Voltage input to BLDC inverters of tethered drone? - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: January 17, 2020, 07:26:39 pm
content: Hi,Regarding tethered drones such as the LIGH-T 4, which is rated at 1.2kW….https://elistair.com/ligh-t-tethered-station-for-drone/Do you know what voltage the BLDC motor inverters are likely to take?...presumably a lot more than a battery powered drone?Also, do you know what is the kind of power ranges that these tethered drones tends to be?presumably the voltage sent up the tether is high, but do you know how high?

author: ocset
author_rank: Super Contributor
timestamp: January 19, 2020, 11:57:47 am
content: HiWould you agree that the old addage that “Motor torque is proportional to current” is not entirely true?....rather it is only true for an individual, particular motor.In other words, as long as I can provide the power, it doesn’t really matter what voltage I have to drive a BLDC with. Its not the current which produces the Torque…it’s the magnetic field….and I can have a large magnetic field with a low current if I have enough motor windings…..so Torque being proportional to current is a misleading generalisation. Would you agree?I this way, would you say that its very easy to make a 200W BLDC motor…..with a voltage input to its inverter of either 1000V, …or 20V? (assuming I have drive transistors available which can handle this high voltage)

author: Siwastaja
author_rank: Super Contributor
timestamp: January 19, 2020, 12:47:56 pm
content: Quote from: treez on January 19, 2020, 11:57:47 amExactly, for a particular motor.For example, if you rewind the same motor core with double the number of winding turns, the torque (with the same current) naturally doubles, because each turn provides the same magnetization as before. But, inductance rises by 4x. From V(L) = L*di/dt, you can see that to get the same torque as before - i.e., half the current, you need double the voltage. Double the turns - half the current - double the voltage - same power - same torque; makes perfect sense.Note, with motors, you can't arbitrarily increase on-time, because the cycle time is defined by the rotational speed. So if the inductance is higher, you need higher input voltage to charge it to your desired current level in the time available.The motor designer can freely trade between voltage and current requirements, by varying the number of turns in windings: P = U*I holds. Torque = k1*I, and Speed = k2*U, but the exact constants k1, k2 only hold for a particular motor. You can freely choose.Total core area (in mm^2) and maximum magnetic flux density (e.g., some 1.8 Tesla for the laminated iron cores) defines the maximum torque available from the motor core. Trying to exceed it leads in core saturation, diminishing torque returns per invested current, and lower efficiency.Similarly powered (and torqued) motors exist with variation in voltage and current rating by order of magnitude. Naturally, a 1000V 1A, or a 1V 1000A motor won't make much sense, but 20V 50A or 200V 5A motors would be perfectly sensible and show similar performance in all parameters, if designed properly. To ease the physical winding work, and reduce skin effect losses, high-current motors use paralleled strands of magnet wire. Hence, the cross-section of the winding of these two examples could beidentical, and current through each wire would beidentical. The only difference would be, the 200V 5A motor might use 2 strands, and 20V 50A 20 strands in parallel. The winding of the 200V motor would belonger.Then why people make the generalization? It's because we often have to work whatever motor happens to be available for the job. We seldom have the luxury to be able to choose from different motors designed for different operating voltage. Often, it's just one specific motor which was chosen for availability and mechanical reasons, and electronic designers have to work with that.

author: ocset
author_rank: Super Contributor
timestamp: January 19, 2020, 02:41:46 pm
content: Thanks, we are looking for a offtheshelf 200w-500w BLDC motor which takes Vin of 400V or more but cannot find one.There are millions with voltages up to 48V, but almost none above this.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 19, 2020, 06:30:29 pm
content: Chances are the switches/mosfets in the controller have to withstand something like 2-2.5 times the operating voltage.So it does not make much sense to wind a motor for more than 400 V as this would likely need FETs for more than 1000 V which are not that common. At too high a voltage one would also need extra insulation on the wires - this is no included in the simple same total area calculation.Usually the nominal voltage is no a brick wall. It is just a values for the nominal RPMs. Especially for something flying one may opt to use a motor at higher than it's nominal RPM / power at the cost of reduced working hours.

author: ocset
author_rank: Super Contributor
timestamp: January 19, 2020, 08:59:13 pm
content: QuoteThanks, you mean due to when braking it might "generate" back to the inverter and cause overvoltage like that?

