"Hard starting" 120 VAC motor

The Dude · Jan 21, 2016

We are rebuilding a glass grinding machine which has a grinding spindle. Rebuilt, turns very smoothly, grinding wheel weighs maybe 5 lb. at the most. Spindle is belt-driven (v-belt) by a 120 VAC motor that sits on top. Motor is controlled by a relay. When starting, the motor shudders (I don't think there's anything wrong with the motor other than we maybe didn't select the correct type) enough to move the spindle carrier anywhere from .001-.004" (dove-tail slide, the ways are about 4" apart). The whole slide is fairly heavy duty but being driven by an amce thread (about 1/2") and it does have some backlash (from what I understand, that's normal and unavoidable). We can work around that as we typically will drive in from only one direction and "touch-off" each time we make a new part (very low production specialty parts for now). Also, I suspect we could tighten up on the way at the bottom (it's adjustable) but maybe only a thou or two.

Bottom line, is there an AC motor that's designed to start smoother that's economical and, if so, would it likely produce any benefit? We will have to do a "touch-off" and I'm thinking if we do it when the spindle is off, turn it on and then re-adjust after it moves I'd think we'd be good to go? The is all kind of "first-time" for me so I have really no idea what's normal.

Thanks,
The Dude

DMF_TomB · Jan 21, 2016

The Dude said:
We are rebuilding a glass grinding machine which has a grinding spindle. Rebuilt, turns very smoothly, grinding wheel weighs maybe 5 lb. at the most. Spindle is belt-driven (v-belt) by a 120 VAC motor that sits on top. Motor is controlled by a relay. When starting, the motor shudders (I don't think there's anything wrong with the motor other than we maybe didn't select the correct type) enough to move the spindle carrier anywhere from .001-.004" (dove-tail slide, the ways are about 4" apart). The whole slide is fairly heavy duty but being driven by an amce thread (about 1/2") and it does have some backlash (from what I understand, that's normal and unavoidable). We can work around that as we typically will drive in from only one direction and "touch-off" each time we make a new part (very low production specialty parts for now). Also, I suspect we could tighten up on the way at the bottom (it's adjustable) but maybe only a thou or two.

Bottom line, is there an AC motor that's designed to start smoother that's economical and, if so, would it likely produce any benefit? We will have to do a "touch-off" and I'm thinking if we do it when the spindle is off, turn it on and then re-adjust after it moves I'd think we'd be good to go? The is all kind of "first-time" for me so I have really no idea what's normal.

Thanks,
The Dude

View attachment 160673 View attachment 160674

.
induction motors typically pull 4 times rated amps and horsepower on startup until up to minimum rpm that the centrifugal switch turns off the starter windings. if supplied voltage drops because of high amp flow on startup typically the lower starting voltage means less hp and the starting takes even longer to get up to speed.
.
i have seen motors connected with a hydraulic or magnetic coupling. they slip on overload and as driven item gets up to rpm and load drops they stop slipping. look for different types of couplings.

sfriedberg · Jan 21, 2016

If possible, find a way to better decouple the motor mount and tension on the drive from exerting force on the slide. Maybe you can arrange things so the startup-jolt takes the backlash out of the slide rather than putting it back in?

If that's not possible, replace the single-phase motor with a three-phase motor and use a motor controller with slow-start capability instead of a relay/contactor. A typical modern variable frequency drive (VFD) has that feature built-in, but you can get dedicated slow-start controllers if you don't need variable speed in general. For a mere 3/4HP motor, you can run most three-phase VFDs on single-phase input. Don't think that's the case for dedicated slow-starts, but may be wrong.

atomarc · Jan 21, 2016

While this doesn't answer your direct question, I will agree with post #3. A small TECO drive is available for just over $100 dollars and there are 3/4hp motors of the proper frame size and RPM available on eBay for just under $100 dollars. So for this modest investment you get your controlled soft start, the ability to vary your RPM, motor overload protection and the added bonus of a controlled stop which could be helpful when you have a 5 pound rotating mass and want to change the wheel or walk away from a fully stopped machine.

Stuart

The Dude · Jan 21, 2016

Hey thanks guys, definitely some things to check out. Right now, the 3 phase isn't an option (not in this building) but, if the machine proves out and we load it up with work, we are planning a move to a new facility that definitely will have 3 phase. In the meantime, I'll look into the coupler.

Thanks again,
The Dude

atomarc · Jan 21, 2016

The Dude said:
Hey thanks guys, definitely some things to check out. Right now, the 3 phase isn't an option (not in this building) but, if the machine proves out and we load it up with work, we are planning a move to a new facility that definitely will have 3 phase. In the meantime, I'll look into the coupler.

Thanks again,
The Dude

Dude,

The TECO inverter of which I speak takes 120 volt single phase in and outputs the 3 phase for the motor...it abides!!

Stuart

sfriedberg · Jan 21, 2016

The Dude said:
Right now, the 3 phase isn't an option (not in this building)

To quote myself:

For a mere 3/4HP motor, you can run most three-phase VFDs on single-phase input.

The Dude · Jan 21, 2016

atomarc said:
Dude,

The TECO inverter of which I speak takes 120 volt single phase in and outputs the 3 phase for the motor...it abides!!

Stuart

Whoa...dude!

garyhlucas · Jan 22, 2016

This is a high inertia load. It would be a good idea to get a VFD drive with a braking resistor, that will prevent faults due to trying to stop too fast, and it will allow you to stop the wheel much faster too. A three phase motor doesn't have a starting switch so is much better suited to start stop operation with a VFD.

Search

Largest Manufacturing Technology Community on the Web

Search

"Hard starting" 120 VAC motor

The Dude

Hot Rolled

DMF_TomB

Diamond

sfriedberg

Diamond

atomarc

Diamond

The Dude

Hot Rolled

atomarc

Diamond

sfriedberg

Diamond

The Dude

Hot Rolled

garyhlucas

Stainless

Similar threads