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I am demonstrating a soft-starter for a technical report, and I am having trouble finding the correct motor and power source.

I want to use SCRs in a reverse-parallel configuration to limit the in-rush current and torque. The problem is that this soft-start setup would require a 3-phase voltage supply, along with a 3-phase induction motor, which is quite costly for an experiment.

My question is would I be able to demonstrate a soft-start of a motor by using a single phase AC source? OR is there a better way to demonstrate this experiment besides using the reverse-parallel setup?

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What are the requirements for your project? It sounds like you've got a lot of flexibility...help us understand your constraints, if any. Can you soft-start a DC motor, for example? – HikeOnPast Dec 4 '12 at 3:38
You're right about the flexibility. I want to focus on the benefits of a soft starter in an application. What I plan on doing is comparing a motor's inrush current/torque with and without a soft starter and expand from there. To answer your question; yes I could soft start a DC motor if it is the only solution to cheap cost. As long as this application demonstrates it's benefits..there's room for flexibility. Thanks for your thoughts. – Maraldz Dec 4 '12 at 3:52
up vote 1 down vote accepted

For your experiment, I presume that you want to demonstrate peak current with and without your soft starter. I'd start with a low voltage DC motor, a fixed flywheel mass, a current shunt, and possibly a speed sensor. You'll also want a fixed voltage power supply with a low source impedance and adequate peak current; a battery will also work.

The flywheel attached to the shaft will extend the "area under the curve" so that your measurements can be done slightly more leisurely. Without it, the motor might go from full stop to full speed too quickly to measure repeatably. The speed sensor may be helpful to compare current at particular shaft velocities, and also to indicate when terminal velocity has been reached.

The DC motor can be driven with 1) a low-impedance DC source to show peak current without limiting 2) a plain resistor current limiter (though top speed will be reduced) and 3) solid state current limiting, presumably doing cycle-by-cycle limiting or whatever your project requires.

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Demonstrating peak current with and without my soft starter is definitely my goal. Now that I know the proper motor and power supply to purchase, I could take the rest from here. However, I wont be able to use the SCR configuration anymore because of the DC power supply, but other soft-starting concepts could be learned. What did you mean by cycle-by-cycle limiting? I don't want to reduce the motors top speed either. I still want it to be reached, just more gradually instead of spiking upon startup. – Maraldz Dec 5 '12 at 22:57
Search for "cycle by cycle current limiting" - it shortens the on time when the current limit has been reached. I haven't seen it done with SCRs, however, as it would require that they be forced off after they were already in conduction. – HikeOnPast Dec 6 '12 at 0:02

There is no reason why you couldn't develop a soft starter for a single-phase AC induction motor.

There are already several soft starters available on the market which are suitable for single phase motors. You must take care however to ensure that your motor is compatible with the soft starter.

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Perfect. Using a soft starter in single phase will definitely reduce costs. However, I want to actually create the soft-starter, rather than purchase one. I want to purchase a power supply and a motor, then I'll take care of everything in between. – Maraldz Dec 4 '12 at 15:48

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