400hz motors, whats the deal?

Discussion in 'General Electronics Chat' started by strantor, Nov 14, 2011.

  1. strantor

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    in this video, the guy basically says 400hz motors have 7-10X the power of 60hz motors for the same physical size. Is that true? if so, why has the modern world (except for aircraft) standardized on the 60hz (or 50hz) motor? What would be the down side(s) of using a 400hz motor as an EV motor? I've yet to hear of any EVs using a 400hz motor, which would seem idea if the guy in the video is right.
     
  2. thatoneguy

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    Lower frequencies need much larger cores for transformers, motors, and generators. Using 400Hz means more efficient magnetics due to lighter cores that do not saturate as easily.

    400Hz was born in the UK with early WW II Aircraft when the British had an alternator designed for Aircraft instead of the DC Generator which was capable of only 500W. Airborne Radar was desperately needed, but there wasn't much spare power to work with. So a couple of scientists went to a plant and stated they wanted at least 50% more power in the same size, with the same mountings, case, and shaft, as a DC Generator. They got 800W at 400Hz, which gave them 300 Watts extra to power their Radar with. The alternators were retrofitted into existing airplanes with rectifiers and regulators. This idea also moved to automobiles, which had also used DC Generators/dynamos for electric power. When Radio and other power hungry electric devices were available post war, the more efficient variable frequency alternator + rectifier and regulator were used in vehicles.

    The Power Grid in the US was designed a couple decades earlier, when all electric machines and devices were of a size you could evaluate without a magnifying glass. Rather than rebuild all the cabling, the US stuck with 60Hz, and Europe stuck with 50Hz, even after WWII, when more and more power plants and wiring were installed.

    To switch now would be a huge nightmare. Instead, some providers are looking to HVDC transmission. This gives many benefits over a new mains frequency, while keeping the current low by keeping voltage extremely high. With modern electronics at end user points, any frequency desired can be created at point of use. The hitch is 3 phase current, which is what industry runs on, though more efficient inverters are always being designed and produced.

    I suspect we will see HVDC become common in the next 20 years, maybe not in the US, but in Rapidly Developing countries that do not currently have any sort of stable grid would see it as the best choice to run a "universal voltage", with inverters as needed. Many of the newer Switch Mode power supplies will run on 120VDC as well as 120VAC. With HVDC, they could be simpler as well, since there wouldn't be power factor correction to worry about.
     
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  3. Teri

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    I have played with surplus 400 Hz motors and found that they run just fine on 60 Hz if you reduce the voltage on the motor to 60/400 (~1/7) of its rated voltage. A 115V 400 Hz motor will run on 17V at 60 Hz. The current draw increases rapidly and the motor gets hot if you try to go much above 17V. And of course, like in the video, the motor only runs at about 1/7 the speed. The torque is about the same and since power is speed times torque, the output power is about 1/7 of the 400 Hz rated power.

    There are down-sides to 400 Hz power distribution. Long 400 Hz power lines would act like antennas to some extent and radiate some of the power into space. And 400 Hz transformers although smaller than 60Hz units, require expensive special metal alloys and structures in their cores to keep losses to a minimum.

    AC motors would make good EV motors if practical controllers were possible. Since the AC motor speed is related to the frequency and the torque is related to the voltage, the controller would have to be capable of varying both frequency and voltage. To make the motor instant starting, some form of phase control would also be necessary. DC controllers can be built so much more cheaply that we are pretty much stuck with DC motors for EVs. That is not a bad thing -- DC motor technology has improved greatly over the years.

    Teri
     
  4. shortbus

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    @Teri - you should try telling that to Tesla Motors and GM(Volt)
    This, AC induction motors and controls is the place where EV's are headed.

    @Strantor - with your back round, Navy and oil field, the 400Hz should be right up your alley. Its used in both.
     
  5. crutschow

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    I believe you will find that the AC motors used in EVs operate with a maximum frequency much higher than 60Hz.
     
  6. Crispin

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    Possibly over simplistic then but why stop at 400Hz? Where is the drop off?
     
  7. Lundwall_Paul

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    It's all about filtering. Much easier and cheaper to filter 400 cycles vs. 60 cycles. You get cleaner power.
     
  8. crutschow

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    At higher frequencies you will get more losses due to eddy-currents and other parasitics. You will need higher grade magnetics and thinner laminations to control these loses which adds to the expense of the equipment. So it's a trade-off between cost and size.

    Some commercial devices do operate at higher frequencies. Automotive alternators can have 6-7 poles, for example, and generally run at 2-3 times the engine speed so their output frequency can be upwards of 1kHz (that's the alternator whine you sometimes hear in the car radio). That AC output, of course, is rectified to get DC.
     
  9. crutschow

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    Not really. The main reason for 400Hz instead of 60Hz is the significant reduction in size and weight of the magnetics, which is very important in aircraft applications.

