skogsgurra, or others more help with a DC drive

[snaggletto]

Hello,
I would like to get some more information on a drive for this motor;

http://members.cox.net/snaggletto/Siemensdcmotor

Skogsgurra was very nice to answer my question before;

http://www.eng-tips.com/viewthread.cfm?qid=100427

however, this is more of a budget minded setup. The motor is the original motor on a small Emco cnc lathe. I bought the machine with dead controls.

I was hoping to get some leads on more of a no frils, affordable type DC drive that would work with this motor, and accept a 0-10V analog signal from my PC based cnc for speed control. I'm sorry to double post, but I need some leads. Thanks.

 

[UKpete]

Depending on your available ac supply voltage, you may have difficulty in getting 400V DC, for example see:

http://www.sprint-electric.com/340.html

They quote a typical armature voltage of 180V dc. With full-wave rectification you can only get a dc voltage out (on full load) approximately equal to 0.9 x AC RMS input voltage. To get a higher dc voltage you need a voltage boost circuit, this may not be included in the dc drive.

Sorry to be negative (and others may have better suggestions) but it doesn't look low-budget compared to an ac drive unless you are lucky and find someone with an unwanted dc drive.

 

[snaggletto]

Hey,
Thanks for the reply. When we first got the machine, we tried fixing it. It does have a large 5kva transformer mounted on the side of the machine. We wired the transformer with 220 3ph. on the primary and I *think* the secondary that the machine used was 380. The control worked somewhat, but had some type of fatal error reported to the screen. No spindle or axis activity was possible.

The max programmable RPM was 4000RPM according to the manual. The 2 pulley belt drive ratio was approximately 1:1.25 reduction. I calculated this to be ~5000RPM at the motor. Correct me if I'm wrong, but won't the motor run with lesser voltage, just not achieving it's rated max RPM of 6000RPM?

If I'm right, it seems logical that with only 380V, 5000RPM should be achievable, compared to 6000RPM at 400V. So, even if I can get 380V with the step up transformer, the problem is still finding a ~400V DC drive?

 

[Skogsgurra]

There are many old analogue 400 V thyristor drives (scrapped) available for next to free in Europe. Where are you situated?

 

[snaggletto]

I'm in the US, Pensacola, Florida. I did some more searching on the 'net and found some digital and analog DC drives for ~400V armature. To my disbelief they were all in the $2000+ range.

I already bought a 5HP VFD and 3HP 'inverter duty' motor for around $600. However, I wanted to give another shot at finding a DC drive for the current motor. If I could find an affordable DC drive (looks to be impossible now) then I could use the VFD/AC motor on another project I've been planning on.

 

[Skogsgurra]

I have some over here. Come and pick them up! ;-)

 

[DickDV]

check out ABB's DCS400 DC drive line. The same unit is capable of 230-480V operation. Just need a proper control transformer for 120V control power.

 

[ScottyUK]

Have a look at the Eurotherm - now SSD - range of drives. Their AC drives are pretty good. I haven't used one of their DC units before, but the quality should be comparable.

http://www.ssddrives.com/usa/Products/DcDrives/DcDrives.php

The 512C is capable of 320V armature output from a 380VAC input. Cost is £200 for the 8A output version.

The 590 series is considerably more expensive, but offers a lot more.





----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[UKpete]

snaggletto, in answer to your follow-up question - yes the motor will run on lower voltage but it will be at a lower speed. Speed is approximately proportional to voltage in a dc motor assuming the field current is kept the same.

Incidentally, what you have is a separately-excited dc motor, so-called because the field circuit is separate from the armature circuit to give extra control. At starting the field current is set to maximum (0.74A), giving high torque, and the voltage is increased from 0 to 400Vdc gradually so that the maximum armature current level isn't exceeded whilst the machine is accelerating. When maximum voltage is reached, further increase in speed is achieved by reducing the field current, and the maximum speed of 6000rpm is achieved the field current is 0.11A (armature held at 400V). If you take the field current any lower in order to increase speed, you risk a damaging commutator flashover. A controller normally does all this for you, as well as regulating the speed. But it can be done manually with a suitable variacs/rectifiers if you know what you are doing and know you can do it safely.

 

[snaggletto]

Thanks for the replies.

Like I mentioned before, I already have a VFD/AC motor replacement. However, the DC motor is large and was originally spec'd for the job. I'd hoped to find an affordable drive replacement, but it appears that I'm looking in the $2000+ range for the type of drive I would need. This is way too much. It's a shame beacause I don't expect the AC motor and VFD to perform very well at low and high speeds, and I really didn't want to use a step pulley system to maximize torque in various speed ranges. Thanks for the help.