Am I wrong here? - Page 1
Forum section: electronics / general

author: RogueBotix
author_rank: Contributor
timestamp: January 04, 2018, 11:40:00 pm
content: I have been fixing a machine at work with a stepper driven centrifugal pump. The pump has been suffering from low inadequate flow. The tech over the phone says its friction causing it to slow down slightly, he is very adamant about that. I have always though a stepper can only spin its driven speed else it will just stall. I am wrong?? I don't want to seem like a dick arguing over the phone. I am not sure how to handle this. And to clarify there is no feedback on the stepper, its just driven.Thanks

author: wraper
author_rank: Unknown
timestamp: January 04, 2018, 11:43:04 pm
content: QuoteIt may lose steps as well.

author: rstofer
author_rank: Super Contributor
timestamp: January 04, 2018, 11:52:39 pm
content: Here is a summary of stepper motor torque:http://www.nmbtc.com/step-motors/engineering/torque-and-speed-relationship/It's important to know that torque declines as RPM increases.If the pump used to work and now it doesn't, something has changed. The bearings in the pump may be seizing up, the impeller could be dragging OR the motor doesn't have as much torque as it used to have. This could be caused by low winding voltage. Maybe the power supply doesn't put out the required voltage. Or maybe the stepper driver now loses more voltage than before. If the RPM the same as it was before? Did somebody try to make the pump move more <whatever> by increasing the RPM?

author: mc172
author_rank: Frequent Contributor
timestamp: January 04, 2018, 11:53:41 pm
content: A centrifugal pump driven by a stepper sounds a bit strange. Of all the motor choices, a stepper? Are you sure it's a stepper motor?

author: Ampera
author_rank: Super Contributor
timestamp: January 04, 2018, 11:59:18 pm
content: Quote from: mc172 on January 04, 2018, 11:53:41 pmI agree. Steppers tend to be used in more precise applications, like printers.

author: RogueBotix
author_rank: Contributor
timestamp: January 05, 2018, 12:04:02 am
content: Yes I am sure it is a stepper, it is on a selective soldering machine. Everything spins freely, I even put a new motor on, and still nothing. We got desperate for pump flow, so I made a quick fix by designing a new driver circuit to drive it up to 2000 rpm.(The original one could only go to 1300) It is a band aid fix for now, but it works pretty well. I have heard steppers skip steps or stall, and it is not a nice sound. But this motor sounds happy, its runs quietly. I want to say the pump is just worn out, but the tech insists the motor is slowing down. I should put a tach on the spindle......

author: Circlotron
author_rank: Super Contributor
timestamp: January 05, 2018, 12:05:47 am
content: Quote from: mc172 on January 04, 2018, 11:53:41 pmIdeal motor for centrifugal pump that is a square law load would probably be a series wound dc motor.

author: RogueBotix
author_rank: Contributor
timestamp: January 05, 2018, 12:10:27 am
content: A stepper is used so that you can accurately set rpm to get repeatable wave height for different nozzle sizes.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 05, 2018, 12:49:29 am
content: Quote from: RogueBotix on January 05, 2018, 12:04:02 amNote that significantly increasing the step rate may put the motor beyond the point where it can properly step without "slipping". How does it work back at 1300 RPM?

author: james_s
author_rank: Super Contributor
timestamp: January 05, 2018, 12:50:48 am
content: That's a bizarre design decision, a brushed or brushless DC motor with an optical or magnetic encoder can provide the same functionality and do a much better job, quieter, at lower cost than a stepper. Even printers where steppers were once the norm pretty much all use permanent magnet DC motors with optical encoders now.

author: RogueBotix
author_rank: Contributor
timestamp: January 05, 2018, 01:03:41 am
content: Quote from: Richard Crowley on January 05, 2018, 12:49:29 amIt works terribly at 1300, thats why I built a high speed driver.

author: amyk
author_rank: Super Contributor
timestamp: January 05, 2018, 01:10:48 am
content: Averybizarre design decision. Perhaps it was meant to be a peristaltic pump (which is commonly stepper-driven for precise metering) but then changed to a centrifugal?

author: RogueBotix
author_rank: Contributor
timestamp: January 05, 2018, 01:31:57 am
content: Quote from: amyk on January 05, 2018, 01:10:48 amNo, It has always been a centrifugal pump. I'll take a picture at work tomorrow. but this is a similar model to the unit.http://ace-protech.net/assets/templates/advance/img/photo/portfolio/2in-square-wave-HMP-tank-pump.jpg

author: Nusa
author_rank: Super Contributor
timestamp: January 05, 2018, 01:46:54 am
content: Have other causes of poor performance been eliminated? For instance a worn or damaged impeller with excessive clearances in the housing? Or partial blockages anywhere in the fluid circuit?

author: RogueBotix
author_rank: Contributor
timestamp: January 05, 2018, 01:58:04 am
content: I replaced the impeller, cleaned all the passages. I think if its worn, there is liquid blowing around the impeller. There aren't any seals because it runs at over 300C its pure speculation but I believe it relies on tighter tolerances to keep the blow by to a minimum.