Soft start solutions for 1P induction motors?

Fred B

Senior Member
Location
Upstate, NY
Occupation
Electrician
"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.
Had my start in automotive electrical work, DC, generator, starters, alternators. Old school points and condensers, condensers would take a large charge quickly from the coil, and dissipate quickly as points closed to create a spark for engine to run. Very high voltage, low amperage, have in neighborhood of 16,000 to 30,000 V, HEI would develop some in excess of 80,000V. Unless AC voltage capacitor is totally different than DC. Here is one simplified source quote about capacitors:
"A capacitor (originally known as a condenser) is a passive two-terminal electrical component used to store energy electrostatically in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors (plates) separated by a dielectric (i.e., insulator)."
Thus my comment about hard start capacitors. Again unless AC capacitors are totally different operationally than DC.
 

kwired

Electron manager
Location
NE Nebraska
Had my start in automotive electrical work, DC, generator, starters, alternators. Old school points and condensers, condensers would take a large charge quickly from the coil, and dissipate quickly as points closed to create a spark for engine to run. Very high voltage, low amperage, have in neighborhood of 16,000 to 30,000 V, HEI would develop some in excess of 80,000V. Unless AC voltage capacitor is totally different than DC. Here is one simplified source quote about capacitors:
"A capacitor (originally known as a condenser) is a passive two-terminal electrical component used to store energy electrostatically in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors (plates) separated by a dielectric (i.e., insulator)."
Thus my comment about hard start capacitors. Again unless AC capacitors are totally different operationally than DC.
Capacitors are storage devices for DC.

They are just a reactive component for AC current, with opposite effect as an inductor. Effect is a change in relationship of voltage to current on AC. With capacitive reactance the voltage leads the current, with inductive reactance the current leads the voltage. This phase relationship change is what makes single phase induction motors work. With a single phase source you must change the reactance in one of the motor windings to cause a moving magnetic field within the motor. With three phase motors you already have three inputs that reach different peaks at different times.

The change in phase relationship is same regardless the size of the capacitor, but more capacitance can support more current which means more work is potentially done if voltage stays the same. Yes there are some inefficiencies that factor in.
 

kwired

Electron manager
Location
NE Nebraska
PFC capacitors would help in keeping the voltage up a bit but I have no idea how they would work in OPs situation.
They work because the reactive power that otherwise flows back and forth between the motor and the source is now flowing (or at least a certain amount is flowing) between the capacitor and the motor instead of all the way back to the source - this essentially lessens the voltage drop on the rest of the run to the source, or on a small local source lessens voltage drop across a higher impedance source.
 

kwired

Electron manager
Location
NE Nebraska
Lets not forget a power factor capacitor serves different function than a motor starting or running capacitor. PFC is connected directly to the supply lines and functions like I mentioned in my last post.

Motor starting and running capacitors are usually in series with a motor winding, and change the phase relationship of that winding to the other motor winding to create a differential in timing of magnetic forces being applied to the rotor. The start capacitor is normally a much higher value as additional value is needed during starting than during running, particularly when driving loads that require higher starting torque. A PSC motor has no start capacitor and usually is only used on loads that don't need high torque at starting, though some may still be a little slower at acceleration because of this than if they had a capacitor start motor.
 

retirede

Senior Member
Location
Illinois
Had my start in automotive electrical work, DC, generator, starters, alternators. Old school points and condensers, condensers would take a large charge quickly from the coil, and dissipate quickly as points closed to create a spark for engine to run. Very high voltage, low amperage, have in neighborhood of 16,000 to 30,000 V, HEI would develop some in excess of 80,000V. Unless AC voltage capacitor is totally different than DC. Here is one simplified source quote about capacitors:
"A capacitor (originally known as a condenser) is a passive two-terminal electrical component used to store energy electrostatically in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors (plates) separated by a dielectric (i.e., insulator)."
Thus my comment about hard start capacitors. Again unless AC capacitors are totally different operationally than DC.
The “condenser” in an automotive ignition system was to dampen the voltage rise across the points caused by the collapse of the magnetic field in the coil when they opened. It’s purpose was to prolong the life of the points, not store energy to be used in the ignition cycle.
 
  • Like
Reactions: MTW

kwired

Electron manager
Location
NE Nebraska
The “condenser” in an automotive ignition system was to dampen the voltage rise across the points caused by the collapse of the magnetic field in the coil when they opened. It’s purpose was to prolong the life of the points, not store energy to be used in the ignition cycle.
True there, the sudden high voltage comes from the collapsing field in the coil and not from the capacitor.
 

fastline

Senior Member
Location
midwest usa
I feel like my PFC comment was completely misunderstood! I used that as an example of scams that are in the market. PFCs have their place, but one thing they do NOT do is save a residential home owner any money on their electric bill.

So far, it sounds like no one here has any experience with soft starting a 1P motor of any sort? Yes, I understand the relationship of capacitors in an AC circuit. Some here say if a motor takes too long to start, it could be damaged. That is nonsense. A motor will not burn out if currents are held to FLA. Where risk comes in is that range between FLA and inrush. Jraef knows well that VFDs start motors over many seconds at or even more than FLA. Many VFDs will do 2x FLA for motor starts and it rarely causes issues.

That being said, one of the issues I brought up was frequency. When a 1P motor is energized, it gets a 60hz hit, against a 0hz rotor. That is massive slip and a stall condition, which is VERY hard to get out of, thus one of the reasons motors pull big girl power trying to start. But in the case of a VFD, the frequency is largely reduced so slip is reduced to optimize for peak torque at basically 0rpm. In short, a huge current decrease just my reducing the frequency.

