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ahidley
05-15-2009, 10:12 AM
I'm looking to upgrade my lathe to a variable speed motor.
I'm looking at about 1.5 hp. I only have 110vac single phase available.
It needs to be reversable.
Can someone educate me on what I need?
What kind of torque/power do variable speed motors put out at low RPMs?

camdigger
05-15-2009, 12:15 PM
I dunno nothing about variable speed motors. All I've ever worked with have been 3 phase synchronous motors (speed determined by input frequency) hooked to a variable frequency drive from a pase converter/inverter.

See here http://www.phaseconverters.co.uk/boost/ for a brief discussion on 110 V single phase input inverter generating a variable frequency 3 phase out put. Then a different 3 phase motor in place of your existing 1 phase motor.

SGW
05-15-2009, 12:36 PM
Single-phase induction motors don't take well to variable speed. It messes up their starting circuit. You could get a variable-speed DC motor. I know little about them, but I guess some people have use variable-speed DC motors off treadmills.

What I would suggest, though, is getting a 3phase motor and a variable-frequency drive (VFD) that accepts 120V single-phase input and produces variable-frequency 3-phase output. Such VFDs do exist for fractional HP, but you may have trouble finding such a VFD that will handle 1.5 hp though.

Running a 1.5 hp motor on 120V is a dubious proposition, anyway. I think you're going to need to put in a 240V single phase line, at which point your choice of VFDs expands greatly.

Oh -- a VFD ought to take care of the reversing issue, too.

camdigger
05-15-2009, 12:45 PM
A quick calc of power requirements indicates that a 1.5 hp motor has a running amperage in the 10.5 amp range. Rule of thumb starting current will be twice that, or 21 amps.

1.5 hp x 746 watts per hp = 1119 watts

1119 watts / 110 v = 10.17 amps => +/- 10.5 amps running current

Most starting currents act for +/- 1/2 to 1 1/2 seconds way longer than the average time for a breaker to pop. So you'd need at least a 110 20 amp circuit and then you may find some breaker trip issues if you try to start the motor during times of low voltage. Every service has some voltage drop in peak demand periods. Voltage drop is a bigger issue in rural services simply due to longer transmission lines.

Most phase inveters I'm familiar with do not have these issues because their circuitry has been designed to limit starting current loads (or soft start technology?)
IMHO, YMMV
Cam

JeffKranz
05-15-2009, 01:13 PM
Andy,

In a heartbeat, I would find a 2HP 3-phase motor that would work either on E-Bay or local motor rebuilders and use a VFD to run it. I currently do that on my lathe & bridgeport clone and you can run the motor at twice the RPM. The reason for 2HP is you loose HP when you deviate from 60 cycles.

If you need help on a 2HP VFD send me a PM or send me an email, and I might be able to help you out.

Jeff

lakeside53
05-15-2009, 09:01 PM
A quick calc of power requirements indicates that a 1.5 hp motor has a running amperage in the 10.5 amp range. Rule of thumb starting current will be twice that, or 21 amps.

1.5 hp x 746 watts per hp = 1119 watts

1119 watts / 110 v = 10.17 amps => +/- 10.5 amps running current

Most starting currents act for +/- 1/2 to 1 1/2 seconds way longer than the average time for a breaker to pop. So you'd need at least a 110 20 amp circuit and then you may find some breaker trip issues if you try to start the motor during times of low voltage. Every service has some voltage drop in peak demand periods. Voltage drop is a bigger issue in rural services simply due to longer transmission lines.

Most phase inveters I'm familiar with do not have these issues because their circuitry has been designed to limit starting current loads (or soft start technology?)
IMHO, YMMV
Cam




Most motors are about 80- 85% efficient at full power.. so add that to your numbers.


Do you really need a 1.5hp motor? if not... Teco sells 1hp VFD that can power a 230 volt motor 3 phase motor from 115volts. You can also get a surplus DC motor and controller...

Low speed torque. With a VFD, power is roughly proportional to the ratio of rpm verse 60 hz. An 1800 rpm 2hp motor will develop about 1 hp at 900rpm, 0.5 at 450 etc etc... keep you pulley/gears and use then as required.

No 230 volt? if you have room in your panel, a second breaker will convert a 115 volt circuit to 230... just make sure it is dedicated to your needs, and not shared with other outlets.

Bill Pace
05-15-2009, 09:19 PM
What are you putting this on? If a smallish lathe, say like the 9x20s, then a DC from a tread mill functions very well on that size, and could be put together for, in the neighborhood of $150-175. When you get up into the larger sizes, say the 12x36s etc, then youre better off to go the 3ph/VFD route and will probably be in the $300ish range - will depend on how good a scrounger you are;)

ahidley
05-15-2009, 10:27 PM
I dont know a whole lot about 3 phase applicatins. Will all the VFDs make any motor run fwd and rev?

Its going on a lathe with an 18" swing and about 36" bed, light duty chikong

I see some are saying 208, 220, 230. Is this significant?

lakeside53
05-15-2009, 10:32 PM
208 ,220, 230, 240 - same thing to a vfd... except 208 is generally 3 phase.

18 inch swing - you really need a decent motor.. 3-5 hp... which mean you need 230/240 single phase, or 208 3 phase...


Forward reverse - yes, all vfd's so that simply...

Paul Alciatore
05-16-2009, 02:15 PM
......

I see some are saying 208, 220, 230. Is this significant?

230 is twice 115 which are the nominal Voltages for home service. A small tolerance or varience is needed on these due to many variables in the distribution system, so anything from 110 to 125 is considered the same as 115 and anything from 220 to 250 is considered the same as 230.

208 comes from three phase and although it is not considered the same as 230, it is often used interchangably with it. I have a 230 motor on a mill at work but the building has three phase power so it is running on 208. This has been going on for about 10 years with no signs of any problem. The power available may be 10 to 20 percent less, but it is hard to notice 1.2 to 1.35 hp instead of the rated 1.5 hp.

But if you can afford it, the VFD thing looks pretty good.

Note on 208 Volts: The value of 208 comes from two 120 legs that are 120 degrees apart. From trig the factor is 1.732 so 120 X 1.732 = 207.84. If you use the more common 115 as the nominal Voltage, you get 115 X 1.732 = 199.18 as the nominal three phase Voltage. This would actually be about 15 percent lower than a corresponding single phase nominal 230 Volts.