The Mono Drive is a single phase drive and the Sub Drive is a three phase drive. Only difference between these and most other drives is that they now use a go/no go switch instead of a pressure transducer but, the effect on the power consumption is relatively the same. Systems with pressure transducers can be slow reacting and not very reliable, usually sticking in the full speed position. So the go/no go switch solved some of the slow reaction problems but, caused a few others. I was told by Franklin Electric that their Mono and Sub drive units are made to vary the flow rate of a pump for intermittent use only. Intermittent being taking a shower or filling a washing machine. Any long term water uses such as heat pumps and irrigation needs to be matched to the max output of the pump. When you are using low flow rates the amps are bobbing up and down 45 times per minute between say 2.6 and 3.2 amps. This is showing you that the (go/no go) switch used with the Mono Drive instead of a pressure transducer is causing the pump to ramp up and down about 45 times per minute. Then once per minute the unit does a "bump" as Franklin calls it, which increases the pressure about 4 PSI and completely shuts off the pump/motor. Then because water is still being used the pressure quickly drops and the pump restarts after only a couple of seconds. With a Mono or Sub Drive this process is repeated over and over as long as the flow required is less than the max output of the pump. When you are using maximum flow the amps stay steady. That is because this is the only amount of water that the pump is "designed" to pump for any long term water use. The continuous bouncing and weaving is just how it maintains the speed required for low flows.
What I am hearing and seeing in the field is that the continuous ramping up and down of the Mono Drive and Sub Drive controlled pumps is causing the wire down hole to wear out quickly and even wearing holes in the side of the pump or motor from touching the casing. This motor torque is happening 45 times per minute which is a million times every 21 days of use. I am also hearing that the fan in the Mono Drive needed to cool the electronics is drawing in lint and other debris which reduces the air flow and causes overheating of the Drive itself. Not to mention the ones I have heard about that where destroyed from this air flow area being taken over by fire ants and other critters. Then there is the "stray voltage" caused by the Mono Drive or any VFD. Do a search for "VFD stray voltage" and it will give you other problems to worry about. If you have to purchase a new pump/motor and or Mono Drive every 2 or 3 years, energy consumption is the least of your worries. If your pump system last 15 or 20 years as it should, you can afford to use considerably more energy and still be better off. Save 10 bucks a month on energy and spend $1,000.00 on a new pump every 2 years, or pay 10 bucks more a month for energy and have that $1,000.00 pump system last 20 years, do the math. Even if the VFD can save $10.00 per month in energy, considerably more money would be saved from the pump simply lasting longer. The actual amount of energy saved by a VFD if any, is closer to $3.00 per month on average, which makes saving energy even less important than pump system longevity.
The problem with any variable speed control is that you lose head by the square of the speed, so you can’t slow it down enough that saving energy by the cube of the speed makes any difference. In other words the power consumption between a regular full speed pump and a variable speed pump is basically the same. Lift or pressure required from a well pump or booster pump is constant, so it can’t even lift water out of the well if it is slowed down enough to save energy. Simply restricting the water from a regular pump with a valve will reduce power consumption as much as a VFD.
All a Cycle Stop Valve or CSV does is restrict the flow from a normal full speed pump to match the amount of water you are using. The CSV basically mimics much of the way a VFD pump works but, will triple or quadruple the life of a pump system compared to a VFD. There are many advantages to using a CSV instead of a VFD. A few of which include;
* Same constant pressure as a VFD
* Being able to use a normal full speed pump
* Regular pressure switch and small tank so pump does not start for the ice maker or single glass
* Cost less and makes pumps last longer
When restricted to low flow, some pumps will reduce in amperage more than others. Some pumps use a floating stack impeller design. When the flow is restricted, all these floating impellers drag on each diffuser causing the amps from lowering any further. A pump with a fixed impeller design (locked to the shaft), such as many of the pumps with stainless steel impellers, will not allow the impellers to drag and the power consumption at low flow will be much less than floating stack impeller pumps.
An example would be a 2 HP pump with stainless steel impellers that when restricted with a CSV, the amps will drop from 12 to 5.5. A floating stack pump would only drop from 12 to about 8 or 9 amps. Choosing a pump with good brake horse power at low flow can make a big difference in power consumption.
People who use their well pump to operate a heat pump are usually more concerned about energy consumption than those who only use their pump for water use in the home and/or irrigation. An $80.00 per month electric bill for the well pump sort of defeats the purpose of saving energy with a heat pump. Using a smaller well pump that more closely produces the amount of flow needed for the heat pump is the best way to reduce energy consumption. An additional booster pump or storage tank and a booster pump to supply water to the house when needed will keep the well pump from having to be oversized. What I hear from most Heat Pump customers a few years down the road is, that the energy consumption quickly becomes less important than the life of the pump system. A normal speed pump controlled by a Cycle Stop Valve is proven to last longer than most VFD pump systems. It is virtually impossible for a VFD pump system to save enough energy to payback it’s own cost, before the pump system fails and must be replaced.
