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?

  • \$\begingroup\$ I don't see any diagram - and how the pot is connected will affect the answer. \$\endgroup\$
    – brhans
    Apr 28 '16 at 16:05
  • \$\begingroup\$ You are going to give either a schematic (edit your question and use the schematic editor), or post a diagram. \$\endgroup\$
    – gbulmer
    Apr 28 '16 at 16:06
  • \$\begingroup\$ Could you please draw a circuit diagram and edit your question (press the edit button at the bottom of your post), and then press the circuit editing button. The more specific you are the easier it is to get an answer to your question. Thanks \$\endgroup\$
    – Voltage Spike
    Apr 28 '16 at 16:13
  • 1
    \$\begingroup\$ You would be better off changing the belt sheave sizes so that the motor's top speed is the desired maximum speed. Or is the power supply wrong for the motor? \$\endgroup\$ Apr 28 '16 at 16:31
  • \$\begingroup\$ Can you at least give us the type of motor and controller? It could be that the maximum speed that you see in when the lathe is not under load, and might slow down (and will need you to turn that pot up higher) when you are actually cutting. \$\endgroup\$
    – Mark
    Apr 28 '16 at 23:40

enter image description here

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.


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.

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


I need to know how can I increase the range of the pot so that the motor reaches the top speed with nearly a full turn of the pot. Bigger Pot?

Nonono. Bigger pot will make it worse. You would as you said, need a pot quarter of the 5M (5000k) pot.

This only applies if your potentiometer that you mentioned goes through some sort of sophisticated circuitry, with one end connected to Vcc or GND and the middle connected to the switching device, most likely to the gate of a MOSFET.

But this makes it complicated as in some situations and wiring layouts, changing the pot makes no difference in how far you turn the knob of the potentiometer to control how fast your motor spins (but may change other factors which could endanger your circuit).

You did not provide a circuit. Thus, if your circuit is laid out like this:


Change to this:



If your circuit looks like this:


Change to this:


  • \$\begingroup\$ This is all true if the motor is a brushed DC motor. On a lathe, however, I suspect that it would be an AC induction motor. \$\endgroup\$
    – Mark
    Apr 28 '16 at 23:40
  • \$\begingroup\$ You mean 3 phase AC motor? \$\endgroup\$
    – Bradman175
    Apr 29 '16 at 2:20
  • \$\begingroup\$ No, I was thinking a single-phase (with capacitor start). But a 3-phase is even more of a possibility if the lathe is big. \$\endgroup\$
    – Mark
    Apr 29 '16 at 2:25
  • \$\begingroup\$ Well i don't believe I have to change much as I could use a TRIAC instead and if 3 phase have 3 TRIACs connected to one pot. \$\endgroup\$
    – Bradman175
    Apr 29 '16 at 2:36
  • \$\begingroup\$ Wait one thing doesn't make sense about my circuit, adding resistance in series with the motor will generate a huge load of heat. I think I should change my circuit so that the pot goes to a PWM modulator before going to a transistor of some sort. \$\endgroup\$
    – Bradman175
    Apr 29 '16 at 2:38

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