- Thread starter Lewis_RX8
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Ubique
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But you still have to ramp up enough energy to start the machine moving - which is the same amount regardless or rate of application of energy. For example, I have a spring gauge for measuring the breakaway force required to turn a wheel hub, the force is the same whether I pull on the gauge slowly or quickly. I appreciate the way a VFD applied 'energy' is different than plain old fixed voltage & frequency mains, but the energy required must be the same, just over a different timespan.
daleyd
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Yes, but you don’t need to build in an “allowance” to give you a safety margin which you would need to do with a dol starter, or to a lesser extent a star delta starter on a larger motor.
When you start a motor direct on line you provide it with full mains voltage instantaneously, while the motor is stationary. This effectively acts like the motor is stalled for an instant so the starting currents are very high. Once the rotor begins to move then the current gradually reduces, but on a load with high inertia this can be some seconds until it settles down.
To alleviate this star delta starting provides an improvement by starting in a lower power configuration initially then switching over after some delay.
But with a vfd you have none of that, if you set some sensible parameters within the drive. As it ramps up the frequency starts of at 0, as does the voltage and smoothly ramps up - you don’t get the massive start up spikes of current you get with the above mentioned methods. Of course if you set some stupid parameters in the drive you could potentially see big spikes on start up, but even then a decent drive will protect itself if it sees an out of spec current by slowing itself down - I’ve actually seen this happening on some big fans (~100amps) that pulled too much current when cold and would only run at max of about 45 hz, when the air they were moving was hot they were able to run at 50 hz as they weren’t limited by current.
The fact you are imparting the same energy is neither here nor there, it’s the fact that you don’t see the big start up currents is what matters.
Kram
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For old motors, its recommended to program it for a lower switching frequency so the transient voltages will be lower. The problem with that is, its then in the audiable range and could be annoying.
I would say you also want one with a braking resistor ability, which are a bit harder to find.
The amazon one I linked on first page is a 220-380v boost version but without braking resistor output.
Voltage doubling is usually a low current method and would need huge capacitors to scale up.
I assume boost vfd's do it with a high frequency switching transformer, that is the most compact way to do it.
I would say you also want one with a braking resistor ability, which are a bit harder to find.
The amazon one I linked on first page is a 220-380v boost version but without braking resistor output.
Voltage doubling is usually a low current method and would need huge capacitors to scale up.
I assume boost vfd's do it with a high frequency switching transformer, that is the most compact way to do it.
Lewis_RX8
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That's what I was hoping use the VFD function for dual speeds
Think it depends on the windings so will open the box and have a look when I'm home
daleyd
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Depends what you want to spend and how much effort you want to put into it - you could potentially get a vfd with 2 or more parameter sets and use it with a 2 speed arrangement with contactors or existing switchgear. It’d take a fair bit of thinking about and you’d probably want to put some interlocks in to avoid inadvertent switching of either motor windings or the vfd parameters when in operation!! The other issue with this is that the step up vfd are unlikely to come with multiple parameter sets (I don’t know however, that’s just a guess?) and the industrial brands that do come with them don’t do step up vfd!
Easiest option (IMO) would be a static or rotary phase converter and keeping the original switchgear.
Failing that you could run vfd on either of the 2 speeds, depending on your usage higher or lower speed might suffice with some speed adjustment from vfd. You’d need a 230-400 vfd though.
Or replace motor for single speed and use a standard 230v vfd.
Easiest option (IMO) would be a static or rotary phase converter and keeping the original switchgear.
Failing that you could run vfd on either of the 2 speeds, depending on your usage higher or lower speed might suffice with some speed adjustment from vfd. You’d need a 230-400 vfd though.
Or replace motor for single speed and use a standard 230v vfd.
Hood
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Replacing the motor with a 230v one wouldn't really be of much benefit as you would still just have a single speed motor, exactly the same as you would if you chose one of the speeds from the current motor.
