Run central AC unit on generator

[gathermewool]

Only if you are using a VFD in a soft start role. A basic soft start is based on thyristors and does not act on the frequency.

Agreed. Just stating the obvious. This thing isn’t gonna to ramp at a set v/f ratio, I wouldn’t think. My knowledge is admittedly limited when it comes to all of this black magic and sorcer.

 

[KrisZ]

Yes the solid state ones do use Thyristors to ramp the voltage up gradually. They do this by controlling the firing angle of the thyristors. combine this with ohms law and you have an easy way to control current. They don't directly act on the current.

What is Soft Starter? Its Working, Diagram and Applications

Motor Starter. Soft Starter Circuit Diagram. Working Principle of Soft Starter. Advantages & Disadvantages of Motor Starter Applications of Soft Starter

www.electricaltechnology.org

See a bunch of diods on that diagram? They act directly on current, not voltage. A resistor acts on voltage. Voltage regulation on alternating current makes very little sense in this application.

 

[AZjeff]

This unit spreads out the start up load with electronic magic. This vid illustrates it.

 

[Spectre694]

See a bunch of diods on that diagram? They act directly on current, not voltage. A resistor acts on voltage. Voltage regulation on alternating current makes very little sense in this application.

Did you read the article? That's not how any of this works. Those "diodes" are the thyristor (SCRs) that make up the soft start. They are switches that allow voltage through them. They look similar to diodes because they work in one direction. That's also why they are back to back in pairs. That setup allows them to work on both sides of the AC waveform. The entire reason for this device is to keep the inrush current low until the motor acts less like an inductive load and more like a resistive load. Also no diodes don't act on current they are essentially electric check valves some of which like SCR's can be turned on and off.

From the article:
The soft starter is a type of motor starter that uses the voltage reduction technique to reduce the voltage during the starting of the motor. The soft starter offers a gradual increase in the voltage during the motor startup. This will allow the motor to slowly accelerate & gain speed in a smooth fashion. It prevents any mechanical tear & jerking due to sudden supplying of full voltage. The torque of an induction motor is directly proportional to the square of current. & the current depends on the supply voltage. So the supply voltage can be used to control the starting torque. In a normal motor starter, applying full voltage to the motor generates maximum starting torque which possess mechanical hazard to the motor. Therefore we can say that a soft starter is a device that reduces the starting torque & gradually increase it in a safely manner until it reaches it rated speed. One the motor attains its rated speed, the soft starter resumes the full voltage supply through it. During motor stopping, the supply voltage is gradually reduced to smoothly decelerate the motor. Once the speed reaches zero, it breaks the input voltage supply to the motor.

 

[Spectre694]

Agreed. Just stating the obvious. This thing isn’t gonna to ramp at a set v/f ratio, I wouldn’t think. My knowledge is admittedly limited when it comes to all of this black magic and sorcer.

That makes sense. This thread is just really getting alot more detailed than the OP really requested. For a soft start though the freq is equal to the line freq the entire time. In the US that's 60Hz. The soft start starts at a lower voltage and then as the motor begins to move faster and faster the SCR on time increases until the motor is directly connected to the line with the SCR's on all the time. All this does is turn an inductive load into a resistive load more or less and removing the inrush current.

VFD's are much more advanced and the Freq is different than the line freq. This allows for full speed control at full power which is not possible with the soft start.

 

[KrisZ]

Did you read the article? That's not how any of this works. Those "diodes" are the thyristor (SCRs) that make up the soft start. They are switches that allow voltage through them. They look similar to diodes because they work in one direction. That's also why they are back to back in pairs. That setup allows them to work on both sides of the AC waveform. The entire reason for this device is to keep the inrush current low until the motor acts less like an inductive load and more like a resistive load. Also no diodes don't act on current they are essentially electric check valves some of which like SCR's can be turned on and off.

I did read the article and just like you it talks about controlling voltage in one sentence only to talk about current in another.
In bolded section , you just explained it how I said it works, on limiting current. Diodes pass current in one direction, and SCR is a diod that can be turned on or off, that’s the difference and hence similar symbol. How is all of this not acting on current?

 

[Spectre694]

I did read the article and just like you it talks about controlling voltage in one sentence only to talk about current in another.
In bolded section , you just explained it how I said it works, on limiting current. Diodes pass current in one direction, and SCR is a Dior that can be turned on or off, that’s the difference. How is all of this not acting on current?

Current as in power not current as in amps. Diodes pass alternating current or direct current in one direction. They don't control current as in amps. There aren't any devices that I'm aware that can directly control amperage except as a function of using it for work

This confusion is probably on me for not making sure we use the correct terminology.

Soft starts control those SCRs by turning them on and off. That controlls the flow of alternating current through the device. The lower initial voltage out lowers the maximum amperage (current) available to the device. As it then ramps up the voltage by allowing the SCRs to stay on longer more amperage (current) is available but at that point the motor is mostly resistive and doesn't require the massive inrush amperage.

 

[zzyzzx]

That's more or less a hard start without the disconnect relay. They don't actually lower the current needed just the amount of time that current is needed for.

So it's a soft start with power factor correction!

 

[gathermewool]

So it's a soft start with power factor correction!

For the very short ramp-up/soft-start period, I would think. Don’t most inductive loads have capacitors for passive PFC anyway?

 

[Spectre694]

So it's a soft start with power factor correction!

Or overcorrected if its too big for the load. You do actually want something to remove the start cap from the circuit. PTC or relay are the most common.

 

[Spectre694]

For the very short ramp-up/soft-start period, I would think. Don’t most inductive loads have capacitors for passive PFC anyway?

You would hope but not always and even the usually not as good as it could be.
Most residential stuff treats power factor as an afterthought if it's addressed at all. That's because in most areas at least in the US charge by the KW father than KVA like most commercial and industrial.

KVA is higher than KW with poor PFC.

 

[gathermewool]

You would hope but not always and even the usually not as good as it could be.
Most residential stuff treats power factor as an afterthought if it's addressed at all. That's because in most areas at least in the US charge by the KW father than KVA like most commercial and industrial.

KVA is higher than KW with poor PFC.

Where I am, it’s by the KW mother. It costs more and they call all the time asking if I can turn the heat up.

 

[dlundblad]

I’ve thought about installing a soft start in my heat pump/ AC. My generator can do it, but it doesn’t like it.

I’m not too worried about AC. If I’m home, I’d turn the heat to emergency heat (gas). If I’m not home and the generator kicks on with mild temps, it’ll have to start the heat pump eventually.