorporation of the excitation 
system in the main synchronous machine itself in such a way that 
independent control of the field current is possible. 
An earlier patent granted to me (U.S. Pat. No. 5,012,148 of Apr. 30, 1991) 
also describes a brushless scheme for providing the DC field current of a 
synchronous machine without the use of brushes or an exciter machine. The 
new scheme has the following definite advantages over the earlier 
described schemes. 
The technique for providing the DC field current in the present invention 
is different from all the three described in the earlier patent referred 
to above (U.S. Pat. No. 5,012,148 of Apr. 30, 1991) and has the following 
advantages over those. 
(i) The power supply to the motor does not need to have more than one 
frequency component. Therefore, the motor can operate from the standard 
utility three phase supply, which is essentially a single frequency 
supply. For adjustable speed applications the motor may be used with an 
adjustable frequency inverter. But such an inverter does not have to 
provide a combination of frequencies in its output. 
(i) There is no unbalance in the stator circuit of the motor and it does 
not need any unbalance in the power supply. Therefore, since the standard 
utility power supply and conventional inverter outputs are essentially 
balanced ones, the motor can operate from all such power supplies. 
(iii) Normally, in three phase AC motors of the induction and synchronous 
types, the stator phase windings are designed in such a way as to minimize 
spatial harmonics in the mmf distribution. This statement is also 
applicable to the new invention. No special steps are necessary, as 
regards the spatial mmf distribution of the stator windings in this 
invention, different from the conventional machines. This is because 
spatial harmonics in the mmf distribution of the phase windings are not 
necessary to create the auxiliary rotating field responsible for the 
creation of the DC field current. 
The field current adjustment is totally independent of the main power 
circuit and is decoupled from it. This makes it easy to implement 
automatic closed loop adjustment of the field current and thereby the 
power factor, during the operation of the motor. This also makes it easier 
to implement vector control for achieving fast dynamic response in 
adjustable speed drive systems. 
Further objects and advantages are that the motor works on the induction 
principle and can be used for applications where an induction motor is 
usable, but with a considerably better power factor, and without any slip. 
Further it is usable as a synchronous motor or as an alternator without 
the need for a separate exciter. It can be used as a "brushless DC motor" 
in combination with a power inverter and appropriate timing circuit for 
commutation. Still further objects and advantages will become apparent 
from a consideration of the ensuing description and drawings.

REFERENCE NUMERALS IN DRAWINGS 
1. Main three phase stator winding 
2. Auxiliary stator winding 
3. Auxiliary rotor winding 
4. Diode bridge rectifier 
5. DC field winding 
6. Damper winding 
7. Field current regulating elements 
DETAILED DESCRIPTION OF THE INVENTION 
In accordance with the present invention the synchronous machine consists 
of a three phase main winding and a three phase auxiliary winding, both 
being on the stator, an auxiliary winding on the rotor, a diode bridge 
rectifier on the rotor, the DC field winding on the rotor and an optional 
short circuited damper winding on the rotor. 
A typical embodiment of the synchronous machine of the present invention is 
illustrated in FIG. 1. The machine has the main three phase winding 1 on 
the stator which is shown Y connected in the figure. Alternatively it may 
also be delta connected. The stator additionally has another winding 2 
designated as the auxiliary winding which is shown Y connected, but may 
alternatively be delta connected. Although both the main and the auxiliary 
windings are housed on the stator, the magnetic coupling between the two 
sets of windings is completely eliminated. This elimination of magnetic 
coupling is achieved by the appropriate choice of the pole numbers of 
these two sets of windings. In the machines that were built and tested to 
confirm the validity of this invention the pole number for the main 
winding was 4 and the pole number for the auxiliary winding was 2. For 
convenience we will use these pole numbers in the present description. But 
any combination of pole numbers which eliminates the magnetic coupling 
between the two sets of windings can be used in this invention. With the 
above stated choice of pole numbers, the zone of one pole, say a north 
pole of the two pole winding spans the combined zone of two poles (one 
north and one south) in the four pole structure. Therefore the resultant 
magnetic flux due to the four pole structure linking with the two pole 
coils will be zero. Conversely the resultant flux from the two pole 
structure linking with the four pole structure is also zero. Thus the main 
and auxiliary windings, although they use the same magnetic structure, 
ideally have zero magnetic coupling between them. 
