Vfd advice please

Hi all i power all the machines in my work shop through a static converter. However I have a 3/4 hp motor which I wish to vary the speed with.

Question do I need an exact match inverter for it I.e. .55kw vfd. Many of the ones on e

Fleabay appear to be .75kw

Question 2 will the use of a vfd reduce the torque of the motor, meaning I should go for a 1hp in the first instance?

Many thanks

You can use a bigger inverter than the motor rating, best to adjust the settings to match the lower current.

The inverter you buy is likely to be designed for delta connected motors, or "230 volt". "415 volt" ones are available but cost more. If your motor is already wired in delta, or you can rewire it from star to delta, you should get the same torque, in fact possibly better as the drive will have accurate phase relationships and voltage balance.

Did you mean change to a 1 hp motor or use a 1 hp VFD? There would be no need to do the latter.

I currently have two IMO iDrive 2 inverters, one of which runs a 3/4 hp motor on my VMB with no sign of a lack or power or torque.  Newton Tesla supply them.

Edited By John Haine on 13/02/2021 09:49:56

Correct, but it is pertinent to note that as the frequency decreases below 50Hz the torque will stay roughly constant, unlike a belt or gear change arrangement. So the motor output power will go down in proportion to the frequency decrease.

Andrew

A 0.75 kW inverter will be fine but its prudent to adjust the parameters to suit the smaller motor. Partially for safety by setting the maximum output current lower to match the smaller motor and partially because any inverter of decently modern design has its internal programming adjusted to suit the size of motor being driven. Especially so if its one of the vector drive variety. Theoretically inverters run a motor at constant power above its rated speed but if you simply hook a 0.75 kW inverter to a 0.55 kW motor without suitably adjusting parameters it will try to run the motor at 0.75 kW at higher speeds. Which may be embarrassing even though its unlikely to fully succeed.

Best to go to a recognised supplier, like Inverter Drive Supermarket, for a decent branded inverter rather than an economy E-Bay special. For the low power inverters the price differential in real £ between uber economy and branded is not really significant. For starters the instructions will be in understandable English so you will be able to set it up easily, a not inconsiderable benefit. Secondly an economy import VFD will almost certainly be of the simple V/F (voltage/frequency) control type which simply don't work as well as more modern vector types. Basically power drops off faster as the rpm reduces and they can struggle to get heavier loads moving.

Clive

This might seem a bit of a cack-handed reply, as I can't answer your specific question from an electrical point of view - I'm not qualified to do so. What I can do, is tell you of my practical experience of doing a similar motor swap on my Perfecto shaper.

As part of my lock down boredom relief, I decided to re-motor my shaper and lathe with 3-phase motors and VFDs. The lathe was going to be uprated to 1HP from the standard ¾HP as I considered it lacked sufficient grunt. The shaper, on the other hand was powered by a ⅓ HP single phase motor. I found a replacement 3-phase motor of the same power rating but not a matching VFD. I subsequently purchased a super-cheap 0.45kW VFD online and fitted that. It works fine with no apparent ill effects on either piece of equipment.

There is a short video of the set up in action here: Shaper in action.

So, from a purely practical point of view, I'd say "Go ahead."

John

2

g head."

Thanks guys,, I am building a 3" x 72" belt sander, so I am especially glad about the torque answer, and I will steer away from fleabay

If you plan to run the motor much below normal speed for extended periods, consider motor cooling.

The built in motor fan is less effective at lower motor speed.

The VFD may deliberately limit it's output to effectively de-rate the motor when speed drops in order to protect the windings (that might be a user selectable option), it's also possible to add an external fan to the motor to force cool it regardless of speed.

First question, a 0.75kW will be fine. A VFD much larger than the motor might not work well, but a bit more capacity than the motor just means the VFD will run a bit cooler, which is good.

