Originally Posted by PB_Smith
can someone please explain to me how an electro-magnet that is using nominal house current and has no rheostat or other means of adjustment is going to consume more or less energy by virtue of the flow rate???
the current and amperage is the same and constant, the only difference between pumps with more or less flow from these type of pumps/impellers is by increasing or decreasing the impeller size.
I really think some of you are off target because you are extrapolating information from other types of systems and applying it to a completely different type of system.
Again, magnetic drive pumps have a more or less free floating impeller and I fail to see how adjusting the water flow is going to increase or decrease the power consumption when the power is being supplied at a fixed, constant rate.
One of you electricians need to do an actual test to see if there is any increase in the electrical current being drawn/supplied when the flow is restricted or full open.
My gut tells me that in the small pumps we use, there will be no discernible difference in power consumption either way.
We need to see real numbers in action to settle this very crucial, vital and important piece of information once and for all. The future of the free world depends on it!!!
Well, here goes. [flash back to 2nd year electrical apprenticeship - 10 week course all compressed into a couple paras]
First off, your obviously not well informed about electrical. Not a jab, but you will have to accept the fact that much may be over your head. there is NO such thing as nominal electrical current. Electrical current varies greatly depending on the load applied. nominal electrical voltage is 120v (for arguments sake). the current will vary depending on the impedance. Impedance, what the heck is that???
Its a combination of resistance caused by wires and other physical devices [Resistance is constant for a motor] and inductive reactance. Reactance can change greatly in a motor under changing loading. Reactance has a much greater effect on impedance than resistance. In fact, resistance has a negligible effect on the motors load characteristics.
When a motor is under greater loads, the phase angle between the voltage and the current increases greatly due to the electro magnetic force applied. This increased phase difference is responsible for the increased impedance
(by cos[theta]). Because the impedance increased, the current also increases proportional to impedance, drawing MORE power (I=V/Z). BTW, current and amperage IS the same (second para), because they mean the same thing. Current is measured in amperes. But it is NOT constant.
Now, as for the pumps, FORGET about pressure. Pressure is constant in a throttle valve situation. What
. Look, water is not compressible. Pressure is proportional to the pipe diameter and flow rate squared. If the pipe diameter before and after the throttle valve is the same, and, because the throttle valve cannot add or remove water, then the flow rate before and after the throttle valve is the same. Therefore, if both factors are the same on both sides of the throttle valve then the pressure is the SAME. Now, I think everybody can appreciate that shutting a throttle valve some DECREASES the flow, then it also DECREASES the pressure. I know it sucks, but there it is.
If you have decreased the flow, you have also decreased the work required because, as Jeff has pointed out, moving less water the same distance requires less work.
BUT, and this one is for Jeff, by throttling back the pump/motor you have rendered it less efficient, and they will heat up somewhat. They will draw less power, but they will convert a greater percentage of the drawn power into heat, which is why throttle valves are not used temperature situations