Using VFD to overspeed a motor
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    Default Using VFD to overspeed a motor

    If there is an application where a motor needs to run at a range of 30-120 hz, is it better for the motor to keep the correct hz/v ratio vs oversizing the motor so limiting the voltage above 60hz to 230v and reducing torque? If I run a 230 v motor with a 460 vfd and program the 230 @ 60hz, The motor will receive up to 460 when speeded up. If I run with a 230v vfd and run a larger motor for high end torque, will starving the motor voltage have any effect on the windings over the long term? Assume the motor is inverter rated, 230/460, and severe duty. I can go either route as I have a 240-480 transformer. Just want what the motor likes best before I choose a vfd. [/B] Thanks, Dave

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    The motor ALWAYS receives the full voltage of the VFD, it just gets that in narrower spikes if set to a lower voltage. So you always want to rate the motor for the VFD nominal voltage.

    You also need to always respect the V/Hz limit. You can always go lower, but should not go higher.

    Setting a 230/460V motor up for 230V allows overspeeding to 120Hz, at double power and double speed, without exceeding the V/Hz ratio.

    You set up 230V at 60 Hz, and the V/Hz will automatically be correct at 460 (460V/120Hz = 230V/60Hz)

    The question is, do you need the higher TORQUE at 120Hz, or is the higher POWER ok for you? If direct drive, or otherwise unable to do a 2:1 reduction, the torque will not change, but the speed and power do.

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    VDF s have a wide range of input voltage these days
    I heard it being done here about 12 years ago already That is 240 volt motor on 400 volt net and going constant torque up to 400 volts 86 Hz

    Peter

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    My lathe could live with reduced torque at 120hz with a 5 hp motor vs the 2.5-3hp it now has at the high speed. My fear is running a motor at that speed with a 240v limit and how it might affect the windings. If I go with a 460 vfd I can get by with a 3 hp motor, a little less torque down low than the 5 hp but still more than what I have currently on both the high and low. The 5 hp limited to 240 will have slightly less high end torque than now but still OK. I expect the vfd will need to be uprated either way, both for the 120 hz and for the additional braking needed to stop 35 lbs at 3000 rpm. Dave

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    Can't say which is better, but I ran the 240 V motor on a 440 VFD for several years . Lathe application.

    I found I just didn't need the speed range from the motor. The head strock has a pretty useful gearbox. But... some head speeds were quieter than others, so the vari-speed came in when I was feeling "sensitive". ;-)

    When the VFD died due to control board failure (A good sized Toshiba TOS-Vert), I replaced it with the 240V equal. Cleared out the transformer when I did, so WIN-WIN!

    I would not hesitate to do a similar set up again. It was fun running the rubberflex set up at 120 hz knowing I could peel of 15 HP worth of steel if I wanted to. In reality, speed and power seldom was needed at the same time ;-)

    Maybe a mill application would open up possibilities...

    eta

    There is little difference in construction between a 1725 and a 3450 motor, 240/440 V motors are the norm.

    So don't even consider "saving the motor".

    Now VFD frequency and carrier freq. harmonics etc. can be an issue. but a different issue.

    ETA 2

    If what you want is to run below about 20hertz, and still have power, GET A BIGGER MOTOR!

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    Asa long as you have a quality motor (good insulation system) its not an issue. Its a trick that is used fairly often to get more torque out of a motor when oversped.

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    There MAY not be any difference between a 230 and a 460V motor as far as insulation, just due to economies of scale and using the same for all. One does not generally KNOW that.

    In the engineering world, one must go by the published "specifications". Stuff "the factory guy said" is probably true, but does not help if the warranty claims go up.

    It is usually better to go with what is known for sure (specs), but YMMV, some settling may occur in shipment, etc. Whatever floats your watch.


    If you are powering a lathe, you might be able to change pulleys to get more power at the double motor speed, but since you want to overspeed, it is likely you want the extra RPM.

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    The additional torque at 120hz provided by the higher voltage is nice but not really necessary if I go with a larger larger motor. What I don't know is whether a 240 motor running at 120 hz fed with 240v could develop issues vs one run keeping the v/hz ratio of 4-1. Because I can do either, I'm willing to do whatever is more safe for the motor. If there are no motor issues from running at 120 with lower voltage, I'll take the easy route and avoid the transformer. Baldor ECP 3665 for the motor. Dave

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    Speeding the motor over the design V/F ratio will run out of HORSE POWER.

    i.e, You get full rated HP from the motor, but friction and other losses result in little useful output at highest frequency.

    Some motors won;t even run unloaded at F x 2 when supplied at base F voltage.

