Question about 2-wire centrifugal pump, VFD and too small whole house generator

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Jim Goodman

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I have an 800' deep well that serves my house, with a 2-wire, 1hp Grundfos pump. This forum was very helpful in solving a problem I had a few years ago with the prior 1-1/2hp pump that was too large and quickly burned up. The advice I received led to the current 1 hp pump, along with a Cycle Stop Valve and Cycle Sensor. This combination has worked flawlessly. Please hold that thought.

My house is 7 years old, not large (2400 square feet), and very energy efficient (Passivhaus). I also have a Generac 16kw Guardian whole house generator that runs on natural gas. If I were to do it again, I would get a 22kw generator but the 16kw is the one I have and I'm stuck with it. We lose power pretty often.....I'd say once every couple of months. The big electric draws, any of which that could come on at any time without my physical actuation, are the heat pump, electric water heater, and the well pump. The generator has a load shed module for the heat pump. If the HVAC is on and the water heater is off and the well pump comes on, there's no problem. If the HVAC is off and the water heater is heating and the well pump comes on, there is no problem. But if the HVAC is running AND the water heater is heating AND the well pump kicks on, the generator cannot handle it. The reason is because of the inrush current spike as the well pump tries to turn on, which is about 5X the full load amps. Please hold that thought.

So, I'm in the process of setting up my woodshop and I have a 5hp, 3-phase table saw that I will be running off a VFD. I've been on other forums asking for advice about this and one guy mentioned that with a VFD I will not have an inrush current spike because a VFD will ramp up the motor. This got me thinking about my well pump/generator problem. NOTE: I am NOT talking about or considering a pump controller that would vary the speed of the pump and the corresponding flow rate. What I am wondering about is........could I use a VFD simply to ramp the startup of the pump so there would be no spike in the inrush current and thereby permit the pump to start when under generator power, even when the HVAC and electric water heater are under load? My pressure tank and pressure switch are in my utility room in the house, so there is a nice, clean, dry, conditioned space in which I would install this VFD. So, this VFD would not be a part of a pump control system. Its sole purpose would be to ramp up the 2-wire pump, which I also could not do with a pump controller because I believe they are always used with a 3-wire pump. My intention would be to install the VFD between the pressure switch and the well pump.

Thanks for any advice on this
 
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Fitter30

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Probably not, two wire pump has all the starting components built in to the pump. Have the brand and model of pump then you call their tech line. Could install a two relays that when pumps calls shuts off the water heater or the condenser with a time delay 1-2 minutes to keep water heater from short cycling or 5 minute for the condenser if it doesn't have one when on gen power
 

Jim Goodman

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Probably not, two wire pump has all the starting components built in to the pump. Have the brand and model of pump then you call their tech line. Could install a two relays that when pumps calls shuts off the water heater or the condenser with a time delay 1-2 minutes to keep water heater from short cycling or 5 minute for the condenser if it doesn't have one when on gen power
Thank you.
 

Reach4

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I presume the Grundfos pump is a "4 inch" type. 2-wire 4 inch pumps take more current to start than 3-wire pumps with a control box.

Is your water heater a classic electric unit with resistance elements, or is it a heat pump "hybrid" type? If conventional type, short cycling would not bother the WH. So that idea to interrupt the WH power when the well pump is on and the generator is on sounds like a good one.

If hybrid, you might try putting the unit into heat-pump-only mode. I am wondering if that might reduce the max current consumption, tho the WH would hot recover as fast. Maybe use a bath rather than a shower when the generator is running.
 

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I have an 800' deep well that serves my house, with a 2-wire, 1hp Grundfos pump. This forum was very helpful in solving a problem I had a few years ago with the prior 1-1/2hp pump that was too large and quickly burned up. The advice I received led to the current 1 hp pump, along with a Cycle Stop Valve and Cycle Sensor. This combination has worked flawlessly. Please hold that thought.

My house is 7 years old, not large (2400 square feet), and very energy efficient (Passivhaus). I also have a Generac 16kw Guardian whole house generator that runs on natural gas. If I were to do it again, I would get a 22kw generator but the 16kw is the one I have and I'm stuck with it. We lose power pretty often.....I'd say once every couple of months. The big electric draws, any of which that could come on at any time without my physical actuation, are the heat pump, electric water heater, and the well pump. The generator has a load shed module for the heat pump. If the HVAC is on and the water heater is off and the well pump comes on, there's no problem. If the HVAC is off and the water heater is heating and the well pump comes on, there is no problem. But if the HVAC is running AND the water heater is heating AND the well pump kicks on, the generator cannot handle it. The reason is because of the inrush current spike as the well pump tries to turn on, which is about 5X the full load amps. Please hold that thought.

