Two-stage polyphase generator is usable as induction motor operable over range of speeds while powered from constant frequency source. It requires neither slip rings nor special adjustable-frequency power supplies or external reactive sources

Riaz, M.

English

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NASA TECH BRIEF 
.
October 1970	 Brief 70-10478 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issuedto encourage commercial 
application. Tech Briefs are available on a subscription basis from the National Technical Information Service, Springfield, 
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nology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 
Induction Generator Produces Constant-Frequency
Voltage from Variable-Speed Drive 
14	 Frequency	 06
	 2	 P4	 Stage 1 —4 
I	 Converter	 I	 Rotors	 .1 
 D. C.	 I 
Output 
Inverter	 Rectifier	 I
Shaft 
I
 W
DC.	 Stators Sou rce
Controls 
Load 
or 
Supply 
A two-stage, polyphase induction generator has 
been designed which will produce constant-frequency 
ac from a variable-speed mechanical drive. It can also 
be used as an induction motor which will operate over 
a range of speeds while powered from a constant-fre-
quency source. Major advantages of this device over 
previous designs are that it requires neither slip rings 
to connect external circuitry to the rotor windings nor 
special adjustable-frequency power supplies or 
external reactive sources, and that, as a motor, it oper-
ates over a wide speed range with a reasonably high 
power factor. 
The device is composed of two standard polyphase 
(usually three-phase), wound-rotor induction genera-
tors mounted on a common shaft. The windings of the 
two rotors are connected in reversed phase sequence, 
and the two sets of stator windings are connected 
in parallel through a dc-coupled electronic frequency 
converter. A schematic diagram of the device is shown 
in the figure, with the two generators labeled Stage I 
(continued overleaf) 
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	 information contained in this document, or warrants thàtsuch use 
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and Stage 2. Normally, the common shaft is driven by 
a variable-speed prime mover (not shown). The rectifier 
circuit converts the variable-frequency output of 
Stage 2 into dc, which is reconverted to constant-
frequency ac-by the inverter circuit. The dc source is 
connected temporarily to provide starting excitation 
to Stage 1. 
The constant frequency ac is supplied to the stator 
of Stage 1 and to the output load. Because of the 
reversed-phase connection of the two rotors, the net 
effect in operation is that Stage 1 acts as an induction 
generator driven above synchronous speed, while 
Stage 2 is driven below synchronous speed. After 
frequency conversion, the power generated by Stage 2 
is added to that of Stage 1 to supply the output load 
with constant frequency ac regardless of the speed of 
the prime mover. 
In operation as a motor, Stage 1 is fed with constant 
frequency ac from the supply. Slip-frequency power 
generated in Stage 1 supplies the rotor losses and feeds 
Stage 2, which thus operates as a second induction 
motor supplied through its rotor. Power induced in 
the stator of Stage 2 is fed through the frequency con-
verter and returned to the power supply. 
The power level produced in the generator mode, or 
the output shaft speed in the motor mode, is controlled 
by adjusting the voltage ratio of the frequency convert-
er, thereby controlling the amount of power coupled 
between the stages. 
Reference: 
Riaz, Mahmoud: Energy Conversion Properties of 
Induction Machines In Variable-Speed Constant-Fre-
quency Generating Systems. AIEE Transactions, vol. 
78, Part II, 1959, pp. 25-30. 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
Headquarters 
National Aeronautics 
and Space Administration 
Washington, D.C. 20546 
Reference: TSP70- 10478 
Patent status: 
This invention is owned by NASA and a patent ap-

plication has been filed. Royalty-free, nonexclusive 
licenses for its commercial use will be granted by

NASA. Inquiries concerning license rights should be
made to NASA, Code GP, Washington, D.C. 20546. 
Source: Mahmoud Riaz
Electronics Research Center

(ERC- 10065)
. 
Brief 70-10478	 Category 02


Induction generator produces constant-frequency voltage from variable-speed drive

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