power dimming of a mag Drive pump

[press9761]

We have a device that, amongst other things provides power dimming for pumps (AC sine wave phasing for inductive loads). things were going extremely well until we tried to dim the power of a mag Drive pump.

This did not go very well at all. The pump started vibrating very intensely. It was as if we created an earthquake machine. Within a few moments the pump was broken.

What appears to have happened is that the impeller sheared off of the "magnetic rod"-- forgive me, I am not a pump engineer.

Anyway, I was hoping that someone may be able to point me in the right direction. I am having a hard time researching information on how these pumps work. Clearly I have significantly interrupted normal operation by cutting the AC sine wave. however, I do not see any information on this topic. I suspect it may be that I do not know how to ask the question properly.

If anyone knows anything about mag-drive pumps please point me in the right direction.

 

[itsmoked]

You cannot use that method with AC motors at all...

You created some very nasty stuff exciting that poor thing. If the pump hadn't failed the motor would've in seconds or the so called "Dimmer" would've eaten it.

There is only one type of motor that would accept that electrical abuse and it would never be paired with a mag pump.

You either need to acquire a mag drive pump with a designed-in speed controller.

Alternatively use a three phase motor and run it with an inexpensive VFD. This would allow you to vary the speed of the motor without harming it.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[press9761]

before we begin, I looked at your website. Nice work!
(incubator?)

well, I appreciate your advice however I'm a bit confused.

we have done extensive testing with inductive load phasing "dimming a pump" and have had great success. We can operate "traditional" pumps over a wide power range. When we do this, we see nothing but smooth operation. The pump blades simply spin slower. Our circuitry is designed to handle power phasing at up to 20 continuous amps.

as far as using a pump for this application. That is not up to us. We are providing a user interface but not the actual equipment. It was up to me I would get a DC motor and use pulse width modulation however, it's not up to me it's up to the end user. And all of their pumps are simple single phase AC.

So, perhaps we are on the brink of disaster and we just don't know it. That's always lovely news.

But still, what was the physical process causing the impeller to act so violently during our "dimming" experiment. (we were cutting AC sine wave X seconds after zero cross)

Thank you for your response

 

[itsmoked]

press9761;

There are a lot of reasons that is a poor idea. A whole lot of reasons.. I will only touch on a few.

Most motors, and especially single phase induction motors, are designed to have smooth alternating voltage applied to them. If you apply anything else to them their magnetic circuits have much larger losses than normal.

Magnetic circuits found in motors expect to have no DC provided to them. If you feed them with diced up power their magnetic circuit can saturate because diced up power can represent DC. This causes the current draw to rise limited only by the actual wiring resistance instead of the inductance which is the normal and much larger current limiter. This greatly heats the motor and can damage the switching elements in the control unit.

Single phase motors generally have start switches. The start switches are mechanical and centrifugally actuated. If the motor doesn't exceed the speed needed to open the start switch,(about 80% of nameplate), the motor remains in start!! This normally destroys the motor quickly.

If you turn the motor speed down you also reduce the cooling dramatically.

If there are capacitors in the system they will be interacting with all this causing nasty resonances.

I would not buy a product run this way..

You would be better off throttling the output in this case not "dimming" the pump.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[EdStainless]

Let's assume that you don't have any problem or risk with controlling motor speed this way (the motors will run hotter and that will shorten their life, but that is a long term issue).

The problem with mag drive pumps is that if you turn them down too far then the drive will slip. The resulting 'cogging' will either overheat the motor and burn it up, or overheat the magnets and kill them, or the vibration will ruin pump and/or motor bearings.
Without a mag drive you would just have more motor phase slip and unless you burned the motor all would still work.

= = = = = = = = = = = = = = = = = = = =
Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
eblessman@plymouth.com
or edstainless@earthlink.net

 

[press9761]

Thank you all very much for your responses!!! It is greatly appreciated.

I would hate to come off being stubborn or arrogant however, I have to say that there are extensive articles written about inductive load phasing using single phase AC. free scale semiconductor has a very nice flash animation on how this is done. Essentially, you're just holding the gate of a triac X seconds after zero cross, this in turn delivers less power.

Okay, from what I have just read it appears that we were seeing the drive slipping. the pump vibrated very intensely. I'm surprised it still works it felt like an earthquake( seriously!!).

my concern was that by cutting the AC sine wave we were causing the impeller on the mag Drive pump to be rapidly ejected and then drawn back in from the "magnetic field chamber" -I don't know what it's called.

Perhaps it is that when using the mag drive pumps the phasing must be significantly less.

Does anybody know of place where I can get detailed engineering information on how a mag Drive pump works. In our stage of operation it is too late to remove the power phasing capability of the product. The only thing we can do is modify the operation and put in a disclaimer.

Thank you all very much

Any further input would be much appreciated!!!!!!!!!!

 

[EdStainless]

I honestly don't know of a good source of info. All that i ever did was design and build them. Our rules were all empirical. (hey, it was 1981)
My guess is that you need a stronger coupling, and a way to make the motor run smoother. The couplings have some 'give' in them. Similar to a motor there has to be a slight phase shift to create any torque. Think of like trying to drive a pump through a springy coupling. Harmonics will kill you.

= = = = = = = = = = = = = = = = = = = =
Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
eblessman@plymouth.com
or edstainless@earthlink.net

 

[EdStainless]

They were mostly custom and R&D jobs. The production units were all military. It was all with RE magnets.
The problem with mag couplings is once they slip they will not re-engage until the rotational speeds match. Usually this means everything is stopped. The slippage causes large amounts of Eddy Current heating in the magnets and structure. As well as vibration.
This is an interesting control systems problem with the variation in motor speed and the compliance of the coupling. I can picture some high frequency torque spikes.

= = = = = = = = = = = = = = = = = = = =
Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
eblessman@plymouth.com
or edstainless@earthlink.net

 

[sreid]

Without extensive research, the Freescale website refers to "Lamp Dimming" and "motor speed control." Thes are two very different things.

Universal motors can be AC phase speed controlled but single phase AC induction motors cannot. The speed of induction motors is basically set by the supply frequency. SCR Phase control can reduce the voltage but not the frequency.