Variable speed blues...

S

stevejack

Joined
Oct 24, 2005
Messages
25
Hey guys, I need some expert advice here.
I have a Coote running with a variable speed 750w motor (cap-start), with a high voltage control box and wall control. These are all components used on evaporative air-conditioners, all brand new. For the life of me I can't get the variable speed to work... It is either running at full speed, or stopped. If I switch it on with the dial on low speed, it starts slow, but any pressure on the grinding belt stops the motor. I seem to be able to turn the speed up, but once it gets to speed, I cannot turn it back down with the dial. So I figured maybe it needed back pressure right? If I apply grinding pressure, it just stops the motor if the dial is turned down to say half-way, but if it is turned up to full speed, it will keep running at that speed (ie. the pressure on the belt will not bog it down - as it shouldn't). I have had 2 electricians look at this and neither can work out why it won't work - but neither have tried this setup it on anything other than an air-conditioner.
Does anyone have any ideas which might explain why this is happening? :confused:
Thanks guys,
Steve.
 
Simple voltage controls on AC motors are notorious fo "all or nothing" torque. That is why variable speed grinders invariably use either a dc motor or a 3 phase with a variable frequency drive. The latter is cheaper but I'm starting to wonder because these forums are full of stories about ones that failed.

Rob!
 
stevejack said:
Hey guys, I need some expert advice here.
I have a Coote running with a variable speed 750w motor (cap-start), with a high voltage control box and wall control. These are all components used on evaporative air-conditioners, all brand new. For the life of me I can't get the variable speed to work... It is either running at full speed, or stopped. If I switch it on with the dial on low speed, it starts slow, but any pressure on the grinding belt stops the motor. I seem to be able to turn the speed up, but once it gets to speed, I cannot turn it back down with the dial. So I figured maybe it needed back pressure right? If I apply grinding pressure, it just stops the motor if the dial is turned down to say half-way, but if it is turned up to full speed, it will keep running at that speed (ie. the pressure on the belt will not bog it down - as it shouldn't). I have had 2 electricians look at this and neither can work out why it won't work - but neither have tried this setup it on anything other than an air-conditioner.
Does anyone have any ideas which might explain why this is happening? :confused:
Thanks guys,
Steve.
Click to expand...

:confused: Steve
If memory serves me, most capacitor start motors are induction type motors and are by their nature synchronous. This means they want to operate at a speed determined by the frequency of the input voltage. If your controller is of the variable voltage variety, made for a universal (AC?DC) type motor, all that happens when you turn down the speed, is you reduce the voltage going to the motor there by reducing the power of the motor until it stalls and overheats if you don't shut it down immediately. Check out the name plate data on the motor to see if it is an induction motor. Also check the controller to see if it is a variable frequency or variable voltage type.
Give me yell if you need any additional help.

JIm A.
 
R
Rob! said:
............. That is why variable speed grinders invariably use either a dc motor or a 3 phase with a variable frequency drive. The latter is cheaper but I'm starting to wonder because these forums are full of stories about ones that failed.

Rob!
Click to expand...

Rarely do you hear of factory matched units, purchased from regular distributors, and hooked up properly, per the instructions, failing. Mine have been in use for 15+ years, one since 1978, and no problems at all.

It's when people mix and match from ebay, surplus center, etc. that you get failures. The only saving grace is, they usually don't have much money in those units.

Personally, I'd rather pay a little more up front for proper equipment, and have it running whenever I flip the switch. Your time is also worth something!:eek: ;)
 
The VFD's that seem to be getting fried here, are the ones that are open housing and are getting metal dust in them. Causing them to short out.
I know that one person here, in bladeforums, said that the controller should be located away from the grinding area.

The grinder makers are selling controllers that are nema 4 (closed). I guess they are correct in their recommending closed housings.
 
My little exposure to VFD had a lot of variable control functions (not in english but some insider jargon) that had to be set in a program. The directions had several warnings about errors and the results of them, (most related to transforming the motor and/or VFD into a smoke machine)

I survived this replacment only because 1. the machine manufacturer had the program settings in the manual, 2. we bought an exact replacement motor and drive and 3. the original failure was caused by the user not keeping up on the lubrication and maintainance of the machine for 3 years (dry & rusted bearings).

If the stuff is wired correctly and there are no mechanical problems, the settings might be wrong for your application (should be close enough). If the VFD was unpluged for a while, the controller battery might have died and you lost the settings. Hard to tell from here.

good luck
 
Mike Hull said:
R

It's when people mix and match from ebay, surplus center, etc. that you get failures. The only saving grace is, they usually don't have much money in those units.
Click to expand...

Thanks Mike.. Where do you source yor macthed units?

Rob!
 
Rob! said:
Thanks Mike.. Where do you source yor macthed units?

Rob!
Click to expand...


Two of mine are DC variable speed Leeson motors and MM control units from Minarik Electric. They have stores all over the States.

The old one is a Grainger shunt wound DC unit, but be prepared to pay top $$ if you go to Grainger. No deals there for sure. If it ever goes out I'll replace it with a Leeson.

