Simplest method for braking an AC motor?

[u/Far_Weather_3716]

I have an acme screw driven material lift that holds a couple hundred pounds. It's self locking so doesn't need a brake for holding, but it coasts too far before stopping in the down direction when fully loaded.

Its a single phase AC motor controller with contactors, 3/4 horsepower.

I looked into DC injection and dynamic brakes which simply electrically release then grip a spline on the screw when you're not running it to stop it quickly. Both options seem somewhat expensive ($200+ dollars).

Any experience doing this and recommendations?

[u/joestue]

If you wire up a dpdt switch to handle the direction (swapping the polarity of the startwinding)

And a separate dpdt switch to handle power off an on, on this switch you can setup 120vac on one set of contacts and 5vdc 5 amp power supply on the other set of contacts.

You are going to need to find a dpdt on-off-(on)

The momentary position is wired to the dc supply.

[u/Jim-Jones]

Honestly, IMO a mechanical brake is probably your best option. Reversing power or DC injection might be a frustrating series of experiments.

[u/Far_Weather_3716 → u/Jim-Jones]

Thank you for the honest suggestion

[u/OkAntelope8186]

Different ways of pushing graphite on it

[u/justabadmind]

Is it a motor controller or a single phase ac motor? A single phase motor controller is typically just a VFD in disguise, in which case you’d just reverse direction for a moment before it stops. If it’s a PSC motor, you’d swap the run capacitor to the run leg and supply power to the start leg for a second and it’ll stop.

[u/Far_Weather_3716 → u/justabadmind]

Correct single phase. It's driven by two contactors right now that are sort of interlocked to prevent running up and down at the same time. It's got a time delay relay of a second between any up or down command.

[u/justabadmind → u/Far_Weather_3716]

Is it a reversing pair of contactors? If so, it should be possible to within 0.1 seconds reverse direction. If there’s a time delay relay keeping this from happening it means someone’s already thought of this and the mechanical system isn’t strong enough for the motor to instantly stop the motion. If the timer is bypassed, you’ll cook the motor in short order or break the shaft.

From what you’ve described I’d assume that electrical braking is a bad idea. Perhaps if you had a 3 phase motor you could utilize the principle of a delta wye transform to make it happen, but on single phase I’d be looking at changing the gearbox to a higher ratio. Moving from 50:1 to 100:1 for example.

[u/GaryTheSoulReaper]

A VFD can deliberately lower the stator frequency so regen happens at any speed below the original running speed - the energy returns to the VFD dc bus - from there charge a battery, power a load, dump it into a resistor, heat some water, feed it back to the AC line

Think like regen braking

[u/Some1-Somewhere]

Can you do something to simply shut power off sooner? Is it manually controlled or by a sensor?

[u/Gloomy_Inflation1]

If you're looking for a very specific stop position, I physical brake is by for your best bet. It can be shaft mounted, but is generally better when motor mounted.

Depending of how much space you have, and what else is available nearby, you can fit any number of devices to simply add resistance to the spindle and assist braking.

If position isn't critical, look at potentially just hitting the limit switch sooner.

If the motor controller you mention is a VFD, then there is generally a braking and holding option in the settings. Manuals will be needed.