View Full Version : DC Motor speed control

12-05-2011, 08:54 PM
I bought 2 5HP DC motors & want to make them variable speed control. I know a VFD works on AC but what'sthe best/least expensive on DC motors? They are 1725RPM. These things are Beasts. :D

Forrest Addy
12-05-2011, 11:22 PM
I assume the motor has a permanant magnetic field. Your motor may be shunt wound but I doubt it. Holler if it does although most single board DC drive have a field output.

I also assume the motor is honestly rated. A 2 1/2 HP DC motor draws ~12.5A @ 180 VDC or ~9.8 Amps @ 230 VDC. This assumes the usual losses between actual electrical demand and mechanical output.

Many consumer motor driven products have inflated ratings. Sears, for example, hawks their 12 Amp 115 Volt shop vac as having "6 1/2 HP" a clearly fraudulent claim intended to deceive customers in their pruchase decision. If your DC motor came from a treadmill it may not be accurately rated.

Look at the motor name plate for Volts and Amps. These are ratings mandated by fire safety underwriters and are more likely to be on the level. HP is not. Once you have that information restrict your search to controls that match. This may save you from overbuying. Single phase SCR or PWM based motor controls take a big jump in price over 2 HP.

Also look at your application. Most drives have built-in current limit protecting the motor and control. If you are going to drive a drill press, 2 1/2 HP is overkill. A smaller HP control may suit you and will be less expensive. If you are going to run a 12" lathe you may need a full rated DC control.

Dart and KB are two sources. Look in eBay under "DC motor coltrols" DC motor drive*" for likely canidates. Note. DC controls come in many models and variants: the selection is confusing. Shop carefully

12-05-2011, 11:35 PM
Also Baldor sell the KB relabled, they have a few variations, simple 2 quadrant SCR bridge, 4 quadrant SCR bridge which gives you dynamic braking and bi-directional control without the need for reversing relay, or PWM models.
The open chassis models are a bit cheaper than the complete enclosure model.

12-06-2011, 07:12 AM
They show 23.7 amps @ 180V,1750rpms. He also has the controllers that went with these but the look like a very complicated nighhmare. Turned out to be Hoosier racing tire I bought them from. So I'll get to see them make tires when I pick them up.

12-06-2011, 09:34 AM
How are you planning on powering them?

12-06-2011, 11:18 AM
They show 23.7 amps @ 180V,1750rpms. He also has the controllers that went with these but the look like a very complicated nighhmare.

KB/Baldor have the SCR KBMM-225 or the more expensive PWM KBCC-225, these will run a 3 HP 180vdc motor off a 30amp 240v 1ph circuit.

If the motors were used in servo application they could be 3ph SCR drives if older units, but if 1ph they may have an analogue control input that would accept a pot.
Also if the motors have a DC tach, the KB units also accept a Tach for better RPM control.

12-06-2011, 07:30 PM
Best is a regenerative DC drive. Watch it though, some of these are truly three phase only as they use the third leg in braking.

12-07-2011, 07:45 PM
They are shunt wound. I lucked out & bought the original controllers for these. They look very complcated for me. I'll have $250 total in the 2-5HP motors & 4 controllers, all guaranteed to work as new. I later bought his last 2 for $25/ea & I'll get a tour of Hoosier Racing Tires. From what I read these really should have a lot of torque. As for powering the I have 120/240 sng phase & 230/460 three phase. Again any suggestions,advice,cmments or free installation is welcomed, encouraged & greatly appreciated. :D :D
PS The drives are Emerson 5.0 / 2.0hp DC Drive Model RG2600 Part Number 262-8001. Hope this helps as it's all Greek to me but I'm learning. Thanks again!

12-07-2011, 08:10 PM
It may be possible to resurrect them, do you know the application they were used in, servo? spindle, or general purpose?
If wound field as it sounds, there is most likely a field supply with field loss detection included.
It may be worth dropping an email to Emerson to see if they can offer a manual.
There is a good chance you can control them with a pot. as they more than likely use 10vdc analogue control, do the motors also have a tach?

12-07-2011, 09:37 PM
Not sure on the tach,I haven't picked them up yet. The Ebay numbers I listed show pictures. I believe they were general purpose. I mave not have needed the drivers but my thinking was if they controlled the very motors they should work. I looked at other 5HP DC motors & controllers & they seemed crazy high in price for used & bought the extra 2 because they took my $25/ea offer. I hope I haven't bit off more than I can chew but if I have I'll just have to find a bigger dog. LOL!

12-07-2011, 11:38 PM
I don't see any ebay #s?