Electronic – DC Motor Speed Control Board

dc motormotor controller

Speed ControllerI have a 120VAC DC motor speed controller (diagram attached as picture) it works very well, however in the circuit there is a 5000K POTENTIOMETER that controls the actual speed/voltage – The controller output range is from 0VDC to 120VDC to the brushed 1.75 HP motor. I am controlling a LATHE with this motor and circuit.

The problem is that if I rotate the pot about a quarter of the way around; the motor is already at the maximum speed I require (200RPM) or 50VDC. If I continue turning the pot the voltage could reach 110VDC and the lathe will spin over 4000 RPM and this is a nono.

I need to know how can I change the circuit (or the pot) so that the voltage output to the motor is not higher than 50VDC with a full turn of the pot. Bigger Pot?

Best Answer

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Figure 1. The area of interest in the speed controller.

Figure 1 shows that the speed control is a 5 kΩ (5000 Ω, not 5000 kΩ) and that it's output will increase from 0 to 12 V as it is adjusted from L (low?) to H (high?) speed.

... if I move the pot about a quarter of the way around; the motor is already at the maximum speed I want.

It sound as though your drive is at full speed at 3 V out (25%). To increase the useful span of the pot we need to have 3 V out at full rotation.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 2. Modified speed controller. Options A and B.

It may be as simple as adding 15 kΩ in series with the top of the pot as shown in Figure 2, Option A. This will result in 3 V at the top of the pot.

Alternatively put in a fixed 10 or 12k resistor and a 5 or 10k preset to allow you to tweak the max more closely to suit as shown in Option B.

You might see some non-linearity as the pot is loaded by R21, etc. If that's a problem we may need to scale all the resistance values down.

Just a caution: it might be worth checking what would happen should the pot go open circuit. Here's how to do it safely:

  • Disconnect the motor.
  • Measure the voltage on the W wire with the pot at mid-point.
  • Unsolder the wire from the pot wiper and measure the voltage on the wire (not on the pot).
  • If the voltage falls to zero then the motor should stop if the wire falls off.
  • If the voltage rises past 3 V (your maximum) then you need to consider if you need to put a pull-down resistor on W. You could try 100k or more and see if that's enough to bring it back down to a safe value.
  • If that doesn't work you may need to monitor the motor itself and put in independent speed monitoring and safety cut-out.
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