28-06-2012, 12:28 PM
Solid State Control Solutions for Three Phase 1 HP Motor
solid state control of three phase induction motors.PDF (Size: 252.76 KB / Downloads: 120)
INTRODUCTION
In all kinds of manufacturing, it is very common to have
equipment that has three phase motors for doing different
work functions on the production lines. These motor
functions can be extruders, fans, transport belts, mixers,
pumps, air compressors, etc. Therefore, it is necessary to
have equipment for controlling the start and stop of the
motors and in some cases for reversing them. Actually, one
of the most common solutions for performing this control
functions is by using three phase magnetic starters. It
consists of a block with three main mechanical contacts
which provide the power to the three main terminals of the
motor once its coil is energized. However, the magnetic
starter has a lot of disadvantages and the most common
appear when they are driving high current levels that can
cause arcing and sparks on their contacts each time they are
activated or de−activated. Because of these kind of effects
the contacts of the magnetic starters get very significantly
damaged causing problems in their functionality. With time
it can cause bad and inefficient operation of the motors. This
is why, thyristor should be considered as a low cost
alternative and indeed a powerful device for motor control
applications. Thyristors can take many forms but they have
certain features in common. All of them are solid state
switches that act as open circuits capable of withstanding the
rated voltage until triggered. When they are triggered,
thyristors become low impedance current paths and remain
in that condition (i.e. conduction) until the current either
stops or drops below a minimum value called the holding
level. Once a thyristor has been triggered, the trigger current
can be removed without turning off the device.
DEFINITIONS
Three phase induction motor.
A three phase induction motor consists of a stator
winding and a rotor of one of the two following types: one
type is a squirrel−cage rotor with a winding consisting of
conducting bars embedded in slots in the rotor iron and
short circuited at each end by conducting end rings. The
other type is a wound rotor with a winding similar to and
having the same number of poles as the stator winding, with
the terminals of the winding being connected to the slip
rings or collector rings on the left end of the shaft. Carbon
brushes bearing on these rings make the rotor terminals
available at points external to the motor so that additional
resistance can be inserted in the rotor circuit if desired.
Three phase voltages of stator frequency are induced in
the rotor, and the accompanying currents are determined by
the voltage magnitude and rotor impedance. Because they
are induced by the rotating stator field, these rotor currents
inherently produce a rotor field with the same number of
poles as the stator and rotating at the same speed with
respect to the stationary rotor. Rotor and stator fields are
thus stationary with respect to each other in space, and a
starting torque is produced. If this torque is sufficient to
overcome the opposition to rotation created by the shaft
load the motor will come up to its operating speed. The
operating speed can never equal the synchronous speed of
the stator field.
The following figure shows a three phase 1HP motor
controlled through a conventional magnetic starter which
has an over−load relay for protecting the motor against
over−load phenomena.
APPLICATION NOTE
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2
Power Schematic
3 Phase
Motor
1 H.P.
L1 L2 L3
220 Vrms 60 Hz
A A A
NC OL OL OL
A
A
Start Stop
When the start button is pushed on, the coil of the
magnetic starter (A) is energized, thereby, the mechanical
switch contacts close allowing current−flow through the
motor which starts it to operate. If the stop button is pushed,
the coil (A) will be de−energized causing the motor to stop
because of the mechanical switch contacts opened. In
addition, if an overload phenomena exists in the circuit of
the motor, the switch contact (NC) of the overload relay
will open de−energizing the coil and protecting the motor
against any kind of damage.
Magnetic starters have a lot of disadvantages like arcing,
corrosion of the switch contacts, sparks, noisy operation,
short life span, etc. Therefore, in some motor applications,
it is not useful to control the motors by using magnetic
starters since the results can be undesirable.
On the other hand, the following schematic diagrams
show how thyristors can perform the same control function
for starting and stopping a three phase 1HP motor. In addition,
the diagrams below show an over load circuit for protecting
the motor against overload phenomena.