Rotary (or static with motor to make rotary) would be the best solution especially if wishing to run multiple machines. I had one years ago, just a static one but fitted an idler motor and it worked great for my Student lathe with the 2 speed motor and third shaft control (no clutch, motor stopped/started/reversed with the spindle)
That being said a Static and motor or a rotary will set you back a lot more than a VFD, that is why I am curious how @m_c uses his VFD in place of a rotary as that could be your best option.
Rotary (or static with motor to make rotary) would be the best solution especially if wishing to run multiple machines. I had one years ago, just a static one but fitted an idler motor and it worked great for my Student lathe with the 2 speed motor and third shaft control (no clutch, motor stopped/started/reversed with the spindle)
That being said a Static and motor or a rotary will set you back a lot more than a VFD, that is why I am curious how @m_c uses his VFD in place of a rotary as that could be your best option.
Kram
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The lower speed on your motor is half the power and half the speed. You will loose nothing by connecting it up for the higher speed and using the vfd to adjust speeds.
Often the lower speed is done to get a higher torque and similar overall power, but not the case with yours.
A new 230v capable 3ph motor is not going to be cheap even if the vfd was cheaper. You may find something used on ebay tho... Lots of motors that are too big for home use.
Often the lower speed is done to get a higher torque and similar overall power, but not the case with yours.
A new 230v capable 3ph motor is not going to be cheap even if the vfd was cheaper. You may find something used on ebay tho... Lots of motors that are too big for home use.
daleyd
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My thinking was that you would get a bigger choice of cheaper vfd to run a dual voltage motor rather than having to use a step up vfd to get 400v out. Probably go some way to offsetting the cost of a motor, plus you’d get a new inverter rated motor as well.
that was what I was thinking, ok you'll lose torque as the frequency / speed drops, but seeing as you only need half the the power/speed anyway, it would probably still be sufficient. Worth a try anyway.
daleyd
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Yes, it was the bottom of my list of options but an option none the less. Not forgetting you could almost certainly recoup some cost of a new motor by selling the old one.
Hood
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True
Just set it to ramp up to 50Hz. The issue is finding one that can handle having nothing attached to it (I still have the idler motor from my rotary attached for this reason), and isn't overly sensitive to shock loads.
The 'Digital Phase Converters' that the likes of Drives Direct supply, are just VFDs with a voltage doubler attached.
Do you really need the low speed?
How slow does the lathe go with the gears in high speed?
I'd be reasonably happy to run the original motor at 50% speed via a VFD, as that's what it's rated for on the low speed.
Typical electric motors theoretically are constant torque, so you don't lose torque as speed drops, you lose power as the motor isn't spinning as fast (power = torque * speed).
In electric motor terms, torque = current, and VFDs adjust voltage proportionally to speed, to ensure the motor windings stay within their rated current.
You can get the impression of losing torque, as you no longer have as much inertia to get through any tough cuts, but if you try taking the same consistent depth of cut, at the same feed per revolution, the motor will likely stall at the same depth/feed if you're relying on a standard VFD to control speed.
Where the basic theory falls down is at very low speeds, cogging can be an issue, and torque does drop off.
IIRC it's typically below 10Hz for a typical squirrel cage motor, but I would never advise running a motor that slow, as you can easily overheat the windings due to lack of cooling flow, unless you fit an additional cooling fan.
Most VFDs do have the ability to compensate for lack of torque at low speeds by boosting the current, but you are risking overheating the motor if it's done for a prolonged period.
not sure about constant torque throughoout the lower frequencies. Pretty sure it drops off, or is it the other way round? if I remember rightly there's a ratio between the frequency and the motor output torque, and you can calculate that loss fairly accurately. When I get an hour or two, will have to refresh my memory as it's a while since I've commissioned any. Did plenty of them on the last HVAC job, big AHU's and fan sets, those were all danfoss drives, but the theory was the same..
daleyd
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It’s as much to do with the type of load rather than the drive, most of my work has been with fans and pumps rather than machine tools so have been the variable torque loads. There’s usually a setting in the drive so you can optimise it for which type of load it will be connected to.
Most drives are capable of providing constant torque though, if the load requires it - usually up to the base frequency after which it will drop off.
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.
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.