So, I'm in the process of setting up my woodshop and I have a 5hp, 3-phase table saw that I will be running off a VFD. I've been on other forums asking for advice about this and one guy mentioned that with a VFD I will not have an inrush current spike because a VFD will ramp up the motor. This got me thinking about my well pump/generator problem. NOTE: I am NOT talking about or considering a pump controller that would vary the speed of the pump and the corresponding flow rate. What I am wondering about is........could I use a VFD simply to ramp the startup of the pump so there would be no spike in the inrush current and thereby permit the pump to start when under generator power, even when the HVAC and electric water heater are under load? My pressure tank and pressure switch are in my utility room in the house, so there is a nice, clean, dry, conditioned space in which I would install this VFD. So, this VFD would not be a part of a pump control system. Its sole purpose would be to ramp up the 2-wire pump, which I also could not do with a pump controller because I believe they are always used with a 3-wire pump. My intention would be to install the VFD between the pressure switch and the well pump.

Thanks for any advice on this
The 2 wire motor is most likely the problem as they have a pretty hard start. A 3 wire motor with a capacitor start box has much less inrush current on start up. Then with a 1HP pump using 630' of #10 wire or 990' of #8 will also help with Inrush currents. The longest length of the smallest wire possible for that size motor will result in 5% voltage loss, which causes a 20% reduction in starting current and 36% reduction in torque. The motor just can't reach full inrush current if the wire is not large enough to supply it. But the pump will still start just as well.

These things will require some changes to be made as you would need to replace the motor and wire. However, there is one thing you can try without making any changes to the equipment.

Starting a pump against a closed or almost closed valve cannot do anything about the amplitude of the inrush current, but can greatly help with the duration. The CSV1A is pressure controlled. When adjust to 50 PSI it is only closed to the 1 GPM position when the pressure is higher than 50 PSI. So, with the CSV set at 50, turn the large screw in the pressure switch about 5 full turns to the right. Setting the pressure switch to 55/75 means the CSV will be in the 1 GPM position when the pump starts. This by itself may not be enough to help with the generator start, but it is worth a try and I would like to know if you can tell any difference?
 

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Soft starting a regular submersible motor requires caution and may not change the inrush current as much as you think. The Kingsbury type thrust bearing in the motor needs at least 50% of full speed to cause the hydroplane effect needed to put a film of water between the bearing surfaces. In other words from zero to half speed the bearing is running dry and wearing quickly. The motor must reach 50% speed or 30hz in less than a second or the bearing will be damaged. So, a soft started needs to be able to quickly get the pump to 50% speed, then it can slowly increase speed from 50% to 100% as needed. But the necessity to get to 50% speed in 1 second causes any reduction is inrush current to be limited. Not all soft starters can do this.

The best I have found for low inrush current on start up is the Grundfos SQ pump. It has an oil filled motor and ball bearings making it possible for the controls in the motor to take 5 seconds to get the motor up to full speed. The SQ pump also has a very good drop in current or horsepower when restricted and works very well with a Cycle Stop Valve.
 

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They do make 2 wire VFD's these days. They are expensive, and I don't think any more reliable or longer lasting than a 3 wire VFD, but they are convenient for 2 wire pumps with problems like yours. When using a VFD you should not need the CSV as they do the same thing but in different ways. You can set the set point of the VFD to 50 PSI same as with the CSV. However, as with the 3 wire VFD's, they will cost you a lot more in the long run. Most people are finding that VFD's will need to be replaced about every 5 years on average, which can get expensive compared to a CSV system than should last 30 years without any maintenance. A VFD also runs the pump on pulsing DC voltage with lots of voltage spikes, harmonics, and other problems that you don't have with normal sinusoidal power as when using a CSV.
 

Jim Goodman

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They do make 2 wire VFD's these days. They are expensive, and I don't think any more reliable or longer lasting than a 3 wire VFD, but they are convenient for 2 wire pumps with problems like yours. When using a VFD you should not need the CSV as they do the same thing but in different ways. You can set the set point of the VFD to 50 PSI same as with the CSV. However, as with the 3 wire VFD's, they will cost you a lot more in the long run. Most people are finding that VFD's will need to be replaced about every 5 years on average, which can get expensive compared to a CSV system than should last 30 years without any maintenance. A VFD also runs the pump on pulsing DC voltage with lots of voltage spikes, harmonics, and other problems that you don't have with normal sinusoidal power as when using a CSV.
Thank you, Cary. You've answered my questions. I had my Generac dealer out today to do PM on my generator and to investigate a possible load shed module for the water heater (Rheem Marathon, 4500 watts, resistance elements). As I said earlier, if I were to do it again, I would have gotten a larger generator, but I'm stuck with my 16kw unit. The options I had were to program the load shed module to completely turn off the water heater while on generator power, for however long the utility outage lasted, or to implement a 5-minute delay at the loss of utility power before the water heater would come back on under generator power. I looked for some clarification about Generac load shed modules on an electrician's forum website and if I understood correctly, the Generac load shed modules are not true load shed devices. The kind of device I was hoping for would detect changes in load while the generator is running and shed and reconnect loads as the load draws varied in the course of normal daily activities. This is NOT what the Generac modules do. Their purpose is to just shed loads at the initial startup of the generator so that the generator is not overloaded. Priority load 1 would come back up after 2 minutes. Then priority load 2 would come back after 5 minutes, then priority load 3 would come back up after 10 minutes.....something like that.....and then all 3 loads would remain drawing power regardless of whether the generator was overloaded or not. I have not had a problem with my well pump starting under generator power (48 LRA) with the heat pump running, but if we had an ice storm and my heat pump resistance strip heaters kicked in, I'd have a problem. I'm not too concerned about the daytime when I'm home, in that if utility power goes out, I can run to the panel and flip off high load breakers. But if we're sleeping or out of town, and the power goes out and the high loads are running, I cannot do anything and could damage my generator and who knows what else.

