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.

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