
AN1003
Application Notes
http://www.teccor.com
+1 972-580-7777 
AN1003 - 6
2002 Teccor Electronics
Thyristor Product Catalog
Permanent Magnet Motor Control
Figure AN1003.16 illustrates a circuit for phase controlling a per-
manent magnet (PM) motor. Since PM motors are also genera-
tors, they have characteristics that make them difficult for a 
standard triac to commutate properly. Control of a PM motor is 
easily accomplished by using an alternistor triac with enhanced 
commutating characteristics.
Figure AN1003.16
Circuit for Phase Controlling a Permanent Magnet 
Motor
PM motors normally require full-wave DC rectification. Therefore, 
the alternistor triac controller should be connected in series with 
the AC input side of the rectifier bridge. The possible alternative 
of putting an SCR controller in series with the motor on the DC 
side of the rectifier bridge can be a challenge when it comes to 
timing and delayed turn-on near the end of the half cycle. The 
alternistor triac controller shown in Figure AN1003.16 offers a 
wide range control so that the alternistror triac can be triggered at 
a small conduction angle or low motor speed; the rectifiers and 
alternistors should have similar voltage ratings, with all based on 
line voltage and actual motor load requirements.
SCR Phase Control
Figure AN1003.17 shows a very simple variable resistance half-
wave circuit. It provides phase retard from essentially zero (SCR 
full on) to 90 electrical degrees of the anode voltage wave (SCR 
half on). Diode CR
1
 blocks reverse gate voltage on the negative 
half-cycle of anode supply voltage. This protects the reverse gate 
junction of sensitive SCRs and keeps power dissipation low for 
gate resistors on the negative half cycle. The diode is rated to 
block at least the peak value of the AC supply voltage. The retard 
angle cannot be extended beyond the 90-degree point because 
the trigger circuit supply voltage and the trigger voltage produc-
ing the gate current to fire are in phase. At the peak of the AC 
supply voltage, the SCR can still be triggered with the maximum 
value of resistance between anode and gate. Since the SCR will 
trigger and latch into conduction the first time I
GT
 is reached, its 
conduction cannot be delayed beyond 90 electrical degrees with this 
circuit.
Figure AN1003.17
Half-wave Control, 0° to 90° Conduction
Figure AN1003.18 shows a half-wave phase control circuit using 
an SCR to control a universal motor. This circuit is better than 
simple resistance firing circuits because the phase-shifting char-
acteristics of the RC network permit the firing of the SCR beyond 
the peak of the impressed voltage, resulting in small conduction 
angles and very slow speed.
Figure AN1003.18
Half-wave Motor Control
DC
MTR
115 V ac
Input
1.5 A
3.3 k
250 k
15 k 1/2 W
0.1 
μ
F
400 V
HT-32
Q4006LH4
100
0.1 
μ
F
100 V
0.1 
μ
F
400 V
G
MT1
MT2
+
-
R1
IAC
SCR1
2.2 k
R3
R2
CR1
Load
IN4003
IN4003
IN4004
IN4004
IN4004
120 V ac
60 Hz
120 V ac
60 Hz
240 V ac
60 Hz
240 V ac
60 Hz
240 V ac
50Hz
0.8 A
8.5 A
0.8 A
8.5 A
2.5 A
500 k
100 k
1 M
250 k
1 M
1 k
Not 
Required
1 k
Not 
Required
1 k
EC103B
S2010F1
EC103D
S4010F1
T106D1
R2
R3
SCR1
CR1
AC
Input
Voltage 
AC
Load
Current 
M
R1
R2
C1
D1
SCR1
HT-32
3.3 k
AC
Supply
Universal Motor
CR1
AC
Input
Voltage
120 V ac
60 Hz
240 V ac
60 Hz
240 V ac
50 Hz
AC
Load
Current
8 A
6.5 A
6.5 A 
150 k
200 k
200 k 
IN4003
IN4004
IN4004
S2015L
S4008L
S4008L
0.1
μ
F 200 V
0.1
μ
F 400 V
0.1
μ
F 400 V
R2
CR1
SCR1
C1