Thoughts on this VFD

[Pete.]

If that were my machine, I'd bang a 4 pole 2.2KW 3 phase motor in it and fit a VFD sized to suit. 3HP is going to be plenty for you at home. Weigh in the old 3 phase motor (or donate it as a rotary converter idler to someone who needs it).

 

[daleyd]

Since I'm struggling to concentrate on what I'm supposed to be doing, I'll explain a bit about motors, current, and voltage.

As per my earlier post, most common motors are constant torque up to their rated speed. The torque may vary due to various factors, but for all intents and purposes, torque output is constant.
Torque is a result of the strength of the magnetic field, which is controlled by the amount of current flowing through the windings.
The amount of current flowing through the windings is limited by the voltage applied, and the amount of back emf (electro motive force - in simple terms, the voltage generated by the motor spinning).
As the motor speed increases, the back emf increases, so you need to apply more voltage to ensure the correct current flows.

Off course, for an unloaded motor, very little current will flow, as the back emf generated by the motor will very closely match the supply voltage. What happens when you apply load to the motor, is the motor slows slightly, the back emf produced reduces, and the current able to flow through the windings increases.

As voltage is proportional to speed, but torque constant, the actual power output from the motor is proportional to motor speed.
So a 1Kw motor running at 50% of rated speed, will only be theoretically capable of outputting 500W.

For DC motors, this is quite a simple concept, but the same concept applies to AC motors, but with AC motors, speed is controlled by frequency, not voltage.
Basic VFDs handle this by varying voltage according to the motor speed. So at 50% of rated speed, 50% of rated voltage will be applied.
If the voltage wasn't reduced in proportion to speed, then you would have a motor producing 50% of back emf, but still receiving 100% of supply voltage, which would force far too much current through the windings, quickly converting them into a smouldering mess.
The same applies, that if you load the motor up, the rotor starts to lag behind where it's expected to be, reducing the back emf generated, and allowing more current to flow for any given voltage.

That theory applies up to the rated speed, which is the speed the motor is expected to run at from the specified voltage and frequency.

However, many AC motors can be driven beyond their rated speed, but it then means you enter in to constant power operation.
Beyond the rated speed, you no longer have the additional voltage difference to drive the rated current through the motor windings, but due to using AC, you can still spin the motor faster, but with a proportional reduction in available torque as speed increases.
For example, if you have a 1kw motor that produces say 5Nm and 1000 RPM at 240V/50Hz (totally random figures for simplicity, which defy physics!), if you then apply 100Hz to that motor, the motor will now spin at 2000RPM, but only produce 2.5Nm torque. It will still be outputting 1Kw, hence being called constant power operation.


It's pretty common practise now to specify a lower rated speed motor, then spin it faster with a VFD to get a greater useable speed range. Quite a bit of machinery you'll not typically use more power at higher speed, but more torque at lower speeds is more beneficial, so why use a 2 pole 2.2kw 3000RPM motor running at 50% speed and torque, when you can use a 4 pole 2.2kw 1500RPM motor that produces twice the torque at 1500RPM, but will output the same power and torque as the 3000RPM motor at 3000RPM?


In the above I have used round figures, but 50Hz motors don't actually spin perfectly. They're asynchronous, which means they don't perfectly follow the changing electromagnetic fields. They 'slip', which is why although 50Hz should theoretically spin a 2 pole motor at 3000RPM, and a 4 pole at 1500RPM, the rated speeds are actually lower.
If you want an example of a synchronous motor, that's what a stepper motor is, as it perfectly follows the changing magnetic fields.


Off course, that's the basic theory.
Most VFDs let you boost current at low speeds to increase torque, but as I said earlier you have to be careful about overheating the motor, and most VFDs do have an overload tolerance as standard, even the Chinese VFDs. IIRC most will handle 200% for around a second, with 150% for a bit longer, before faulting out and stopping things all together.
Then there are things like vector control VFDs that monitor the output far closer to see how the motor is reacting, which mostly produces benefits when running at slower speeds, and you're pretty much running a squirrel cage motor like an AC/Brushless servo motor, but without the low speed stability a servo motor is designed to achieve.