I know in our CNC world, we have countless VFD driven motors doing start/stop hundreds x/day. Motor accelerations are betwee1 1-3sec usually. Motors are not failing, BUT I do understand that these are 3P motors too but the principal is the same.
 

drcampbell

Senior Member
Location
The Motor City, Michigan USA
Occupation
Engineer
... A motor will not burn out if currents are held to FLA. ...
That's not true of hermetic refrigeration & air-conditioning compressors. They rely on refrigerant flow over the windings to remove heat, and if the motor's not turning, the refrigerant's not flowing and the motor's not being cooled. They'll probably be okay for a minute or two, but will eventually overheat if they're carrying full-load current and not turning.
 

fastline

Senior Member
Location
midwest usa
That's not true of hermetic refrigeration & air-conditioning compressors. They rely on refrigerant flow over the windings to remove heat, and if the motor's not turning, the refrigerant's not flowing and the motor's not being cooled. They'll probably be okay for a minute or two, but will eventually overheat if they're carrying full-load current and not turning.
I was referring to the motor 'start', ie bringing it up to full rated rpm. I questioned my own use of "never" because this is the internet where that is usually met with some armchair opposition. Yes, obviously 'nearly' any motor that sits at FLA current draw and does not start is likely to die as 'nearly' all motors have fan cooling. I do get what you are saying. Just saying that reduced power does not really mean a burned motor. Just need to get this balanced out.
 

fastline

Senior Member
Location
midwest usa
Your solution in post #19 came and went, as other conversations continued, which is typical during the OP's absence
Not really! I am examining all solutions but I firmly agree with you! They were specifically designed to limit the inrush hit with simple inductance! We have them in many industrial machines to reduce the nasty hit from large VFDs charging up at start up.

The only issue I see from a reactor is cost. They are copper and iron. Great idea though! I have something I can use to run some tests with that just to see. A compressor even.
 

Besoeker3

Senior Member
Location
UK
Occupation
Electrical Engineer
Not really! I am examining all solutions but I firmly agree with you! They were specifically designed to limit the inrush hit with simple inductance! We have them in many industrial machines to reduce the nasty hit from large VFDs charging up at start up.
Why would there be a "nasty" hit from a VSD charging up at standstill? Surely that's the purpose of the VSD?
 

Besoeker3

Senior Member
Location
UK
Occupation
Electrical Engineer
I would guess he is referring to the initial charge of the capacitors when the unit is first powered up. Not during motor startup.
Yes, initial start.
There are two variations for charging. One arrangement was using thyristors (SCRs) for initial charging. The other was resistances with a contactor after starting then closed. In either case, there was not nasty starting hit.
 

paulengr

Senior Member
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.
An electronic soft start is literally using the same tech as lamp dimmers, just bigger and optimized for motors.

That’s the gold standard. In both single and three phase though the goal is just cutting the starting voltage. Resistors, capacitors, reactors, and transformers can all do similar things combined with contactors. As an example a capacitor start, capacitor run single phase motor IS a soft start. When it starts the start capacitor creates a leading phase shift and boosts the voltage to the start coil. Manipulating the capacitor size changes the current/torque. Once it gets up to speed, a centrifugal switch, a voltage switch (potential relay), or a timer disconnects the starting coil. The run capacitor decreases current by improving power factor.

The obvious war to reduce starting current in the start coil is use a smaller capacitor. It works but you will reduce torque too. At some point you can’t get the load to start. These things are pretty optimized though. Most likely if you can do this just use a smaller motor.
 

kwired

Electron manager
Location
NE Nebraska
An electronic soft start is literally using the same tech as lamp dimmers, just bigger and optimized for motors.

That’s the gold standard. In both single and three phase though the goal is just cutting the starting voltage. Resistors, capacitors, reactors, and transformers can all do similar things combined with contactors. As an example a capacitor start, capacitor run single phase motor IS a soft start. When it starts the start capacitor creates a leading phase shift and boosts the voltage to the start coil. Manipulating the capacitor size changes the current/torque. Once it gets up to speed, a centrifugal switch, a voltage switch (potential relay), or a timer disconnects the starting coil. The run capacitor decreases current by improving power factor.

The obvious war to reduce starting current in the start coil is use a smaller capacitor. It works but you will reduce torque too. At some point you can’t get the load to start. These things are pretty optimized though. Most likely if you can do this just use a smaller motor.
I don't think so. Start capacitor is different value than run capacitor to create more torque to accelerate the motor quicker. It can not run indefinitely with start capacitor in the circuit. If the capacitor holds up it eventually overheats the winding, but my experience if the cut out switch is hanging up it usually takes the capacitor out, they are not designed to remain in the circuit that long at the current they usually end up carrying.
 

StarCat

Industrial Engineering Tech
Location
Moab, UT USA
Occupation
Brewery Engineering Plant Technician - HVACR Electrical and Mechanical Systems
Here you go:
I have never seen one in the field in 32 years HVACR experience, but apparently they exist. No idea when they may have come into limited usage. I have been in facilities work since 99.
I can tell you that going through the advent of scroll compressor Technology in the early 90s, the story was the same. Correctly selected hard start components solve a major lot of problems both real and imagined.
 

drcampbell

Senior Member
Location
The Motor City, Michigan USA
Occupation
Engineer
Is this a new project being designed from a blank slate, or trying to adapt things to existing equipment?
Are there higher-end air conditioners available that already have a soft-start or variable-speed drive integrated?


OK, so I watched one of the videos -- the one with the nerdy-looking guy in an electronics shop with at least one oscilloscope in the background. I wasted four minutes learning absolutely nothing more than I could have learned from one (still) frame.
Screenshot_20210301_092707.jpeg

The deceptively-simple wiring diagram there raises more questions than it answers. If the start winding isn't used, how is any starting torque developed? With only one winding in use, there would seemingly be only an alternating magnetic field, not a rotating one.
sofstartinfo.jpg
 
Top