    That fact that it's easier to filter is just a fortuitous side effect of the higher frequency. The higher price of larger capacitors required to filter 60Hz is not a significant cost factor.
     
  10. shortbus

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    As the frequency in a motor or for that matter a transformer goes higher the metal laminations become the problem. A 400Hz motor need to have much thinner and a different steel to keep the lamination and the motor from over heating. Due to 'eddy' currents. As the laminations are made thinner it gets harder for them to take the torque with out distorting and stripping away from the shaft on the rotor.

    Oops crutschow types faster than me:)
     
  11. Crispin

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    Thanks folks:)
     
  12. strantor

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    What is it about thinner laminations makes a 400hz motor cost more? Are the thinner laminations really that much more difficult to make? It seems to me that the 50% (or more?) reduction in building materials would make 400hz more economical than 60Hz (talking motors only, not power system infrastructure). As far as 400hz being in the navy and the oil field; If I ran across it in the Navy, I probably didn't know enough about electricity to care (or remember). The majority of the motors I encountered in the oil field were these GE 1500hp 60Hz motors. Somehow I made it through both without every learning about 400hz.
     
  13. crutschow

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    Nobody is arguing that 400Hz magnetics are not smaller (and possibly cheaper) than 50-60 Hz devices. But that's a moot point since no country will ever change the operating frequency of its power grid for numerous and obvious reasons.

    This Wikipeda article states that 400Hz can not be economically transmitted long distances (apparently due to greater parasitic loses).
     
  14. thatoneguy

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    Electric drills, and Permanent magnet DC motors have used the concept since their invention. The fewer poles the motor has, the faster it will spin for a given voltage. A DC commutator is sort of an "mechanical inverter" to continually switch the polarity of the poles. So any given voltage can produce the desired RPM when the correct number of poles are built.

    Where we see this coming in small phase now is Brushless DC motors, which use a 3 phase driver, and a BLDC controller is simply a 3 phase generator, for faster speeds, the frequency of the 3 phase power increases. So multi-frequency has been around for a good while.

    From Electric cars to trains, DC Motors are typically used.
     
  15. strantor

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    @ crutschow I don't think we're on the same page; I'm thinking big motors and (I think) you're thinking small houshold motors.
    As I said, I'm not talking about the power grid or transmitting power. I'm looking into different types of motors used in EVs & in industry, and different types of motors that DIY guys are putting into EV conversions. So far when I've searched for 400hz motors, they cost way more than 60hz motors, and I am wondering why the motors themselves cost more.

    Outside of the EV world, in the industrial world where I reside, VFDs with 60hz motors are the norm. They vary the frequency from 0 to 60hz (or higher up to 400hz, for most drives) so we no longer need to worry about what the line frequency is. I just installed a 15hp 60hz motor that weighed about 500lbs, and it is controlled by a VFD. Why are we still using these massive 60hz motors when 400hz motors would do the same job in a much smaller (and cheaper, I would think) package?
     
  16. shortbus

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    The last price I saw for a motor and inverter from 400Hz.com was $25,000 for the pair! It was a used test system that IIRR was 150hp rated.

    The laminations in both the stator and rotor are around0.003-0.005" thick compared to 0.025-0.030" for a 60Hz motor. Plus they need a special grade of steel and a special coating between the laminations. There isn't much need for them so the price stays high. Most of the 400Hz motors in airplanes are fairly low hp.
     
  17. Adjuster

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    One issue affecting bigger induction motors may be that a large number of pairs of poles will be needed to keep the speed to an acceptable value. Whereas the synchronous speed for a simple two-pole motor is 3000rpm at 50Hz, or 3600rpm at 60Hz, which are practical speeds, at 400hz it would be 24000rpm which is too fast for all but a minute motor.

    Obviously moderate sized 400Hz motors can be made with a few pairs of poles, but the rotational speed has to drop for bigger sizes to keep the stress on the rotor in check. That implies a large number of poles for a big industrial machine. Perhaps the extra complexity costs more?
     
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  18. shortbus

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    Another reason is that existing machines are made with the mounting for the 60/50Hz frame sizes. To switch to a smaller 400Hz motor you wold need adapters to the machine a new drive/inverter and maybe even power transmission to the existing machine. Then the cost of scrapping all the old spare parts and buying new spares. It would be a logistical nightmare.
     
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  19. John P

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  20. strantor

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    Lets I wanted to rewind a 60hz motor for 400hz; I could use the existing laminations as a template to cut new, thinner ones. I'm assuming I would reduce the number of turns by the ratio of 60hz/400hz (n-turns * 0.15) and increase the wire gauge to fill the slot? oh, and upgrade to high speed bearings? Would I need to do anything to the windings in the rotor?

    Is what I'm asking foolish? I know people sometimes have motors rewound for a different voltage, but never heard of one being rewound for a different frequency.
     
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