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    Having done quite a few lathe VFD installs, I think you might want to factor in costs and complexity vs. actual performance. I would not go with a smaller motor and run it to 480V as outlined, when there are so many other simpler options that will perform just as well. First I assume you are doing a retrofit to a stock lathe, so you have multiple gears ratios to put you in a higher power range if you were to need it. If you look at most manual factory installed VFD's they use a 2 speed gear head, may oversized the motor and will have a motor speed range operate the lathe spindle at something like 50-400 in low and 400-2000/2500 in high. So the motor is operating over a wide range. In manual mills they will over speed the motor often to 200 Hz, lathes usually a bit less, but no reason one could not go higher with the right type of motor.

    I can guarantee you that you will never run out of power in this scenario with most 13-16" swing manual lathes. You have a lot less complicated setup of stepping up the voltage with a transformer and then having to oversize the 400V VFD 2X assuming you are running it off of single phase. I would suggest you look at two options, one is to use a 3Hp Inverter/Vector motor (I have used the Baldor IDNM, Marathon BlackMax/BlueMax and also the Lincoln vector motor series and a few others), in a 3 or 5 Hp range these will easily go to 3X their base speed with full Hp. Torque does fall off, but you also gain a mechanical advantage when over speeding the motor. High speed, torque becomes less of an issue on a lathe. You also could go with a standard inverter rated 5Hp motor that are inexpensive and easily go to 1.5-2X the base speed with no issues. Now the major limitation of using the motors at the higher speeds will probably be the lathe motor pulley size, typically the motor pulleys are already pretty small and sometimes the manual brake is incorporated into the pulley. So measure up what you have and see if a the higher motor speed is even feasible. The maximum motor Hz on lathes that I have been able to do on my lathe installs has been 125Hz, the motor pulley size was only reduced about 25% from stock. So in top gear one could easily exceed the safe limit of the headstock/chuck, but the advantage was a much wider useful motor range of around 15-125 Hz in the lower speed ranges. All VFD lathe installs I have done, I never have been able to stall the motor and typically they maintain their RPM withing +/-2 RPM under all loads. If it is a heavy duty application then just put in a larger motor assuming the rest of the lathe can handle the power.

    Another factor is cost, there are surplus inverter/vector motors, just helped another gent pick up a 5Hp 240/480VAC Allen Bradley CM202 NOS vector motor for $250 shipped. Pair that with a decent VFD, doubt you will have any power issues. That being said, 3 Hp VFD's single phase input are inexpensive and plentiful, native 5 Hp single phase inputs VFD's are about twice the price, I have installed a number of Yaskawa drives and they work well. Otherwise you will need to use a 3 phase input VFD derated for the motor Hp/amperage and add a DC buss choke. I also do that quite routinely, not a big deal. Certainly a lot simpler than taking a smaller motor and mapping the V/Hz it to 480VAC in this application. One aspect I do not follow with mapping say a 3 Hp motor to 2X it base speed to 480V in this manner is your are dissipating the heat of a 6 Hp motor in a smaller frame and operating a motor outside it designed speed specs unless you are using something like a vector motor. Although doable, I have seen very little published as to a side by side comparison and any discussion of longevity/reliability.

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    Above base speed with motor terminal voltage equal to drive input voltage, torque drops linearly with increasing frequency while breakdown torque drops at the square of the increase in frequency.

    If a 230V drive connected to the 230V motor leads is running at 120Hz, you will have 1/2 the torque, but rated motor power. Your breakdown torque will be at 1/4 of its nameplate value. Generally NEMA B motors are rated at 300% breakdown torque and you typically never want to run a motor at more than 2/3rds your actual BDT.

    AT 120Hz, BDT is 75% of rated torque while actual torque is 50% of rated torque. In this case 50% / 75% = 2/3 . . . so no problem. Going above 120Hz means you now need to limit your torque to 2/3 of the decaying BDT.

    In answer to your question on whether operation at reduced V/Hz is hard on a motor, No it will be perfectly fine running at reduced flux density.

    To properly evaluate the optimum motor drive selection, you should seek to match motor inertia to your lathe drivetrain inertia at the highest spindle gear ratio (lowest numerical reduction ratio) and make your decision on drive / winding configuration based on that.

    As mentioned above, Baldor/ABB IDNM series motors are good candidates as are Marathon Black Max motors. These are invariably TENV designs with encoder mounting features built in.

    At last count we have deployed over 4000 IDNM and Black Max motors in ACVector configurations. Fewer than 10 percent have been set up as 480V drives connected to 230V leads. These are mostly 10HP Black Max motors set up on centerwind mandrel for high speed winding of towel and tissue paper. They see excursions to over 5000 rpm and back down to 800 rpm every 6 seconds, then decel to a stop to strip the mandrel. Hard accelerations and then aggressive decelerations 24 hours a day, 7 days a week. I have seen these motors last for 10 years plus with this duty winding EnMotion paper towel hitting 17 hp peak several times a minute and we are holding better than +/- 0.1% velocity control.

