Starter vs VFD

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texie

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Fort Collins, Colorado
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Electrician, Contractor, Inspector
Customer bought a used 10 HP 3 phase sawdust collector. As per usual they don't understand the details. Unit factory wired for for 480 volt with an IEC starter. Need to convert to 208 and get a NEMA size 2 starter as the existing starter will now be to small. I noted that the motor is inverter rated and since the customer also does not understand air flow either and I don't know the details on the ducting I'm wondering if it might be just a easy to just use a VFD so he can adjust as needed. Thoughts?
 
Low voltage on that motor is going to be 240. Can you get a VFD that will do 208 on line side and 240 out?
 
I have wired hundreds of collectors, flow control is best achieved with a damper on the discharge, and blast gates on the inlets to the machines. Slowing down the CFM flow can cause the system to plug up. A VFD is normally a waste of money, and an additional point of failure. Lack of flow, is a bigger problem than excessive flow. When the filtration gets loaded up, it will reduce flow and effectiveness. Give it full flow and adjust the building pressure with the outlet damper.
 
MTW said:
I have wired hundreds of collectors, flow control is best achieved with a damper on the discharge, and blast gates on the inlets to the machines. Slowing down the CFM flow can cause the system to plug up. A VFD is normally a waste of money, and an additional point of failure. Lack of flow, is a bigger problem than excessive flow. When the filtration gets loaded up, it will reduce flow and effectiveness. Give it full flow and adjust the building pressure with the outlet damper.
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Adjusting the flow with the outlet damper(s) will still affect the overall flow. The building pressure is determined by the net air flow into the building (supply of outside air) and the leakage rate to the outside. Given that, you cannot decrease the building pressure without decreasing air flow or opening addtional outlets from the building.

(I am sure you understand this, but it might not be clear to someone with no experience reading your post.)
 
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GoldDigger said:
(I am sure you understand this, but it might not be clear to someone with no experience reading your post.)
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When the collector is installed properly the discharge is piped back into the building, therefore the majority of the pressure differential goes away and flow is maintained and the building pressure is kept at a reasonable level. If it’s not piped correctly it can become a serious hazard. Pressure can become so negative that just trying to open an exterior door can be a problem, you could loose appendages with the door slamming behind you as soon as you release it. I have seen this condition so bad where it actually destroys doors and hardware when not corrected, not to mention the effect it can have on the heating bill, when huge drafts are sucked in at every door opening and every small crack or hole. The larger the blower the larger the problem can become.

Thats why its so important to get the ducting installed correctly. Once thats done properly then the discharge damper can get minor adjustments to equalize building pressure without choking the system flowrate to cause other problems.

As mentioned previously, adjustable blast gates on the inlet side, one at each machine allows the operator to fine tune the flow to where they need it, and to help control the building pressure. The bigger the collector the more serious this balancing act becomes. 10 HP is not a large collector so having it ducted properly is imperative to getting to do the job that was intended to do, without causing these other problems. Reducing flow by slowing the blower is a bad idea for it’s effectiveness.
 
I've never dealt with one on a VFD. I'm not seeing much difference between controling CFM bu speed of by a damper on the discharge. That being said I see no advantage in using a VFD. When you throttle the discharge you lower the load on the motor, so a VFD really won't save energy.

Not to mention the blower is centrifugal so it's going to have a curve. So small changes above and below rated speed are going to create large changes in pressure/flow
 
Joethemechanic said:
When you throttle the discharge you lower the load on the motor, so a VFD really won't save energy.
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While this is true, flow control via a VFD saves MORE energy compared to throttle control. Energy varies linearly by the flow reduction using a throttle, but energy varies by the CUBE of the speed change. Speed change is not the same as flow change, but still, the motor will consume significantly less energy using a VFD. This is a well documented fact.

In a dust collector it is important however to make sure that the VFD is programmed with a minimum speed that corresponds to what it takes to avoid plugging up the system.
 
Jraef said:
While this is true, flow control via a VFD saves MORE energy compared to throttle control. Energy varies linearly by the flow reduction using a throttle, but energy varies by the CUBE of the speed change. Speed change is not the same as flow change, but still, the motor will consume significantly less energy using a VFD. This is a well documented fact.

In a dust collector it is important however to make sure that the VFD is programmed with a minimum speed that corresponds to what it takes to avoid plugging up the system.
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Yeah but centrifigal fans and pumps, when you lower the speed (lets say 1/2) you reduce the pressure/flow by more than 1/2. I can't tell you an exact figure without knowing the curve of the device.

But yeah I could see the VFD saving more energy, if nothing else you are cutting down mechanical friction and windage losses
 
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