So, I have another idea that would really solve my problem, but I don't know if it's doable. Could I take a momentary switch, like you'd use on a machine in a workshop, and an appropriately sized magnetic starter and install those between the pressure switch and the well (for the pump) and install those between the panel and the water heater to control the water heater? When utility power goes out, there's about a 15-second delay before the generator starts, so that 15-second dead period would shut down the pump, if running and shut off the water heater circuit. Then if I was out of town or sleeping, it would protect the generator from overload and I also would be able to manually control the energizing of those circuits with an on/off switch. The main advantage over just flipping breakers is that the circuits would automatically be broken when utility power went out, whether I was there or not. Is this idea feasible and would it not damage the well pump?
 
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You can certainly put a contactor or two pole relay on the wires between the pressure switch and pump or even before the pressure switch. Wouldn't make much difference. Don't see why one between the water heater and breaker wouldn't be OK as well. Just don't know how you would energize and de-energize those according to having grid power or not. If you can make them stay energized until you loose grid power, then come back on after the generator starts, I don't see why it wouldn't work.
 

Jim Goodman

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You can certainly put a contactor or two pole relay on the wires between the pressure switch and pump or even before the pressure switch. Wouldn't make much difference. Don't see why one between the water heater and breaker wouldn't be OK as well. Just don't know how you would energize and de-energize those according to having grid power or not. If you can make them stay energized until you loose grid power, then come back on after the generator starts, I don't see why it wouldn't work.
This is how I see it working: When utility power goes out, I lose all power for about 15 seconds before the generator starts up on its own. During that time, if the pump happened to be running, it would shut down. If the water heater happened to be heating, it would shut down. The relays would automatically open with the loss of power, just as a motor starter does on a table saw, for example. Then, once power resumes, whether that be generator power or restored utility power, I could physically push the spring-loaded 'on' button of the momentary switch and this would activate the relay to close, and the water heater would again be energized and the same of the pump. Just thinking about that now, I might instead need to have the relay between the panel and pressure switch so that when the power came back, whether generator provided or utility provided, it would energize the relay. If the relay were between the pressure switch and the pump, the pressure switch would need to be closed in order to provide power to the relay, and that would only happen if I also happened to be demanding water at that time. The whole purpose of this scheme is not just to ensure that I can control the usage of the big loads when under generator power (which is possible with throwing circuit breakers, I know) but also to deal with the situation when I am not at home, or the very rare scenario when I am sleeping when a utility power outage occurs, like in an ice storm, and the resistance heat strips in my heat pump are active, and the water heater is heating water after showering at night and someone flushes the toilet and the pressure tank is finally drained to the point that the pressure switch kicks on the well pump. If all of those loads hit the generator at the same time, it cannot handle them. I would be able to restart the well pump but leave off the water heater, for example. The key feature is the automatic shut down with no automatic restart of those relay-controlled loads.
 

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I am sure when the power goes out you also lose Internet or wifi? But if Internet comes back when power comes on you could use it to turn the well pump and heater back on when you want. I would not power the coil in the relays with the same line power as going to the heater or pump. Use a 115V coil in the contactor and connect it to one of those 115V smart plugs. I can turn on lamps and irrigation zones using these little smart plugs, so turning on the well pump and heater would work the same way.
 

Jim Goodman

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I am sure when the power goes out you also lose Internet or wifi? But if Internet comes back when power comes on you could use it to turn the well pump and heater back on when you want. I would not power the coil in the relays with the same line power as going to the heater or pump. Use a 115V coil in the contactor and connect it to one of those 115V smart plugs. I can turn on lamps and irrigation zones using these little smart plugs, so turning on the well pump and heater would work the same way.
Great idea, thanks.
 

Reach4

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You don't have a UPS on the computer and modem?
 

Jim Goodman

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You don't have a UPS on the computer and modem?
I do, but the service is always interrupted despite the UPS. We're on AT&T fiber and we're 800' from the street and there are a couple of other devices that AT&T has on the system besides the gateway and more often than not, we have to do a manual reset. I don't know why, but we do.
 
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