That’s a good description. The linear v/f curve you describe that simple drives produce is a good illustration of why companies like the one I work for (Schneider) and ABB, Danfoss etc spend a lot of r and d time and money developing algorithms for things like pumps and fans that don’t have a linear torque requirement - when you have a pump that at (for example) 50% speed only requires 25% of the torque that it needs at full speed then there are big efficiency savings to be made by tailoring the voltage the drive outputs rather than just sticking to a perfectly linear v/f line. When you have a big vfd, or a lot of them, or both, then the cost/energy savings are considerable (a bit off topic for the home workshop but relevant to the constant torque discussion)

 

[Pete.]

Most certainly a lot of DC motors have failed, in the past, due to overheating at slow speed and high load.

Probably why my 3hp DC motor is open-frame and about 18" diameter

 

[daleyd]

Likely the biggest problem with the home workshop and slowed motors is the cooling aspect. Most certainly a lot of DC motors have failed, in the past, due to overheating at slow speed and high load. It is certainly not so much of a problem with (most) induction motors but the cooling fans on the motors slows along with the motor…

It shouldn’t be a problem if you set a sensible minimum speed figure on the vfd, at work we usually use around 15/20hz as min speed if it’s intermittent low speed operation and up it if we are expecting it to run at the bottom end more often. I’ve used the same at home on my lathe and mill and it works no prob, even at fairly long periods running at the min speed.

If you need lower speed operation then use forced cooling, most vfd will have a contact that you can program to energise when either the drive is running or if it drops below a certain speed - use this to power a computer fan or similar on the end of your motor and jobs a good un.

 

[Kram]

Mine never had a problem with heat, low speed in summer I wouldnt call it warm. Ive done some parts that due to size, couldnt safely be spun any faster and thhere was a few hours of constant heavy load on it.
The thermal trip has never triggered and if you have issues a small fan will sort it.

It's pretty common practise now to specify a lower rated speed motor, then spin it faster with a VFD to get a greater useable speed range. Quite a bit of machinery you'll not typically use more power at higher speed, but more torque at lower speeds is more beneficial, so why use a 2 pole 2.2kw 3000RPM motor running at 50% speed and torque, when you can use a 4 pole 2.2kw 1500RPM motor that produces twice the torque at 1500RPM, but will output the same power and torque as the 3000RPM motor at 3000RPM?

This is very true. I put a 6 pole 960rpm motor in mine but it has no problems spinning at 2200rpm and if I needed too, I'd happily go faster. The bearings wont be anywhere near the limit, I think 3000rpm would be quite safe.

Higher pole count motors are bigger, lower power ratings for their size and cost quite a bit more than a 2 pole. If its worth while really depends what mechanical reduction the lathe has, and this likely has a decent set of gears inside. I generally keep mine at 3:1, giving me a usable speed range of 83-733 rpm. Its a bit of a pain swapping belts at the moment but it gives me 250-2200rpm on 1:1.
Below 12.5hz is unusable on this, it sounds bad and very coggy.

 

[Ubique]

Having read the recent VFD, rotary converter & one-into-three-phase threads, and looked at various pro's and cons, I'm glad I'm not in a position where I need any kind of three-phase, If I did, I'd probably be on the lookout for a second-hand generator, persuade the neighbours either side to 'lend' me a phase through a meter or go for the gold standard and get a supply put in - which isn't as costly as I thought it would be. https://www.ukpowernetworks.co.uk/electricity/upgrade-reduce-electricity/time-and-cost

And then stick a VFD in if need be.

