Why does my VFD overheat when running at low RPM?

[TTalma]

I've got my machine running pretty good. Last night a loaded a new setup. This involved using a metalworking dovetail tool. The tool has 12 flutes on it. So to prevent burning in hard maple I ran the spindle at 5000 rpm, machine at 100ipm.

I got a nice finish with no burning. But after running for about 10 min the VFD errored and shut down. Fortunately the machine was moving (not cutting) and I was able to e-stop before any damage.

I checked the code on the VFD and if shut down due to over heating. I felt the heatsink and it was to hot to touch. I shut the system down and let everything cool off. I loaded a new part and same thing happened.

After cooling off I switched tools and jobs I was running a 3/4" 2 flute at 18000 rpm 100ipm bit for about 45 min. I felt the VFD and it was warm, not hot.

So I switched back to the job with the 12 flute bit. I changed the speed to 10000 rpm 200 ipm, got a nice surface finsih etc. and the machine finished the job (about 15 min). I felt the VFD and it was real hot. I think it would have overheated if I had run it longer.

During everything the spindle got pretty warm, but never what I would consider hot.

The only thing I could figure from the over heating was that the VFD had to work extra hard because of the 12 flute bit to keep the spindle at speed. But I couldn't wrap my head around that since I don't have speed feedback from the spindle.

So why would the VFD overheat?

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[ericks]

A good quality drive would fault on (drive) over current before the drive overheats. Thus if you have a good quality drive then i'd say ventilation problem or drive cooling fan failure if it has one

[ericks]

Without seeing the installation its hard to say for sure. I have seen many installations where improper installation caused over temperature problems. Also something as simple as a blocked filter

[mactec54]

I've got my machine running pretty good. Last night a loaded a new setup. This involved using a metalworking dovetail tool. The tool has 12 flutes on it. So to prevent burning in hard maple I ran the spindle at 5000 rpm, machine at 100ipm.

I got a nice finish with no burning. But after running for about 10 min the VFD errored and shut down. Fortunately the machine was moving (not cutting) and I was able to e-stop before any damage.

I checked the code on the VFD and if shut down due to over heating. I felt the heatsink and it was to hot to touch. I shut the system down and let everything cool off. I loaded a new part and same thing happened.

After cooling off I switched tools and jobs I was running a 3/4" 2 flute at 18000 rpm 100ipm bit for about 45 min. I felt the VFD and it was warm, not hot.

So I switched back to the job with the 12 flute bit. I changed the speed to 10000 rpm 200 ipm, got a nice surface finsih etc. and the machine finished the job (about 15 min). I felt the VFD and it was real hot. I think it would have overheated if I had run it longer.

During everything the spindle got pretty warm, but never what I would consider hot.

The only thing I could figure from the over heating was that the VFD had to work extra hard because of the 12 flute bit to keep the spindle at speed. But I couldn't wrap my head around that since I don't have speed feedback from the spindle.

So why would the VFD overheat?

What is the spindle motor spec's, this determines how slow it can run with out over heating or damaging the VFD Drive

[ericks]

What is the spindle motor spec's, this determines how slow it can run with out over heating or damaging the VFD Drive

This would be typical of a very badly designed drive

[TTalma]

The drive is a TB Woods X2C.

The spindle is a perske vs50

It is passively cooled (No fan). The cabinet is 40" x 50" x 8". It is mounted to a large piece of aluminum. on the back of the cabinet there is about 2' of room behind the cabinet.

[ericks]

The drive is a TB Woods X2C.

The spindle is a perske vs50

It is passively cooled (No fan). The cabinet is 40" x 50" x 8". It is mounted to a large piece of aluminum. on the back of the cabinet there is about 2' of room behind the cabinet.

Is your drive properly sized for the motor?

[ericks]

Just to add.....having a cooling fan on the control panel is always a good thing. We use fans on all our control panels that house drives. Good to have the equipment run cooler....
The drive looks fine, i would make sure the drive size is suitable for your motor....but definitely install a panel fan. I install the filtered suction at the side/bottom, then the fan opposite side top.....never had any issues over many years of doing installs like this

[mactec54]

This would be typical of a very badly designed drive

Not at all

[TTalma]

The drive is properly sized. it is for 2-5 hp. The spindle is 3.3hp

[ericks]

Sorry mate but i have to say you are wrong here. If you run a motor from a drive and you run at a very low speed and lets say your motor stalls....then the motor will go into overcurrent and the drive should fault due to you exceeding the motor FLC. Another option is that you limit the drive current output, in that case if you exceed that value then the drive would fault on drive current exceeded. However these parameters/functions are usually only available on "good" drives.

[mactec54]

The drive is a TB Woods X2C.

The spindle is a perske vs50

It is passively cooled (No fan). The cabinet is 40" x 50" x 8". It is mounted to a large piece of aluminum. on the back of the cabinet there is about 2' of room behind the cabinet.