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    Quote Originally Posted by CalG View Post
    Speeding the motor over the design V/F ratio will run out of HORSE POWER.

    i.e, You get full rated HP from the motor, but friction and other losses result in little useful output at highest frequency.

    Some motors won;t even run unloaded at F x 2 when supplied at base F voltage.
    This is why feeding a 460V to a motor wired for 230V is being suggested. This maintains constant V/f up to 120Hz.

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    Quote Originally Posted by beckerkumm View Post
    My lathe could live with reduced torque at 120hz with a 5 hp motor vs the 2.5-3hp it now has at the high speed. My fear is running a motor at that speed with a 240v limit and how it might affect the windings. If I go with a 460 vfd I can get by with a 3 hp motor, a little less torque down low than the 5 hp but still more than what I have currently on both the high and low. The 5 hp limited to 240 will have slightly less high end torque than now but still OK. I expect the vfd will need to be uprated either way, both for the 120 hz and for the additional braking needed to stop 35 lbs at 3000 rpm. Dave
    I think you do not understand how a VFD works. It does just as it states. It changes only the AC frequency driving the motor. The motor is an inductive motor and its speed will follow the drive frequency change. The motor, however is wound for a given amount of impedance (AC resistance). The amount of impedance a coil has depends largely on the frequency it is being driven with, as it will have a fixed inductance. The formula is XL=2PsquarefL. So, as the frequency increases, the speed increases, the impedance increases, the current decreases along with the motor torque and motor efficiency. If the frequency is dropped lower, the speed decreases, the impedance decreases, the current increases, torque decreases and efficiency decreases. This is why a VFD driven motor is always sized larger, typically doubled. A VFD is NOT an efficient replacement for gears. The driving voltage never changes, only the frequency.

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    Quote Originally Posted by steve-l View Post
    I think you do not understand how a VFD works. It does just as it states. It changes only the AC frequency driving the motor. The motor is an inductive motor and its speed will follow the drive frequency change. The motor, however is wound for a given amount of impedance (AC resistance). The amount of impedance a coil has depends largely on the frequency it is being driven with, as it will have a fixed inductance. The formula is XL=2PsquarefL. So, as the frequency increases, the speed increases, the impedance increases, the current decreases along with the motor torque and motor efficiency. If the frequency is dropped lower, the speed decreases, the impedance decreases, the current increases, torque decreases and efficiency decreases. This is why a VFD driven motor is always sized larger, typically doubled. A VFD is NOT an efficient replacement for gears. The driving voltage never changes, only the frequency.
    Nope, VFDs definitely change output voltage. They cannot raise it above their input voltage (except in the special case of 120>240V models with a voltage doubler), but they can output anywhere between 0V and Vin.

    Consider putting 460V at 5Hz into a motor. It's going to saturate immediately, and you're going to be closer to DC braking than actually motoring.

    So in general (there are more advanced methods), they keep the voltage proportional to the frequency.

    460V/60Hz. 230V/30Hz. 115V/15Hz.

    This maintains a constant voltage:frequency ratio, which is why this mode is usually called V/f or V/Hz. This is also called 'constant torque' - the motor can maintain full rated output torque from 0 to full speed, though they require extra cooling at low speeds as the fan is spinning slower. Note that as the motor is only outputting full torque despite being at e.g. half speed, the output power is only half also. You're correct that unlike gearing, they can't increase the motor torque.

    Fans and centrifugal pumps ('variable torque') are often run in parabolic or square V/f, where the voltage is reduced below this straight line in the middle, as the load uses less torque at lower speed. Reducing the voltage below this linear relationship gives slightly improved motor efficiency as it reduces iron (hysteresis mainly I believe) losses at the expense of more current so higher copper losses.

    The impedance of an induction motor is, unlike a simple coil, not constant. The rotor and slip speed has a significant effect.

    More or less the sole reason VFDs are used in such huge numbers in industry and HVAC is that it is vastly more efficient to use a VFD than almost any other method of capacity control. Running a pump or fan at a lower speed for half flow uses much less energy than running the same load at full speed and 50% duty. Another approach used was to use dampers on the inlets to fans, restricting the flow. Less airflow reduces the motor load and therefore power consumption - but the motor is still spinning at full speed, now against an even higher static pressure. Using a VFD allows the fan to be slowed and the restrictors removed. Less pressure, less flow, more energy.

