Is VFD good or bad for old 3 phase motors?

brandoncosta

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Feb 2011
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I have been looking at a couple of power tools that have a 220v 3 phase motor. I was wanting to avoid having to install a rotary converter. But I have been told the VFDs that some folks use are bad for older motors and can ruin them. Can you share any insight into this? Thank you in advance.
 
The output waveform of VFDs can damage the winding of motors which are not deigned for inverter use. That is mostly older motors but can include new ones also. Adding a reactor between the VFD and the motor will usually solve that problem.
 
i am using the ABB ACS355-01U-09A8-2+J400 3 Hp, ACS355, VFD, IP20 on my lathe and have no problems in the last 2 years. however my motor in new
 
I have been looking at a couple of power tools that have a 220v 3 phase motor. I was wanting to avoid having to install a rotary converter. But I have been told the VFDs that some folks use are bad for older motors and can ruin them. Can you share any insight into this? Thank you in advance.


If you aren't dealing with long motor leads or attempting to overdrive the motor, then it shouldn't be a problem. You should go into it understanding that you'll have diminished HP at lower speeds and that very low speed operation could cause overheating problems for your motor.
 
The more prevalent problems happen at 380V and up. VFDs can cause high voltage reflected wave spikes that are theoretically capable of being 2.5 to 3x the line voltage. So when an old motor was built with 1000V insulation and used on a at 480V VFD, the VFD spikes could be over 1400V, so its only a matter of time before the insulation fails prematurely. But at 240V they still used the same 1000V insulation, so although the reflected wave spikes are possibly 3x the line voltage, that's still below the insulation level of the magnet wire.


There are other possible failure modes though, such as the PWM pulses creating a voltage across the stator and rotor that flows through, and damages, the bearings and races. "Inverter Duty" motors often take measures in the design to prevent this, but you can also find retrofit solutions as well. If the motor is custom or difficult and expensive to replace, I would suggest adding a dV/dt filter to the output of the VFD, not just a reactor (a dV/dt filter is a reactor + additional RC filtering for the spikes and capacitive coupling issues that can cause the bearing damage). You can also get shaft grounding devices to add to the motor that give that voltage a safer path to ground.


A common way to approach this though, especially if it's easy to replace, is to use your existing motor until it fails then replace it with one that is rated for inverter use.
 
Last edited:
My criteria for putting a VFD on an existing motor:
1) 1.15 Service Factor
2) Power demand at shaft below nameplate
3) Class F or better insulation
 
I have installed VFD's on older motors in the past that would include open frame type slip-ring motors maid in 30's 40"s and some worked fine and others burned out the motor. there is just not real way to tell if it will work or not until you try. Advise would be make sure everybody understands that if the motor goes you have to replace it. if it's a large motor send it out to a good motor shop and have them rewind it with inverter duty wires I also find the having the motor VPI ed while its at the shop is worth the cost.
 

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