VFD - General question about frequency and current

[dansawyer]

I am installing a VFD on a 2HP Bridgeport. The VFD is rated at 2.2KV and has a frequency control pot.
My question is how do VFDs adjust for changes in motor inductance due to changes in frequency? Specifically the impedance of a 60 HZ AC motor at 10 HZ is much lower than it is at 60HZ.
Do VFD adjust for this? Do they typically adjust voltage to account for the lower impedance at lower frequencies?
Conversly the impedance is higher at higher frequencies. I assume VFD do not increase voltage to account for this.
Is it safe to assume VFD control voltage to and the low end to limit current and allow current to fall off at the high end.
Is this reasonable?

 

[Reddinr]

Before VFDs motors had a fixed frequency rating. Almost anything magnetic (motors, transformers...) that has an iron core has a maximum voltage that can be applied to it at a given frequency before it saturates the magnetic core. Saturation is just what it sounds like. The core just can't take any more. For example, a 60 Hz. motor, rated for 240V has a saturation voltage at some point near but above 240V. But, if you increase the frequency you can increase the voltage before saturating the motor (and vice-versa). So if you want to run an 1800 RPM, 60 Hz. motor at 450 RPM, you can feed it about 15 Hz. BUT you have to lower the average AC voltage proportionally too, otherwise the motor's core will saturate. That is because its saturation voltage is about 1/4 its 60 Hz saturation voltage at that frequency. You can go higher RPM in the same way up to a point. The VFD has a curve built-in to adjust the V-F ratios. It is what the VF in VFD stands for. On some drives, you have some control over that curve. Mostly, you just let the drive deal with it.

As far as the series impedance of the motor coils, in general it would be higher at higher frequencies and lower at lower frequencies. Technically, the saturation is the main driver for the V-F operation although technically I guess it results in a shunt impedance swing as seen by the power source. It is not linear like in a resistor but drops off quickly above some maximum flux density of the core.

 

[JimDawson]

Back in the day VFDs used what is called V/Hz control. They simply reduced the voltage as the frequency was reduced. The result of this was poor low speed performance, the motor torque was reduced along with the speed.

Modern VFDs can normally be programmed for operation in V/Hz mode, but instead of just limiting the voltage, they use pulse width modulation to limit the current and allow the voltage to go a bit higher. Improves the low speed performance.

Normally today you would buy a VFD that has sensorless vector mode available, especially for a machine tool, this allows constant torque up to the base motor rpm (1725?) and constant HP above that speed. Again the current is controlled by PWM and the voltage is allowed to peak at the maximum value set in the parameters. But the commutation is magically performed by reading the rotor position relative to the windings, and the VFD computer figures how best to apply the current for best performance at the instant operating conditions.

All but the very least expensive VFDs have sensorless vector mode available. This is many times call SV mode, or Vector control, or any other fancy name the marketing department comes up with.

 

[graham-xrf]

Jim is right, and we are getting to it only partly so in the "VFD" naming remnant from the past.

The term VFD for "Variable Frequency Drive" is a hangover from the very early method of getting a measure of speed control by changing the frequency of the AC supply, especially with synchronous motors where the internal magnetics are supplied current to have the motor rotate at a rate locked to the AC line frequency. Most industrial AC motors were the induction type, which operated with a necessary slip, the motor rotating slower than the AC supply alternations as part of the way the rotor gets magnetized.

Providing a sort of AC, basically power switched on and off in sequence to the windings, with energy storage chokes, and frequency slowing down, does have the desired effect, over a partial range, and quite drastic drops in torque. There is a limit to how far you can "slow down" an AC motor in this way. Lowering the switched voltage, and letting it operate with huge amounts of slip also slows it down. None of this is good for efficiency.

Pulse widths and "carriers"
Motor speed control moved on to the pulse width modulation scheme Jim mentioned. This is where the coils of the motor are switched on and off by transistor electronics at much higher frequencies - way too high to have any relationship to complete shaft rotation. The switching on and off ranges from about 2kHz to 20KHz or more. This is known as the carrier frequency, which is a reference to radio frequency modulation for communication, where the term comes from. Higher power motors, and those with very long cables (30m or 50m) need the lower frequencies, but it comes with an very unpleasant buzzing squeal which only goes away when the carrier rate is set to higher than about 8kHz. So if the "noise" coming out of the motor drives you nuts, it may be the VFD menu configuration only needs to have the frequency set higher.

