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Soft Starter with SSR


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I did some tests with a home made softstarter: I'm using three Solid State Relays and I activate them from 0 degree to 180 degree, synchronized on the line. The motor started slowly as intended, but the current drained by the motor is still very high. The peak current is still roughly equal to the motor lock current (current measured with a scope).


I was expecting to eliminate this peak and reduce it at a lower value. Does the fact of not changing the frequency 60Hz explain the results that I got?


Do you have some explaination?




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Dear Francis,


I understand that you are intending to control the speed of a 3-phase induction motor with a soft starter. You are using three separate SSR's.


I have the following points:


1. The gate pulses must be isolated with a pulse transformer or opto- isolator

2. The gate pulses must be out of phase

3. You can start as a try at a triggering angle of 90 degrees and decrease it gradually in, say, a step of 10 degress down to practically 20 degrees.

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Hello Francis


Welcome to the forum.


The starting current for an induction motor when using a soft starter is still many times greater than the ratd current of the motor. There is information on this on my web site at http://www.LMPhotonics.com.

Are you concerned about the maximum "average" current as would be measured by an ameter, or the peak current of each cycle? Remember that the average voltage is being controlled by using phase control, but the instantaneous peak current during a cycle will be higher than the average because you are supplying current for less than 360 degrees per cycle. - a higher current for a shorter time to give a reduced average current.


Best regards,

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My concern in asking if he is using 3 individual 1 phase SSRs is that in doing that, he probably does not have control of each SCR individually, both SCRs in each pole will fire based soley on the phase they are connected to, not based on the 3 phase relationship that needs to exist between the 6 SCRs for true phase controlled firing. In other words, how would he be able to turn on +A and -B, then +B and -C, then +C and -A at the proper times? With 3 separate 1 pole devices, he has no crossover phase measurement, let alone independant firing control. So at best, he can operate it more like a 3-SCR/3-Diode starter, and his peak currents are going to be higher. Am I off base on this Marke?
"He's not dead, he's just pinin' for the fjords!"
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Hi Jraef


The short answer is yes and no!!

Depending on the SSRs and the internal circuitry, it may be made to work OK, but there is also the possiblity of major problems.

1. The voltage rating of single phase SSRs would not be satisfactory for a three phase supply unless it is a 220 volt three phase supply.

2. If the trigger circuit inside the SSR is pulsed, there will be problems getting SCRs to turn ON reliably because you need to turn ON two phases at the same instant. If it is a hard fire technology, it should be OK.

3. SSR needs to be full wave phase control.


It is possible to use a phase Neutral reference as a phase reference, or an input output voltage as a reference, so it could work. can't say that I have tried this as it is an expensive way of going about it!! Certainly, I agree, it is much easier to do using and SCR Diode approach.


Best regards,

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Hello Francis


I assume the switching device is a thyristor.......unless you have a sophistacated circuit the thyristor will only switch off at the zero crosssing of the wafeform. So the switching is pulses are back to front if you know what I mean.

as it was poited out soft starters primary goal is to take tha jolt out of the start condition.

The current is still high...but an average of the current is not as high as a direct on line starter.

If the switch *beep* is at 0 degrees then you basically get the full voltage at the motor

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Hi guys,


Thanks for the feedback.




I'm using 3 single pole SSR, one for each phase (I might forgot to tell you that I'm using a 3 phase motor). The SSR is 2 SCR back to back with a common start input.


To be able to increase the voltage, I detect the zero crossing of the line and wait a delay before to activate the SSR's. By reducing the delay, the voltage increase.


Why do you recommend me to use SCR + diode modules?





The SSR has already an isolated control start input built-in. I just need to provide 12V to the module to turn it ON and remove the 12 and the SSR will turn OFF as soon as the current drops to 0 Amp.


Is it the good way to make a soft-starter?



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Dear Francis


Well, some of our students did like your project. He used three SSR's.

He built a zero crossing detection (ZCT) circuitry and used CT's to measure

line currents of the 3-phase motor.


However, his acheivement was only starting the motor and he may have had problems with the triggering pulses.


Remember that you need 6 pulses or 2 pulses per phase: one pulse for the

positive cycle and the other for the negative cycle. You need to trigger the pulses at proper times. I would prefer to use software rather than hard-wired electronics circuits. But you definitely need to build some.


Ofcourse, you could turn ON the motor by applying pulses to the gates of the SSR's and turn OFF the motor by removing the pulses.


As a conclusion, your approach is okay but you need to design the triggering circuit very carefully.


I recommend you refer to these professional textbooks:


1. Electrical Machines, Drives and Power Systems by Theodore Wildi




2. Power Electronics by Ned Mohan et al




3. Principles of Power Electronics by Muhammed Rashid



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Hello Francis,


There is break away torque, where the magnetic fields become in sinc. You are basically preventing this by limiting the motor torque. By having no load

this break away torque is low.

If you give it a long ramp you should see the point where the motor seems to suddenly run at full speed.

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