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Soft-Start Question

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I know that soft-start controls are needed to limit inrush current at startup of typically large induction motors.

Now, what happens if you want to implement soft-start for triac controlled heater?

Supposedly that at startup you would have to slowly ramp up your firing angle to the desired_setpoint firing angle. But what about the case, when you are at, let's say 60°, and your desired angle is 120°? Do you need soft-start again?

Also, I know that inrush current time depends on the load attached to the system. But when designing a soft-start for some application that uses 120V and 15A max, what ramp-up time do you guys recommend? I just would like to know a ball-park figure, so that I can have a starting point.


Thanks.
 
There is no need for soft start with a resistance heater. The problem with induction motors is that, altho it draws 10 amp when running, it may draw 100 amps starting up. The soft start limits the maximum current. The heater, on the other hand, draws very little more than its normal running current when starting.
 
Sorry, I should have clarified the type of heater.

The heater is actually a self-regulating heat trace cable. Now on Thermon.com (one of manufactures of heat trace equipment), they said that a soft-start is needed for a self-regulating heat trace cable operating at 30A.

I wonder if the same holds for those cables operating at 15A?

Also, I would like to know (in general) whether a soft-start control type is needed after a actual startup sequence is over.

For example, let's say that for a larger induction motor we need soft-start when we turn on the motor. Now, supposedly the motor has been running for a while at very low load. If the load is suddenly increased to a very high load and we want to maintain the same speed as before, will there be a high inrush of current?
Will the soft-start be required?
 
As it's self regulating, I'm presuming it's like a light bulb? - low impedance when cold, and high impedance when warm - as it gets hot it takes less current, so the temperature stabilises.

As such, it's going to take it's highest current when you plug it in from cold, so it's probably a good idea to surge limit it - it's likely to help the life of the cable, and may prevent occasional fuse blowing.
 
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