Soft start solutions for 1P induction motors?

fastline

Senior Member
Location
midwest usa
Dealing with a limited power situation in which the continuous current on these 1P motors is not an issue, but the inrush hit is. I have seen a few soft start products floating around but I want to know exactly what they are doing internally so I can ensure this is a real solution and not a "magic box".

I have seen a couple things out there that indicate they reduce the voltage, but if frequency remains the same, that will drop the start torque of the motor considerably and some of these loads are compressor loads.

I am hoping we can find a real solution here vs switching a few things to 3P motors and have to install VFDs just to get a reasonable soft start.
 

junkhound

Senior Member
Location
Renton, WA
Occupation
EE, power electronics specialty
but the inrush hit is
how so, gen set stalling, lights dimming not desired, inverter on off grid solar array, or ??

need details to conjecture solution, one sized does not fit all esp with positive displacement compressor load.
 

fastline

Senior Member
Location
midwest usa
All won't run together but 3-5HP. This is a solar application so soft start IS required here. This is not a "get a new service" situation.
 

Fred B

Senior Member
Location
Upstate, NY
Occupation
Electrician
That inrush can be a problem on gen circuits. I've installed when a customer had purchased, without consultation, a gen set waaay too small for his demands. Had a geothermal heatpump setup, peak on the compressor pump was a very large load, (if recollection is correct almost 60A) installed a soft start on the motor and cut the starting to nearly the same as the running amperage (about 18A).

AFA OP you may wish to contact the compressor mfg as to soft start capabilities for their respective units. But it shouldn't be a problem. Actually the operating premise of a soft start is beneficial for a motor that starts under a load as it lessens the starting stress on the motor, lengthening motor life.
 

fastline

Senior Member
Location
midwest usa
Fred, that is what my target is. To get inrush near FLA. What would be helpful is to know what product you used for that? The right way to source this is to talk with the OEM of the softstart devices.
 

kwired

Electron manager
Location
NE Nebraska
Dealing with a limited power situation in which the continuous current on these 1P motors is not an issue, but the inrush hit is. I have seen a few soft start products floating around but I want to know exactly what they are doing internally so I can ensure this is a real solution and not a "magic box".

I have seen a couple things out there that indicate they reduce the voltage, but if frequency remains the same, that will drop the start torque of the motor considerably and some of these loads are compressor loads.

I am hoping we can find a real solution here vs switching a few things to 3P motors and have to install VFDs just to get a reasonable soft start.
Soft starting an induction motor is always reduced voltage (and reduced torque) unless using a VFD.

You can still lessen the amount of inrush that occurs just to establish magnetic fields though. Motor may not have sufficient accelerating torque until voltage reaches a certain level though, but by then your fields are established enough that you did have less total surge current. A high torque demanding load is still going to need high torque to accelerate it. This soft starting may be worse with a single phase motor than with a three phase motor as the starting capacitors may not provide sufficient phase shift at less than designed voltage as a three phase motor on soft starter might produce for starting torque.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
While what has been said is generally true of soft starters, there is another aspect to this situation that is crucial; SINGLE PHASE induction motors. Unlike 3 phase, there are multiple types of single phase motors and only a few of them are compatible with soft starting, especially on loads that might be difficult to start (another issue).

Capacitors and Soft Starters do not play well with each other. To the SCRs in the Soft Starter, the charging of a capacitor looks like a short circuit and the SCR can be damaged by dV/dt (too rapid of a change in voltage through them). Then to the capacitor, the phase angle firing of SCRs contains a lot of harmonics that the capacitor absorbs, causing them to over heat. Motors that have a Starting Capacitor (Cap-Start / Induction Run or Cap-Start / Cap-Run) and a centrifugal switch then can be problematic for using with Soft Starters, especially if the reduction in torque causes them to take too long to get to the speed at which the centrifugal switch changes state, typically about 80% speed. Even a Split Phase motor, although having no capacitor, is at risk for the same issue in that the Start winding is not designed for being in the circuit very long by itself, so the lack of starting torque can cause damage there too by leaving it in the circuit too long.

So what happens is that it's usually a race to see which one fails first, the SCRs in the Soft Starter or the Starting Capacitor (or start winding) in the motor. IF the load is something like a centrifugal pump or fan with no flow on start-up (closed valve or dampers) so that the motor can get to speed quickly even with the reduced torque, then they are OK. But on a compressor, probably not. Even with an unload valve, the reduced torque can be a problem for acceleration time and risk the damage.

When people sell Soft Starters for single phase motors, they usually state some sort of caveat saying that it must be a Permanent Split Capacitor (PSC) or Shaded Pole type motor, or a Cap-Start / Split Phase but starting completely unloaded. Unfortunately a lot of on-line sellers fail to disclose that and leave it up to buyers to discover it the hard way, or for the luckier ones, find out when reading the manual.

