# Soft start for single AC phase motor?



## umnik (Aug 28, 2019)

I have an AC induction motor, single phase, 3 HP, 220V. Once it gets running, it consumes noticeably less than 20A. I have 220V, 20A outlet. And when this motor starts it frequently trips circuit breaker on motor startup, but never, once it got running. Obvious resolution is to install 30A outlet and call it a day. Well, may be I will do it one day. But for now I would like to check other options. Is there such thing as soft start device for such device which will prevent high amperage draw on startup?


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## tazzat (Aug 28, 2019)

Cant you convert it to 3 phase (remove cap) and use a VFD?


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## umnik (Aug 28, 2019)

Tazzat, I know about VFDs, but the motor I have is single phase motor. I never heard about conversion of such motors to three phase.


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## tazzat (Aug 28, 2019)

Look in the conection box of the motor.. see if there is a capasitor in there or on the side of the motor.


			https://5.imimg.com/data5/WE/YA/MY-4071295/1-hp-single-phase-ac-induction-motor-500x500.png
		



			https://compressedaircentre.com/wp-content/uploads/2014/01/Electric-Motor-1-phase-Main-300x297.jpg


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## JimDawson (Aug 28, 2019)

As far as I know it's not possible to run a single phase motor on a conventional VFD, nor is is possible to convert a single phase motor to 3 phase.

But there are single phase soft starters available https://www.briskheat.com/single-phase-soft-starter.html


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## tazzat (Aug 28, 2019)

Depends on the motor they used.. often they use a 3 phase motor and just add a cap to run it on 1 phase..
The pump to my steam shower runs on 1 phase but the motor i 3 phase whit a cap.


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## umnik (Aug 28, 2019)

That is the motor






						Shop Tools and Machinery at Grizzly.com
					

Grizzly Industrial®, Inc. is a national retail and internet company providing a wide variety of high-quality woodworking and metalworking machinery, power tools, hand tools and accessories.  By selling directly to end users we provide the best quality products at the best price to professionals...



					www.grizzly.com


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## umnik (Aug 28, 2019)

JimDawson,
Yes, it looks like what I am looking for. Though there is one caveat that I found in tech spec. It says 15 starts per hour, hmm, that the limitation. Interesting, why is that? Will research more on that. And thank you for the link.


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## Janderso (Aug 28, 2019)

It has to have something to do with heat generation I would think??. 15 starts per hour, interesting.
I love this forum.


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## Flyinfool (Aug 28, 2019)

What do you have this motor on? Is there anything you can do to the equipment to reduce the starting load?
The nameplate says 22 amp, you probably should have it on a 30A circuit anyhow. The running amps will be highly dependent on the motor load. As you add load the amps will climb.


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## JimDawson (Aug 28, 2019)

umnik said:


> JimDawson,
> Yes, it looks like what I am looking for. Though there is one caveat that I found in tech spec. It says 15 starts per hour, hmm, that the limitation. Interesting, why is that? Will research more on that. And thank you for the link.



There is some heat generated when starting, so needs time to cool between starts.  With better thermal management they could make it work with more starts.  Doesn't look like there is much heat sinking on that unit.  For it's designed use (HAVC) it probably doesn't need more than that.  There may be others that have a higher cycle rating.


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## umnik (Aug 28, 2019)

Flyinfool said:


> What do you have this motor on? Is there anything you can do to the equipment to reduce the starting load?
> The nameplate says 22 amp, you probably should have it on a 30A circuit anyhow. The running amps will be highly dependent on the motor load. As you add load the amps will climb.


It's Grizzly 3hp cyclone dust collector. Yes, 30A circuit will work, but I explained the reason why I want soft start. I have Grizzly sliding table saw, which is 5HP and never tripped circuit breaker on 20A circuit. Not a cyclone. They put 22A, that is what it draws on startup only.


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## JimDawson (Aug 28, 2019)

Unloaded a 5HP single phase will draw about 15 amps, and the start up load will momentarily be up in the range of 30 or so amps.  The start load on a table saw is pretty low, it only has to spin up the blade.  On the cyclone, the accelerated mass (impeller) is much higher than a saw blade, thus requires a much longer time at high current draw to spin up.  It takes the breaker a little while to read that overload.

All breakers are not created equal, there is some minor differences in the trip time when comparing ''identical'' breakers.  Older breakers will normally trip earlier than a new breaker.  You can also buy breakers with different thermal curves, ''A'' being the fastest, ''D'' being the slowest.  A ''D'' curve breaker is normally used for high starting loads.  I don't know if you can buy breakers with different curves for a standard breaker panel.


