VFD signal problem

Thread Starter

kenwil

Joined Feb 19, 2024
5
Is there a circuit that calms the signal and makes a pure sine wave?

Does anyone know if there is a circuit that would come after the VFD and before the 3ph 3hp motor that would eliminate the interference and leave with a clean, pure sine wave?
 

Thread Starter

kenwil

Joined Feb 19, 2024
5
to Maxheadroom
Not a pure sine wave. So the result is a broken signal to the motor. I am wondering if you can a circuit between the VFD and the motor? if not, no problem.
 

strantor

Joined Oct 3, 2010
6,783
to Maxheadroom
Not a pure sine wave. So the result is a broken signal to the motor. I am wondering if you can a circuit between the VFD and the motor? if not, no problem.
The simple answer is no. It doesn't get any better than an output filter. An output filter is more complex than an output reactor but still doesn't turn the output into a "pure sine wave." The output of a VFD doesn't need to be a "pure sine wave" if you're using it for its intended purpose, to control a motor. I suspect either you're wanting to solve a problem that doesn't exist or you're wanting to use a VFD for some purpose other than to control a motor, in which case I recommend using an appropriate sine wave inverter.
 

Thread Starter

kenwil

Joined Feb 19, 2024
5
thank you, I appreciate any response. And no, I am running a 3hp motor and a VFD. I am worried that the VFD generates such a distorted signal the level may be working to motor problems in the long term. I know they recommend a special grounding bearing, but I haven't been able to find it.
I am also sorry about using the phrase, "pure sine wave", I am not familiar with speaking with people. I just want something more consistent with a regular wave function.
 

MaxHeadRoom

Joined Jul 18, 2013
28,669
The 3phase choke is generally recognized device for what you are trying to do.
In all the VFD's I have installed, I have never needed anything else usually.
 

strantor

Joined Oct 3, 2010
6,783
thank you, I appreciate any response. And no, I am running a 3hp motor and a VFD. I am worried that the VFD generates such a distorted signal the level may be working to motor problems in the long term. I know they recommend a special grounding bearing, but I haven't been able to find it.
I am also sorry about using the phrase, "pure sine wave", I am not familiar with speaking with people. I just want something more consistent with a regular wave function.
It is true that the output of a VFD is more harsh than mains power but it is not a problem for modern inverter duty motors. They have good insulation that is not bothered by VFD power. The insulation of older motors was not designed to cope with the continuous HV spikes that a VFD produces, so in some cases it is said that they may have their life shortened by operating from a VFD. So when installing a VFD, most would recommend also installing a new, inverter-duty motor. But my recommendation is different. I recommend to not spend money until it is necessary to spend it. I have installed VFDs with old motors many times and I find that in most cases it works just fine for a long time. I find that even though the motor is old, and was not designed for use with a VFD, its insulation is still "good enough" to use with a VFD, for a while, or forever. If it fails later then replace with an inverter duty motor. Any attempt to "clean up" the VFD output beyond what an output reactor can do, is almost always money wasted (cost more than just replacing the motor), unless the motor is large (75+kW) or a very old, very special kind, that should be left unmolested for historical or sentimental reasons.
 

MaxHeadRoom

Joined Jul 18, 2013
28,669
I recommend to not spend money until it is necessary to spend it. I have installed VFDs with old motors many times and I find that in most cases it works just fine for a long time. I find that even though the motor is old, and was not designed for use with a VFD, its insulation is still "good enough" to use with a VFD, for a while, or forever.
Agree, My experience also.
The oldest I converted to CNC was 30ft RR rail planer, circa 1950's, The motors were checked out by a local motor Co. and they said they were some of the best motors they had seen for that era.
Chokes were fitted and it is still running today, 20yrs later!.
 

Thread Starter

kenwil

Joined Feb 19, 2024
5
It is true that the output of a VFD is more harsh than mains power but it is not a problem for modern inverter duty motors. They have good insulation that is not bothered by VFD power. The insulation of older motors was not designed to cope with the continuous HV spikes that a VFD produces, so in some cases it is said that they may have their life shortened by operating from a VFD. So when installing a VFD, most would recommend also installing a new, inverter-duty motor. But my recommendation is different. I recommend to not spend money until it is necessary to spend it. I have installed VFDs with old motors many times and I find that in most cases it works just fine for a long time. I find that even though the motor is old, and was not designed for use with a VFD, its insulation is still "good enough" to use with a VFD, for a while, or forever. If it fails later then replace with an inverter duty motor. Any attempt to "clean up" the VFD output beyond what an output reactor can do, is almost always money wasted (cost more than just replacing the motor), unless the motor is large (75+kW) or a very old, very special kind, that should be left unmolested for historical or sentimental reasons.
Thanks again, I will proceed, (cautiously), but still more assured than before.
 
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