Start up requirements of repulsion induction motor?

Photowhit

So I have the chance to pick up an early Unisaw (Table saw which most are compared to...) It has a 1Hp 12amp 120v(/220) motor of the repulsion induction type, I've never heard of such but figured some of you electrical types might be able to help me out.

I put it here as I will hope to run this off grid, and I haven't purchased my inverter yet.

I really am curious as it would start with 'no load' of course it weights 80lbs by it's self... can this type of motor use a capacitor start? OK I already feel uncomfortable asking that...

HELP!

niel

Re: Start up requirements of repulsion induction motor?

although i'm not directly familiar with it either, i don't think there would be that much difference between that and one that uses magnetic attraction. kind of 6 of one and half a dozen of the other. as such i don't think there's a problem with using caps, but what do i know?

waynefromnscanada

Re: Start up requirements of repulsion induction motor?

My grandfathers domestic water pump had such a motor, but it was 1/4 HP, and it sticks in my mind, 4 amps @ 110 volts. AC of course. It had a wound rotor, that looks somewhat similar to a DC motor, complete with brushes, but instead of being connected to a power source, the brushes were actually shorted to each other. Depending on motor design, the brushes may, or may not retract once the motor is up and running. The one on my grandfather's pump did not retract it's brushes, but I understand that there was a centrifugal switch that cut power to them once the motor was up to speed, but I could be wrong on that. Standard induction motors can benefit from a capacitor to help them get started, but the repulsion induction type is a whole different kettle of fish and usually had rather high starting torque anyway if I remember right. Another I saw in action several times, also on a water pump, would start at roughly half speed, and after about 10 seconds, the brushes would somehow reposition their angle and the motor would come up to full speed. But all this was several decades ago, and repulsion induction motors have long been relegated to history. They were old when I saw them, so if you have one of these motors, you have history in your hands.

BB.

Re: Start up requirements of repulsion induction motor?

Interesting motors... I have never heard of them before. Anyway, here is a short Wiki Article about Repulsion Motors that confirms what Wayne typed.

-Bill

Photowhit

Re: Start up requirements of repulsion induction motor?

Well, I couldn't sleep tonight, so I hunted around. Apparently they have less of a startup requirement!

Something for off grid tinkerers to think about....

waynefromnscanada

Re: Start up requirements of repulsion induction motor?

Photowhit wrote: »

Well, I couldn't sleep tonight, so I hunted around. Apparently they have less of a startup requirement!

Something for off grid tinkerers to think about....

Kind of suspected that, but had no way of confirming, as there are none left in this part of the country. If you come across one, you'll have history in you're hands.
In general, I would not expect then to be nearly as reliable as induction motors, because of wound rotor, brushes etc.

Photowhit

Re: Start up requirements of repulsion induction motor?

I was worried it would be greater, as the 1HP motor weights more than my washing machine(not really sure about that, but nearly 90lbs) but the unisaw's motors have a reputation for being reliable, and I guess the spinning mass, helps with cuts as they claim no comparison between this 40's 1HP and a current 1HP rating.

waynefromnscanada

Re: Start up requirements of repulsion induction motor?

Photowhit wrote: »

I was worried it would be greater, as the 1HP motor weights more than my washing machine(not really sure about that, but nearly 90lbs) but the unisaw's motors have a reputation for being reliable, and I guess the spinning mass, helps with cuts as they claim no comparison between this 40's 1HP and a current 1HP rating.

I've found that the old motors had a whole lot more iron and copper in them, and thus had lower amps draw per hp. I often see a 1940's 1 hp induction motor, still in occasional use, that belonged to my grandfather. Massive and runs very cool. One thing bothers me though, my uncle views it as just another tool that benefits him. The bearings are getting stiff, haven't been greased in decades and rust is slowly replacing the black paint. To me, it's a work of art that I'd have to restore to like new and treasure. Most modern, cheap, standard inductions are small, light, power hungry and usually run too hot to hold you're hand on.

westbranch

Re: Start up requirements of repulsion induction motor?

Wayne, get out the grease gun when he isn't looking... at least you can keep it from seizing up.

think he would notice if you sprayed it with LPS3? would stop the rust.

waynefromnscanada

Re: Start up requirements of repulsion induction motor?

Gonna have to try something. Yeah, he'd notice the change in color and probably think I was after it. lol

Ken Marsh

Re: Start up requirements of repulsion induction motor?