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## mksj (Aug 28, 2019)

The 22A is not what it draws on start-up, it is actually much higher than that, 22A is for full Hp output. Motors have various different rating depending on the actual Hp vs. peak and the service factor. You can install different breakers with different thermal curves that will not trip on start-up (typically D-curve) and code allows you to  oversize the breaker by 125% to allow for start-up. So technically this would be a 25A breaker, but this assume that the running amps (under all loads) does not exceed the wire ampacity which is not the case here. In addition, although the breaker is 20A the wire capacity is not designed to run at a full 20A continuously.

They do sell single phase soft starts, but they are not inexpensive and if I recall they are limited to certain types of motors.


			https://www.alliedelec.com/product/carlo-gavazzi-inc-/rgts24250gv00/71547043/


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## umnik (Aug 28, 2019)

mksj said:


> The 22A is not what it draws on start-up, it is actually much higher than that, 22A is for full Hp output


But if it 22A constant draw, which means about 4.5HP power, not 3hp as it rated. But I will argue about that, I am not that well educated in motor intricacies. I just gathered some info what people observed. Folks who tried to run that dust collection report that cyclone trips standard breakers frequently, but once it started it works without tripping until you turn it off. That is why I got the idea to use soft starter.


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## mksj (Aug 28, 2019)

You are not accounting for the efficiency and performance factor of a single phase motor which is often around 60% or less. The motor nameplate is the full load running amps. You can always put a clamp amp meter on the line and see what it is drawing, but I wouldn't want to be pulling 17+ amps from a 20A circuit for an extended period of time. A fan/pump also has a logarithmic type load so it is running at load when on at rated speed/output (assuming there is some load variation based on suction), vs. a table saw which would be running at a lot less amps when turned on and the laod would be based on the material being cut over what time frame. The basic idea is not to burn down the house. Not an electrician, but I would look at the actual motor load and then see if I could get a 20A panel breaker with a D curve which allows for a longer start-up period without tripping, otherwise a soft start is an alternative but these are limited to the number of start/stops in a given time period.


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## JimDawson (Aug 28, 2019)

I think the bottom line here is that the supply to the cyclone is not large enough for the use.  If it were mine, I would install a 40 amp breaker and #8 wire to power it up.  Should be a lot less cost than buying a soft start, will end the problems and is safe.


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## umnik (Aug 28, 2019)

Yep. It is beter to be healthy and wealthy rather then poor and sick ;-)


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## Downunder Bob (Jan 5, 2021)

A simple work around could be to wire it up through a start/run switch put a 30 or even 35A fuse in the start side and then switch to run using your 20A breaker.


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## Flyinfool (Jan 5, 2021)

We have one machine at work that has a 1 hp motor, but that poor motor has to start against a tremendous load, I have measured it at an 80 A spike while starting, Granted that spike only lasts a couple of milliseconds, and it has never yet popped a 15A household breaker. Buy the time the motor makes a half of a revolution, the starting load is gone. Your cyclone can take a second or more to get up to speed All that time it can be drawing more than FLA of 22A. That may just be to much for the breaker. Efficiency of the motor makes that 3HP at 22FLA believable.

Another thing you can do is to measure the amp draw while running under worst case scenario, Maybe that motor is bigger than needed for the job?

The cheapest and easyest and safest route is probably to rewire to a proper 30A circuit and have it done right and safe with no bandaids.


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## RJSakowski (Jan 5, 2021)

To control inrush current, I have used a varistor.  A varistor is a resistor with a negative temperature coefficient. They are most commonly used to limit inrush current to switching power supplies.   At start, when the varistor  is at room temperature, its resistance is high, limiting current.  It quickly heats up which drops the resistance to a fraction of a percent of the cold value. 

In my case, I bought one that has a 10 ohm cold resistance  and a 15 amp capacity.  On a 240 volt line, the initial current draw would be less than 24 amps and probably disregarding the resistance of the load.  As the varsitor heats up, its resistance will drop to around .05 ohms which mean a .75 volt drop.

I purchased mine from Digi Key for $2.40. https://www.ametherm.com/datasheetspdf/SL3210015.pdf  A m,multitude of varistors with different specs are available.  I would use a clip on ammeter to determine your steady state current draw and order accordingly.

There is a drawback in that the varistor needs some time after shutdown to cool down before restarting to obtain maximum inrush limiting.  If you can allow several minu6tes between shutdown and startup, it shouldn't be a problem.  Starting up after a shorter cool down period isn't a disaster.  It just means the inrush current will be higher.  Worst case, it won't exceed what the original inrush was.


