Is there any harm done to a motor by setting a VFD to very low frequency?

I have a Hitachi WJ200 set up on a lathe pushing a 1 hp three phase motor and wish to use very low frequency at times.
Is there any harm to the motor from this practice if done for short periods of time?

This is the motor:



The lathe is set up with a single dog clutch & I am trying to get some way of hitting a accurate start for threading when starting from a shoulder.
I would like to just spin down the rpms to a craw when close to my start.

No foul?

Thanks all

Heat is the mechanism for harm.

Monitor amps on your VFD to determine loading.

From your description of application, I would not worry about the motor unless it started to smell "hot". (and that is NOT BURNED!)

Use whatever gear or belt speed reduction you can.

Act confident, the electrons are watching you ;-)

Agreed. The only thing that will damage the motor in this use is excessive current or running so slow that a cooling fan doesn't cool. You can have the motor almost stopped with rated amps flowing and appropriate cooling airflow without hurting it. Zero frequency will put all the current between two legs, which is not a good idea but all right for a while. You should be able to set the current limit of the voltage that will push that much current through on the VFD. A load meter is a good idea. A short run such as you describe without cooling will not hurt it.

Note that this refers to a three phase induction motor. The above is not true of a single phase capacitor start or brush type DC motor. The starting capacitor is not designed for continuous service and a stopped DC motor will be putting all the current through a single armature winding instead of distributing it among all of them.

Bill

If the motor is in good shape, 10 hz isn't a problem, as long as the motor's heat is managed.

You can install a "muffin" fan on the end bell when/if you get worried about heat.

If you had given the desired frequency (10hz?), the desired duration (10 minutes?), and the desired intensity (simply a positioner, or cutting 6 inch diameter threads to a shoulder?), you would get better answers.

If the VFD trips out (overcurrent) at 5 hz or so, when it happened to me, I blamed the condition of the motor, since other motors didn't trip the VFD.

Thanks all.

This is just positioning and I imagine just a few seconds run time as I glare at the tool closing in on the shoulder....
I have two back gears and use the faster of the two for threading due to too fast approach to shoulders- still too slow for the threading operation.
Just dropping down the frequency will help & I will step up the speed for threading.

I will have to play around with rpm and see what frequency gives me enough time for comfort.

fella can cut a lot of air getting comfortable with a thread to shoulder situation, and it costs nothing! ;-)

Oh the lack of courage is worse than that.
I am running away from the shoulder cutting and just trying to hit a close start point on return.
I do naturally run it back 'in air' but still fuss trying to get the damn thing spot on.
Dog clutch so engaged screw as I run back as in cutting.

Incompetence didn't run in the family but traditions need to start somewhere eh?

9100 said:

Zero frequency will put all the current between two legs, which is not a good idea but all right for a while... Note that this refers to a three phase induction motor.

The above is not true of a single phase capacitor start or brush type DC motor. The starting capacitor is not designed for continuous service and a stopped DC motor will be putting all the current through a single armature winding instead of distributing it among all of them.
Bill

Click to expand...

Bit off topic, but just for reference, an induction motor at 0 rpm does not put all current into 2 leads... by definition, there is slip.

So if the motor is in v/hz mode, there will be no significant current into any leads as the voltage will be too low to make a magnetic field, while if vfd is in sensorless vector mode w/o feeback it MAY be making a magnetic field depending on how 'real' the vector mode is (probably none folks here work with), and if it is sensor feedback real vector mode, it will be making a full magnetic field. At 0 rpm this field will be rotating at the slip frequency. A clamp on ammeter will show this as equal current into all leads, and since magnetizing current is usually about 1/2 the motor nameplate amps, it will be the significant highest component. This assumes the motor is not loaded heavily at this point: if it is, then the load current will still be only about 1/2 the total - possibly mostly into 1 of the 3 phases (not possible to get ALL current into 1 phase - see how 120 degree 3phase current overlaps, even when 1 phase at peak), so again, no issue.

On DC motors, yep of course all current into armature will go into 1 or 2 windings (say brushes are half on 2 comms). Motor mfgrs take this into when they rate them. If they are really honest they show Istall lower than Icontinuous while rotating (to distribute the heat over all armature). But the derate is usually not so much - I am used to about - swag w/o looking up many motors - about 15% derate.

I've seen wash down SS motors with no fan run 24/7 at 8Hz.
You can fry eggs on them but they keep running.
For some reason!

