# Thorn DC motor controller wanting Schematic for the controller.

#### campa.marine.electrical

I am a maintenance electrician and have a good knowledge of electronics the issue is that the machine I am working was built in the 1970s.
The variable speed control is from 3 phase AC input to a DC output to drive the DC motor. When the first broke down I checked the control board there a number of swollen capacitors on the control board I replaced those and the machine come back to life and the current draw also dropped.
Everything seemed good as far as operation when but now there is a report that the machine has stopped and I am on my way back to Auckland to find out the reason why. The unit from what I can gather uses SCR control to regulate the speed and torque but a schematic would be a real help. The machine is a Bettol Twin-screw extruder of plastic materials.

#### Carl

Hi Campa
I used fix DC drives pcb's ...usually with no schematics...if the drive is that old then it going to be op-amps.....

Yes that would most likely be scr for the armature...and the rectifier bridge is for the field....that would be the power element....make sure you have a DC field..that's the first thing I would do is check that you have a good field just to be safe, you don't want to be running DC motors with a funny field( stator winding ) that can be dangerous....also be careful of the zero volts on the board....that is most likely a floating zero volts...it'll be the zero side of the rectifier bridge..this was normal for dc drive circuitry in days gone by, unless you have a fancy modern processor DC Drive

Yes the SCR would be the part that controls the speed...(unless you have field weakening as well )... it's pretty basic really ...you change the firing angle in the thyristor to change the voltage going to the armature.

If you need some guidance I can probably help...sorry no scheme..think you will be lucky there...but you never know.

.

#### campa.marine.electrical

Hi Campa
I used fix DC drives pcb's ...usually with no schematics...if the drive is that old then it going to be op-amps.....

Yes that would most likely be scr for the armature...and the rectifier bridge is for the field....that would be the power element....make sure you have a DC field..that's the first thing I would do is check that you have a good field just to be safe, you don't want to be running DC motors with a funny field( stator winding ) that can be dangerous....also be careful of the zero volts on the board....that is most likely a floating zero volts...it'll be the zero side of the rectifier bridge..this was normal for dc drive circuitry in days gone by, unless you have a fancy modern processor DC Drive

Yes the SCR would be the part that controls the speed...(unless you have field weakening as well )... it's pretty basic really ...you change the firing angle in the thyristor to change the voltage going to the armature.

If you need some guidance I can probably help...sorry no scheme..think you will be lucky there...but you never know.

.
I was very fortunate that it was not a controller problem which I was happy to find out it ended up being a temperature controller. It was not even related to any motor problems. In the next few weeks while the factory is shut down I will need to remove the other DC motor due to the fact it is running really hot. I removed the opposite one a couple of years ago for the same reason and sent it away for an overhaul. I guess these motors have been in service since the 70's. Thanks for your input it was great you responded to my question fairly quickly which helped a lot. What is a good way of stopping contact burning on a resistive load like a heating element on a plastic extrusion machine? The relay contacts are rated at 10 amps and the switched load current is between 3 to 6 amps. What I was going to do with these Omron relays is to wire both sets of contacts in Parallel to try and increase the service life of the relay. The load for the elements are all single phase-type of loads when it comes to the heating elements but switched frequently by the heat controller.

#### David_2

Let us know how paralleling the contacts works. In all these years, I've never tried that.

'Snubbers' are used across the control relay contacts on inductive loads to absorb the back EMF that can arc and burn the relays. But the heater elements are not likely much of an inductive load.

Let the temperature controller relay output drive a Solid State Relay (SSR) that switches the current load. But you'll have to have the controller's relay 'switch' a DC voltage to the SSR's coil. The SSR's typically have 3-32Vdc 'coil' and are sized by the current that their 'contacts' can carry. The key to long life of an SSR is to make sure that it is mounted on a heat sink with some heat transfer compound so the the heat generated by the SSR get dissipated. Also, using the next size up is probably only an additional \$10.

Some stand-alone temperature controllers can be ordered with a SSR output (as opposed to an electromechanical relay) to drive the heater elements.

Or use an electromechanical interposing relay so the controller's relay output is only driving the low current of interposing relay.

#### Carl

Hi Campa

That worked out for you then ...panic over. I would agree with Dave SSR is most likely a solution, Make sure that you have some sort of fusing in that line usually SSR's blow open circuit but you never know. Don't forget the heat paste compound on the SSR ( that if it is the small black one that's mounted on the backplate of the panel )

The problem is most likely not the inrush current, although you quote 6amps, I suspect it will be higher than that if you tested it with scope.

Unfortunately, it does not take a lot of inductance to generate an arc across the contact. If you switch the lights off and view the contactor in the dark you will most likely see a small flash as the contactor switches on and a larger flash as it switches off....

You can also sometimes feel this effect when you switch a heater of via a wall switch like a kettle .....the on feel has a crispy feel while the off feel has a sort of sponginess about it....

There is arc suppressing contactors on the market but that's  compared to an SSR and usually used on high power/voltage loads.

A quick fix might be to give the temperature controller a slower reaction time,,, but that sounds unlikely on an extruding machine I would imagine that needs to be a reasonably tight loop.

If this is a marine application make sure the panel is sealed properly...this can cause all sort of longevity issues.

The last point... if it's an old panel with old switchgear and relay logic....then this is an issue in its own right and will give you endless problems. Sell them a new panel with PLC and upgrade all the DC drives to inverters...overhauling a dc motor might not outweigh the cost of an inverter, the smaller inverters are as cheap as chips and have DC like performance.