# VFD Questions



## ConductorX (Mar 20, 2014)

Everyone,  I am curious about VFDs.  

How do they work?

Do I need to buy a special motor to use with a VFD?

What if I have a machine with a three phase motor?  I need a three phase VFD?

What brand gives the best results?

As you know I have an 11 x 36 Rockwell lathe I am attempting to restore and the variable drive mechanism for it was ruined in a fire.  I hope to use a VFD to run the lathe and provide me with safety mechanisms such as an E-Stop.

Thanks much,
"CX"


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## Terry_S (Mar 20, 2014)

Hello Gary,
Not sure how deep you want to go into this, but here's a starter.

VFD's make use of the fact that a standard induction AC motor will try to match the "synchronis" speed for which it is wired. In the US, at 60 Hz, 2 pole motors try to run 3600 rpm, 4 pole motors try to run 1800 rpm, etc. They never actually get up to this speed because "slip" is required to develop their power. Hence, 2 pole motors are usually rated around 3450 rpm, 4 pole motors around 1750 rpm, etc. When you cut the frequency to 30 Hz, the motors all try to run about 1/2 speed. The VFD takes incoming AC power and makes it into smooth DC power on a bus. Then, using the magic of electronics, it chops the DC power up into something which looks like AC power to the motor. By changing the frequency, the motor speed changes.

VFDs work with 3 phase motors. You need a 3 phase motor. In theory, any 3 phase motor will work, but in today's world, there are motors made with higher rating insulation to resist possible voltage spikes that can come with VFDs. Most of the experts will say that a regular 3 phase motor will hold up as long as the leads between the VFD and the motor are short (say 10 feet or less), and you run the VFD at its lowest chopping frequency (means it might make an obnoxious hum).

You don't need 3 phase power to run a VFD as long as the rated HP is 3 HP or less. In the 3 HP size and down, most manufacturers make a single phase version. So, the VFD ends up serving as a phase converter as well as a speed controller. You do need a 240 VAC connection of ample capacity.

There are a number of low cost versions on the market that work well for hobby machines. My personal favorite is Teco (made by Westinghouse). Another popular brand is Hitachi. Both of these are available in the 3 HP size for less than $300. There is at least one Chinese version for about 1/2 the cost, but I have heard mixed reports on this one.

For your lathe application, you need to be aware of one short-coming of VFDs. They are not "constant horsepower" devices like the mechanical drives in older machine tools. For example, when you slow to 1/2 speed, you no longer have the full rated torque and horsepower of the motor available. With a "sensor-less vector" version of a VFD, you get to keep a fair amount of torque down to about 30% speed, but it is reduced from full speed. Another issue is motor cooling. Regular AC motors count on full speed to have the cooling fan do its job. Having said that, I have a 3 HP VFD on a Clausing lathe. I routinely run in the 50% speed range without issues, but I use the mechanical belt system to get speeds lower than that.

If you are thinking about going down the VFD path, and you need a new motor - choose an over sized motor, maybe even 3 HP. That way you subject the motor to less than its rated load - and there is more torque at low speed and less chance of overheating. Unless you take a lot of deep cuts at 10" diameter, the VFD will probably do well.
Terry S.


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## spinningwheels (Mar 20, 2014)

A VFD will give you controll how fast the motor spools up and also how fast it stops, In have mine set to a 0,25 sec stop from max rpm to zero.
Also I have set the max Frecuentie a bit higher to get a bit higher spindle rpm.
I use a 7.5 KW hitatchi sj 100, realy need to RTFM but than it is simple to setup .


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## kvom (Mar 20, 2014)

If you want a fast stop you will need a braking resistor.  When the frequency goes towards 0 the motor feeds energy back to the VFD.  If it's more than the VFD can absorb then it will signal an overcurrent situation and essentially stop until reset.  The motor, and hence the spindle, then free wheels until it coasts to a stop.

Having a braking resistor allows the energy of the braking motor to be absorbed.  VFD manufacturers will supply braking resistors, but I've seen other high-resistance things used as well (i.e., heater elements).


