Page 117 - Servo Motors and Industrial Control Theory -

6.4 Frequency Converter 111

Vm
TH1 on TH4 on V 4
ωt
0 180° 360°
Vsn
TH3 TH3 on TH6 on
on ωt
0 120° 300°
Vtn
TH2 on TH5 on TH2 on
ωt
0 60° 240°

Vrs
-Vsn-Vrn
180° 300° V 4
ωt
0 120° 360°

Vst
-Vsn-Vtn
ωt

Vtr
-Vtn-Vm

ωt

Thyristor
gating
sequence TH2 TH4 TH6 TH2
TH1 TH3 TH5 TH1 TH3

Fig. 6.9 The triggering sequences of a simple frequency converter

conditions. The system suffers from non-sinusoidal output current waveform and
the problems associated with harmonics.
Among the three types of converters described, the PWM usually gives smooth
control of speed from zero to maximum with satisfactory low frequency torque
capability. In all converters, a time delay between output voltage and input con-
trol signal protects the electronics and the motor from overload and sharp current
surges. There must be also current limiter in the circuit to protect the windings of
the motor. Therefore, the output voltage and frequency are related to the input signal
approximately by a first-order transfer function in the form of
a
V:= 1 e (6.8)
o τ +
s1