I have found that even with higher power servo motors (70kW) with cables as long as 20m, 12.5kHz was perfectly OK, and much nicer to live with.

How is the speed changed?
This is by altering the energy delivered during a high frequency cycle by changing the ON time proportion compared to the OFF time. It might change (say) from 10%ON 90%OFF to 50%ON, 50%OFF. This is PWM, Pulse Width Modulation, and you hear the term "duty cycle", which is another way to describe that fraction. If going at (say) 12500Hz, the whole period is 80 microseconds, of which some time is ON, and the rest is OFF

Feedback
It's not enough to just vary the energy in that way. To get a good speed control that will deliver a good high cutting torque, even when turning slowly, or to keep the speed constant, regardless of the load (within limits), needs a way to ramp up the duty cycle as needed. This is done with a feedback control loop. You set the speed demand, and the system compares that to the measured speed, and adjusts accordingly until the loop is satisfied. The ways the speed, acceleration, braking, etc. are measured can vary. Precision servo motors use sensor encoders, but for straight speed control, it is sufficient and accurate to get the information from the currents going to the coils, which have to be measured anyway. This is the "sensorless vector mode" that @JimDawson refers to. It's not magical, but I allow it can seem that way.

Inside the electronics, there are more feedback loops. A Torque loop, which controls at MHz, Position loops, and more. Current limits, tripouts, braking, etc. You can imagine what happens if all power is cut because a torque limit is reached. The system might let go of a few tons load, instead of at least keeping on delivering what it can. Even when you switch off, the braking can snap shafts unless the energy is delivered over the correct milliseconds into a resistor to bring the motor to a swift, yet non-violent stop.

The cost of this kind of kit has fallen dramatically over not many years. The term "VFD" is now generic, like "Hoovering". Not to be take literally. Yes, it still uses frequencies, but they are much higher, and we don't vary them much anymore. Instead, we vary the mark-space ratio of the PWM waveform, and we sense the speeds from ferrite toroid coils monitoring the currents.

 

[Reddinr]

I agree with much of Jim and Graham's further discussion of motor drives, which is probably a more accurate name to use than "VFD" to be sure. To answer the original question though has most to do with motor core saturation and its avoidance. The vector control / torque control is "on top of" the V-F operation in a manner of speaking. The V-F operation occurs over one or many cycles where-as torque control (voltage/current control) happens more cycle by cycle or actually within each cycle. In any case the frequency must be controlled as well as instantaneous voltage and current in order to control speed. For most drives you can just turn off the vector control and you are left with the still necessary V-F control bits. [edit] I'm not implying that you should turn it off!

 

[tcarrington]

Unless you buy a more expensive drive, the VFD will have a fixed V - Hz curve. Simply put the voltage is more or less a function of the frequency. That works for many applications, machine tools usually being one of them. If you have higher starting loads, you can boost the curve through a range of frequencies to supply a little more current, which is done by imposing more voltage. The less expensive drive may not have a V - Hz curve that is greatly adjustable. Frequently, pumps and conveyors will need a boost to start well.

So, the basic concept is the FREQUENCY output by the drive is the speed at which the shaft will want to rotate (less the slip as mentioned in previous posts). The VOLTAGE is directly proportional to the current that will flow through the windings. Since the voltage is reduced at the lower frequencies, the current will be kept near the rated running current of the drive. The maximum running current can be set on the drive. The control of the VOLTAGE is done through PWM (pulse width modulation), turning the voltage off and on at a rate usually 4 to 20 kHz range. You will be able to "hear" your drive on the AM radio unless you shield and suppress the effects.

You will be cautioned by many, including me to be careful with running your motor below 45 Hz or so as the cooling fan on the motor may not provide enough flow. Many motors, especially larger ones may have integral cooling motors with fans to counter that problem. For the small motors, up to around 5HP, a muffin fan or two might be enough for extended running under load.

