Soft starter vs Single phase motor

Pierre

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Join Date
Apr 2002
Location
Montreal
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Recently, I was involved in a problem with peak intensity at startup of a
water pump. This pump was pulling over 50 Amps on 120 Vac/ 1 phase supply.
To lower the surge, I was wired with the 220 Volts windings. OK, we now
pull about 25 Amps and run at about 6 Amps. To even further reduce this peak, we want to install a
Softstarter and we have found the Sqaure D Model # LH4N112LU7.

It looks OK to me BUT...

A friend told me about single phase motor not being able to be soft-started
because the all pumps have starting capacitors which will not permit it. He
says that I will not be able to make any movement under 30 Hz and that at
some point in time, the motor will most certainly melt...

Can anyone teach me a little lesson or two on those single phase motors and pumps?

If we where to control this motor with a VFD, would we have to set the min Freq. at above 30 Hz?

Tanks in advance.
 
If the motor has a start capacitor you may have a hard time using a soft-starter. The start capacitor is only rated for a very short acceleration time (it is usually removed from the ciruit by a centrifugal switch, hence the 30hz reference). Yes you can still soft start, but watch the time rating of the capacitor.

If the motor does not have a start capacitor (i.e. a universal AC/DC motor) then you should be able to soft-start it with no problem. This is why Square D offers the L4HN single phase soft-starter (which I have used). Motortronics also offers a single phase unit.
 
"...but watch the time rating of the capacitor."

OK, OK, I've got my E-Tech diploma somewhere... but what is this... Time rating on a capacitor?

Never heard of it.

See! This is why I wanted to be teached a lesson or 2 :)

Tanks for the info.
 
My apologies, there is no such single thing as a time rating for a capacitor. I was refering to a time-current rating for the motor and capacitor as a unit.

You need to contact the motor manufacturer and find out what acceleration time is acceptable for their motor. Or contact the capacitor manufacturer and ask them: how much current can the capacitor be subjected to and for how long.

Normally a motor will draw 600-800% of FLA during startup. The centrifugal switch removes this current from the capacitor after a few seconds, which often happens around 50% speed.

Your soft-start will reduce the amount of current the capacitor is subjected to but it will also increase the amount of time it takes the motor to reach the cutout speed for the centrifugal switch. If your soft-starter allows the motor to reach the cutout speed fast enough, then you will have no problem.

This timing problem will also occur if you use a VFD on a capacitor start motor and you run it below the cutout speed.
 
As a general rule, VFD's are intended for use on three phase motors only. I have experimented with them on single phase motors and have had some success varying speed on dual 50/60Hz labeled motors within that frequency range but any further than that and the motor torque falls off rapidly.

Another problem I found is that the direction of rotation is usually unpredictable when starting at 0Hz so some way of getting the motor running across the line first is needed.

The bottom line is that it doesn't work very well. I don't recommend using a VFD on a single phase motor, period.
 
Say this pumps peak intensity is 25 Amps in the first second... it reaches top speed at about this time...

Do you thinck we could reduce this if we set the soft starter to begin say at 30Hz and to reach full speed in say ... 3 seconds.

I hunderstand that this is purely an rough guess, cause we don't have the pump in front of us to test... but why would those single-phases soft-starter exist if they where not ment to work.

I also read that this soft starter work with three phase but the hatching is done only on phase # 1 so it can be used with single phase motors. Beats me, but thats what they say at SquareD.

I have also found other single-phase soft-starter (Baldor and others)
 
A soft-start works by controlling the voltage and limiting the current to a motor, not by controlling the frequency.

The Square D LH4N1 device uses an SCR to control the voltage in one phase conductor, which allows it to be used with a single phase motor. Starting torque can be adjusted from 10-70% of normal.

If you are looking for a total acceleration time of only 3 sec, I believe you should have no problem even if you have a capacitor start motor, think how long it takes for a band saw to come up to speed. Of course, you can ask the motor manufacturer if a 3 sec start up time is acceptable.
 
Thats the thing, this pump is very far away. I talk trough a satelite phone with the man in charge... And did not have any other contact with him since then.

I wanted to know the basics.

I will get back when we will meet... in a month or so.

Tank'ya'll
 
I was just going to check and see if the motor was rated for the pump (matched) or if something had happened to change the characteristics of the motor/pump relationship. Thinking maybe someone had changed out the motor or pump and replaced it with what ever was lying around. Maybe a bad bearing or a packing gland too tight etc.....you know, the usual things but you probably went through that list already.

Good luck
Keep us posted on what you do.
Roger
 
Yes we did, this system was on a small grid where other loads where not online. Now that they are, it creates a problem. Also, the breaker from the distribution panel is not a B type (motor rated) so it was tripping. Now that we have the motor running on 220 Volts, the breaker does not trip. I was curious about softstart for single phase because I never used such a beast.

Tanks
 
rsdoran

I've heard of this trick but never tried it myself. Have you done any investigating into the difference in power consumption between running a single phase motor or a three phase motor of the same horsepower with this method.
 
Basically this is a trick to use when 3 phase isnt available like when you have a 3 phase tool machine at home. As far as current goes you will definitely pull more because of the fact you have 3 motors BUT you will have more torque power (but at a lower HP than rated) and less surge on start(s).

I have done this before. On many occasions I have bought lathes, mills etc from business auctions and used this method to power them because of not having 3 phase at home.

Using Pierre's situation as an example (depending on motor HP used) its possible running current will increase to maybe 10 to 12 amps but starting current wont be as high, less than 20 amps. This is application specific so you would have to test it to determine optimal performance but if you ever wanted a lathe etc in your garage and it was 3 phase this is an alternative.

I dont know if I would do this in an industrial application but the theory behind it is sound.

Note in this application that there are 3 motors but one is being used more or less as a generator. There will be a cumulative factor for current (the motors current draw will add together)that is why I stated a higher running current but this also allows a stronger start factor but one that is less than the rated HP of the motor.

Rules Of Thumb (Approximation)
At 1800 rpm, a motor develops a 3 lb.ft. per hp
At 1200 rpm, a motor develops a 4.5 lb.ft. per hp
At 575 volts, a 3-phase motor draws 1 amp per hp
At 460 volts, a 3-phase motor draws 1.25 amp per hp
At 230 volts a 3-phase motor draws 2.5 amp per hp
At 230 volts, a single-phase motor draws 5 amp per hp
At 115 volts, a single-phase motor draws 10 amp per hp
Motor Formulas
 
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