Motor query for 3 phase VFD conversion

[Martin Connelly]

On Martin Connelly Said:

Clive, the advantage of a higher power motor is that when running at lower than standard frequency you regain some of the torque lost by not changing belts/gears.

Martin C

Indeed so but probably not a great issue in this case as enough power is enough power and you have belt speed changes to help out. Going to a 4 pole 1400 rpm motor is the appropriate change if the existing 2800 rpm motor wimps out. I set up a Bridgeport for a friend using a VFD on the standard 1400 rpm motor. Belt stayed pretty much permanently on one of the middle speed ranges. He drove his machine harder than I do mine too.

CNC VMC machines typically have much more powerful spindle motors, 10 or 20 hp and up, than you’d expect when comparing to manual machines purely to maintain high torque at lower spindle speeds.

Clive

[Mark SlatterParticipant @markslatter13251]

Thanks again for all the input everyone, much appreciated!

Old Mart you mentioned the VFD you provided a link to was screened to prevent interference, can I ask how you determined that? The reason I ask is the following VFD was offered by Newton Tesla and I was wondering if it too offers some form of screening:

https://inverterdrive.com/group/AC-Inverter-Drives-230V/Mitsubishi-FR-D720S-070SC-EC/

Newton Tesla will offer the above VFD, pre-programmed along with a pendant for £370 VAT incl. Not pocket change but then its a name brand, ready to wire into my motor and the manual is excellent.

Clive I only mentioned a motor change should it be necessary in the future, the mill comes fitted with a 4 pole motor which I intend to use, and I’m sure in practice it will be ample for what I need. Your comments on swarf being the limiting factor are very true!

You did a cracking job on that conversion b2…and probably saved a couple hundred quid in the process, hats off to you!

[Julie AnnParticipant @julieann]

A few notes on induction motors, and VFDs, might clarify a few characteristics.

The plate on an induction motor will have a rated voltage and a rated current. In the UK these will normally be specified at 50Hz. The rated voltage is usually the supply voltage, nominally 380VAC for 3-phase in the UK. If the rated current is drawn at the rated voltage this gives the rated power of the motor. Since the speed is fixed by the 50Hz supply the rated torque can be calculated. The theoretical synchronous speed of the motor, known as base speed, is set by the supply frequency and number of poles.

Now consider a VFD driving the motor at a variable frequency, and hence variable speed. First assume that the frequency increases above 50Hz. By definition the rated current is drawn at the rated voltage at 50Hz. As the frequency increases more voltage is needed to maintain the rated current. But a higher voltage is not possible. So the voltage stays constant and as the frequency increases the current drops. To a first approximation motor torque is proportional to current, so the torque drops too. To summarise, as the frequency rises the current and torque fall in proportion. But since the speed has increased, and power is torque times angular velocity (aka speed), the drop in torque is cancelled out by the increase in speed and the power is constant. So above base speed the motor is constant power, but not constant torque.

Now consider the case where the frequency drops below 50Hz. If the applied voltage is constant the current will rise above the rated current and the windings may well overheat due to I2R. To keep the current at the rated value the voltage is reduced, hence the V/F characteristic (also known as scalar control) described by Clive. Since the current is kept at the rated current the torque stays constant. But since the speed has decreased in proportion to the frequency the power decreases. Below base speed the motor is constant torque but not constant power.

Also alluded to by Clive more complex VFDs use vector control, or field-oriented control. The use of vector control allows more control of the motor torque characteristics compared to scalar control. The mathematics of vector control is fairly complex and involves transforming the 3-phase rotating field to a stationary two dimensional space with two orthogonal currents, which can be represented by a vector, hence the name. One current controls magnetic flux and the other controls torque. These are then transformed back to a rotating field to control the currents in the motor winding.

Interesting fact: The transform between rotating field and stationary vector is called the Clarke transform, after Edith Clarke the first professional female electrical engineer in the US.

Julie

[old martParticipant @oldmart]

The VFD’s are either unscreened or C1 or higher. The screening feature does add cost, but being in a museum, I did not like to chance getting an unscreened one. There should be information online concerning VFD screening.