For small HP motors fed by limited sources like solar systems, a small VFD is a better choice. Up to and including 3HP, most mfrs offer a version that will accept single phase and give you three phase output to the motor and above 3HP, you can usually just buy a three phase VFD that is de-rated for use with a single phase input.. That of course means changing to 3 phase motors, but if your solar system can't start the motors, this may be the only viable option.
 

fastline

Senior Member
Location
midwest usa
This is why I want to dive into soft options. They are certainly out there but no one can tell me what is inside them. A motor can develop very strong torque at low voltage IF the frequency is reduced. But in the case of a VFD, the V/Hz scalar relationship is maintained but with high end drives, it will also look at the back EMF to determine slip and optimize that to ensure max torque is seen at all RPMs.

Obviously a VFD has way more control of the load but I want to explore the 1P soft options first to compare pros/cons. Converting equipment to 3P is not an option on at least one compressor load. In the case of an air compressor, that may be an option.
 

fastline

Senior Member
Location
midwest usa
Just to drive a bit deeper, I think some people are stuck in the past thinking solar means limited power. It is pretty much unlimited these days. However, since I am in this field, I can attest to the complications of it's application to the old school electrics of across the line starts. That starting method is hard on everything, even though it is common as dirt. In the solar sector, it ends up causing a serious overbuild just to handle motor starts. Mostly because you can't size just on that one motor, but you have to assume other loads are already running. A typical inverter might offer 2x FLA current for 1-5sec, but a cap start motor could pull 6x FLA at inrush. This can cost thousands to build to that need.

In the case of the air compressor load, that may be better handled with a 3P motor/VFD. In another case, you have say a 1HP cased well pump. I work with those daily and they are not very efficient so a 1HP can pull around 1500W. But they are a serious hit even to a 5000W generator. Obviously if you can calm that down to allow a 3000W supply to handle it, that is helpful.

Another common load is HVAC compressors. Residential is commonly 1P and you won't easily swap that to 3P. As well, that is certainly something you want to get right as a burnout will not be a cheap repair! I hear what Jraef is saying too and that is why I am reviewing this.

Here is one "magic box" but I am not going to install or recommend anything until I know exactly how it is reducing inrush and that it is indeed safe for the motor.

This might compare to the idiotic "energy saving black box" or box of power factor correction caps people sell for homes. They show off reduced amps and people think that means reduced energy use. We all know a kWH takes power factor in consideration.
 

Besoeker3

Senior Member
Location
UK
Occupation
Electrical Engineer
While what has been said is generally true of soft starters, there is another aspect to this situation that is crucial; SINGLE PHASE induction motors. Unlike 3 phase, there are multiple types of single phase motors and only a few of them are compatible with soft starting, especially on loads that might be difficult to start (another issue).
Excellent post, Mr Jr.
(I just truncated it just for brevity.)
 

kwired

Electron manager
Location
NE Nebraska
Just to drive a bit deeper, I think some people are stuck in the past thinking solar means limited power. It is pretty much unlimited these days. However, since I am in this field, I can attest to the complications of it's application to the old school electrics of across the line starts. That starting method is hard on everything, even though it is common as dirt. In the solar sector, it ends up causing a serious overbuild just to handle motor starts. Mostly because you can't size just on that one motor, but you have to assume other loads are already running. A typical inverter might offer 2x FLA current for 1-5sec, but a cap start motor could pull 6x FLA at inrush. This can cost thousands to build to that need.

In the case of the air compressor load, that may be better handled with a 3P motor/VFD. In another case, you have say a 1HP cased well pump. I work with those daily and they are not very efficient so a 1HP can pull around 1500W. But they are a serious hit even to a 5000W generator. Obviously if you can calm that down to allow a 3000W supply to handle it, that is helpful.

Another common load is HVAC compressors. Residential is commonly 1P and you won't easily swap that to 3P. As well, that is certainly something you want to get right as a burnout will not be a cheap repair! I hear what Jraef is saying too and that is why I am reviewing this.

Here is one "magic box" but I am not going to install or recommend anything until I know exactly how it is reducing inrush and that it is indeed safe for the motor.

This might compare to the idiotic "energy saving black box" or box of power factor correction caps people sell for homes. They show off reduced amps and people think that means reduced energy use. We all know a kWH takes power factor in consideration.
A power factor correction box (capacitor) is not going to do anything to soft start a motor. There is more than that in the black box if it is a soft starting device.

I didn't look at your link but the the title indicates soft starter for RV AC compressors - Outside of newer inverter or vfd driven types of compressors those are normally a PSC motor and would be a motor type that can handle soft starting without too many issues. You would definitely want either time delay or pressure sensing so it has a chance to equalize pressure in the system before attempting to start or it very well may not have enough torque to get started.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
A power factor correction box (capacitor) is not going to do anything to soft start a motor. There is more than that in the black box if it is a soft starting device.