9100 said:

Agreed. The only thing that will damage the motor in this use is excessive current or running so slow that a cooling fan doesn't cool. You can have the motor almost stopped with rated amps flowing and appropriate cooling airflow without hurting it. Zero frequency will put all the current between two legs, which is not a good idea but all right for a while. You should be able to set the current limit of the voltage that will push that much current through on the VFD. A load meter is a good idea. A short run such as you describe without cooling will not hurt it.

Note that this refers to a three phase induction motor. The above is not true of a single phase capacitor start or brush type DC motor. The starting capacitor is not designed for continuous service and a stopped DC motor will be putting all the current through a single armature winding instead of distributing it among all of them.

Bill

Click to expand...

Bill,

With a couple of mills and lathes I have developed the habit of turning the knob down from the running frequency all the way down to zero Hz - either before or after hitting the stop button. I never had thought about it until I read your comment that "Zero frequency will put all the current between two legs...".

I'm not educated enough in electrical matters to understand your comment. But is what I'm doing a bad idea?

In other words, can the practice of cranking the motor down to zero hertz without turning off the VFD lead to overheating or other damage?

If so, I need to mend my evil ways.

Thanks,

VT

The damage mechanism as it relates to speed is indeed heat, but really, heat over time. Every motor has a "thermal damage curve", meaning the damage does not happen immediately, because the heat is always flowing from hot to less hot, radiating away from where it will do damage. The problem is that when the rate at which it moves away is exceeded by the rate at which it is created. What creates heat in a motor is mostly current flow, what's called "IR" heating, not Infra Red, but "I" for Current and "R" for Resistance. The winding wire itself has resistance (plus the motor itself has AC inductive circuit equivalent, impedance, but from a heating perspective, they re the same) so the amount of heat produced is basically the same no matter what, as long as the current is the same. Once a motor is running at slip, current and torque are equivalent. So regardless of the SPEED you run a motor at, if you are requiring full TORQUE from it, it will draw full current, and produce full heat.

If your motor is dependent upon shaft mounted cooling fans to move heat away from it, those fans move less air at slow speeds, you can end up with a thermal runaway and cook the motor. But if, at the same time you lower the speed you also lower the TORQUE required by the load, then that can act to mitigate the heat build-up. Also, if your motor is designed to NOT require cooling fans, as is the case with a TENV (Totally Enclosed Non Ventilated) motor, then slow speed should not matter. TENV motors use more metal in the frame and rotor shaft as a means of acting like a heat sink to draw heat away from the windings and rotor bars without the need for fans. ODP (Open Drip Proof) motors MIGHT be like that too, but it was not really the intent of the design criteria, meaning ODP motors might still need fans, it depends. If you SEE fans, it NEEDS fans and if it NEEDS fans, then assume they will not work at low speeds so you are at risk.

Less sure but often possible is that old motors tended to have more iron in them than new motors, some people take that risk and are fine with it. But at the same time old motors have old insulation, so using a VFD can accelerate the end of usefull life as well. It's kind of a crap shoot.

Bottom line: If it is fan cooled watch the current when running slow and under your expected task load. If it is less than 1/2 of the motor FLC, don't sweat it. If it is between 1/2 and 3/4 of FLC, but only for brief periods, just keep an eye on it and let it cool off by running it unloaded for a while (fan cooled motors cool off quicker running unloaded than they do turned off). Over 3/4 FLC or running for more than a few minutes under slightly less load, consider a separately powered muffin fan mounted to the end of the motor pulling air across it whenever the VFD is energized.

But DO NOT wait for the smell of roasting insulation before deciding on taking action or not. By the time you can smell it, it's already compromised.

And no, the current does not flow in only two legs at low speed, that comment must have come from experience with cheap forms of soft starters or old devices known as "starting torque controllers" that were sold years ago. In both cases, that is a horrible thing to do to an AC motor, but both were only supposed to be used temporarily on start-up, not under continuous loading.

PS: another wrinkle is what's called the "carrier frequency" in the VFD settings. This is the rate at which the transistor pulses are fired, having nothing to do with speed. The higher the carrier frequency, the lower the audible noise (or really, the less we hear it because it moves out of our spectrum), so people sometimes increase that carrier frequency to get rid of that annoying whine sound from the motor. But the problem is, that INCREASES the switching losses in the circuit, both at the VFD end and at the motor end. Most motors are fine with up to 4kHZ, but above that it starts having a noticeable affect, and you dont really move out of the range of human hearing until you are above 10kHz (lower as you get older, I'm at about 5Hz now). So if you must do that because you can't stand the whine in the motor, understand that it will increase the motor heat.