[Robert Atkinson 2Participant @robertatkinson2]

Hmm,

Not sure you mean screening, I think you mean filtering. Some VFDs have no or limited filtering, some have full filtering. A VFD will not meet EU/UK conducted emissions without some kind of filtering either built-in to the VFD, or external. Some VFDs may need both especially to meet the domestic limits. The Mitsubishi-FR-D720S-070SC-EC Mark was offered by Newton Tesla does not have an internal filter. It’s not clear, but I expect N T’s offering is for a packaged VFD with correct filtering etc.

150 to 3000RPM is quite a speed range. You will need at least two different mechanical ratios for that including step up. What is the current speed range of the mill?

I agree with others that you should stick with the existing 4 pole motor for now. A 2 pole will have a lot more torque ripple. This will produce more noise, vibration and poorer surface finish.

Robert.

[Pete RimmerParticipant @peterimmer30576]

<p style=”text-align: left;”>I’ve had (still have most) probably a dozen vfd-driven motors on equipment in my workshop and I’ve ran them down to 15hz. I’ve nver had a motor even get warm.</p>

[old martParticipant @oldmart]

What I meant to say was that I bought a VFD with EMC compliance of C2 rating. This allows for up to 5 metre long cables. The domestic C1 is ment for up to 1 metre long cables. All of the cables were screened, the VFD to motor and the remote cables. The metal box for the VFD was fully earthed, including the door. Many domestic users buy the cheaper VFD’s without the EMC (electromagnetic compatability).

If you are going to carry out your own wiring, then remember to write down the motor speed and amps used, they may be required when programming.

[old martParticipant @oldmart]

I have had to resize this photo and hopefully it will show the VFD fitted to the Tom Senior mill. Since Itook the picture some years ago, we have proper mains sockets behind the mills. The drill is 3 phase.

 

 

 

[Robert Atkinson 2Participant @robertatkinson2]

Very nice Old Mart. Good to see it done properly.

Robert.

[Mark SlatterParticipant @markslatter13251]

There has been some fantastic information shared here, thank you everyone! its great to see that old Tom Senior up and running Old Mart, lovely machine and set up.

Well I have bitten the bullet and ordered the Mitusbishi VFD and pendant from Newton Tesla, with the intention of using my existing 4 pole motor. According to Newton the D700 range of VFD’s are rated to C3 EMC with “a degree of filtering built in”. They did offer a RFI filter to go with it for £22.50 which for the money I thought why not. No idea if it’s needed or not but it’s done now.

Here is the speed range and belt set-up as displayed on the side of my machine:

So if I use position 1 as the belt arrangement and increase the frequency of the drive to 90Hz, I should be in the region of 2600 rpm I think. Does that sound right? How does one determine the max frequency that can or should be used?

[Robert Atkinson 2Participant @robertatkinson2]

As that series of motors includes a 2 pole version that does 2800 RPM I think you are probably safe at 90Hz for the motor. It is difficult to get exact details of the motor as it seems to be a generic type sold by many companies.
The spindle is another matter you might want to check the ratings of the bearings.

Robert.

[old martParticipant @oldmart]

Starting with 50Hz being standard in the UK, then 90/50 X 1550 = 2790rpm and the same with the lowest speed assuming you chose 20Hz, then 20/50 X 240= 96rpm.

I chose 25 to 75Hz for the Tom Senior and haven’t bothered to change it so far.

 

 

[duncan webster 1Participant @duncanwebster1]

Where did 1550 come from. There is always some slip, 4 pole motor on 50hz is normally quoted as 1475 rpm

[john fletcher 1Participant @johnfletcher1]

Regarding VFD and possible interference, has any one fitted an ex washing filter block, capacitors and chokes, there are plenty around and what was the result ?

[old martParticipant @oldmart]

I was calculating from the spindle speeds chart on the side of Mark’s mill.