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## RonGinger (Mar 21, 2014)

The manufacturers seem to think their resistors are gold or something- they charge nearly as much as for the VFD itself. Have you any idea of the resistance and power values needed? I have no idea where to start with this and have been afraid of blowing the VFD if Im wrong. Has anyone got a factory resistor they could measure the resistance?


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## jschoenly (Mar 21, 2014)

Ron, 

I have a factory braking resistor.  I'll try to measure it if i can this weekend.  I think it may actually even be listed on Automation Direct or other sources.  I was reading about these and read some articles where people use old electric stove or water heater elements for braking resistors.  Once I got around to looking for an old one, pricing new ones, sizing them, being in a rush, etc...  I just bit the bullet and bought the factory recommended one.

Jared


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## mu38&Bg# (Mar 21, 2014)

kvom said:


> If you want a fast stop you will need a braking resistor.



Most inexpensive (read non-regenerative) VFD do not brake the motor by reducing the frequency. They do it by disconnecting the frequency input and supplying DC power to the motor coils. This is called DC injection braking. This turns the motor into a hysteresis brake. All of the energy is dissipated in the motor, in addition to the DC power. A braking resistor can allow shorter stop times because the VFD will have a limit to how much DC injection current is available.

My VFD will overvolt if I reduce the frequency too quickly. This could be avoided by increasing the ramp down time for speed changes. My ABB VFD is rated at 6 start/stop cycles per minute. This can be increased by adding a braking resistor. I don't have one and sometimes I exceed the limit without issue.

VFD are great and it allows the use of 3PH motors in a residence without phase converters with the benefit of being efficient, variable speed, and small.

Greg


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## MachineTom (Mar 21, 2014)

I have a Rockwell 10" and the drive of the 11 is similar. In addition my Monarch has a VFD I installed a few years ago. 

In a fire the AL pulley could melt, but all the iron remains, seem like an easy fix as there are several guys parts out 11 DR machines on ebay.

I'll share some ideas. Any three phase motor will work with a VFD -voltage must match of course.But an Inverter Duty motor has special wire insulation that resists the higher temps found in VFD service and the ability to overspeed the motor without the rotor coming apart. The black Max motor  in the Monarch is rated at 1750 rpm, max rpm is 5300. -I have run regular 3 phase motors at double rpm without problems.

In use a VFD driven motor will have constant torque below base rpm, and constant HP above base rpm. To get good use out of the DR lathe buy a 1.5-2HP 1750 rpm motor, set the pulley sizes to give a spindle speed of 7-800 RPM at 1750 motor RPM. You will likely use backgear a bit more often than in the past when parting off or large diameter work. 

Hitachi makes a good VFD, and make specific units for single phase 220 input. For ease of use plan on putting together a remote control box, with on/off Fwd/Rev and a speed control switches.I like Square D control switches, and mount them in a a Plastic enclosure box. There is a photo of my box in the disability forum, Shop Tricks.

You will spend a $7-800 so if all items are purchased new. It is likely cheaper to repair your current drive system than refit. to a vfd.


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## Sshire (Mar 21, 2014)

When I installed the Teco VFD on the Grizzly G0602 lathe, I bought a new (bigger) motor. It has worked flawlessly for nearly 2 years and I don't miss changing belts.

That said, I got a Bridgeport last summer and put a 220v 3 phase Hitachi VFD on the original BP motor. The BP has a 2J ( vari speed) head which I leave at 1750 rpm most of the time. With the head at 1750, and speed control with the VFD, the rpm display on the VFD is accurate. The Hitachi has a remote front panel with 6' cord which I've mounted in a pocket in .75 aluminum with magnets epoxied to the bottom. It usually sits on the BP table.
I have Tom's idea high on my list of "stuff to do" as I'd like bigger buttons.

Don't know how the BP motor will hold up in the long run but I assume I could have it rewound/rebuilt if there is an issue.


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## Wizard69 (Mar 23, 2014)

You already have excellent answers but hey more info never hurts.  



ConductorX said:


> Everyone,  I am curious about VFDs.
> 
> 
> 
> How do they work?


