Thread: Calling all electrical experts - DC motor conversion

Originally Posted by neevo

Cheers Simon, appreciate the help. I've been in contact with a treadmill repairer in Penrith who may have a replacement controller board.

If not I will let you know and see if you have something that can help.

Appreciate the offer.

No worries. Happy to be a backup plan.

Simon

Originally Posted by neevo

I'm really annoyed with myself. After receiving all the necessary parts to build my new box, building the box, chasing out the lid to incorporate the control board and speed indicator leds... I accidentally let the smoke out of the board by shorting it on my welding plate!....................
.

You are a FOOL!!!!

Sorry to offend you but I have to.

Only a fool can be so foolish, as to test a board with life mains components on a STEEL PLATE COVERED BENCH. Be very grateful to still be alive. You should NOT do this sort of work as you are clearly not suitably qualified.




PS: you did a nice job on the housing. But it is still not worth to risk your life. Go and buy a ready made speed controller.

I will take my chastising as its due. Appreciate the comcern though.

New plan. Turns out the lead I had to replace the board wants $250 for it, maybe not.

For that price I can buy a KB Electronics DC motor controller which will let me use a pot on it and get total variable speed control... So that's the plan.

Question: where is best to buy them from? Also what else do I need to set it up for my 2.5HP peak DC motor?

On a side note I finished motor mount. I didn't have any large plate so I cut 2 x 75mm wide lengths and TIG'd them together. I cut 2 x 10mm holes and used a Drexel with cutting disc to join them:



Then I welded the motor mount to the plate:



Fits perfectly and has about 1" of adjustment, plus it's much smaller than the AC motor on there now. Just needs a nice coat of gloss black paint.

Also got the remainder of my enclosure bits but now I'm changing the design again I may need another box unless I can use the original one again.

Originally Posted by neevo

New plan. Turns out the lead I had to replace the board wants $250 for it, maybe not.

For that price I can buy a KB Electronics DC motor controller which will let me use a pot on it and get total variable speed control... So that's the plan.

Question: where is best to buy them from? Also what else do I need to set it up for my 2.5HP peak DC motor?

..........................................................

KB DC controllers are obsolete designs. 15 to 20 year old fossils. They have not designed a new DC controller since probably 2000.

In my honest opinion, you would be better off selling that DC motor, and invest instead in a 3-phase motor and VFD.

I don't disagree cba, but I don't have the budget at the moment for a 3 phase motor and VFD, whereas I currently have all I need to get the DC motor running, bar the PWM.

Is there another PWM you could recommend?

If this experiment with the DC motor doesn't work, a VFD is definitely next on the cards.

Originally Posted by neevo

I don't disagree cba, but I don't have the budget at the moment for a 3 phase motor and VFD, whereas I currently have all I need to get the DC motor running, bar the PWM.

Is there another PWM you could recommend?

If this experiment with the DC motor doesn't work, a VFD is definitely next on the cards.

I agree with cba_m, a suitable used 3phase motor will set you back ~$30 and a a budget Level VFD is ~$140.
Since you only paid 99c for your DC motor it's not like you have a lot to lose.

Everyone talks about cheap 3ph motors but I've never found one for less than a hundred plus.

Here's one for $50,.
http://www.gumtree.com.au/s-ad/horns...ase/1066053005

400v, don't I need 240v?

Originally Posted by neevo

400v, don't I need 240v?

Technically yes but if you run a 420V 3Phase motor, connected in "star" or "Y" connection at 240V you will get about half the power.
So 3HP becomes ~1.5 HP but for a small Hercus like yours 1HP is more than enough.

More explicitly, that motor looks like its a 420V "Y" , or 400V "∆" 3HP
If you don't know what "y" and "∆" mean you will need to do a little bit of homework.

Running in Y Mode will give 420/240 = ~1.7HP
Running in ∆ mode = 400/240 = ~1.8HP

To get the full 3HP at 240V the Y connection inside the motor will need to be converted to ∆ connection.
This requires internal motor surgery and is not for the faint hearted or inexperienced to tackle

The ideal sort of motor for your lathe will have a nameplate like this.
These specs are the same as what I have on my hercus and I have only felt the need for more power than this a couple of times a year.

Got it. Thanks Bob.

Keep looking for 3 phase motors. They pop up from time to time real cheap. My motor on my VFD mill conversion was $40 brand new, the one for my lathe was $30. Both are 3 HP. I did need to re-wire them internally to bring out the star point though. The first time was interesting, after the sixth time it's routine.

Just have to keep looking.

Simon

Ok, on the look out for 3ph stuff now to see how that would pan out. Made an offer on that motor and waiting to hear back. Pricing up the DC motor drive and it's the same as a 3HP VFD delivered.

Also checking VFD options and have some questions.

1. Would I buy a VFD based on listed HP of the motor? Eg 3HP?
2. I would look to mount the VFD under the bench and then build a control box for it with my RPM display, will that work? Assume I can run a pot for the speed control.

Originally Posted by neevo

Ok, on the look out for 3ph stuff now to see how that would pan out. Made an offer on that motor and waiting to hear back. Pricing up the DC motor drive and it's the same as a 3HP VFD delivered.

Also checking VFD options and have some questions.

1. Would I buy a VFD based on listed HP of the motor? Eg 3HP?
2. I would look to mount the VFD under the bench and then build a control box for it with my RPM display, will that work? Assume I can run a pot for the speed control.

1.

Modern VFD's (those not older than say 15Y) are very intelligent and adaptable. It is customary to have the VFD rating match the motor rating. Depending on application, it can be of advantage to have the VFD rated one or two size steps larger than the motor, especially if the rated motor power is required at elevated room temperatures and/or it is desired to run the VFD at the highest possible chopper frequency in order to reduce motor noise (or motor singing). Also Motor braking can be improved this way. Motor protection always remains fully assured. Downside of doing this is actually only the extra cost for a larger VFD.

That said, it is also perfectly possible to run a motor one or two size steps larger than the VFD. The motor output is then of course limited to the VFD rating. The downside of doing this is the extra cost, extra weight and extra inertia of the oversized motor, meaning reduced accelleration and braking performance. It will still perform better overall than the overrated treadmill motor setup you were looking at before. This is certainly not something you would do on purpose when buying new components. It is something that you can do in order to reuse existing components, or to make use of components that by coincidence just happen to be available to you at an irresistible price.

2.

Yes that would work very well.


PS: when buying a motor, also make sure the number of poles match your application. A 2-pole motor has about 3000rpm at base frequency, a 4-pole motor about 1500rpm, a 6-pole motor about 900rpm. The selection is a little different than it is with a DC motor. For example, a 4-pole motor will have the best power at about 1700rpm, but will remain usable up to at least 3000rpm, and it will provide constant torque down to about 500rpm with a cheap VFD and down to near standstill with a slightly more expensive "vector" type VFD.