 

[daleyd]

Having read the recent VFD, rotary converter & one-into-three-phase threads, and looked at various pro's and cons, I'm glad I'm not in a position where I need any kind of three-phase, If I did, I'd probably be on the lookout for a second-hand generator, persuade the neighbours either side to 'lend' me a phase through a meter or go for the gold standard and get a supply put in - which isn't as costly as I thought it would be. https://www.ukpowernetworks.co.uk/electricity/upgrade-reduce-electricity/time-and-cost

And then stick a VFD in if need be.

The hardest part is deciding which route to go down initially, after that everything is pretty straightforward. (I’m speaking from the point of fitting a vfd, I have no experience of using a static/rotary converter) - but my experience of switching out my lathe from a single phase motor and replacing with a 3 phase/vfd has made it a hugely more useable/user friendly machine. My mill already had a 3 phase motor so I just added my own vfd, but again it’s super easy to use and the speed adjustment is great.

 

[Lewis_RX8]

Just ordering a 5.5kw boost vfd 240v to 380v 3 phase thanks to @m_c recommendation

Arriving at some point about 14th November hopefully which isn't too big a deal plenty of time to sort the rest of the lathe bits out.

Will keep this thread updated for any future owners etc post the install.

Do not think I will use the low speed function of the lathe as that also takes the power output down so will just vary with frequency

 

[Kram]

The amazon one is the same but next power up for near the same price, and you'd have it tomorrow.. neither look to be the B version for braking though.

Variable Frequency Drive, 220VAC Input 3-Phase 30VAC Output 7.5KW, High Performance Drive Single Phase Variable Frequency Drive Converter Speed Controller Converter : Amazon.co.uk: Business, Industry & Science

Shop Variable Frequency Drive, 220VAC Input 3-Phase 30VAC Output 7.5KW, High Performance Drive Single Phase Variable Frequency Drive Converter Speed Controller Converter. Free delivery on eligible orders of £20 or more.

www.amazon.co.uk

Nflixin 9100-1T-00750G or 9100-1T-0550G
Manual here

Product information download – NFlixin

nflixin.com

anyway both should do the job

 

[Lewis_RX8]

The amazon one is the same but next power up for near the same price, and you'd have it tomorrow.. neither look to be the B version for braking though.

Variable Frequency Drive, 220VAC Input 3-Phase 30VAC Output 7.5KW, High Performance Drive Single Phase Variable Frequency Drive Converter Speed Controller Converter : Amazon.co.uk: Business, Industry & Science

Shop Variable Frequency Drive, 220VAC Input 3-Phase 30VAC Output 7.5KW, High Performance Drive Single Phase Variable Frequency Drive Converter Speed Controller Converter. Free delivery on eligible orders of £20 or more.

www.amazon.co.uk

Nflixin 9100-1T-00750G or 9100-1T-0550G
Manual here

Product information download – NFlixin

nflixin.com

anyway both should do the job

bugger

New thread - How to cancel banggood order

 

[Onoff]

bugger

New thread - How to cancel banggood order

I have. Check the order status, if it hasn't shipped you should be able to.

I've ordered something, cancelled it then reordered.

Another time I got the order, queried that seemingly 1 out of 4 solar lights didn't work. Got an apologetic "we'll refund you for the one. They then refunded for all four.

Then the one "faulty" one started working.

I've nothing but praise so far for Banggood.

 

[Lewis_RX8]

Big thanks to @Kram there then

Arriving tommorow and no worries about import etc ,,, also look at the price now just shot up ater that was worried I paid that when it changed but not paid lower price

I have. Check the order status, if it hasn't shipped you should be able to.

I've ordered something, cancelled it then reordered.

Another time I got the order, queried that seemingly 1 out of 4 solar lights didn't work. Got an apologetic "we'll refund you for the one. They then refunded for all four.

Then the one "faulty" one started working.

I've nothing but praise so far for Banggood.

Yea managed to cancel thankfully as it wasn't even in stock nevermind sent out and only bought last night. had to talk to some a Chinese person apparently called russel however he offered me some bangood points but had to say no and just ask to cancel order a few times before he did it instead of offering other services.

Was cancelled so cant complain