Vfd for an old 3hp lathe

[Batmanacw]

I believe you're intending to power the VFD single phase? A three phase motor drawing 10 FLA is drawing a sustained 10 amps on each of three wires. When supplied by 2 wires (single phase), the equivalent power is supplied by approximately 17.3 amps. A VFD with a maximum of 13 amps input on single phase cannot sustain 10 amps on three phase to the motor. Cheap import VFDs often falsely claim otherwise (and my 120V vacuum cleaner claims peak 5HP). Since VFDs are not 100% efficient, you'd need 18+ amps single phase to sustain 10amps. Sizing VFDs by HP is an approximation. VFDs are really sized by output current to match the motor FLA.

An electrical motor may significantly exceed FLA briefly on startup, or briefly under heavy load. If you go strictly by electrical code, circuits aren't suppose to be loaded to 100% capacity especially for loads like motors where a brief high startup current occurs. 30A is the next step up.

All that said, I run a 3HP lathe w/ VFD on a 20a single phase circuit and have never blown the breaker. I do have the VFD programmed to ramp up somewhat slowly to avoid high startup current, and to allow the RPM to slow down when the current draw gets too high. So I'm potentially sacrificing some performance by not using a 30a circuit.

So the vfd manufacturers knowingly sell vfds rated for more than they can handle?

All the folks who have posted here and elsewhere that you don't have to oversize the vfd are wrong?


I need a 5hp plus rated vfd to run a 3 hp motor and an $82 wire instead of a $38 wire even though 12-2 will support a 20 amp circuit with margin for error?

I need all that and you are running a vfd on a 20 amp circuit without issues? What is the rating on your vfd?

 

[Batmanacw]

Can I run a reversing switch that is already on the lathe between the vfd and the motor? I'm sure it's just reversing two leads to reverse the rotation.

 

[Flyinfool]

NO on the reversing switch.
You can wire the machines reversing switch to the VFD to have the VFD reverse the motor. Ideally you do not want any switches of any kind between the motor and the VFD. Any switches you want can be wired to the VFD so it still feels like you are using the equipment as before.

Many Chinese (and some USA) companies rate at PEAK theoretical calculated power, this is what they can do for less than 1 second. Continuous power is a lot less but is not as an impressive number for advertising so they don't mention it, Many do not even mention it in the manuals.
This is not just VFDs, it is all electronics.

 

[rabler]

So the vfd manufacturers knowingly sell vfds rated for more than they can handle?

All the folks who have posted here and elsewhere that you don't have to oversize the vfd are wrong?


I need a 5hp plus rated vfd to run a 3 hp motor and an $82 wire instead of a $38 wire even though 12-2 will support a 20 amp circuit with margin for error?

I need all that and you are running a vfd on a 20 amp circuit without issues? What is the rating on your vfd?

Cheap import VFDs lie, or at least play numbers games. It is called marketing. Saving money on a cheap imports is not always a win. Decent quality VFDs tend to be rated more accurately, so those folks aren't wrong.
I'm using a Teco L510-203, specs are for 3HP, 10.5amps to the motor and 21amps input current on single phase.

 

[B2]

As far as the wire to feed 220v to the vfd, do I need 12-3 with ground or is 12-2 w/ground good enough?

Depending upon how long a run you have from the breaker box to the lathe you might use 10 or 12 gauge wire. For my 3HP motor and Hitachi inverter I used 10-3 with ground even though I was not going, but about 20 foot with the wire (standard non-metallic/NM romex). However, if you use a 30amp-220V breaker then you normally should use 10 gauge wire. If you are only going a short distance (10-15 feet) then the 12-3+gnd is ok, but the breaker should be a 20Amp- 220V breaker.

The motor may only take 20Amp at 3 phase, but the inverter probably needs 30Amp as it is single phase converting to 3 phase. This should be in the inverter manual. Yes, I know that the motor is supposed to be a balance 3 phase load. That would imply that a neutral is not needed and there may not even be a terminal on the inverter to connect the 3rd wire too. However, there are a couple of reasons I used the 3 wire plus ground. 1). Life is not perfect and who knows about your old motor/lathe. The insulation in the motor may not even be very good. If anything is out of balance (different impedance) then his means different voltage/currents can flow in the windings. This would mean that the center point connections might not be at the mid point in voltage. This is not very important until things get well our of whack. However, an even more convenient reason to have a neutral is that you may want to connect some 120 volt devices (coolant pump, lighting, small compressor, other motors, or even an electrical outlet) in your circuits and for this you need a neutral (3rd wire) for the power/current to return to the breaker box. You would not want to have to run a second wire pair from a second breaker just to hook up a extra motor. The ground should go to your lathe for safety reasons. Should ever one of the high voltage wires touch the lathe or insulation just breaks down and it did not have a ground then the lathe would be at this voltage. Image a barefooted child walking by, on a damp concrete floor, and the child touches the lathe. Current might flow from the lathe, through the child, and into the concrete/earth. I experienced this as a child, in a different setting, and I am lucky to be alive after this event. I still have the burn scar on my thumb to show for it. So I try not take chances like this!