A gross power mismatch isn't a good idea because VFDs are configured to match their output to a particular motor, and big motors have a different profile to small ones. VFDs are quite intelligent, and a high-power VFD might mistake a small motor for a faulty big one and refuse to start.

Running a motor slowly will reduce torque. Most VFDs compensate somewhat by putting more current into the windings, but beware the motor doesn't overheat if run slowly for long periods. If persistent low speed and high torque are needed, better to get it by fitting gears or pulleys so the motor can run fast in it's comfort zone rather than fitting a bigger motor. Big motors still heat up, it just takes longer because they're heavier.

My lathe has a permanently on fan cooling the main motor - it works well for the work I do, but I don't spend hours making heavy low rpm cuts.

Dave

Oh dear, that's very disappointing, I thought SoD was smarter than that. He should go and stand in the corner and contemplate his misdemeanor!

In a simple model of an induction motor torque is proportional to current. As the frequency, and speed, decrease from base speed the VFD will decrease the applied voltage to keep the current constant. So the torque is essentially constant below base speed. For the purposes of this thread we'll take base speed as that when the motor is running from a 50Hz supply. As the motor speed drops the torque stays constant but the output power drops in proportion. Gear and pulley arrangements increase the torque in proportion to the speed reduction so the power stays constant.

Newer VFDs, that use vector control, can increase the current, and hence torque, below base speed. To some extent that ameliorates the drop in output power. But it is normally intended to counteract short term overload, not for continuous operation. The motor is being run beyond its design current and will quickly overheat, even with a fan running at normal speed.

I'd agree with the above that a larger VFD will have no problem running a lower power motor. The only real problem occurs when the VFD is capable of much more power than the motor needs. The internal current sensors will be sized according to the rated motor power. If a smaller motor is used naturally the currents will be lower and the sensor will be operating over a small part of it's range, leading to a loss of resolution. That may be a problem for the more complex control algorithms.

I'm not sure why a belt sander needs variable speed control?

Andrew

Hi John

The shaper looks like a useful tool and quite in action, I notice you have mounted the VFD in an accessable position when using the machine so you have easy access to the controls, all looks pretty neat to me and working as intended.

Emgee

Emgee

There is a short video of the set up in action here: Shaper in action.

Nothing to do with the original question, but a shaper use query from the video : why are you incrementing the traverse on the cut stroke ?

I usually set my Boxford to do the feed increment on the return stroke, that way the cut is perpendicular to the traverse - your cut is at an angle, as can be seen at the end of the pass on the block where the last bit to be cleared isn't parallel to the edge.

I appreciate that for the operation shown it doesn't matter, but for shaping up to a shoulder it would. Not intending to be nitpicky, just curious.

Nigel B.

Edited By mgnbuk on 13/02/2021 14:29:13

As already mentioned, the inverter drive supermarket is a good place to buy a VFD. They have a printable "quick start guide" for many of their products which is so easy to follow that it is worth its weight in gold.

This link is for their cheapest VFD which is a good one. The speed control for it is by using 2 buttons rather than a potentiometer. Other of their products have the potentiometer for speed control. You won't need to overspecify the VFD power it is just wasting money.

**LINK**

Wot me make a mistake on the forum? Surely not!

In this case yer honour I was thinking of this graph:

But Andrew's quite right, I should have explained the context. And maybe the graph doesn't cover how a 3-ph motor behaves when it's speed is reduced by altering frequency! It's another pause for thought moment.

What I think I know doesn't help much. How different types of electric motor behave during start up and low speed varies. A single-phase motor suffers low torque until it builds up speed, but a stepper motor has high torque even when stopped. I'd be in trouble if asked to program the output voltage, frequency and current of a VFD because none of my books describe how 3 phase motors behave at anything other than 50 or 60Hz. (Or if they do, I don't understand it.)

Here's a question. Is there an easy way of measuring the torque available at a lathe's chuck at various speeds? If so would be interesting for members to see what their treadle, single-phase, DC-PWM, 3-phase, VFD, or brushless motor is actually delivering.