Short form:   The convert AC voltage into DC voltage and then convert that DC voltage into variable frequency AC voltage. 


> Do I need to buy a special motor to use with a VFD?


No, it does need to be a three phase motor though.   That isn't a bad thing because good quality three phase motors are often cheaper than single phase motors.  

That being said inverter rated motors have insulation that can withstand high voltage transients that can be seen with these drives.  For light users of machine tools that particular feature isn't of much use.  Some inverter rated motors supposedly maintain torque over a wider range of operating speeds but I sometimes think that the manufactures are playing funny with the numbers.  


> What if I have a machine with a three phase motor?  I need a three phase VFD?


All VFDs I know about are three phase drives.    Don't sweat this though, you actually want a three phase motor in many applications.  The VFD drive, if you purchase the right one, converts single phase to three phase no problem.  


> What brand gives the best results?


That is a tough one!   We use various brands at work and few have ever failed.   Sized properly and installed as recommended they are very reliable these days.  


> As you know I have an 11 x 36 Rockwell lathe I am attempting to restore and the variable drive mechanism for it was ruined in a fire.  I hope to use a VFD to run the lathe and provide me with safety mechanisms such as an E-Stop.
> 
> 
> 
> ...




Well a VFD can help with some of that but it shouldn't be seen as a solution to an E-Stop situation.  For one thing in an E-Stop situation the drive may be shut off or disconnected.  You might want to consult with an engineer if the lathe is going into manufacturing or even a job shop with one employee, but if an E-Stop function is a requirement you will likely need a clutch brake assembly.  

A VFD can give you a fast ramp down in normal operation though which is a very very good thing to have.  You can also moderate ramp up which can greatly reduce stress on the machine that might come from an across the line starter.  In any event there is a lot of good associated with VFDs, just don't think of them as an E-Stop solution.  

In any event somebody already pointed out that VFDs are not perfect, the big problem being the loss of torque at low speeds.   You will not get away from the need to use a gear box or belt reduction.  What you should do though is some sensible engineering to fit the right motor to the machine.   Often this means a 4 pole motor (1800 RPM).     You will need to know the motors maximum rotor speed which is not its nameplate rated 60HZ speed.   Most motor rotors can handle at least 3600 RPM so you want a VFD drive that can run that 4 pole motor at 120 HZ.    

If you follow me so far we have a 1800 RPM motor being driven by the VFD to run at 3600 RPM.   In this example that becomes your maximum RPM design speed.   At this point you can figure out a belt ratio that will give you the safe operating speed for the spindle and any chucks you may have.  This is a metal lathe so that will likely be well below the 3600 RPMs.    In any event what we have done here is to make use as much as possible the constant horse power range of the motor.  At the motors rated speed of 1800 RPM your belt reduction will have the spindle running at well under that speed, possibly a 2:1 reduction, meaning 900 RPM.  That puts your motors rated horse power into a very nice RPM point for a lathe this size.   You will loose torque below that speed but it won't be too bad down to around 500 RPM.  

In any event I hope you see this as an example and not a well worked out solution.   A lathe that might be used for bar work with collets may need a different solution as would a lathe that can handle different RPMs max for the supplied chucks.  The point is if you do a little leg work you will likely be able to get very good results.  As such reading up on three phase motors and VFDs should help you with understanding what you need to do.  Also realize that the numbers above basically have rolled off the top of my head.   For example many AC induction motors have rotors rated for more that 3600 RPM but you need to know what your specific machine is rated for.   Also VFDs have operating ranges that vary with model, some can go up to 400 HZ others top out at 120HZ.  

I hoping this helps and doesn't add confusion, the concepts are pretty straight forward, it might take a better explanation than I just delivered though. 


Sent from my iPad using Model Engines


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## jschoenly (Mar 24, 2014)

I'm testing a playing around with a single phase VFD at work.  However, it's not like the 3 phase units.  The single phase motor needs to be a Shaded Pole or a PSC motor.  It really doesn't seem to start a motor well unless you can go to full speed right away rather than a set point.  You can then vary it from there.  Still working with it, but I'm not convinced its going to work for my application.  3 phase versions are still best.

Jared


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