Dave L.

 

[Eddyde]

12/2 w a ground is fine for powering the VFD-3hp motor. 3hp = about 2,250w / 230V = about 10 amps.

 

[Batmanacw]

Depending upon how long a run you have from the breaker box to the lathe you might use 10 or 12 gauge wire. For my 3HP motor and Hitachi inverter I used 10-3 with ground even though I was not going, but about 20 foot with the wire (standard non-metallic/NM romex). However, if you use a 30amp-220V breaker then you normally should use 10 gauge wire. If you are only going a short distance (10-15 feet) then the 12-3+gnd is ok, but the breaker should be a 20Amp- 220V breaker.

The motor may only take 20Amp at 3 phase, but the inverter probably needs 30Amp as it is single phase converting to 3 phase. This should be in the inverter manual. Yes, I know that the motor is supposed to be a balance 3 phase load. That would imply that a neutral is not needed and there may not even be a terminal on the inverter to connect the 3rd wire too. However, there are a couple of reasons I used the 3 wire plus ground. 1). Life is not perfect and who knows about your old motor/lathe. The insulation in the motor may not even be very good. If anything is out of balance (different impedance) then his means different voltage/currents can flow in the windings. This would mean that the center point connections might not be at the mid point in voltage. This is not very important until things get well our of whack. However, an even more convenient reason to have a neutral is that you may want to connect some 120 volt devices (coolant pump, lighting, small compressor, other motors, or even an electrical outlet) in your circuits and for this you need a neutral (3rd wire) for the power/current to return to the breaker box. You would not want to have to run a second wire pair from a second breaker just to hook up a extra motor. The ground should go to your lathe for safety reasons. Should ever one of the high voltage wires touch the lathe or insulation just breaks down and it did not have a ground then the lathe would be at this voltage. Image a barefooted child walking by, on a damp concrete floor, and the child touches the lathe. Current might flow from the lathe, through the child, and into the concrete/earth. I experienced this as a child, in a different setting, and I am lucky to be alive after this event. I still have the burn scar on my thumb to show for it. So I try not take chances like this!

Dave L.

Everyone keeps predicting higher and higher starting loads but the vfd doesn't say anything about being able to handle it. I'm thinking there is a bunch of guessing and confusing of the reality. I'm getting lots and lots of different views that are hard to sort through. Pretty soon I'll be told I have to rewire my entire shop to handle the load.

I'm going to go off the vfd specifications and deal with it accordingly. I already have 120v plugs right behind the lathe so the extra expense is not necessary.

There are people in this thread successfully running a 3 hp lathe on a 3 hp rated inverter on a 12-2 wire. Good enough for me.

 

[matthewsx]

Everyone keeps predicting higher and higher starting loads but the vfd doesn't say anything about being able to handle it. I'm thinking there is a bunch of guessing and confusing of the reality. I'm getting lots and lots of different views that are hard to sort through. Pretty soon I'll be told I have to rewire my entire shop to handle the load.

I'm going to go off the vfd specifications and deal with it accordingly. I already have 120v plugs right behind the lathe so the extra expense is not necessary.

There are people in this thread successfully running a 3 hp lathe on a 3 hp rated inverter on a 12-2 wire. Good enough for me.

You’ll be fine with what you’re proposing, I have a similar setup at my shop.

Do wire the VFD directly to the motor, you’ll be able to run the machine straight from it then wire the controls in at your convenience.

Seems to me there are several different levels of VFD conversion talked about on the forum. You can start with a basic setup then take it as far as you want, if you want to upgrade to a better VFD eventually just keep the cheap one. There’s always another project you’ll need it for.

John

 

[Batmanacw]

You’ll be fine with what you’re proposing, I have a similar setup at my shop.

Do wire the VFD directly to the motor, you’ll be able to run the machine straight from it then wire the controls in at your convenience.

Seems to me there are several different levels of VFD conversion talked about on the forum. You can start with a basic setup then take it as far as you want, if you want to upgrade to a better VFD eventually just keep the cheap one. There’s always another project you’ll need it for.

John

If my 12-2 wire isn't enough I'll turn it into a spare 120v line. Nothing is lost. It will be on a proper breaker.

 

[Batmanacw]

From VFDS org

Variable frequency drives also optimize motor starting characteristics. Variable frequency drives bring motors up to full speed quickly and by drawing only 100% to 150% of full load amps (FLAs). This ability to start at normal FLA is very important if the power supply cannot withstand the normally six times FLA across-the-line starting draw, or even the 350% FLA soft-start device current. Variable frequency drives do this by managing the magnetic flux of an induction motor. Magnetic flux is directly proportional to the voltage and inversely proportional to the frequency. By keeping the flux constant, the inrush current does not exceed the FLA rating of the motor, and full torque is maintained. This is a significant improvement on a soft-start, which has significant voltage drop problems and cannot start under full load.