If you want to stay with AC VFD, then you might try this place, they sell them as a set.
http://www.dealerselectric.com/
 
Thanks for the help guys, I will double check the wiring diagram when I get back to work to make sure everything is connected in the right place, never know maybe we all missed something. I hope it's not varying the voltage - that is exactly why we got the control box etc - but I guess if it is wired in wrong it could be doing anything. I'll stop using it for now until I can get it sorted. If that fails, I guess I will have to go with a whole different setup. There should be no reason why the controller/box/motor shouldn't work for this application as far as I can tell - but I don't know much! :D
Thanks guys
Steve.
 
stevejack said:
Thanks for the help guys, I will double check the wiring diagram when I get back to work to make sure everything is connected in the right place, never know maybe we all missed something. I hope it's not varying the voltage - that is exactly why we got the control box etc - but I guess if it is wired in wrong it could be doing anything. I'll stop using it for now until I can get it sorted. If that fails, I guess I will have to go with a whole different setup. There should be no reason why the controller/box/motor shouldn't work for this application as far as I can tell - but I don't know much! :D
Thanks guys
Steve.
Click to expand...

I could be wrong here, but it almost sounds like you're attempting to put one of those speed controls on a larger motor than it was really meant for. Those things work on fractional HP motors. Someone is always trying that but it just won't work with motors over 1/4HP or so. Things like Foredom and Dremel tools, routers, etc. work well with them.
I could also be wrong about this and may have misinterpreted your original post too.
Are you running a three phase motor from a single phase control? I'm just not quite clear on it.:confused:
 
Mike Hull said:
I could be wrong here, but it almost sounds like you're attempting to put one of those speed controls on a larger motor that was really meant for fractional HP motors. Someone is alwaysd trying that but it just won't work with motors over 1/4HP or so. Things like Foredom and Dremel tools work well with them.
I could also be wrong about this and may have misinterpreted your original post too.
Are you running a three phase motor from a single phase control? I'm just not quite clear on it.:confused:
Click to expand...

Sorry mate, I realise I haven't been too clear on all this - I am no motor expert but here is what I know; I am running a 750w Single phase Variable speed motor 'CMG' is the brand. I have a high-voltage control box (Tekelek 297) and a wall control to match (Tekelek 288). All that I really know, is that this same combination works successfully for variable speed air-conditioning systems, which use pulleys/belt to operate the fan. This exact combo is used in hundreds of units over here but of course I either got a faulty component or have just done something wrong somewhere (more likely) :rolleyes:
Technically, if the wiring in the control box is correct, this setup should vary the speed of the motor in the same way, it is just running the coote grinder instead of a fan.
I will have to wait until tomorrow when I can bring home the wiring diagram and double check everything, it does seem to be simply varying the voltage somehow in the same way that wiring in a dimmer switch would do - which is obviously bad, and is not *supposed* to be the function of this control box :)
I originally wired in a Low voltage box/controller for safety reasons and had the same problem, but was told that low voltage controllers of this type need significant back-pressure to vary the speed (ie. a fan sucking in air from outside). He suggested I switch to the high voltage setup to solve this - but I have the same problem :eek:
Oh well, its all good fun as long as nothing breaks (and I don't get hurt). I'll ask one of the sparkies to come and have another look at it next weekend and try to figure out what might be wrong.
Thanks again, sorry for the long post.
Steve.
 
Steve

Single-phase, capacitor start motors are not normally connected to speed controllers except in the rare instance where a fairly small (less than 1 hp) motor is connected to a low torque load. I had asked the R&D Manager of our Motor Design group at work about this a few weeks ago. A fan would require much less torque than a grinder.

If you want to go the three-phase route, call around your local electrical wholesalers (I think you have Mastertrade in Oz too) and see if you can get an acceptable price on a PDL brand drive. The "Microdrive Elite" product line (www.pdlelectronics.com/Products/MElite.htm) is designed to be dust-proof and splash-proof (IP-54). I recall the mechanical designers doing a lot of testing to make sure that they met the standards that they claimed. I also don't recall any coming back for repair because of dust getting in and many were used in pretty dirty places like mines. They usually failed due to some form of (electrical) abuse such as a lightning strike, or being connected incorrectly (output connected to the mains). Otherwise, get a common and inexpensive NEMA 1 drive, mount it in another (dust-free) room and use the remote control features that come with virtually all modern drives.

The "matching" of a motor drive to a motor is typically a marketing strategy to generate greater revenues for a company that sells motors or motor drives by offering convenient, one-stop-shopping for their customers. PDL, my former employer did this too. All competently designed motor drives will protect themselves, without suffering any damage, when connected to a load that is too large. Please see the thread "Why VFDs Blow Up" for for an explanation of why they blow up.

Hope this helps.

Phil
 
I agree with PSO. It is a torque problem.A fan does not give much resistance to starting to turn.A grinder does.Start up torque is very different.I have been watching this thread and waiting for someone to mention the torque.Your setup may not work for a grinder.
Stacy
 
Ok, thanks for the info guys. I will look into alternative setups. The fans which these setups run are those big barrel type ones, basically like a large cylindrical cage, not just the normal flat bladed fan like a desk/ceiling fan. Anyway, I am sure you are all probably right, just the wrong kind of application for the equipment. I'll take all the info you have provided and look into getting something that is actaully designed to work with a grinder :o
Thanks again,
Steve.
 
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