Questions with regulating DC output

Thread Starter

skeer

Joined Oct 28, 2022
45
So I know there's many ways to skin this cat but I'm wondering about two:

SCR and DC PWM

Here's the situation: I want to retrofit a rated 2hp treadmill motor (120vdc @ 15A) to a 1954 Delta drill press.

Are there any real pros or cons to setting it up like: AC -> SCR -> rectifier -> motor. Or AC -> rectifier -> PWM DC controller?

I'm currently using an SCR in the first example on a benchtop drill press and other than the pot being a bit too high in resistance, using it's been... ok. I am seeing lately the torque in low speeds a bit lackluster.
But I've never used an actual PWM Controller so I figured I'd ask here.

Thanks!
 

MaxHeadRoom

Joined Jul 18, 2013
28,513
There is a couple of reverse engineered TM schematics here, search for schematic MC-60 . SCR bridge, or the PWM MC2100.
The PWM control is of course quite a bit superior to the SCR version.
 

Thread Starter

skeer

Joined Oct 28, 2022
45
I know the mc2100, you are talking about reusing the original controller. I was referring to aftermarket in this case. I wouldn't try to buy an OG controller, things are priced as if they are solid gold.
The MC60.. was'nt that the controller that you could hot wire with just a pot?


FWIW I do have an MC2100 all wired up for a different project. All that guy needs is a PWM generator of some kind and its good to go.
 

MrAl

Joined Jun 17, 2014
11,263
So I know there's many ways to skin this cat but I'm wondering about two:

SCR and DC PWM

Here's the situation: I want to retrofit a rated 2hp treadmill motor (120vdc @ 15A) to a 1954 Delta drill press.

Are there any real pros or cons to setting it up like: AC -> SCR -> rectifier -> motor. Or AC -> rectifier -> PWM DC controller?

I'm currently using an SCR in the first example on a benchtop drill press and other than the pot being a bit too high in resistance, using it's been... ok. I am seeing lately the torque in low speeds a bit lackluster.
But I've never used an actual PWM Controller so I figured I'd ask here.

Thanks!
Hi,

To get constant speed you have to incorporate a speed regulator which requires feedback. The speed is measured and that signal is fed back to the control circuit. If the speed decreases the control circuit senses that and increases power to the motor to keep the speed constant.

There are a few different ways to measure the speed. The simplest uses a measurement of the back EMF. A more direct method is to use a small generator. Still yet other methods are to use a magnetic sensor with a toothed wheel or a photo speed sensor.
To measure the back EMF you have to measure the current to the motor and combine it with the input voltage with some given ratios in mind. This depends on what you call the motor constants.

When you do incorporate a speed regulator like this that is to be used in an application where the human more or less needs to be able to monitor the operating conditions as the product is being used, you would design the controller such that the speed does not stay perfectly constant, but is allowed to decrease a little as the load is increased. That gives the human user a little sensory feedback so they get a feel for how the operation is progressing. Without that they may end up breaking a lot of bits.
 
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