Just a small note about DC motors. The armature current flows through all the windings, no matter whether the speed is high, low or almost stopped. The armature windings are all connected together and all contribute to the magnetic flux. What the commutator and brushes do is to make the current flow in the right direction for all the individual windings to generate torque in the same direction.

We will now return you to your regularly scheduled programme.

Mark Rand said:

Just a small note about DC motors. The armature current flows through all the windings, no matter whether the speed is high, low or almost stopped. The armature windings are all connected together and all contribute to the magnetic flux. What the commutator and brushes do is to make the current flow in the right direction for all the individual windings to generate torque in the same direction.

We will now return you to your regularly scheduled programme.

Click to expand...

Cheers back at ya Mark! Obviously your statements are 100% correct. But the comment was STOPPED, not ALMOST STOPPED.

So of course while NOT stopped, there will still be current thru all the armature windings - each sequentially. But actually STOPPED, there can only be current thru 1 or at most 2 armature coils..... armature coils are ALL SEPARATE - not connected to each other. Totally independent coils, only connected to opposing commutator bars.

I COULD say that this side of the pond it is 'We will now return you to your regularly scheduled Program' Theater not theatre

Jraef said:

The damage mechanism as it relates to speed is indeed heat, but really, heat over time. ....called "I^2R" (typo fixed) heating

Click to expand...

And there are TWO different thermal time constants involved, not just one like the old fashion rules.... There is the long term get the heat out of the thermal MASS of the motor - like 30-60 minutes, but there is ALSO a very short seconds long thermal time constant based on heating the I^2R coil itself - so short that it cannot get the heat into the surrounding motor mass fast enough to dissipate it before the copper melts down itself; we are now calling this the 'coil thermal time constant' and it is in the 5-10 sec area. This time constant overrides the 30-60 minute constant between about 1.3 & 2.0 times motor rated current. Stay tuned and soon you will see a new definition of how long you can put more than continuous current into a motor before it fails. We are not done with the equations yet but soon.... watch for it in Machine Design mag....

We had an application where I used to work that needed speed regulation better than .1% from 10 to 165 RPM(through a gearbox) on a precision pump. The pump drew less than 5 HP at full speed, and proportionally less at lower speeds.

To get the .1% regulation at 10 RPM, we had to use specially built 25 horsepower frame motors with independent forced air cooling. Even though the load was low, very high currents were necessary to make the drive "stiff" enough for the speed variation specification at the low speed. It was a high end drive from Reliance, with very special tuning. The normal V/Hz curves were pretty much ignored.

Thanks for the lesson, to all that participated.

Love the depth of knowledge here, much appreciated...

SAF Ω

mike_kilroy said:

But actually STOPPED, there can only be current thru 1 or at most 2 armature coils..... armature coils are ALL SEPARATE - not connected to each other. Totally independent coils, only connected to opposing commutator bars.

Click to expand...

Hmmmmm

That might depend a lot on the winding method. A common lap wound armature has conductivity to many windings from any commutator segment. Whether any significant current will flow in some of the connected windings is another matter entirely.

mike_kilroy said:

Cheers back at ya Mark! Obviously your statements are 100% correct. But the comment was STOPPED, not ALMOST STOPPED.

So of course while NOT stopped, there will still be current thru all the armature windings - each sequentially. But actually STOPPED, there can only be current thru 1 or at most 2 armature coils..... armature coils are ALL SEPARATE - not connected to each other. Totally independent coils, only connected to opposing commutator bars.

I COULD say that this side of the pond it is 'We will now return you to your regularly scheduled Program' Theater not theatre

Click to expand...

The reason I used the 'almost stopped' wording was that normally there'd be no armature current at all in a stopped motor. If the motor was holding against a torque then stopped applies as well.

To reiterate whether the armature is lap wound or wave wound, the windings are continuous. The individual segments are either connected to each other at the commutator segments or at the risers from the commutator, depending on design. All of the coils carry current together.

To elaborate on Thermite's comment:- The purpose of inter poles (compensating poles of compoles), is that the field from the armature distorts the field from the field poles (that's how the torque is generated), so the compoles are placed in between the field poles and carry windings in series with the armature. As the armature current rises, the field in the compoles rises and moves the magnetic field back to where it should be. This is very important on large variable load or bi-directional motors as without it, you'd only get clean spark-free commutation at one load.

Yeah, this was my degree and early employment, before that part of the business got wiped out by the closure of the UK coal mining and steel industries destroying much of the market for large electrical machines in the UK.

There's a brief, reasonable explanation here


Sorry for the thread drift.