[duncan webster 1Participant @duncanwebster1]

Ah, silly me

[noel shelleyParticipant @noelshelley55608]

I’m sure I must have missed some thing – but the spindle and motor chart seems strange. Its a 5 step pulley so step 3 should be about 1:1 ? If so then a 4 pole 50 Hz motor will be 1450ish RPM  and step 3 would be 1450ish . Where has the 2.4 reduction come from ? Is it a geared head ? Noel.

[old martParticipant @oldmart]

Now you mention it, Noel, it does seem odd, an eight pole three phase motor would work the closest in that case.

[PeteParticipant @pete41194]

Thank you Julie, that was the absolute best and clearest description I’ve seen anywhere about power verses torque with VFD controlled motors, and the difference with those vector controlled VFD’s. Extremely well explained so that I learned quite a bit I didn’t know or properly understood. 🥂

[john fletcher 1]

On john fletcher 1 Said:

Regarding VFD and possible interference, has any one fitted an ex washing filter block, capacitors and chokes, there are plenty around and what was the result ?

Not yet!  I think it’s worth trying and have the bits waiting in a box.    As it’s a filter, I can’t test the idea with a quick lash-up.  Has to be assembled inside a properly earthed metal box because interference can escape in myriad ways.

I predict the result won’t be perfect, but it should reduce the problem.  Whether or not it’s worth the effort is unknown!   Like as not, the basic washing machine filter can be improved by adding some capacitors, but these increase earth current and risk tripping an RCD.   I don’t believe there are any H&S concerns provided the usual electrical guidance is followed.

Dave

[Pete]

On Pete Said:

Thank you Julie, that was the absolute best and clearest description I’ve seen anywhere about power verses torque with VFD controlled motors, and the difference with those vector controlled VFD’s. Extremely well explained so that I learned quite a bit I didn’t know or properly understood. 🥂

I thought it very good too.

Might be useful to explain Power and Torque, both slippery concepts, especially as they relate to each other!  My attempt:

• Power is the Rate of Doing Work.    That is, given a blob of work such as raising a 10kg weight 1 metre high, a powerful motor will lift the weight faster than a weaker motor.   Time is a factor.
• Torque is rotational force.  If the load on a motor is too great, the motor will stall. Time is not a factor, and force is not power, and torque can exist without power.

Clear as mud!  Can anyone do better?

When starting, and at low speeds, many motors have low rotational force, but torque increases with speed, up to a point.  Stepper motors are quite interesting because they can hold the rotor in a fixed position.  In this state power output is zero, but there’s high torque.  Considerable external force is needed to turn the rotor.  Power input is not zero.

Graphs are the best way I know of visualising the relationship between power and torque on a particular motor.

Jeremy Clarkson pokes fun at the confusion by asking on Top Gear, “how many torques does this car have?”  Unlike “how many HP does this car have?”, his question makes no sense.   It’s a joke.

Dave

 

 

 

 

[old martParticipant @oldmart]

The most likely reason for wanting to use a slow speed would be if drilling a large hole. We have R8 to MT2 and 3 sockets which can hold up to 1 1/2″ drills, which need slow speed and power. Power tapping can have similar requirements. A large fly cutter, likewise.

90% of our milling uses the top belt speed and 50-75Hz, but having all of the belt speeds is well worth retaining.

[PeteParticipant @pete41194]

There’s very good reasons for retaining the original mechanical step pulley drive. Thinking there isn’t going to be anymore belt changes isn’t how this works. Visualize drilling a .125″ / 3 mm hole in something like aluminum or brass and keeping it up around the listed rpm for efficiency. Then the same for tools with far larger diameters. Those bigger drills, hole saws, annular cutting tools, boring larger diameters in steel etc. A lot could still be done just using the VFD and taking lighter depths of cut due to the reduced torque. Far better in my opinion, just to take a few seconds and switch belt positions.

