Old 3-phase motors and Variable Frequency Drives

[vinnygalbo]

I was working on a job this week where they were adding VFDs to some equipment with 25 year old 3 phase motors. These motors have been in daily use and working just fine since installation. Here comes the risk of using a VFD on an old motor; one of them worked OK, but the other tripped out showing ground fault as soon as the drive was started up. It was temporarily reconnected to line voltage and a new drive rated motor ordered.

Sometimes you win and sometimes you loose using drives on old motors. The motor insulation is seeing voltages as high as 1,000 volts. It was designed for 480. Of course the switching cycle of the transistors can be in the megahertz range. That doesn’t help the older motor either.

I just thought I would mention this for those who are considering old three phase equipment when we live in a single phase world.

That motor would probably run on a mechanical rotophase. It is anybody's guess as to how much longer it would have run. An electronic mega ohm meter shows a dead short, but it works on the same principle as the drive. A regular ohm meter shows it being just fine.

[cvillewood]

Post this on OWWM and see what they say. 25 years old is from the 90’s, that’s not old. Most of the motors in my shop are 50 years old and happily being run by a VFD.

[lukebarber]

Here's a good article on the subject.

https://precisionautomation.net/2017/02/...ng-motors/

[EricU]

If a motor from the '90s is old, then I have an ancient motor being driven by a vfd on my metal lathe, '50s or something?  I think if a motor had a problem when first hooked up to a VFD, then it showed up with a problem already.  I think that motors probably got worse in some respects in the '70s, but they have also improved quite a bit since then so it's a bit of a trade-off.  Certainly, they are a lot smaller now, which is a bonus.

[Tapper]

If a motor from the '90s is old, then I have an ancient motor being driven by a vfd on my metal lathe, '50s or something?  I think if a motor had a problem when first hooked up to a VFD, then it showed up with a problem already.  I think that motors probably got worse in some respects in the '70s, but they have also improved quite a bit since then so it's a bit of a trade-off.  Certainly, they are a lot smaller now, which is a bonus.

Will be putting a VFD soon on two tools in my shop with three phase motors - 1) 3 HP Dewalt GA RAS, circa 1960's and 2) South Bend metal lathe, circa 1940's. I expect both to perform just fine IF the existing motors are in good shape (yet to be determined). VFD's are a wonderful solution to old classic machines with 3 phase motors for the home shop. They typically work well with little to no problems (depending on the HP) and can offer features not originally on the machines. It's important to order the correct VFD for the machine it will control. EE's here as well as the reps for the online companies that sell them can be a tremendous help.

For instance, on the RAS you can dial in the blade braking time in seconds on the VFD and it will automatically brake the blade when you turn the saw off, i.e. if you dial in 15 seconds, the blade will brake to stop in 15 seconds.

Doug

[BloomingtonMike]

No switches after vfd line out. Wire the lathe switch functions like reversing to the vfd smart inputs.

Big tools require an external dc braking resistor so keep that in mind when choosing your vfd. Some cannot accept an external braking resistor. The built in dc braking is somewhat limited on many vfds and will overcurrent if you get aggressive on your baking speeds.

[Tapper]

No switches after vfd line out. Wire the lathe switch functions like reversing to the vfd smart inputs.

Big tools require an external dc braking resistor so keep that in mind when choosing your vfd. Some cannot accept an external braking resistor. The built in dc braking is somewhat limited on many vfds and will overcurrent if you get aggressive on your baking speeds.

Need a little help here: The first two sentences seem to be in direct conflict with each other??? What does "overcurrent" mean?

Thanks,

Doug

[KyleD]

Need a little help here: The first two sentences seem to be in direct conflict with each other??? What does "overcurrent" mean?

Thanks,

Doug

I am not a VFD expert but am a multiple VFD user and so have had to learn enough that think I can answer your question.

BoomingtonMike is saying the on off switch of the machine cannot be left in the power wiring between the VFD and motor. A sudden disconnect while the VFD is working like opening the machine switch inline will damage the VFD. The VFD should be hard wired to the motor and the VFD controls should be used. A VFD has options to incorporate the machine switch into its controls.

When the motor is switched off at the VFD a resistor is used to shunt the motor and brake it in a very controlled manner. Big motors or motors with a large momentum require heavy duty resistors so as to avoid an over-current that can fry the lighter resistor installed on the VFD from the factory. Some VFDs have the option to add that heavier resistor.

If the VFD is for a saw one probably does not want to use heavy braking as it can cause the blade to spin loose from the arbor. I do not have to tell everyone how bad this could be.

[BloomingtonMike]

If the VFD is for a saw one probably does not want to use heavy braking as it can cause the blade to spin loose from the arbor. I do not have to tell everyone how bad this could be.

Actually it tightens on many tablesaws when stopped. Table saw arbors that come through the blade from the right are left-handed threaded and arbors that come through the blade from the left are regular threaded.

The VFD has DC braking terminals if it can use an external braking resistor and they are also NOT in between the motor out of the VFD and the motor.

Also do any duirection reverse switching of the lathe through the VFD terminals as well.

Also pay attention to your gearing (on gear head lathes) or pulleys used for speed and the frequency settings you need to obtain full torque at speed.

All this being said, VFDs on lathes are very awesome.

[Bobby Thompson]

I was working on a job this week where they were adding VFDs to some equipment with 25 year old 3 phase motors. These motors have been in daily use and working just fine since installation. Here comes the risk of using a VFD on an old motor; one of them worked OK, but the other tripped out showing ground fault as soon as the drive was started up. It was temporarily reconnected to line voltage and a new drive rated motor ordered.

Sometimes you win and sometimes you loose using drives on old motors. The motor insulation is seeing voltages as high as 1,000 volts. It was designed for 480. Of course the switching cycle of the transistors can be in the megahertz range. That doesn’t help the older motor either.

I just thought I would mention this for those who are considering old three phase equipment when we live in a single phase world.

That motor would probably run on a mechanical rotophase. It is anybody's guess as to how much longer it would have run. An electronic mega ohm meter shows a dead short, but it works on the same principle as the drive. A regular ohm meter shows it being just fine.

I have installed a lot of VFD's in my career as a industrial electrician. I have two on my surface grinder which allowed me to use single phase. I have never seen a motor that would not run on a vfd unless it had a previous problem. In your case, I would have sent the motor to a repair shop and had it cleaned, dipped, and baked. A good motor shop can detect any problems and could have even rewound the motor if necessary. That depends on the size of the motor since the cost can exceed the price of a new motor. I am used to ac motors from 5hp to 2000hp and dc motors up to 1500hp. I have installed dc drives, soft start drives, wound rotor motor drives, and synchronous drives in my 15 years as a maintenance electrician and 20 years as a maintenance supervisor.