Power circuit for single phase drive - low speed control mode
At low speeds, motor voltage V should not have lower-order harmonics. An ideal would be a pure sinusoidal voltage but a compromise is acceptable.
The square wave voltage used in the high speed mode contains lower order harmonics of order 3,5,7,9...etc. So we con not use it for low speed operations. If attempted we will get some wobbling speed perturbations at low speeds.
We use switching strategy known as PWM (Pulse Width Modulation) to deliver near sinusoidal voltage for the motor. We have two operations of PWM.
(a). Bipolar PWM
(b). Unipolar PWM
In this PWM, we treate Q1 and Q2 as one pair and Q3, Q4 as the other pair. These pairs are operated in complementary way, so only one switching signal is necessary.
Bipolar switching signal
Switching signal is generated by a PWM modulator. This contains a high frequency triangular waves generator (carrier), a low frequency sinusoidal wave generator (reference signal) and a comparator.
Bipolar PWM modulator circuit
Ec - Carrier amplitude
Er - Reference amplitude
fc - Carrier frequency
fr- Reference frequency
Ec is constant and fixed.
Er is adjustable such that 0 <= Er <= Ec.
fc is fixed (but provisions exist to change it in steps).
fr is adjustable (fr will be frequency of output voltage V, so a range about 0 -200 Hz will be adequated).
Modulator has two important parameters
1. Depth of modulation m
2. Carrier ratio p
m = Er/Ec
p = fc/fr
Range of m is 0 to 1. Value of p should be an odd integer for best results and its values should be as large as possible. (A figure around 100 would be sufficient).
Vd if S is High (e.g. Vr > Vc)
V (Motor voltage)
-Vd if S is Low (e.g. Vr < Vc)
Fundamental components of V
The motor voltage V has a dominant fundamental components and very high order harmonics (which are less influencing the motor operation). Current wave form I produce by V is almost sinusoidal due to the filtering by motor induction L.
We can quote mathematical expressions for the amplitude of fundamental V, and the order of harmonics in V.