This
project attempts a new speed control technique for the single-phase a.c.
induction motor. It presents a design of a low-cost; high-efficiency drive
capable of supplying a single-phase a.c. induction motor with a PWM modulated
sinusoidal voltage. The circuit operation is controlled by an 8051 family microcontroller.
The device is aimed at substituting the commonly used triac phase angle control
drives.
The
circuit is capable of supplying a single-phase a.c. induction motor (or general
a.c. inductive/resistive load) with varying a.c. voltage. The same as in triac
control, the voltage applied to the load can be varied from zero to maximum
value. On the other hand, it uses a pulse width modulation technique (PWM), and
when compared with the phase angle control used for triacs, produces much lower
high order harmonics.
Thus,
it suits EMC/EMI regulations much better. Because the circuit is aimed at
low-cost, low/medium-power applications, it does not use a conventional
converter topology to produce the output voltage waveform. It directly
modulates the mains a.c. voltage. Compared with costly converter, it requires a
lower number of active and passive power components. In summary, the device
attempted here takes advantage of both the low price of the phase angle control
and the low harmonic content and high efficiency that we can get with standard
converter topology.
The
drive uses a PWM controlled MOSFET and the load in series with a bridge
rectifier. This drive based on this new control technique is targeted for use
in consumer and industrial products: washing machine, dishwashers, ventilators,
compressors, and wherever the system cost is a consideration.
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