Your spindle is rated to have a minimum speed of 9,000 RPM @150 Hz, so running it at 5,000 RPM will cause the VFD to over heat, the spindle will draw a high current at the lower RPM that is why they set a minimum speed

[ericks]

The drive is properly sized. it is for 2-5 hp. The spindle is 3.3hp

Good....

[mactec54]

Sorry mate but i have to say you are wrong here. If you run a motor from a drive and you run at a very low speed and lets say your motor stalls....then the motor will go into overcurrent and the drive should fault due to you exceeding the motor FLC. Another option is that you limit the drive current output, in that case if you exceed that value then the drive would fault on drive current exceeded. However these parameters/functions are usually only available on "good" drives.

Not wrong at all

Even the cheapest VFD Drives have those features you are talking about, all have current control Parameters to set

[ericks]

Not wrong at all

Even the cheapest VFD Drives have those features you are talking about, all have current control Parameters to set

so why would running the motor too slow damage these drives??? I know they all have motor current parameter....
I was referring to specialized parameters

[TTalma]

Your spindle is rated to have a minimum speed of 9,000 RPM @150 Hz, so running it at 5,000 RPM will cause the VFD to over heat, the spindle will draw a high current at the lower RPM that is why they set a minimum speed

This must be the issue. I feel a little foolish as I had never thought to check the min speed!

[Eldon_Joh]

basically for a given amount of torque you need a certain amount of line current and this doesn't change much with rpm. you may need to reduce or even increase the boost volts in order to get the motor running right.


larger motors need less boost volts than smaller ones, so if your drive is set for a 2.5 hp spindle but you have a 3.3 hp spindle, it will be too much. if your drive is for a 2-5hp spindle then who knows what its programmed with, but at 5 hp it should be able to deliver enough amps to burn out the 3.3 hp spindle without overheating the drive.

by default a lot of drives don't apply any boost volts once you get to 20% of base speed. you need to go through the parameters and modify the boost volts until the spindle current at no load is constant from 2hz up to 400 hz. if it drops then the volts/hz is too low (not a bad thing if the spindle is lightly loaded) if it increases as the rpm decreases then the volts/hz is too high at the lower speeds and the saturated motor will draw a lot more current than it needs to, to deliver the same torque. waste heat is proportional to amps squared, so it matters quite a bit.

so the drive is overheating because the current is increasing and it should not be. the motor is overheating because the current is increasing and it shouldn't be.. and its overheating because its only running at 5K rpm. adding another fan may be a solution, as is reprogramming the drive to deliver the optimal volts/hz to the motor.

[TTalma]

Thanks, That makes sense as why it was heating up. The VFD was programmed at the factory.

I will be swapping out the spindle and VFD soon for another, and will pay attention to the min settings as well as the max when I hook up the new one.

I have a job to finish, and vacation before I can get around to doing that though!

[reg.miller]

basically for a given amount of torque you need a certain amount of line current and this doesn't change much with rpm.

A good motor will have a very low wire resistance to minimise resistive heating in the windings. So at very low rpm it can draw a massive current. The VFD will not be able to supply that. When a motor spins at a good speed, the inductive impedance is greater than the wiring resistance, and so becomes the dominant factor. There is also back EMF which reduces the effective voltage driving the current.

Start up current is several times the running current on all inductive motors and is sustainable for short periods to spin up the motor. That can not be maintained as a working current. In most cases neither the motor not the drive are designed run in that regime.

[Eldon_Joh]

A good motor will have a very low wire resistance to minimise resistive heating in the windings. So at very low rpm it can draw a massive current. The VFD will not be able to supply that. When a motor spins at a good speed, the inductive impedance is greater than the wiring resistance, and so becomes the dominant factor. There is also back EMF which reduces the effective voltage driving the current.

Start up current is several times the running current on all inductive motors and is sustainable for short periods to spin up the motor. That can not be maintained as a working current. In most cases neither the motor not the drive are designed run in that regime.

short answer is no.


there is uncoupled leakage inductance which decreases the power factor of the motor as the frequency increases, but that effect is on the order of maybe 10% across the 0-60 hz range. for a 400hz spindle it is significant depending on how large the motor is. however, this is not why the current increases as the rpm decreases. and, even if that was a primary cause, you could reduce the "boost" volts to correct it.

i would urge you to play around with an induction motor and vfd and learn this all for yourself.

the reason startup current on an induction motor is about 6 times the full load amps is because you're sending the motor 240v/60hz when the rotor isn't turning.
if you deliver a 5hp 60hz 240v 1800rpm motor on the order of 20volts at around 2hz it would deliver full load torque of 15 foot pounds with the rotor stalled, and the line currents would not be very similar to full load nameplate amps. however, you can modify both the flux density and the slip to deliver the same torque by changing the frequency and the voltage.

for a given load there is an optimal voltage and frequency for every induction motor.