In this post we discuss a few innovating and simple soft start circuit examples which may be implemented with heavy duty motors so that they are able to initiate with a soft start or a slow sluggish start instead of a sudden, bumpy start
Why Soft Start is Crucial for Heavy Motors
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 arcing across the pump relay contacts causing corrosion and reduction in its life due to stress, and wear and tear.
The high current arcing 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.
Designing a Soft Start using Triac Phase Chopping
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.
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.
Upgrading Triac Soft Start with Relay
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 be selected such 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,
Simplified Soft Start Design
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.
The above design can be even further simplified by removing the relay, as shown below:
An ultimate PWM based motor soft start circuit can also be tried for getting a better control, a better torque and a reliable startup for the connected motor, even for 3 phase motors.