AND8008/D
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3
510
MOC3062
510
MOC3062
510
MOC3062
3 Phase
Motor
1 H.P.
L1 L2 L3
Line Voltage
220 Vrms 60 Hz
To Over Load
Protection Circuit
for Line 3
To Over Load
Protection Circuit
for Line 1
Current
Transformer
Current
Transformer
Diagram 1
BTA08−600CW
BTA08−600CW
BTA08−600CW
Diagram 1 shows how three triacs BTA08−600CW3G or
BTB08−600CW3G (Ref: BTA08−600CW) substitute the
mechanical contacts of the conventional magnetic starter
(shown previously) for supplying the power to the three
phase 1HP motor once the triacs are triggered.
It is important to mention that the optocoupler devices
(MOC3061) will supply the signal currents to the triacs and
hence the motor keeping the same phase shifting (120
electrical degrees) between lines. This is because these
optocuplers (MOC3061) have zero crossing circuits within
them.
Another important thing must be considered as a
protection for the triacs (BTA08−600CW) against fast
voltage transients, is a RC network called snubber which
consists of a series resistor and capacitor placed around the
triacs. These components along with the load inductance
from a series CRL circuit.
Many RC combinations are capable of providing
acceptable performance. However, improperly used
snubbers can cause unreliable circuit operation and damage
to the semiconductor device. Snubber design involves
compromises. They include cost, voltage rate, peak
voltage, and turn−on stress. Practical solutions depend on
the device and circuit physics.
Diagram 2 shows an electronic over−load circuit which
provides very reliable protection to the motor against over
load conditions. The control signals for the two electronic
over−load circuits are received from the shunt resistors
connected in parallel to the two current transformers placed
in two of the three main lines (L1, L3) for sensing the
current flowing through the motor when it is operating. The
level of the voltage signals appearing in the shunt resistors
is dependent on the current flowing through each main line
of the motor. Therefore, if it occurs, that an over load
condition in the power circuit of the motor, that voltage
level will increase its value causing the activation of the
electronic over−load circuits which will stop the motor by
protecting it against the over−load condition experienced.
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+
-
LM324
22 k
+12 Vdc
+12 Vdc
220 F
2 k
4.3 k
1 k
Output Signal Connected
to OR Gate’s Input One
+
-
LM324
+12 Vdc
+12 Vdc
25 k
1 k
+
- LM324
+12 Vdc
10 k
+
-
+12 Vdc
10 k
Over Load Protection Circuit for Line 1
10 k 2 k
220 F
MUR160
0.1 MUR160 1 k
Shunt
1 k
1 k
−12 Vdc
−12 Vdc
−12 Vdc
MUR160
−12 Vdc
+
-
LM324
22 k
+12 Vdc
+12 Vdc
220 F
2 k
4.3 k
1 k
Output Signal Connected
to OR Gate’s Input Two
+
-
LM324
+12 Vdc
+12 Vdc
25 k
1 k
+
- LM324
+12 Vdc
10 k
+
-
+12 Vdc
10 k
Over Load Protection Circuit for Line 3
10 k 2 k
220 F
MUR160
0.1 MUR160 1 k
Shunt
1 k
1 k
−12 Vdc
−12 Vdc
−12 Vdc
MUR160
−12 Vdc
Wire Conductor
Line 3
Wire Conductor
Line 1
LM324
LM324
Diagram 2
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5
MC14013
2N2222
510
Start/Stop Control Circuit
1.5 k
+12 Vdc
SD
CD
MOC3062
MOC3062
+12 Vdc
1 k
1 k
Stop
Button
+12 Vdc
MC14075
Start
Button
Q MOC3062
Output Signal from
Over Load Protection
Line 1
Output Signal from
Over Load Protection
Line 3
Diagram 3
Diagram 3 shows the main electronic control circuit for
controlling the start and stop of the motor each time it is
needed. If the start button is pushed on, the Flip Flop
(MC14013) is activated triggering the transistor (2N2222)
which turns on the optocoupler’s LED’s which in turn the
three triacs (BTA08−600CW) get triggered and finally
starts the motor. The motor will stop to operate, whenever
the stop button is pushed or any overload condition occurs
in the power circuit of the motor.
The following plot shows the motor’s start current
waveform on one of the three phases when the motor starts
to operate under normal operation conditions and without
driving any kind of mechanical load: