What happens to the current under different loads?
What happens to the current under different loads?
(OP)
despite my previous beliefs, i've been informed a fan in a duct would draw less current if a damper down stream of it had been closed. I assumed the fan running would build up air pressure between the fan and the damper causing the motor to work against a higher load and in turn cause a higher current to be drawn. why is this not the case?
also - whatever the explanation for the above is - does it also relate to pumps and the currents been drawn under different loads?
also - whatever the explanation for the above is - does it also relate to pumps and the currents been drawn under different loads?





RE: What happens to the current under different loads?
RE: What happens to the current under different loads?
In practice, some work is being done to overcome the losses in the fan. But, for the most part, it's just spinning the same air around inside the housing which is much less work than moving air through the fan.
RE: What happens to the current under different loads?
RE: What happens to the current under different loads?
BK
RE: What happens to the current under different loads?
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: What happens to the current under different loads?
Similarly I have throttled discharge valves on water pumps to reduce the current draw. Throttling the discharge on a centrifugal water pump does increase the pressure, but the flow drops faster than the pressure increases and less work is done. Motor current drops.
Check some flow-versus-dynamic-head charts for centrifugal pumps.
respectfully
RE: What happens to the current under different loads?
RE: What happens to the current under different loads?
and I appreciate the electrical analogy stevenal.
RE: What happens to the current under different loads?
To that end, I too appreciate the analogy stevenal, wish I had thought of that. Electricians can understand that (well, at least some of them).
On one large pump system I did it got even more bizarre, considering I had to have this conversation with a registered PE who was also the local IEEE group leader. He designed a 2300V electrical system with soft starters, but he didn't design the mechanical/civil side. The pumps had to lift from a well site to a water tank on the side of a hill 3 or 4 miles away and the total elevation was something like 390 feet. The pumps were working fine, but after about 15 minutes, the load would drop off significantly. So they all immediately blamed the soft starters (because they didn't understand them). I immediately said they had no more flow, so the pumps were no longer loaded. They would open a local valve, the water would gush out, the current would spike and they were all baffled. Having done a LOT of pump systems, I said "not enough head", to which they replied, including the electrical PE, that if the pumps couldn't create enough head, the motors would stall!
When it all settled down, it turned out the pump system as a whole was only able to put out about 385 feet of head. So they could literally see the water in the pipe, but it wouldn't go any further. No flow, no full loading on the motors.
RE: What happens to the current under different loads?
Bet there were some red faces in that crowd.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: What happens to the current under different loads?
I gave them a consolation prize though, the soft starters were already rated for 1250HP. Whoop-de-doo!
RE: What happens to the current under different loads?
RE: What happens to the current under different loads?
RE: What happens to the current under different loads?
Great analogy TheBlacksmith. Gotta remember that one.
RE: What happens to the current under different loads?
To some I have had to deal with, they never got it. Even though they were watching an amp meter as well as the discharge pressure gage.
Oh well....
rmw
RE: What happens to the current under different loads?
Closing a valve may be akeen to increasing the resistance, but it does reduce the flow (current) as the centrifugal machine have only certain head (pressure=voltage) and hence the reduced power (work).
RE: What happens to the current under different loads?
I turned down the discharge valve until the amps dropped to the full load rating. Then I wired the valve in position.
The plant maintenance guy knew better. He removed the wire and opened the valve wide. Then I was called back to look at a burned out motor. I was out of patience with this customer.
"Sorry, I don't really know anything about pumps. Get your guy to fix it. He knows a lot more than I do about pumps."
Bad attitude? Probably, but I earned it. Too many years of being the second one called to this plant after the cheap guy didn't fix the problem but caused a lot of secondary damage.
respectfully
RE: What happens to the current under different loads?
I avoided speaking of current sources, voltage sources, and other idealized elements. Seems an invitation to trouble as folks here like to argue. Instead I went straight to the real source, where current is affected by R and goes to zero as R goes up just as you said. I stand by my explanation as stated.
RE: What happens to the current under different loads?
The motor load demand and finally current depend on the driven machine power demand.
As he explained, the pressure on centrifugal machines driving fluids is almost a constant, then reducing the fluid displacement or volume rate reduces the power demand.
Power = k*Pressure*Volrate