Help me understand dust collector motors and amp draw.

[Michael W. Clark]

I have the same experience as most of the others. More flow = more amps. Motor speed and voltage are considered constant regardless of load, so the amps vary with the work being done.

One thing that throws people is that the fan will often make more noise when it is deadheaded (no flow). They think it is working harder, but the opposite is true.

Whether or not the motor will overload when running the fan by itself depends on the motor/fan performance. Some are non-overloading and the motor can handle the HP generated with a bare fan.

One short story to illustrate, I was working on a 200HP scrubber system (~40,000 CFM) and we were attempting to take an airflow reading between the scrubber and fan. The bottom of the duct was full of mud due to water carryover from the scrubber (they are not supposed to do that if working properly). We shut the fan down and opened an inspection door (~18"x18") to inspect the amount of buildup. We finished and the maintenance guy went to start the fan. I told him we need to close the door before re-starting the fan, but he said it would be ok. The fan did not get up to full speed before the overloads tripped. The open door allowed much more air to be drawn in and therefore a higher amp draw. We shut the door, and we were able to re-start the fan.

Mike

[Steve Peterson]

I have never had the wide open overload problem that Steve speaks of and I have installed quite a few Clearvue cyclones so I don't know if it is an urban myth or only some systems suffer from it. Possibly those that run a motor close to its limits for the system would have an issue.

I got the message from the original owner of ClearVue that suggested plugging the bottom of the cyclone during the initial power-on test. A CV 1800 is OK with the inlet wide open and no filters (venting outside). The cyclone itself provides enough resistance in normal operation of air spinning around, but not when it is a straight shot from the bottom.

The CV 1800 has a bit of margin in the motor size. It has a 5hp motor, but really only needs 3.5hp. Some of the other brands of $700 cyclones may be overloaded if they are vented outside, since the small filters provide enough resistance to limit the load on the 1.5hp motor.

Steve

[Steve Baumgartner]

My real-world experience: I measured the current draw before and after I changed my ductwork from 4" to 6". Afterward, the motor amps increased.

The behavior of blowers runs contrary to many people's intuition. It can take some effort to get your head around it.

[Ian Scofield]

Haha well I stand corrected. Looks like its time to grab the multimeter and test. Lots of good info in this thread though!

[joe milana]

Ok, got the dust collector up and running and here are my observations so far:
I seem to be running about 12.4AMPS with one blast gate open. When I open all of them I get a reading of 13.7.
The motor plate reads (5hp)
VOLT 208-230
FLA 13.6-12.4
SFA 16-14
I assume that reads FLA is 13.6 @ 208V and 12.4 at 230V. Is that correct?
What is SFA?

[Todd Willhoit]

Service Factor Amps.

It appears that your motor has a Service Factor of 1.15, meaning that you can draw 115% of rated FLA without causing immediate damage to the motor. 12.4 A * 1.15 = 14.26 A (Rounded to 14 A.) The motor can provide 115% of the nameplate HP, but the temperature will increase, and likely the motor life expectancy will decrease.

[joe milana]

Thanks Todd. That helps. Am I correct on my interpretation of "FLA is 13.6 @ 208V and 12.4 at 230V", and if I am correct, how do I know how much voltage is going to the motor so I know if I am near or at FLA? ~hope that makes sense~

[Marty Tippin]

how do I know how much voltage is going to the motor so I know if I am near or at FLA?

If you can measure the amps, you must be able to measure the voltage...

[David Kumm]

Joe, if you are running off your vfd, you can set the voltage on the output side to any number lower than what the utility or rpc generates ( within the limits of the vfd ). My power is at 240 and the Phase perfect balances out at 240. Motors like a little higher voltage which in turn gives you a little more room to reduce the amp draw. Dave

[joe milana]

If you can measure the amps, you must be able to measure the voltage...

My vfd has a monitoring function for amp draw, but not one for voltage. I measured the voltage across the two busses in my service panel and i get 231V. Not sure if this stays constant through the vfd and on to the motor.

[joe milana]

Thanks Dave. I found out that the default setting of the wj200 drive is 200 volts. I bumped the setting up to 230V ( which is what I am reading at the service panel) and now I am pulling only ~12.5A at 60hz.

Joe, if you are running off your vfd, you can set the voltage on the output side to any number lower than what the utility or rpc generates ( within the limits of the vfd ). My power is at 240 and the Phase perfect balances out at 240. Motors like a little higher voltage which in turn gives you a little more room to reduce the amp draw. Dave

[David Kumm]

The vfd volt thing has happened to me too. Couldn't figure out why the motor ran warm but low voltage will do that. Does your torit have a radial impeller? If so you can speed it up a little to get amps closer to full. 63-65 hz works pretty well if you can stand the noise. Dave

[joe milana]

Yes, radial impeller. I lined my filter plenum with carpet and I am quite pleased with the reduced noise level. Don't see much difference in AMP draw even with three gates open. I may play with the hz and see what happens.

[Mike Henderson]

Like he said above, if you let it run with all the blast gates open, aka with zero resistance, then the impeller will continue to speed up and speed up since there's no force working against it. When that happens, the motor burns up because it's spinning way faster then it's designed.

Conversely, if there's way too much load, IE all the blast gates are closed, you're creating a vacuum because the dust collector is attempting to pull air through your ducting but nothing is moving because there's no new air being pulled into this system. Therefore the dust collector draws more and more current until it trips the breaker.

In a 60 Hz system, as in the United States, the maximum speed of a two pole induction motor is going to be somewhat less than 3600 RPM. The reason is that with 60 Hz (and two poles), the field is rotating at 3600 RPM. In order to function, the armature must rotate less than 3600 RPM. Current is induced in the armature by the wires of the armature cutting the magnetic lines of force from the field. The difference between the rotation of the field and the rotation of the armature is known as "slip".

When the motor is running with a very light load, the armature will rotate faster because less armature current is needed to overcome the resistance of the motor and the load. As the load increases, the slip increases and the armature turns slower. So the "rated" speed of a two pole induction motor in a 60 Hz environment is 3450 RPM, but that is generally only seen at full load. At lighter loads, the speed will be somewhere between 3450 RPM and 3600 RPM (never getting to 3600 RPM).

For the second part of your post, I expect you're assuming that the fan on the motor will overload the motor if it is run "in the open" - without any resistance in the duct work. And when you open sufficient blast gates, it is essentially equivalent to running the motor/fan combination in the open.

Mike

[Mike Henderson]

Yes, radial impeller. I lined my filter plenum with carpet and I am quite pleased with the reduced noise level. Don't see much difference in AMP draw even with three gates open. I may play with the hz and see what happens.

I would not try to go over 60 Hz. Going with a lower Hz will cause the motor to run slower and less air to be moved. Going over 60 Hz could easily lead to overheating of the motor.

Mike