    Motor-VFD combinations can be upwards of 80% efficient across most of their speed range. Sure, you can do better with gears or belts - but not a huge amount, and only if you're using a single speed.

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    You will also find VFDs described as VVVF, particularly in traction applications. Variable Voltage, Variable Frequency.

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    I'm kinda confused! Am I reading that a 480Vac VFD wired I assume to 480VAC is connected to a motor wired 249VAC?

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    Thanks, I'm learning. I'm unclear as to the difference between the Baldor IDM, IDNM, and ECP series for my application- assuming I run a 5 hp motor at 230 rather than step up the vfd to 460 ( I had the transformer which is three phase so the vfd would not be receiving single phase input ) The IDM series has a longer footprint and I will have to measure my space to see if it would fit. I'll have to learn what the encoder referred to is all about.

    While using a 460v vfd added little complexity for me, selling the lathe down the road would probably entail me swapping to a 230 vfd anyway. I had decided to go with the larger motor as it would work with either scenario. The breakdown torque issue is also a good thing to know although I'd never use that much torque on the S and B 1024. When I need torque I have a 10 hp Monarch 60. Doesn't run fast though. Dave

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    When you overspeed at max voltage, you lose torque, but typically maintain power for a while (power, torque and speed are inter-related). You lose torque because the V/Hz is dropping, so the current is dropping.

    When you overspeed AND provide the voltage to maintain the V/Hz, you maintain torque, and double speed, so HP goes UP, without any downside as long as the motor insulation is rated for the higher voltage, and there is no issue with the RPM.

    You do not HAVE to do that. The only advantage of the voltage deal is that you get the speed with no downside, you could get the power with a larger motor if you did not need the speed.

    It depends on what the goal and need is.

    Speaking of which, maybe you could just clearly state what the goal is, and what the need is that is to be satisfied.

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    Quote Originally Posted by JST View Post
    When you overspeed at max voltage, you lose torque, but typically maintain power for a while (power, torque and speed are inter-related). You lose torque because the V/Hz is dropping, so the current is dropping.

    When you overspeed AND provide the voltage to maintain the V/Hz, you maintain torque, and double speed, so HP goes UP, without any downside as long as the motor insulation is rated for the higher voltage, and there is no issue with the RPM.

    You do not HAVE to do that. The only advantage of the voltage deal is that you get the speed with no downside, you could get the power with a larger motor if you did not need the speed.

    It depends on what the goal and need is.

    Speaking of which, maybe you could just clearly state what the goal is, and what the need is that is to be satisfied.
    I though I had but here goes. I want to replace my three speed motor with a single speed controlled by a vfd, ranging from a low of 1200 rpm to a high of 3600 as the previous motor had done. The original motor was wired constant torque, 1.5-2-2.5 hp ( approx )along the rpm range. I could use a 3 hp 1800 rpm 230v motor and plug it into my 480v three phase transformer with a 480v vfd programmed for 230v @ 60hz OR use a larger 5 hp motor, 1800 rpm with a 240v vfd. Both motors would need to run from 30-40 hz to 120hz to achieve that range. The first method gives me a little more high end torque but the second gives me more low end. My concern was if either method could damage the motor and what motor choices would be best for either method. If those with experience had a preference, that would be helpful. If neither can hurt the motor, it might come down to what good quality NOS motor I can find that fits the need for the least cost.

    I also questioned if either method allowed for better braking using the vfd given the approximately 35 lbs of the chuck and stock. Dave

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    Quote Originally Posted by beckerkumm View Post
    I though I had but here goes. I want to replace my three speed motor with a single speed controlled by a vfd, ranging from a low of 1200 rpm to a high of 3600 as the previous motor had done. The original motor was wired constant torque, 1.5-2-2.5 hp ( approx )along the rpm range. I could use a 3 hp 1800 rpm 230v motor and plug it into my 480v three phase transformer with a 480v vfd programmed for 230v @ 60hz OR use a larger 5 hp motor, 1800 rpm with a 240v vfd. Both motors would need to run from 30-40 hz to 120hz to achieve that range. The first method gives me a little more high end torque but the second gives me more low end. My concern was if either method could damage the motor and what motor choices would be best for either method. If those with experience had a preference, that would be helpful. If neither can hurt the motor, it might come down to what good quality NOS motor I can find that fits the need for the least cost.

    I also questioned if either method allowed for better braking using the vfd given the approximately 35 lbs of the chuck and stock. Dave
    OK, NOW we are at the meat of it...

    You are correct on your choices, but you left one out; use a 3HP 2 pole (3600RPM) motor to begin with. You seem hell bent on using a 4 pole 1800 RPM motor and artificially boosting the speed. Just start off with a 3600RPM motor and turn it DOWN with the VFD.


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