Dave

The reduction of torque is a factor, I set the minimum frequency to 25hz for the mill which halved the default speed without loosing too much output. With a belt sander, the lower speeds would tend to coincide with delicate work which would not be so demanding torque wise. I would be happy to run from 20-80 hz with a sander which gives a 4:1 ratio.

My Milling machine came with a 1/2 hp motor fitted with a Toshiba 0.75kw inverter. All has been well since owning it & not had any torque issues, until.

I got hold of a auto tapping head & was doing some tests with it. I was running the inverter at 12hz, rather than changing belt speeds. At that frequency the torque was pretty low. I could not even drive a 6mm tap through 10mm alloy plate.

On my lathe i have a Omron inverter with a 1.5hp motor. I often run this at 1.5 hz when putting glue on rings. It runs happily at that frequency & seems to have bags of torque. That inverter has a setting called torque boost, which maybe explains the power. + the fact the motor is much bigger than on the mill. Or was. I have just changed the Mill motor for a 1 hp. Not done any more tests as yet.

In answer to your question , yes it will work fine. As others have already said, go through the settings & match the motor current .

Steve.

Yes there is.

Hold a brake disk in the chuck and rrange the brake caliiper so you can measure the force it reacts. This could be a bearing held in the tail stock with an arm holding the caliper. This, if unconstraned,would allow the caliper to rotate concentrically with the disk. constrain the movement with a one foot arm and spring balance. If the brake is applied so that the balance reads 10lbs you have 10 lb/ft torque. scale to suit lathe power.

Robert G8RPI

I can see a few lathes with bruised beds coming up.

Steve.

If you go on the inverter drive supermarket website and look at the various 3 phase motors, most have their torque figures at varying input frequencies. Looking up the figures from a comparable size motor to yours will give you a reasonable idea of what to expect.

Here is a typical 4 pole motor of 3/4hp output:

https://inverterdrive.com/group/Motors-AC/ac-Motor-550W-550-Watts-4-pole-Foot-Mount-71-TEC/

Edited By old mart on 13/02/2021 16:51:10

Like the title says the graph shows the torque-speed curve of a 3-phase motor. What it doesn't say is that it is at a fixed frequency. Let's assume that we have a 3-phase motor running off a 3-phase supply with no load and no internal losses. The motor will run at the synchronous speed, for a 4-pole motor and 50Hz that is 1500rpm. At this point we are at the bottom right of the graph. Of course a practical motor will have some internal mechanical losses which require torque to overcome. To provide this torque the motor has to slow down below synchronous speed; it slips below synchrnous speed. So we move up the torque curve with a slight decrease in speed. If we now add a mechanical load to the motor then we continue up the curve. The speed drops more and the torque produced rises as the current rises. When we reach the rated power of the motor the current will be the rated current and the speed will have dropped to the rating plate value. For most motors the speed at rated power is a few percent less than the synchronous speed.

If we now continue to increase the load the speed will drop further, the currents will rise above rated current and the torque will also rise. At some point we reach the maximum torque - pull out torque. This is normally 250% or so more than rated torque and interestingly is determined by rotor resistance. A VFD can mimic this behaviour in a controlled manner by trading speed for increased torque during a short term overload. My CNC mill does this.

If the motor load continues to rise the speed drops further and the torque also drops. Of course from full rated power onwards the motor is overloaded and will probably have thermal issues.

To summarise, the graph is nothing to do with how the torque varies with frequency for a motor driven by a VFD and within it's rated power.

Measuring torque on a rotating shaft is involved, and done properly requires expensive inline sensors and modification of the drive train. Less accurate estimates of the torque in a rotating shaft can be made by measuring the twist of the shaft under load.

Andrew

If i scroll through the readout with the motor running, it shows current being drawn. Not exactly the torque but an indication of what is going on.

Steve.