Soft start for single AC phase motor?

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.
Click to expand...

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.
 
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.
Click to expand...
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.
 
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.
 
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.
 
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
Click to expand...
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.
 
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.
 
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.
 
Yep. It is beter to be healthy and wealthy rather then poor and sick ;-)
 
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.
 
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.
 
You must log in or register to reply here.
Back
Top