Tell Me About Soft Starts

[jyl]

Thinking about adding a standby generator to the house and trying to figure out which loads are vital and how to minimize those. I gather starting current for many electric motors is substantially more than running current, and that a soft start reduces the former.

How do soft starts work?

What equipment would you put them on?

How much will they reduce starting amps?

Is there a brand or model of softstart that I can just standardize on?

Loads here at the usual household motors - freezer, fridge, AC, and so on - mix of 120 and 240.

[fanaudical]

Am assuming you're looking at all single-phase motors. Here are a couple links that might be useful to you:

https://chintglobal.com/blog/soft-starter-working-principle-applications/

https://www.baldor.com/Shared/manuals/834-602.pdf

Some low-cost options here:

https://www.automationdirect.com/adc/overview/catalog/drives_-a-_soft_starters/ac_motor_soft_starters

I've never used a soft start on a single phase motor - just on 3-phase motors for work stuff. And I've given up on soft starts for 3-phase motors now that there are so many VFD options.

My opinion - Fridges and freezers don't really benefit and may not start with a soft-start device. Air conditioning devices may be a different situation depending on how they're designed/built. My external AC compressor/fan unit seems to have its own soft-starter built in. Others will know more.

[jyl]

Thanks. I have some reading to do!

The price difference between a 8.5 kW and a 12 kW genset is substantial, so if i can fit in the power envelope of the former, that would be good.

Why don’t gensets have big capacitors built in them to provide higher surge power?

[masraum]

They have pills to fix that these days. I think the most well known is the blue one.

[ltusler]

Had a 26kw Generac installed, sized for the whole house and shop. Well and infloor heat included. 15k all in.

[red-beard]

https://www.eltwin.com/hyper

Amazon

Mic drop

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[jyl]

Hmm. With a clamp on multi meter, can I measure the peak current at start up of any appliance by clamping around the power cord? Or is the peak current too brief to reliably measure that way?

[red-beard]

If the clamp on meter has the max recording function. The starting inrush current lasts about 100ms

[red-beard]

Also, the soft starter is good for the motor. Note, a hard start capacitor is not a soft starter.

Don't worry about smaller motors, just your air conditioning. Inrush is typically 5 to 6 times running current. You will see it on the nameplate, LRA - locked rotor Amps. The soft starter limits the inrush current to 150% to 175% of running current. This lower starting current is good for your motor, breakers and wiring.

After one of the hurricanes, I sold an 8kW natural gas generator along with a soft starter for a 3 ton AC. Normally that generator would never be able to start that motor. With the soft starter, it fired right up.

[jyl]

Originally Posted by red-beard

Don't worry about smaller motors, just your air conditioning.

Thanks for the info!

The quoted part makes me rethink the idea. We get power outages in the winter here, so AC isn’t a concern.

[red-beard]

Originally Posted by jyl

Thanks for the info!

The quoted part makes me rethink the idea. We get power outages in the winter here, so AC isn’t a concern.

It is minimal cost compared to the generator and it will extend the life of your AC motor.

[3rd_gear_Ted]

If you are going to use a soft start the AC motor design needs to be rated accordingly.
Recommend a minimum of NEMA Code G
Don't let the drive run the motor at less than 20 Hz.

[red-beard]

These are soft starts, not VFDs. It limits current until the motor starts turning and then gives it full power.

[Rapewta]

Years ago we used VFD's to get around the starting current with motors. Typically a motor that has a running current load of X will draw six times that at starting (locked rotor). So your generator is capable of running fine once it can get by the starting current. The VFD's were inexpensive and worked fine.
The problem with the huge inrush of starting current, trips out the gen set. Easy fix with what is available today. Good luck. (VFD means variable frequency drive).

[red-beard]

Trying to self install a VFD on a home AC unit will be problematic. There do exist single phase VFDs, and I expect I could get one to work on my AC unit. But building proper controls for it would require more effort than it's worth.

The soft start kits are designed with "home installer" in mind. Again, limiting the inrush to ~150% solves most of the starting problems.

On large 3ph motors, I agree. We "forced" one of our equipment suppliers to supply VFDs to meet local utility inrush issues. It also saves huge amounts of energy when running part load vs. how they normally operate. 1500 hp 4160 motor