I didn't look at your link but the the title indicates soft starter for RV AC compressors - Outside of newer inverter or vfd driven types of compressors those are normally a PSC motor and would be a motor type that can handle soft starting without too many issues. You would definitely want either time delay or pressure sensing so it has a chance to equalize pressure in the system before attempting to start or it very well may not have enough torque to get started.
I'm always leery of a company that goes to great length to avoid telling you what their product really is...
They do say that is ADDS a "starting capacitor", which for a PSC motor would seem redundant. So I'm thinking this is basically just a timing relay and another capacitor to, just like a 2 speed ceiling fan, slow the motor down by adding capacitance. Then after a few seconds, it shorts out that capacitor and allows it to get to full speed. But knowing that the RV A/C compressor is probably a scroll compressor that acts like a centrifugal Freon pump, slowing the motor down reduces the load on the motor by the cube of the speed change, so it IN EFFECT looks like a "soft start". That would explain why they are SPECIFIC about calling it an RV Air Conditioner Soft Start unit; it might not work on anything else.
 

Fred B

Senior Member
Location
Upstate, NY
Occupation
Electrician
One one source states this about the action of a soft start:
"Many electrical soft starters also use a series of silicon controlled rectifiers (SCR), or thyristors, to limit the voltage to a more manageable amount for the motor as it begins to start up. These SCRs have an ON state, where they allow the current to flow, and an OFF state, where they control and limit the electrical current. When you power up your machine, these SCRs activate, restrict the voltage, and then relax as the machine reaches full power. This keeps the motor’s heat down and reduces overall strain."
And link also included below that will describe difference between a soft start and a vfd:
Another form of motor starting is what is called a hard start that utilize a series of capacitors that will store a boosting surge to "bump start" the motor rather than relying on a surge from the primary power source. Source indicates that hard start for a compressor motor not usually the best choice, and tends to shorten motor life.
 

Flicker Index

Senior Member
Location
Pac NW
Occupation
Lights

It turns a ISCR motor into a CSCR motor. Sometimes the kit is offered by the compressor manufacturer too and those have a proper uF and cut out voltage defined by the factory rather than one size fits a bunch. Since you can't reliably use a centrifugal switch inside the hermetic compressor, a potential relay is used to detect the rotor RPM and cut out the second capacitor at around the same RPM a centrifugal switch would. When it is put in a box and labeled RV, the price automatically goes up a few times, because RV.
 

StarCat

Industrial Engineering Tech
Location
Moab, UT USA
Occupation
Brewery Engineering Plant Technician - HVACR Electrical and Mechanical Systems

It turns a ISCR motor into a CSCR motor. Sometimes the kit is offered by the compressor manufacturer too and those have a proper uF and cut out voltage defined by the factory rather than one size fits a bunch. Since you can't reliably use a centrifugal switch inside the hermetic compressor, a potential relay is used to detect the rotor RPM and cut out the second capacitor at around the same RPM a centrifugal switch would. When it is put in a box and labeled RV, the price automatically goes up a few times, because RV.
This is correct and is called " Hard Start Kit " and has been spoken about often in other posts on this forum. This is the single best way to trim inrush on that type of load. PTC devices perform mostly the same way.
 

kwired

Electron manager
Location
NE Nebraska
One one source states this about the action of a soft start:
"Many electrical soft starters also use a series of silicon controlled rectifiers (SCR), or thyristors, to limit the voltage to a more manageable amount for the motor as it begins to start up. These SCRs have an ON state, where they allow the current to flow, and an OFF state, where they control and limit the electrical current. When you power up your machine, these SCRs activate, restrict the voltage, and then relax as the machine reaches full power. This keeps the motor’s heat down and reduces overall strain."
And link also included below that will describe difference between a soft start and a vfd:
Another form of motor starting is what is called a hard start that utilize a series of capacitors that will store a boosting surge to "bump start" the motor rather than relying on a surge from the primary power source. Source indicates that hard start for a compressor motor not usually the best choice, and tends to shorten motor life.
"Hard start" kits don't exactly store energy to be used to start the motor. They are simply a higher value capacitor than the run capacitor already used in the motor and a potential relay to switch it out of the circuit after motor reaches certain conditions. The phase angle between main and aux windings remains the same. The added capacitance will increase the current in the aux winding and that is what gives the starting boost. It is not a soft starting method, in fact sort of the opposite. It will get the motor started more quickly presuming the supply circuit can deliver what is demanded.

When used on a typical AC compressor they turn what was a PSC motor into a CSCR motor. If the supply is marginal at being able to deliver the starting current needed, they may still help get the motor started, but is still going to draw high current when trying to start. A true "soft starting" method would ramp the current up as motor speed increases.
 

ramsy

Owner/Operator
Location
LA basin, CA
Occupation
Service Electrician 2017 NEC

kwired

Electron manager
Location
NE Nebraska
Like a single phase reactor. Simple, no switching, and no capacitors.
just an extension cord can give you a certain amount of soft starting effects. Resistance of the cord will limit inrush current, but normal running current is low enough there is no significant effects.

I've found this to work in a pinch with low magnetic trip breakers sometimes when using some power tools.

Square D has low mag trip on standard breakers but doesn't always hold on starting some motorized tools in certain situations. They do have a high magnetic trip breaker that can be used in these applications though. Not finding them at big box stores and may even be special order at some supply houses though.
 
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