add Soft Start on Cyclone?

[Will Blick]

Mr. Prosecutor, your case was well presented, your position is clear....
the judge has ruled on your closing argument -
the forum should be closed, as there is no need for it,
ww's should do their own independent research and refrain from asking ww questions on internet forums,
the risk of learning in such a manner represents too much risk to our intellectual health...

case closed....

next....

[Chris Parks]

I put this up for comment only as it blows a theory of mine and also surprised me with the start up results. I have a VFD driving a ClearVue cyclone so I can read the amp draw and during start up the amps do not exceed the running current draw. I expected the current draw to exceed the running current draw by a substantial amount but it does not seem the case at all. The running current draw at 60hz is between 8.9 and 9.2 amps but the surprise to me was that figure does not vary between all gates open and all closed and that little bit of information surprised me as I thought there would be a noticeable difference with it being higher with all gates open. If it does vary it is only in a decimal point or two. What I have noticed is that as the motor has had more running the current draw has dropped from a figure that was always 9.0 plus generally about 9.3, 9.4 to now it has dropped to just under 9.0, roundabout 8.9, 8.8 but generally 8.9. Comments welcome as I know little to nothing about VFD's except they get my cyclone to run at 60hz and have the potential to do all sorts of magic things if I ever get to understand the thing. I am also going to use one on my DP to vary the speed between the gears.

[Dan Hintz]

Chris,

VFDs can control current draw... it's very likely your VFD has been programmed to soft-start, preventing the draw from surging.

[ian maybury]

I guess it all depends on the duty the motor is designed and specified for - versus how often it's started and stopped.

Start up draws much higher currents than does steady state running, and so produces lots more heat. Plus the cooling delivered by the fan is compromised by the lower rpm run for part of the time.

A marginally specified/cheapo motor that's sailing close to the wind on ability to handle high currents, dissipate heat and tolerate raised temperatures is going to be more at risk of failure than a more robust model. After that it's a matter of the duty cycle, and especially of whether or not it reaches some critical temperature above which stuff (the insulation on the windings?) starts to degrade. i.e. how often you start and stop it. How much time it spends running raised currents, and how much time there is to cool. (it'll probably cool faster if left running at full speed than if stopped given the forced draught created by the fan)

It seems that something like a VFD may help because by ramping the start it reduces the start up current draw from the single phase supply on start up to about x3 the running current versus the x5 or more of a direct started single phase motor. Plus three phase motors are usually more robust anyway.

In practice good quality modern motors seem to have much more tolerance of this sort of thing (higher temperature capabilities due to better insulation materials on windings etc), although it may be that cheapie models are not so good. The big three phase motors used on large (full building) HVAC applications have quite a long start cycle and certainly don't like to be started too often.

As the joke goes - they need the smoke to run. Let it out and they stop...

ian

[george wilson]

I know that DC's draw less current when the pipes are all closed,but WHY does my Oneida manual say to always have a pipe open when starting up the DC?

I think I must have a weak breaker,because I can only start my Dust Gorilla 2 h.p. 3 times each hour,or it cuts off. Could be that I seldom run the DC for more than a few minutes at a time to saw a board,etc.,and don't give the motor time to cool enough? It has a Baldor motor. Its so loud I don't like to leave it running.

[Dan Hintz]

I know that DC's draw less current when the pipes are all closed,but WHY does my Oneida manual say to always have a pipe open when starting up the DC?

Are they using any of the air flow to cool the motor? If so, that's a good reason...

[David Kumm]

Chris, the start up is normal. Vfd's are programmed to avoid the inrush. The fact that the amp draw doesn't change leads me to think your system either has enough leaks around the drops or the piping and bags are maxed out so the blower can only force so much air out and opening gates doesn't change the amount passing through the fan. My amp draw doesn't change alot but more than yours and there is a point where it tops out. That is the point at which the capacity of the pipe and filters is maxed out. You didn't say what size motor you are running, the size uf the pipes and the type of filter. That info would help. Dave

[Noah Katz]

I don't think heat at startup is an issue for the motor windings, as the time startup time is brief and they have a lfair amount of thermal mass.

