Treadmill motor DC 180V Setup

[Chris GoodwillParticipant @chrisgoodwill78822]

I am trying to set up a variable speed controller for a 180 volt 6 amp DC motor.

I have a the mains going to a SCR then to a diode bridge through a choke to the motor, all works fine with the motor. The problem I have is with the potentiometer, on the SCR as I turn the knob and there is dead space before the motor runs and then if I keep turning past the ¾ mark it will supply more than 180 volts to the motor.

The potentiometer fitted is 500k 2 watt, I have taken some measurements and the motor starts turning around 400k ohms and at 100k ohms Supplies 160 volts to the motor.

My plan is to change the potentiometer for one at 330k ohms and put a 100k ohm resistor in series with it.

Will this solve my problem? Reduce the dead space at the beginning of the turn and limit the voltage to around 160 volt or am I missing something.

[Grindstone CowboyParticipant @grindstonecowboy]

Don’t see a problem with that idea, except you might want to have a 100k resistor on each side?

Rob

[Robert Atkinson 2Participant @robertatkinson2]

Do you have a schematic you can share?
SCR power controllers normally use phase angle control. This needs a capacitor to provide a time delay. We would need to know the capacitor value as well. If the design purely relies on a voltage divider to set the gate voltage in relation to the input level you will not get very good control/ For a start the gate trigger point is not well defined, do you have a Diac or zener in the gate circuit? It will also be succeptable to spikes and noise.

What country are you in and what is the mains voltage? I don’t understand why you would have a bridge rectifier ” SCR then a diode bridge”. The bridge will do nothing as it is being fed with DC from the SCR. Do yo have the SCR across the DC terminals of the bridge and the AC terminals to the supply and load? This turns thr SCR into s full wave device but why bother? Assuming 240V mains half wave rectification will give you enough voltage for a 180V motor.

The potentiometer is not the problem, your control circuit is not matched to the load.
Also note this type of circuit is inherently dangerous and needs carefull construction to avoid shock hazards.

Robert.

[Chris GoodwillParticipant @chrisgoodwill78822]

I am using a mix of off the shelf parts and salvaged treadmill bits so I have bought a AC SCR Voltage Regulator rated at 10,000 watts from ebay the AC output of this goes to the bridge rectifier to give me a DC voltage then to a salvaged treadmill choke to the motor. I also have a salvaged EMI filter I can put before the SCR.

I know buying a SCR with a DC output and a voltage limiting would have been better, but I had most of these parts laying around from other projects.

What I am trying to do here is make sure that when this all this stuff is in a control box, turning up the speed control cannot supply the motor with more than 180v DC from a 240v AC supply.

Is this the wrong setup for a DC motor speed control?

[Robert Atkinson 2Participant @robertatkinson2]

Well as the *man said when asked for directions “if I was going there I wouldn’t start from here”
Your set-up will work but is not optimal. Fine if it’s the bits you have though.
We don’t know the cicuit the pot connects too so just changing it’s value is a gamble. That sid the 330K + 100k is not too far out. So should work.
How many connections are there to the potentiometer, 2 or 3 ? If 3 then Rob’s suggestion of a resistor at each end may help but I’d suggest 47k at the “fast” end.
Make sure the potentiometer has a PLASTIC shaft and  use a plastic knob. If the mounting bush is metal either ensure it is fitted to a properly earthed metal panel or connect a separate earth to a lug under the potentiometer munting bush.

Robert.

[John HaineParticipant @johnhaine32865]

Chris, frankly given the dodginess of the approach you are trying and the fact that it’s unlikely to achieve what you want, I would suggest that you just buy an off-the-shelf DC motor controller.  They are available at reasonable price on eBay, such as this one:

https://www.ebay.co.uk/itm/363873694863

They’ve been discussed on here before and people have had good experience with them.

 

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