The machine has on its rotor another auxiliary winding 3 which is wound for 
the same number of poles as the auxiliary winding on the stator, that is 2 
poles, in our illustrative description. For the reasons explained before, 
this winding will be magnetically coupled only to the auxiliary winding on 
the stator and not to any other winding, irrespective of whether the rotor 
is stationary or in relative motion to the stator. The auxiliary stator 
winding is the only winding that can induce emf's in this rotor auxiliary 
winding because it is the only circuit to which it has magnetic coupling. 
The rotor also carries a diode rectifier 4 which converts the AC induced in 
the rotor auxiliary circuit into DC. This DC serves as the field current 
of the synchronous machine and is fed to the main field winding 5. The 
main field winding 5 is wound for the same number of poles as the main 
stator winding. Also shown in FIG. 1 is the winding 6 which we will 
designate as the damper winding. This winding is short circuited on itself 
and serves as an induction starting circuit in the motoring mode and as a 
damper winding ensuring stability of rotation during synchronous running. 
It is optional and may not be provided when not required. In the figure it 
is shown as a three phase winding. Alternatively it may consist of short 
circuited coils with a coil pitch corresponding to the main winding, with 
or without interconnection between the coils. Therefore it will be 
inactive during rotation at the synchronous speed of the main winding. 
The DC field current is provided by the rectified output from the rotor 
auxiliary winding. The voltage induced in the rotor auxiliary winding is 
solely due to the currents in the stator auxiliary winding because it is 
the only winding to which it is magnetically coupled. Therefore the 
adjustment of the DC field current can be done independently by adjustment 
of the input to the stator auxiliary winding. 
FIG. 2 shows one way in which this adjustment may be done when the machine 
is typically working from a utility three phase power bus. A three phase 
adjustable circuit labeled 7 in FIG. 2 consisting of elements labeled Z is 
inserted in series with the input to the auxiliary stator circuit. This 
serves to adjust the input into the auxiliary stator circuit and thereby 
the induced voltage in the auxiliary rotor circuit which is the source for 
the DC field current in the rotor. For manual adjustment the adjustable 
circuit 7 may be a variable ratio transformer, or a three phase adjustable 
impedance. Alternatively the adjustment may be done electronically. For 
this the circuit block 7 may consist of three triacs or bi-directional 
thyristor pairs. In this the firing delay angle (phase control) may be the 
means of adjustment of the field current. Alternatively solid state AC 
switches may be used, configured from suitable semiconductor power devices 
such as power Mosfets, or bipolar power transistors or IGBT's or GTO's. 
Control can be implemented by the pulse width modulated switching of these 
switches. In this case adjustment of the switching duty cycle (pulse width 
modulation), may be the means of adjustment of the field current. 
The machine may be operated either from a fixed frequency bus such as the 
fixed frequency power utility three phase bus, or an adjustable frequency 
inverter such as in an adjustable speed drive system. When the main stator 
winding is energized from a three phase supply it creates a rotating field 
rotating at the synchronous speed determined by the input AC frequency and 
the pole number of the main winding. Since the damper winding is designed 
for the same pole number it functions like the rotor of an induction motor 
and helps to accelerate the rotor to near synchronous speed. When the 
speed is close to the synchronous speed the fixed magnetic poles of the 
main rotor winding lock with the rotating field and the rotor is drawn 
into synchronism with it. The rotor pulls into synchronous speed and after 
this its slip with respect to the main rotating field becomes zero and the 
damper windings become inactive and remains so during subsequent steady 
state operation. If the stator main field is being supplied from an 
adjustable frequency inverter, the initial frequency at starting is always 
very low. Therefore the DC excited fixed poles of the rotor can lock into 
synchronism with the main rotating field of the stator from the beginning 
itself. In this case the synchronization is achieved without the 
involvement of the damper winding 
The adjustment of the DC field current can be achieved by any of the 
methods mentioned earlier. Since the operating power factor is dependent 
on the field current, any of these methods can be used as a means of 
adjusting the power factor. It is also possible to use a separate AC 
source for the auxiliary winding--separate from the main source. In such a 
case the adjustment of the field current can be implemented by the 
adjustment of the auxiliary source itself including its frequency.