Not so the caps.

As a point of interest, I put an ammeter on my 5 HP 1-ph Leeson, and inrush is a bit over 100 A.

[Myk Rian]

I was questioning the assertion that moving air is a significant part of the starting load of a fan.

I thought I explained that, as did Ian.

[Chris Parks]

Chris, the start up is normal. Vfd's are programmed to avoid the inrush. The fact that the amp draw doesn't change leads me to think your system either has enough leaks around the drops or the piping and bags are maxed out so the blower can only force so much air out and opening gates doesn't change the amount passing through the fan. My amp draw doesn't change alot but more than yours and there is a point where it tops out. That is the point at which the capacity of the pipe and filters is maxed out. You didn't say what size motor you are running, the size uf the pipes and the type of filter. That info would help. Dave

No filters or bags, 5hp Leeson motor, six inch pipes. What I did BEFORE I put the ducting on was test it by blocking the inlet then running it with just the open inlet. That takes out any variables and the current hardly changes, remember this is with nothing on the exhaust. How much does yours change and are you using a VFD or a clamp meter to measure it?

[David Kumm]

Chris, What size fan are you running? I'm wondering if you have no bags so there is no outlet restriction, the motor might have enough capacity that the amps don't change on the input side because you aren't close to maxing it out. I run a 5hp with 8" pipe and a 15.75 onieda fan rated at 1950 cfm. It may be that the 6 inch pipe restricts the cfm enough - 6= 55% of 8- that the gates are irrelevant. If you are running a smaller fan your motor may never use more amps. mine runs more fully closed than yours does open. Dave

[Chris Parks]

It is a 15" fan, standard CV. I was always of the notion and it stood to reason, my reasoning any way, that if the fan was pushing air then it was drawing more current than if the air was stalled as occurs if the inlet is blocked off. A funny thing happened the other day, I started it (accidently) in reverse and the amp draw (running) was only 6.5. I am there looking at the VFD, scratching my head and only getting splinters in the end of my fingers trying to work out why and also the different noise it was making. BTW nothing fell off it so no harm done.

[David Kumm]

Chris, I know only enough to be dangerous but think your motor and fan with no outlet restrictions pull basically the same open and closed because the pipe size doesn't allow enough air to cause the fan work any harder. Turn the hz up or down and see what happens to the amp draw. It should move some then. I believe you should rethink your pipe size if possible. You are only taking advantage of about half of your system's potential. Dave

[Doug Palmer]

The typical inrush to start your motor is 6-8 times full load amps for approximately 6 cycles (or 1/10th of a second). If you install a soft start, it will pull about 3 times full load amps for 3 to 5 seconds. As David properly stated earlier, a soft start will not significantly reduce the heat in the motor and in some cases can actually increase it. I have seen many motors burn up in an industrial setting due to over starting. Usually this is because the user has disable the thermal protection on the motor and has excessively jogged a stalled motor one reason or another. This is much more severe than what we are talking about in the shop. Bottom line, IMO, save your money.

5-6 times per hour is indeed the recommended start frequency, but realize that that is the mfg recommendation and it includes a safety factor of approximately 2 times. I would not hesitate to start the motor 10 - 12 times in an hour, but would not want to do that for several hours in a row. If you suspect the motor may be heating up, let the DC run for a while (even if that means leaving a tool running). The motor will cool the best when running. The best inexpensive solution is to install a $30 time delay relay as recommend by others earlier to keep the DC running between stations. Most likely you will not change stations often enough to worry about your DC motor even without the timer. Please note that the effect of heat in the motor is not cumulative. In other words, you are not shortening the life by your method of operation unless you destroy the motor all in one day. You should have thermal protection on your motor to prevent that from happening.

FYI, I am an Electrical Engineer and Licensed Master Electrician with 28 years of heavy industrial experience.

[David Connors]

That motor is capacitor start / capacitor run. I think soft start is applied to straight induction motors.