Oh yes, I remember them well, too well.
I even applied a few years back.
They were used and even required sometimes by the electric Co. in the early days of REMC.
They could start, drawing no more than their full load current.
When they stopped making them 50 years ago, motor rebuild shops would pick them up, rebuild and resell.
Demand for them continued for years after motor companies stopped making them.

They started as a repulsion motor with brushes on the communtator.
On the bigger motors you could adjust the angle of the brushes which controlled how much current it would draw
and how much torque it would have on start up.
Then as the motor approached full speed it would lift the brushes and short all quadrants of the the communtator together with a cone.
This enabled it to run as an induction motor.

Probably the biggest issue with them was reliability.
The shorting cone , after a while, would not make contact on all sections of the communtator.
And of course they were expensive to manufacture.
We would not apply them now with the electronic options we have today.

Photowhit

Re: Start up requirements of repulsion induction motor?

As it turns out, just today I found a "Craigs List" steal on a 1 1/2 HP 120V Delta Contractors table saw, belt driven, in pretty nice shape, w/dado insert and a few extras for $100, only wish it had cast iron extentions.

So I can use my extra Unisaw fence(I had purchased earlier, thinking I'd build an over size table around my tiny direct drive table top saw) with it and leave one set up to face cut barn wood frames. Doubt it would start up on my Prosine 1800, but I had 'sorta' planned on having a larger inverter when I take the new place off grid.

Thanks for confirming the RI motors have such an easy startup. If the local fellow who has the Unisaw keeps dropping the price maybe I'll look into it some more. Word on the Woodworking sites are the old Rockwell/Delta motors are super reliable. If I do happen upon one I'll post a guts photo for your assesment.

Thanks one and all!

Ken Marsh

Re: Start up requirements of repulsion induction motor?

Photowhit,

That 1800 w inverter isn't going to like a 1 1/2 Hp motor.
You need to put the motor back on Craigs list and get a 2 or 3 Hp three phase motor.
Run it with a VFD type phase inverter.
You can start the motor beginning at 0 amps with the amps ramping up as RPM rises.
Even at full speed and full load the amps will usually be less since the 3 phase motors are significantly higher efficiency
and run near unity power factor.
You can also vary the speed of the motor and even over speed it if you want.

One problem however, the inverter is probably 120 Vac and most any 3p motor will want 220.
VFDs want the same single phase voltage in as they deliver to the motor.
This means a transformer, or a different inverter.
You probably could get by with a 500 VA transformer using it in boost mode.
The transformer needs only be half the power rating when used in a boost configuration.

Photowhit

Re: Start up requirements of repulsion induction motor?

Your nuts, Heck I'll go buy the RI Unisaw for $4-500 before I spend $3-500 for a motor, $2-500 for a VFD and another $1000+ for an inverter for this single use. The inverter I'll buy will run the new place in total, and had anticipated wanting, if not needing a larger inverter. This will be run for a hour a day now and then.

Cariboocoot

Re: Start up requirements of repulsion induction motor?

A 1.5 HP motor is less than 1200 Watts running. Since this is on a saw, it will only pull that kind of power when it is cutting wood. Running "free" it will be less. The start-up probably will not exceed the surge rating of the 1.8 kW inverter for this same reason; much easier to get a motor spinning when there's no load to start against (unlike a water pump, for instance).

Someplace on the forum is a thread where I tested some actual saws and an air compressor to see what they used in terms of power. This is what I came up with:

For what it's worth, I just ran my 10" Skil tablesaw through my UPM meter. The saw is rated as 13A @ 120 VAC. The start-up was 4.9 Amps (118 Volts here). I pushed a piece of wood through it and loaded it fairly well: 8.6 Amps / 952 Watts. The UPM doesn't do Power Factor, but calculated it isn't good: 8.6 Amps * 118 Volts = 1015 Watts (this won't be entirely accurate). Still, something like the Exeltech 1100 Watt http://www.solar-electric.com/exxp12vol11w.html would probably handle it for occasional use.

Photowhit

Re: Start up requirements of repulsion induction motor?

Thanks Coot,

I may try it out, to spread out the costs the plan is to go off grid in the fall, and bring the 1800 prosine over here, put the cabin on the backup 1000 watt inverter, then a new inverter, and a mini split before summer's AC season next spring... So I may get a chance to try out the table saw this fall!

God willing and the creek don't rise...

Still have some racking and at least 1 midnite classic and a 60Amp PWM this year, and some stove pipe, though if the low feature Classic comes in with a $100 or more discount.... Might be nice if I still made a reasonable living... at least I'm still working which can't be said by all.