# VFD and old motor



## icore3user (Jun 25, 2013)

My Index 55 mill has a Reliance motor, 220 volts, 3 phase ,2 speed 1HP ( 1710 @ 3.2 amps, 1135 @ 4.4 amps) and is currently run of a static converter, that on occasion, does not always start the motor. I am going to convert to a VFD for 3 phase conversion and small speed ( fine tuning ) control of the motor. On another website a guy converted his Enco mill that had a 1.5 HP  motor and replace with a 2 HP ( 6.8 amps 1725 rpm  Inverter Duty ) motor but sized the VFD for 3 HP ( 10 amps output ) , I know you size the VFD by amps and not by HP .

My question is should I buy a VFD on the premise that my old motor will not last long ( unless I keep it in the 60 hz range ) for a future bigger size motor ( 1.5 to 2 HP inverter duty) motor that is not 2 speed but a 1725 rpm motor and size the amps on the VFD as such like the other guy did? I could buy a smaller VFD ( 1 HP model with 5 amps output ) or even a 2 HP model ( 8 amps ). 

A couple of the reasons he upgraded to a 2 HP motor was more torque, he also kept the belt spindle pretty much 1 : 1 , he mentioned that most of his milling operations were kept at about 2550 rpm or below  and by sizing the VFD for more amps than the new motor drew, he felt he would never tax the max output of the VFD drive.

does this sound reasonable?

also note the Reliance motor frame, CB 240 C, from what I can tell it and obsolete frame mounting, pre NEMA, So I am sure I'll have to make some sort of adapter plate for a new motor, but then that part of the fun.)





- Al


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## 8ntsane (Jun 25, 2013)

I have 2- older machines running on VFD, and have been for yrs. I would not assume your motor is going to give up on you over a VFD install. I don't have any 2-speed motors, but I believe those that do, just wire it up for the hi range, and use the VFD for speed control. VFDs have settings for volts/amps to name a few. You should be able to set it up for the motor your using, providing the motor isn't too big for the VFD.

If your going to up grade to a bigger motor later, just make sure you have a VFD that will work with it. If the motor you have now is working fine, you should be able to use it with the VFD. As long as your not doing things like overspeeding the motor, or running the motor very slow for long periods of time. The motor may not like over speeding for long, and under speeding can cause excessive heat. So damage could occur either way. If you use you VFD to fine tune the speed, then you would do fine.

On a mill, the motor seldom, or never really see,s full load. So your not really working it like a lathe. I usually try to stay between 50-60 hrz for general use. I do at times go lower,or higher, but keep tabs on motor temps when lower, and keep the hi-speed work to short periods of time. My mill has been running on the original 60s motor for years now.

I also have a Bridgeport that has a adapted motor on it. It does require a plate, and some work to get it mounted up, but not really that bad of a task. The main thing to look at , is the motor in question mountable to a plate, and is the shaft the proper size for your needs?


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## Codered741 (Jun 25, 2013)

Hi there, 

Your old motor will work just fine with a VFD.  The VFD does not need to be oversized, most are capable of 150% of their rating for a short duration(2-5sec).  Oversizing a VFD for future motor upgrade will not hurt, but you should be careful not to up-rate too much, or you will damage the motor by overpowering it.  

Older motors have more risk of overheating and burning out due to insulation damage caused by age.  This risk can be reduced by using a Line Reactor on the output of the VFD, which will smooth out the PWM, making it more like a standard 3 phase waveform.  Also, don't run the VFD at low frequencies, say 40 hz, and never above the rated frequency, usually 60Hz.  A shorter accel/decel time will also extend the life of the motor.  

No, older motors were not designed to run with VFD's, as the technology did not exist when they were manufactured.  But the motors made today will have similar problems when they are 50 years old, and the insulation damaged by age.  As with all of our old machinery, they take a certain amount of TLC, but will still function perfectly with modern controls.  

-Cody


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## Kennyd (Jun 26, 2013)

I have VFD's on a 1950 something Bridgeport and my 1969 Clausing lathe, both with the original motors.  If I ever have trouble, and I doubt I will...I shall deal with it then.


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## twstoerzinger (Jun 26, 2013)

You will likely notice a substantial improvement in motor starting, noise level, vibration and torque when moving from the static converter to the VFD. The motor will then be pulling on all three phases instead of two.

You may have to give up the two-speed motor feature. The VFD has to be configured to the volts, amps, and rpm of the windings you are going to use.
You be able to find a VFD with two sets of motor configurations (that you can switch in the software). In this case you could continue to use the 4 pole / 6 pole speeds, but you would want to not do this "on the fly." i.e. bring the motor to a stop, switch configurations (both in software and in power connections) and restart.

Terry S.


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## oldgoaly (Jun 26, 2013)

I'm running a vfd on a 60+ year old motor, the motor does not like to be run slow! One the motor's fan doesn't cool as well at speed and second there is some loss of horsepower at lower speeds but I'm working it just as hard if not harder. Going faster doesn't seem to be a problem, but I have not run it that way for very long. This motor is a cast iron body Euro-motor made by Elmo of Sweden. tt


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