Adding a Soft Start to Water Pump Motors - Reducing Relay Burning Problems
Adding a Soft Start to Water Pump Motors - Reducing Relay Burning Problems
When heavy motor systems or high current motors are involved, initial
switch ON current surge often becomes an issue. This surge tends to
inflict huge sparking across the pump relay contacts
causing corrosion and reduction in its life due to stress, and wear and
tear.
The sparking of the motor not only causes relay contact issues,
but also affects surrounding electronic circuits, causing them to hang
or get disturbed due to large amount of RF interference generated during
motor switch ON.
However safeguarding the costly motor relay becomes the main issue with such situations. Though there are many mechanical
contactors available for controlling motor stress, these system are not efficient and are ineffective against the RF emissions.
The simple
electronic circuit
presented below hopefully is able to eliminate all issues concerned with
heavy motor switch ON surge generation and relay contact protection.
The figure shows a simple dimmer switch circuit incorporating an
ordinary triac and diac configuration, which can be very effectively
used for adding a
soft start to any high current, heavy
AC motor.
Here the control pot has been replaced with a LED/LDR box. As we know
that in normal dimmer switches, a variable resistance is used for
controlling the fan speeds. Here the variable resistance is replaced
with a LED/LDR arrangement. It means now the speed of the motor, or in
other words, current to the motor can be controlled by controlling the
intensity of the enclosed LED through an external trigger.
That's exactly what is done here. When the motor relay is switched ON,
either by a switch or through an electronic control circuit such as a
water level controller circuit, the LED of the attached dimmer switch is
also switched ON simultaneously.
The LED switches ON the triac and the connected motor.
Being a solid state device the dimmer switch acts a little faster than
the relay and therefore the motor is first activated through the dimmer
triac and just after a few milliseconds the triac gets bypassed by
the concerned relay contacts.
The above process completely eliminates any sparking from the relay
contact since the triac has already absorbed much of the current and the
relay only has to softly takeover the already switched ON motor
conduction.
Here the brightness of the opto-coupler LED is crucial, and must be set such that the triac is only 75% ON.
This adjustment will save the triac from initial heavy current transient and help the entire system to last for many many years.
The resistor R4 may be appropriately set for achieving an optimal glow over the LED.
Parts List
R1 = 15K
R2 = 330K,
R3 = 10K,
Diac resistor = 100 Ohms,
R4 = to be adjusted as explained,
C1 = 0.1uF/400V
C2, C3 = 0.1uF/250V,
L1 = 10 amp/220V choke
Triac (Alternistor) = 10 Amp 400V,
Diac = as per the above triac.
Simplified Version
A little inspection reveals that the circuit actually does not require
the opto coupler circuit at all. The circuit may be simply arranged in
the following manner:
R2 should selected suh that the triac conducts only 75% of the power.
When power is switched ON, the triac provides a soft initial start to
the motor until within the next split second when the relay also
conducts enabling the motor the required full power. This completely
safeguards the actuator contacts from the initial current surges and
sparks,
Much Improved Circuit
As rightly suggested by Mr.Jim, an initial torque is imperative
for initiating a motor optimally especially when it's loaded, if this
initial torque is absent. the motor might stall with heavy loads under
its belt and might start smoking within minutes.
The following circuit is designed for solving both the issues together,
it inhibits the initial surge current to the ON/OFF switch and yet
allows the motor to start with a "kick" so that it initiates without
problems even when it's loaded.
Sumber : http://homemadecircuitsandschematics.blogspot.com/2012/09/adding-soft-start-to-water-pump-motors.html