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## macardoso (Jan 5, 2021)

Approaching this from a circuit design perspective, your circuit breaker is just too small. You application is considered continuous duty since the load will be present for more than 3 minutes (the time is a grey area in code). This means that you must comply with NEC 430.22(A). The goal of the breaker is to protect the wiring from short circuit or ground fault current, not to protect the motor (that would be the job of a motor overload protective device). 430.22(A) requires the branch conductors and protective device be sized at least 125% of the motor FLA (so 22A * 1.25 = 27.5A). Since a 27.5A breaker isn't available, you would then size up to a 30A Class C (standard) breaker with 10 AWG conductors rated at 60C (NEC table 310.15(B)). You likely won't trip this properly sized breaker, however you could also opt to select a Class D breaker which will allow a longer duration of overload for high starting current loads.

You should also add a motor overload protective device. This is often an adjustable device that can limit the thermal build up in the motor. Motors rated more than 1 hp without integral thermal protection and motors rated 1 hp or less that are automatically started (NEC 430.32(C)) must have an overload device sized per the motor nameplate current rating (NEC 430.6(A)). The overload protects your expensive motor from catching fire in a sustained overload condition (e.g. heavily clogged filter) (e.g. seized bearing) and also protects your home/workspace from a dust fire.

Motors with a nameplate service factor (SF) rating of 1.15 or more must have an overload protection device sized no more than 125% of the motor nameplate current rating. Motors that don't have a service factor rating of 1.15 or higher or a temperature rise rating of 40°C and less must have an overload protection device sized at not more than 115% of the motor nameplate ampere rating (NEC 430.37).

I will add that soft starting does not change the requirement for more substantial conductors and circuit protection. Soft starting is used in industry to save money when high inertia load are frequently started or to ease mechanical stresses on connected components. In your application, neither of these are a huge concern so the soft starter is likely not needed (unless you want one just because). A soft starter is somewhat similar to a VFD that only operates during the motor initial acceleration and then couples to the AC line once at speed.


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## Flyinfool (Jan 5, 2021)

macardoso said:


> You should also add a motor overload protective device. This is often an adjustable device that can limit the thermal build up in the motor. Motors rated more than 1 hp without integral thermal protection and motors rated 1 hp or less that are automatically started (NEC 430.32(C)) must have an overload device sized per the motor nameplate current rating (NEC 430.6(A)). *The overload protects your expensive motor from catching fire in a sustained overload condition (e.g. heavily clogged filter) *and also protects your home/workspace from a dust fire.



With vacuum motors this is a very common misconception. The work the motor is doing is to move air, when it is not moving air it is not working, A clogged filter means *less* air moving, *less* work being done, therefore *less* load on the motor because it is not moving as much air. That is why with a vacuum cleaner when you put your hand over the hose to block airflow you hear the RPM go whey up, by blocking the airflow you just removed the load so the motor can rev higher. When the hose is blocked you will also measure less amps due to the less load. Max load on the motor is with all hoses and filters and anything else that can restrict airflow removed. This is where you will measure the highest running current to the motor.
Now in the case of a restricted system e.g. clogged filter, the motor will not get hot but the air that is getting beat to death inside of the fan housing will get hot, potentially very hot because there is no cool air coming in, just the same air absorbing all of the motors output power. I have measured 400°+ air temp in a nearly blocked vacuum fan. It melted a lot of things in the machine, but the motor itself was nice and cool, it was running at crazy RPM with no load and that RPM making the cooling fan blow a LOT of air over it. The air temp was still climbing when we ended the test around 430°F due to the plastic of the machine melting. In addition to the thermal protection on the motor that was required for electrical protection, we also had to add thermal protection to the air stream to protect against thermal runaway from a clogged hose.

With an induction motor powering the vac fan, you do not get the real high RPM with no load, but you will still see the amps drop due to no load when the hose is plugged and the fan will still get very hot.


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## umnik (Jan 5, 2021)

Guys, thank you for all suggestion. The problem is resolved since then. When I was looking for softstart device I could only find commercial devices costing almost the same as VFDs, so I just crossed them out. But, I found some electronic scheme which I thought to adopt for my situation. While I was looking for parts, I suddenly found ready made device on eBay (https://www.ebay.com/itm/174219086751) for very good price, so I went ahead and bought it. Works perfectly for me.


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## aliva (Jan 5, 2021)

I believe limiting the the starts per hour is to reduce heat build up. This was the case at my former employer. But the motor was 5000 hp blower.
They were allowed only 2 starts per hour


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