What happens to the current under different loads? 2

[MrCGreen]

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?

 

[sreid]

If airflow to a fan is blocked, the fan is not pumping any air and the motor draws the current needed to churn the air. The current for churning is almost always less than current for pumping the air.

 

[LionelHutz]

The motor will only be run at loaded current levels if it is doing work. If the fan it is spinning does not move any air then there is no work being done.

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.

 

[rbulsara]

Thats the basic characteristics of centrifugal machines, fans, pumps, etc. As explained before, No displacement, no work. The seal around the blades/vanes is not fluid tight as it would be around a piston in positive displacement (reciprocating) pumps, so no pressure builds when the dischagre damper or valve is closed and the fluid keeps churning in the casing. If the churning allowed to persist in pumps (not fans) the liquid may buildup heat due to friction losses and can do damage.

 

[bklauba]

Think about the pitch / speed of a vacuum cleaner (typically a universal) motor: When little air flow is being allowed, the motor speed is high, due to the minimal load on the motor, since it is moving little air. When you disconnect the hose, and allow the room air to enter the VC unrestricted (max load) the motor speed goes down (and the current is high). Your situation is very similar, accept that the motor speed is not affected by the load nearly as much, but the motor current is greatly influenced.

BK

 

[itsmoked]

And THAT bklauba is why everyone thinks more work - higher whine with vacuums. The hose sucks up something that blocks it - motor revs up! It MUST be working harder. LOL

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

I once had to install a damper downstream of large fan to lower the motor current so that the motor would not overheat.
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

 

[stevenal]

Think of your damper like a resistor in a circuit. Closing the damper is like increasing the resistance. Since P=RI^2 the higher resistance should dissipate more power. But in a real circuit, I changes with R. As R goes to infinity, I and P both go to zero.

 

[MrCGreen]

Thanks everyone. That's clarified things for me

and I appreciate the electrical analogy stevenal.

 

[jraef]

I have had the most difficulty getting this message through to electricians doing startups for soft starters used on centrifugal blowers (and occasionally pumps). They always think in terms of opening the vane/valve as unloading the motor and can't understand when the motor overloads. I tell them to close the damper and they look at me like I'm nuts (or at least more nuts than I really am). Then they see the thing work and I have to go through this entire exercise every time.

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.

 

[itsmoked]

So what, on turn on the water would move towards its ultimate head and once that was approached the flow would rapidly diminished until bottoming out?

Bet there were some red faces in that crowd.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Nothing that couldn't be fixed with about 50 more HP per motor. Of course, we were looking at 6 x 1250HP vertical turbine pump motors.... I wonder if GE ever gave them some credit for returning the 1200's?

I gave them a consolation prize though, the soft starters were already rated for 1250HP. Whoop-de-doo!

 

[LionelHutz]

I take it they did some impeller work when they added the extra 50hp to make the pumps capable of producing more head?

 

[TheBlacksmith]

Had the flow versus pressure pump argument with a student years ago. He insisted that flow didn't affect power, only pressure. I told him we could go to the stairwell and he could carry two empty buckets up three flights of stairs and then repeat the process with two full buckets up the same three flights of stairs. He didn't have anything else to say.

 

[jraef]

Lionel, they tried changing the impellers alone, even though Byron Jackson (the pump mfr.) said it wouldn't work, and the motors overloaded after a few hours. So they knew they were right on the edge and then went ahead and changed the motors.

Great analogy TheBlacksmith. Gotta remember that one.

 

[rmw]

It is just amazing how when folks don't get it about flow they just don't get it. To them the discharge pressure is something you can see on a gage and when it goes up their intuition tells them that the pump (fan) power is going up and it boggles their minds when you tell them that when the pressure goes down then the pump is working and the motor is drawing amps.

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

 

[rbulsara]

I think the electrical analogy is flawed as explained...closing the valve or damper here is like open circuit...not flow (no current) no work. For a given source (voltage source), increasing the R, reduces the current and hence reduced power dissipation. Only a constant current source will have more power dissipated at higher R.

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).

 

[waross]

I was called out to look at a 5 HP pump that was overheating.
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

 

[stevenal]

rbulsara,
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.

 

[aolalde]

I agree 100% with Rbulsara opinions. His clear analogy deserves a star.
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