Tips/Techniques 3 Phase motor

[Tomc938]

Question: I've heard tell of 3 phase motors that can be picked up for cheap. I would like to swap out the 220 motor on my King 1236 lathe and add a VFD.

Anyone have any idea where to look for one? The ones I've seen on FB are solid gold covered in paint that simulates age and rust. At least that's what I would figure based on the prices.

Also, does the voltage matter in this application? I have 220 in the shop. I'm just at the start of the learning curve with VFDs.

 

[DPittman]

Question: I've heard tell of 3 phase motors that can be picked up for cheap

I guess that depends on where you live. I heard that there was lots available and cheap also but I couldn't find a suitable one where I am so I ended up buying new 2 times for vfd conversions.

I think you'll love vfd on your 12x36 lathe and I think it is an excellent candidate for such a retrofit.

 

[Tomc938]

Another question comes to mind: the lathe currently has a 1.5 hp motor.

Is that the range I should be looking for? More or less?

 

[DavidR8]

More hp than you have now because when the frequency is reduced so is the torque. I’d go 2hp min. Belts will be the limiting factor.

 

[Tomc938]

More because when the frequency is reduced so is the torque. I’d go 2hp min. Belts will be the limiting factor.

Sorry, dumb question - how will belts be limiting?

 

[DavidR8]

Sorry, dumb question - how will belts be limiting?

The belts will only transfer so much power.
My lathe belts slip but the motor doesn’t stall.

 

[Susquatch]

Another question comes to mind: the lathe currently has a 1.5 hp motor.

Is that the range I should be looking for? More or less?

For a lathe, definitely more, not less. But only 25 to 50% more.

Basically the highest load a lathe normally sees is during the spin-up. A VFD allows you to control the rate of spin up so you can use a bigger motor and still not worry about overloading the lathe gears and hardware. On a mill, you should use the same size motor.

I think @David_R8 meant HP instead of Torque. Most VFDs are constant torque below the 60Hz, but horse power drops off. It's not a big deal in my opinion because you don't need the horse power at lower speeds.

Finding a motor requires a lot of patience no matter where you are. But they are out there and they are out there cheap.

 

[cjmac]

Another question comes to mind: the lathe currently has a 1.5 hp motor.

Is that the range I should be looking for? More or less?

When I went to a VFD many years ago, I went to my local motor supplier (who I bought a lot of motors from) looking for a 3 phase motor to replace the 1.5hp motor originally on the lathe. His advice was to get a 1 hp 1200 rpm motor (6 poles). I went with that and it has been great. Lots of low end smooth torque from the extra poles.

Chris

 

[Susquatch]

When I went to a VFD many years ago, I went to my local motor supplier (who I bought a lot of motors from) looking for a 3 phase motor to replace the 1.5hp motor originally on the lathe. His advice was to get a 1 hp 1200 rpm motor (6 poles). I went with that and it has been great. Lots of low end smooth torque from the extra poles.

Chris

You said that was many years ago. In that context, it's probably true. But I "THINK" that advice predates the latest VFDs that can use systems like feedback sensors or SLV (sensor less vector) mode to supply constant torque at extremely low speeds. I put a 1000:1 turndown motor on my mill that actually produces full rated torque at zero rpm. It is a standard number of poles standard speed motor. It has to be seen to be believed. A 60 Hz 30:1 motor will do that at 2hz, a 20:1 at 3hz and a 10:1 motor at 6 Hz.

The same goes for smoothness. It will make your knees go weak to see how much smoother a regular 60hz 3ph motor can be when running in SLV mode.

 

[Susquatch]

Although I'm certainly not an expert, perhaps it would help if I added a little more detail without getting too technical.

We can all imagine a high speed computer controlling the current flowing through the coils of a 3ph motor. It can certainly emulate the 60Hz power that is in your shop.

But it can do a better job of sending exactly what the motor needs if it knows exactly where the rotor in the motor is. Sensors can be mounted on the motor to provide the computer with that information. In a VFD (which is a computer controlling a motor), that is called sensor control mode. It's sort of like having a motor with a thousand poles in it. The VFD knows where the poles are and adjusts the power to simulate an infinite number of poles.

Unfortunately motors with a huge array of sensors are hard to find and hard to make. So the folks who make VFDs came up with a great way to do it without sensors. They called it Sensorless Vector control mode. This mode learns about the motors voltage, current, and back emf characteristics and then uses that information to figure out where the rotor is at all times, and then feeds it with exactly what it needs to run smoothly, at full torque, at low speeds.

The result is a regular motor that feels like one with a thousand poles.