I also strongly recommend to use a motor rated for "inverter duty" as the windings and magnetics are so designed to work well at lower frequencies maintaining torque. It increases the usable range of frequencies, typically 6Hz and up. They may also be better at above 60 Hz. It is hard to find a better source for torque at speed than a good inverter duty motor and capable VFD.

 

[mksj]

You can tune the VFD to the motor which maps out the motor properties over the operating range you specify, and it can make a significant performance difference. The motor constants are stored in memory, but I do no know what the specifics are. The better VFDs have elaborate mapping programming algorithms and in sensorless vector mode have a form of feedback to compensate dynamically as opposed to statically in V/Hz mode which affords better low speed performance. What this measure and how it operates, I have no idea. Newer VFD;s will also operate permanent magnet motors of various configurations and feedback options which afford tighter positional control and smaller package density.

As far as motor cooling and operating range, older motors you do want to run more conservative operating speeds/range, carrier frequency and overload. Newer 4P motors I typically run in the 20-90 Hz range, cooling in general is more of a problem with TEFC below 20Hz, and not an issue with TENV motors in this type of application. In more of a continuous operating mode, cooling can be more of an issue. I have used a number of inverter/vector motors as retrofits for machines, in mills they can go to 200 Hz, in lathes I usually take them to 120Hz if one can alter the drive pulleys. Inverter 4P motors usually have a CT ratio of 1:10 and readily operate to 2X their base speed, so in theory will run from 6-120 Hz, although there can be some performance drop off in the 100-120 Hz range, the vector motors with a CT of 1:1000 or higher will operate close to 0 Hz and the top end is typically in the 6000 RPM range. They are usually TENV or TEBC, I have used all three types of motors with no cooling issues. My installs/dealings with these motor/VFD combinations is that the speed control under different loads is usually withing 0.1% and with an inverter vector motor, it was usually +/-1 or 2 RPM of the spindle speed.

 

[graham-xrf]

Yes indeed. In the depths of sophisticated motor control, as the demands get further to the extremes of the motor running range, the software can alter the switching regime to suit, and this can involve changing the rate at which the field sequence rotates. Here is where the application of the word FREQUENCY has multiple use, and it's not just semantics. In servo motors, allowing the algorithm to alter the PWM energy by proportion control, at the same time as altering the voltage switched, and at the same time, altering the field rotation rate, allows the shaft can be made to rotate slow, all the way down to a complete stop, still delivering the torque, and then slowly go into reverse, and speed up. You don't need that for a lathe, but you do for half a ton of satellite dish that is changing from pointing straight up, to pointing near the horizon!

Of course such stuff means the simple fan for cooling is no longer OK. I have had scenario where we needed two motors, one on top the other, where one motor spins only to provide cooling for the other.

I was trying to find words to get across a simpler description, and going after the evolved name "VFD" as a catch-all description that has stuck, when "speed" in place of frequency would have been independent of the technology changes over time.

One use of the word "frequency" is to the rotation rate of the field, and loosely, the shaft.
Another different use is to the carrier frequency mentioned.
Yet another is to the frequency responses of internal feedback closed loops.
A feedback loop having some parameter within it that is also feedback loop controlled needs that the inner loop be faster, meaning higher loop frequency, than the outer loop by a couple of octaves, to avoid violent instability oscillations. I am OK with all that.

The cautions from @tcarrington are important, and should be heeded. Thanks also for @mksj 's good stuff about what you can do with the kind of motors we use to drive our machinery. The VFDs we purchase do not have the nicety of special wound 4-pole or 8-pole motors optimized for high frequency switching, and a useful minimum inductance. More likely it is like mine, meaning squirrel-cage AC induction motors that were supplied with the kit in 1947! The VFD has to be robust by design to tolerate the flexibility we demand.

I have two modern servo motors handy, acquired as surplus when a client ordered a bigger system. They are capable of all the servo party-tricks not appropriate for (say) a lathe speed control. I also have the quotation for the four connectors. It comes to $180.16 USD equivalent - just to get the cables connected!