I considered multiple methods of using a solid state drive, rotary phase converter, VFD or even changing to a single phase motor when I bought my mill. I also didn’t then (and still don’t) know very much about most of them. More by luck and guessing, I chose to use a  VFD and have seen no reason since that it wasn’t the absolute best choice. But they can still only do so much. The combined mechanical step pulleys and VFD allows both increasing the torque when needed and the variable speeds between those steps, as well as reducing or increasing the rpm that the mechanical system alone doesn’t allow. The only real trade off is a bit more cost and complexity. Personally and for various reasons, I don’t believe standard 3 ph motors were ever designed to operate too far out of there rated rpm. For that reason I limit mine to about 1/3rd below or above it’s normal 1750 rpm range with the VFD. Anything outside of that, I’ll switch belt positions.

My collection of old Model Engineer magazines goes back far enough that it seems many in the early days of the hobby were using treadle driven lathes, or up from that, was the use of a small steam engine or even low HP gas engines if they could afford it to obtain the required mechanical drive power before most homes even had electricity. Waiting to build up steam or having to start an engine, then switch likely multiple flat belt positions has always seemed much more complex than just spending less than a minute to switch a single belt once in a while. Even having to clean the lantern glass and refill it just so you could see what you were doing took more effort. Most of us including myself don’t fully comprehend just how easy we have it today in comparison to the average conditions back then, and how they had no other methods or choice.

[SillyOldDuffer]

On SillyOldDuffer Said:

On john fletcher 1 Said:

Regarding VFD and possible interference, has any one fitted an ex washing filter block, capacitors and chokes, there are plenty around and what was the result ?

Not yet!  I think it’s worth trying and have the bits waiting in a box.    As it’s a filter, I can’t test the idea with a quick lash-up.  Has to be assembled inside a properly earthed metal box because interference can escape in myriad ways.

I predict the result won’t be perfect, but it should reduce the problem.  Whether or not it’s worth the effort is unknown!   Like as not, the basic washing machine filter can be improved by adding some capacitors, but these increase earth current and risk tripping an RCD.   I don’t believe there are any H&S concerns provided the usual electrical guidance is followed.

Dave

There are significant safety issues with mains filters. Don’t DIY them unless you know what you are doing. If it was easy electronic equipment manufacturers would make their own rather than buying them from Schaffner et al. Even using parts from an existing filter can be dangerous. The capacitors in different parts of the circuit have different, safety related (X and Y), ratings. Fitting one in the wrong place or using the wrong value could be fatal if another fault such as a faulty earth occurs.
If you have to ask how, don’t even think about doing it.

Robert.

[Robert Atkinson 2]

On Robert Atkinson 2 Said:

On SillyOldDuffer Said:

On john fletcher 1 Said:

Regarding VFD and possible interference, has any one fitted an ex washing filter block, capacitors and chokes, there are plenty around and what was the result ?

….

I predict the result won’t be perfect, but it should reduce the problem.  Whether or not it’s worth the effort is unknown!  …  I don’t believe there are any H&S concerns provided the usual electrical guidance is followed.

Dave

There are significant safety issues with mains filters. Don’t DIY them unless you know what you are doing. If it was easy electronic equipment manufacturers would make their own rather than buying them from Schaffner et al. Even using parts from an existing filter can be dangerous. …
If you have to ask how, don’t even think about doing it.

Robert.

Can you explain in more detail please Robert?  John, who if I remember correctly is a retired electrician, is asking about fitting a sealed manufactured EMC filter from a washing machine in front of his VFD.   How is that dangerous, especially if he does the work?

In terms of performance, not safety, my feeling is a washing machine filter won’t be brilliant at suppressing the muck coming out of a VFD.  That’s why I suggested it might be improved by adding capacitors.  I plead guilty to assuming readers know the capacitors have to be mains rated components, not standard fare.  I also failed to explain what values might be suitable, or how to test whether or not the filter is improved by them.

Though John may not know anything about filter design, I’m confident he could wire one up without coming to grief.  And important to note, no-one is expecting a washing machine filter to be as well engineered as one designed to meet an EMC specification.   It only has to reduce the problem by a useful amount – a mere 50% reduction would be helpful.

Any job can be tackled provided risk is managed.  In this example I’m not aware of anything about fitting a washing machine EMC filter that’s high risk or high impact if it goes wrong.   As always I might have missed something though!  What’s the worst that could happen?

Ta,

Dave

 

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