 

[slow-poke]

I just paid IIRC $113 for new old surplus 1.5HP at Lenmark, they advertise ok Kijiji all the time. My neighbor machinist buddy tells me he picked up a inverter rated 2HP from Amazon warehouse for $130. They are out there.

 

[Susquatch]

I think it's a lot easier to find a motor for a lathe than for a mill.

On most lathes, you can modify the mounting to fit and you can get a drive pulley the right size for the motor shaft.

Those issues are harder to to deal with on a mill mostly because of space and housing limitations.

For both, it can be difficult to find one that is VFD rated (has a turn down ratio and electrical protection designed in). But it isn't really a huge issue. Hobby equipment doesn't run 24/7 and doesn't normally see very high loads. Most non-vfd motors can handle a VFD just fine if you limit speeds to +/- 25% of 60 Hz. And you can usually go a fair bit slower on the bottom end by adding supplemental cooling with a computer case fan. It's a fairly common modification that works really well. Basically, VFD Rated motors usually have better cooling (or high temp resistance), better bearings, and better static control built right in.

 

[Ironman]

The belts will only transfer so much power.
My lathe belts slip but the motor doesn’t stall.

V Belts are limited in power transfer in rpm conditions below 500. This can be improved by an idler to increase the wrap around of a small pulley.
The micro-V belts of automobiles are a great improvement, but we are not seeing a migration into other stationary equipment. Think about it...when was the last time you heard a steering pump belt squeal when turning at an idle, since the micro-v has arrived?

 

[cjmac]

You said that was many years ago. In that context, it's probably true. But I "THINK" that advice predates the latest VFDs that can use systems like feedback sensors or SLV (sensor less vector) mode to supply constant torque at extremely low speeds. I put a 1000:1 turndown motor on my mill that actually produces full rated torque at zero rpm. It is a standard number of poles standard speed motor. It has to be seen to be believed. A 60 Hz 30:1 motor will do that at 2hz, a 20:1 at 3hz and a 10:1 motor at 6 Hz.

The same goes for smoothness. It will make your knees go weak to see how much smoother a regular 60hz 3ph motor can be when running in SLV mode.

I couldn't check because it is currently in storage, but the drive I used is a Hitachi WJ200 (definitely Hitachi and had 200 in the model number). It came from Drive Warehouse in Texas and looking at their site now the prices have come down. It likely has SLV. It will run very slow and smooth with the 6 pole motor.

Chris

 

[kstrauss]

My local scrappie has used 3-phase motors. Mostly 550 volt so not suitable but they also sometimes have 208 volt ones. Price is by the pound so probably $15-30 for something of suitable size for your lathe. Other than bearings there is nothing to wear out in a 3-phase motor so it might be worth investigating your local junk yards!

 

[Tomc938]

My local scrappie has used 3-phase motors. Mostly 550 volt so not suitable but they also sometimes have 208 volt ones. Price is by the pound so probably $15-30 for something of suitable size for your lathe. Other than bearings there is nothing to wear out in a 3-phase motor so it might be worth investigating your local junk yards!

I like the sounds of that!

 

[VicHobbyGuy]

Scrap yards that sell to the public and cheap used 208 volt 3-phase motors.....I live in a parallel universe without such things.

 

[mbond]

My local scrappie has used 3-phase motors. Mostly 550 volt so not suitable but they also sometimes have 208 volt ones. Price is by the pound so probably $15-30 for something of suitable size for your lathe. Other than bearings there is nothing to wear out in a 3-phase motor so it might be worth investigating your local junk yards!

Normally, 3 phase induction motors can be run at almost any voltage up to the rated one. Voltage that's too high is a problem because of insulation failure and resulting short circuits. But voltage that's too low (down to some minimum based on the internal impedance of the motor) should just work. Of course all the numbers on the name plate (power, torque, amperage) will be totally wrong. But if you are building something from scrap parts, that's probably not a huge concern. Especially if you will then attach a VFD

 

[Dabbler]

Well, you can use a motor +/- 20% of rated voltage. If the voltage is half, due to lower inductance and lower resistance, the current more than doubles. This results in very fast heating of the windings and insulation failure.

For instance I have a grinder that is 3PH 550V. I can choose a voltage-doubling VFD, but I already have a rotary phase converter and 2 VFDs. I'm looking for an inexpensive 15000 VA 3ph 2:1 transformer instead. If I run it on raw 240V 3PH, I'll smell the magic smoke.

 

[VicHobbyGuy]

If the voltage is half, due to lower inductance and lower resistance, the current more than doubles.

I don't understand. It's an over-simplication, I know, but can't I think of the windings as resistors? If the voltage is halved, won't the current be halved, not doubled?