We will first examine small asynchronous (squirrel-cage) motor as an AC generator with direct connection to alternating current grid.
Test setup connections.
Picture of the test setup.
Video taken during
Purpose of this test was to examine the behaviour of a small fractional horsepower squirrel-cage motor when used as a generator of alternating current, connected to AC electric grid. It was found that AC generation is possible even with small and inefficient 0.05 horsepower motor used. Motor was connected to the electrical grid in this experiment, magnetizing current (reactive power) being provided from the grid.
DC motor was used to drive the AC asynchronous (squirrel-cage/induction) motor.
When speed above 1800 RPM was reached (4 pole motor, 60 Hz), AC motor was connected to the AC grid by pushing the plug into the outlet. AC motor then started acting as a generator, delivering useful power to load. Its speed would immediately drop to little above 1800 RPM synchronous speed, and remained stable thereafter. Direct connection was safely tested up to 4500 RPM. System speed before AC grid was connected was entirely dependent on DC motor running freely “open-loop”, rotating only AC motor rotor and its own armature (rotor).
Table on this page indicates that speed
of the system varies very little for a great increase in driving
power required. Above 1800 RPM synchronous speed active power delivered
(and taken from prime mover) increases very quickly.
This is excellent for wind power
generation, since a
double increase in wind speed produces eight times more power.
Asynchronous generator would still be able to accomodate this wind
speed change, while maintaining nearly constant shaft RPM and output
12 V, 8 A (full load current estimate)
AC motor running as
P=0.05 HP (37.3 W)
Rated speed=1725 RPM
Calculated values for AC
Current lag angle=78 degrees
Required capacitance for Q=180 VAR is 33.2 microfarads
It was shown that even very small and inefficient squirrel-cage motor can work as an alternating current generator. Since AC motors are generally inexpensive and more reliable than DC motors (no brushes), they can be used for remote site electricity generation.
A control system with line disconnect when speed drops bellow synchronous is necessary in this system, to prevent asynchronous generator to operate as a motor.
Best use for squirrel-cage generator would be in micro-hydro electricity generation, where driving speed changes are limited. It is used as such in many installations around the world. Most of those are connected to the grid, and hence require no capacitors for magnetizing current (reactive power), since reactive power is taken from the AC grid.
For wind turbines, it is more difficult to get the system to work majority of the time. However, many commercial wind turbines use induction motors as generators, and special control systems to keep the speed nearly constant.
Electrical Machines, Drives, and Power Systems, 4th edition, Theodore Wildi, Prentice Hall, ISBN 0-13-082460-7, pages 312 to 314.
Next: Small induction motor
running as a standalone generator with reactive power being provided by
Back to homepage