Shunt wound DC motor control help needed

I have a band saw that had a Reliant variable speed DC motor controller that croaked this week. The motor itself is in good shape (less then two years old). I would like to get it running without buying another controller. It is 3.4 HP. The nameplate has a schematic of sorts and it states that this is a shunt wound motor. I tied the shunt winding in parallel with the armature winding and applied varying DC. It runs but does not have a lot of starting torque. I asked a friend and he said that he thought that the controller should energize the field with full voltage (90 VDC) and then vary the voltage to the armature for speed control. Does this sound correct? It kind of makes sense that if you start out with full field voltage then you would have full field magnetism and more starting torque then varying the voltage on the field and armature tied together like I tried. Somebody in the shop suggested to use full armature voltage (100VDC)and vary the field for speed control because the field is high impedance and does not take as much hardware to vary the voltage. I have a variac that is good to 12 amps and access to full wave bridge rectifiers. Can I get away with only one variac on the field or armature and run rectified ac straight to the field or armature? I know there is somebody out there who can offer me some good practical advice on this. Thanks so much in advance.

DC Motor

Crossthread,

I run several DC motors in my shop and I just use variacs and full bridge rectifiers. From your post I assume on the nameplate the Field voltage is 90 VDC and the armature voltage is 100 VDC. Is the motor 3/4 hp or 3.4 hp? To vary the speed you must hold the field voltage at 90 and vary the voltage to the armature. This will vary the speed from 0 to the motor's base speed. The rpm should be on the nameplate. You can increase the speed above the base speed by reducing the field voltage and holding the armature at 100v. This should only be done if the motor is rated above the base speed. Above the base speed if your controller does do have the proper safeguards the motor speed can run away with itself and be destoyed. It is best to just vary the armature voltage.

120 VAC to a full bridge rectifier then to the motor field. Check the field volts it should be close to 90 vdc without a transformer. If you need a transformer it must go on the AC side of the rectifier. 120 VAC to the Variac (check rectifier vdc output and don't go above 100vdc) then to the rectifier then to the armature leads. Vary the variac to vary motor speed. Reversing the armature polarity will change direction. Be sure your rectifier and variac have the amp capacity required for the motor. The rectifier will probably need a heat sink.


Good Luck,

Joe

First off what was the nature of the controller that failed, and how did if fail?

Was it a simple variac or variable resistor, or was it a box with solid state
electronics inside?

If the latter, there was probably some circuitry in there that premitted the
motor to develop torque at low speed, that cannot be replicated easilly
with simple components.

In that case you might want to have the original controller diagnosed and repaired.

Often DC motor controllers have tricks in them, where they 'inspect' the
back EMF on the windings during the triac "off" period to generate a speed
signal, and have a feedback loop to try to maintain that constant under
varying loads.

Jim

Thanks for getting back Jim and Joe. The motor is 3/4 HP not 3.4. Sorry I just saw my typo. Jim the controller was made in the 70's with 70's technology. Made by Reliance and actually very nicely made. The whole system doesn't have 5 hours on it. The controller and the motor sat in a friends garage for 30 years and was never taken out of the box. I put it on my bandsaw maybe two years ago but have used it very little. I have no idea why it failed but I do not need all the bells and whistles that were on that controller. I used it because it was free and came with the motor ready to plug in. It had reverse, jog, speed control of course, slow start adjustment etc. I think that getting it repaired is out of the question. I just want the band saw to be variable speed and don't want to spend the money for a VFD and another new motor. Joe, I think you have told me exactly what I need to know. I am going to full wave rectify 110 AC and feed that to the field. I will take my variac and feed another slightly larger full wave bridge and then on to the armature. If the thing blows up then I will invest in a VFD down the road. Thanks again guys. Nice to hear from you as usual.

They did have semiconductors in the 70s....

It might be possible to repair the original, or inexpensively replace it with a
more modern DC drive, at less cost than retrofitting with a 3~ motor and
VFD.

Food for though.

Jim

You know it could be VERY SIMPLE!

I'd open it up, and see if it's something as simple as a blown fuse. It could also be a dead capacitor, or blown transistor. The NIFTY thing about '70s technology is they didn't use a lot of SMT stuff, and parts are easily identified. Some of the older transistors are hard to get, but if you can find the specifications on them, it's EASY to select a good substitute even if you can't find them in a cross reference. That and parts are DIRT CHEAP. Pop the top and look inside. BE CAREFUL, if it has some capacitors in it, they may store power for a few DAYS after it's unplugged. It's definitely worth looking inside though.

Don't forget the pictures!



Jim

As others have said open the drive up, shunt wound controllers are simple, look on eBay for a KB controller, they usually will run a shunt wound motor. Even a Permanent Magnet controller will work if you provide a bridge rectifier to energise the field.

DC motors are nice and simple to control and their low end torque especially the series and compound wound varieties is excellent.

DC Motor

crossthread said:

I will take my variac and feed another slightly larger full wave bridge and then on to the armature. If the thing blows up then I will invest in a VFD down the road. Thanks again guys. Nice to hear from you as usual.

Click to expand...

Crossthread,

A 35 amp full wave bridge will cost $5-$7. You will need two. With a 3/4 hp motor I doubt if you will need a heat sink. Just bolt them to the metal enclosure.

Joe

Thanks again for all the help guys. I am now actually looking forward to playing around with this. Before it was sort of OH CRAP. I have to install one of those laminated floors in the kitchen at home this weekend but I will probably take a nasty stab at this Monday. I never realized how much I use my band saw until it croaked. I will let you guys know if it blows up.

Thanks to you guys I got my 3/4 hp DC motor up and running on my band saw. I used two 8 amp full wave bridges from radio shack. These were the largest they had. When I rectified 110ac I actually got 109 vdc and the motor nameplate says the field should be 100 but I think it will be ok. I am using a 12 amp autotransformer on the armature with the same 8 amp radio shack full wave bridge and it is working great. I now have my starting torque back and no smoke. Thanks fellas.

Dave

Dave,

Sorry I did not see you post earlier as I have some spare SCR controllers. Mine are 100 volt armature and 180 volt field and with your motor I suspect there are two field windings and all you have to do is connect these in series to make it a 180 volt field.

Hey Toolnut. Good to hear from you. Thanks very much for the offer but she's up and running. I had the auto transformer in my junk pile and the bridges were $4.00 a piece. I got by on this one really cheap. Thanks again.