author: Siwastaja
author_rank: Super Contributor
timestamp: January 21, 2020, 11:20:42 am
content: Quote from: treez on January 19, 2020, 08:59:13 pmSimplified: If 400V supply makes the motor rotate at x rpm, freewheeling at x rpm won't generate more than 400V. So inertia won't cause overvoltage, it's spinningdownfrom the speed you created using the input supply.But, if there is any chance there's anexternal mechanical forcethat can turn the motorfasterthan your inverter can, then there are chances that higher than input voltages are generated. The generated voltage is rectified by the freewheeling or body diodes of your bridge, and feed the DC link.The stored energy (in the motor inductance) is able to make your bus voltage rise, but as any sensible control scheme will detect bus overvoltage and make the motor freewheel, we are only concerned aboutonecycle worth of energy in the motor inductance. By using enough DC bus capacitance, dV caused by discharging L completely into the bus is easily set just a few percent of the bus voltage. TVS clamping can be used, of course, as well, if bus capacitance must be minimized.That leaves us the LC parasitics in the switching loop, like in any switchmode converter. "2-2.5 times the operating voltage" sounds quite excessive to me. I tend to design for about 140-150% of the maximum DC bus voltage.I'm not surprised you can't find a half-kilowatt motor rated for more than 400V. They would need to use a super-thin wire for windings, and pay ultimate attention to the insulating materials. Eventually, the motor efficiency would be reduced because the insulating material would take so much space, reducing amount of copper and iron the motor can hold.Hence, I'd strongly suggest you build a buck converter to convert whatever high voltage you have, down to 400V for the motor. Also note, you can easily step down such low power, high output impedance using a single-phase non-synchronous buck converter, so only need one high voltage switch, and one high voltage diode. Whereas the inverter would require six high-voltage switches.

author: MagicSmoker
author_rank: Super Contributor
timestamp: January 21, 2020, 01:30:55 pm
content: Quote from: treez on January 19, 2020, 11:57:47 amYou're asking (apparently rhetorical) questions which are covered in every textbook on motors. Torque is proportional to the magnetic field intensity in the air gap between the rotor and stator (or armature and field, to use the older, yet more specific, terminology) multiplied by the moment arm of the rotor. Magnetic field intensity is the product of current and the number of turns (ie - amp*turns) and since the latter is always fixed in a given motor, torque is always proportional to current.*Different motors may very well produce different amounts of torque at the same current level but pointing that out is about as insightful as saying that different cars with the same engine power won't necessarily accelerate at the same rate or get the same fuel economy. In other words, not very insightful at all.Quote from: treez on January 19, 2020, 02:41:46 pmYou sure about that? BLDC is just another term for PMSM, or Permanent Magnet Synchronous Motor. Millions of industrial servo drives use such motors and are designed for the usual DC link voltages to be expected when powered by the mains. Of course, if you want a BLDC motor built like a typical drone motor, except with, say, 8x the turns of 1/8th the cross-sectional area running at 1/8th the current and 8x the voltage then, yes, you might very well need to get a semi-custom motor. Note that blindly increasing the motor voltage - particularly in small motors - can be counter-productive both because the proportion of a wire's area that is insulation goes up as wire diameter goes down (the thickness of the insulation coating is roughly the same regardless of the copper wire diameter) while a thicker insulation coating (ie - heavy or triple-build) will needed to achieve a higher voltage withstand rating. So, doubly penalized there.* - EDIT - up until the field structure saturates, anyway, but even then the legendary series field DC motor will continue producing torque proportional to current up until it self-destructs (just at a linear rate of increase, rather than a square law).EDIT 2 - I see Siwastaja already covered much the same in his reply... welp, that's what I get for not reading every post in the thread.

author: f4eru
author_rank: Super Contributor
timestamp: January 23, 2020, 07:56:04 pm
content: Simply rewind the motor.

author: temperance
author_rank: Super Contributor
timestamp: January 24, 2020, 12:32:35 am
content: The capacitance between the winding and the motor stator is about 2...10nF. This creates severe radiated noise problems unless the motor stator is coupled to 0V of your drive circuit in order to close the loop. Just like it's being done in an SMPS.-In a mains fed converter, your are limited to just a couple of nF between 0V of your drive circuit and the motor stator which is probably not enough to control the radiated emission. The motor measures 2...10nF while you want pico farards in order to control the radiated emission.Tip: look at the construction of BLDC motors for white goods. Those are meant to be driven with 100V...200V PWM signals. You will notice how the construction is very different.

author: ocset
author_rank: Super Contributor
timestamp: January 24, 2020, 09:37:33 pm
content: Thanks, there seems to be some concensus that 400V is not too high a voltage to feed to a BLDC motor’s inverter…but going much above that can be problematic in terms of winding insulation coating thickness.Supposing you were designing a 56Kg tethered drone, and you were told to do it with an 800V supply on the ground. What voltage would you then use for feeding to the BLDC motor’s inverters?The drone will consume 12.5KW…so 15A going up the cable.(i was told it was "800v", but it may have been 800VAC of course ...but i somehow doubt it...unless it was 3-phase 800vac)

author: f4eru
author_rank: Super Contributor
timestamp: January 26, 2020, 03:59:49 pm
content: 56kg drone with 800V onboard.dangerous stuff.What is the use for that drone ?

author: ocset
author_rank: Super Contributor
timestamp: February 02, 2020, 12:49:13 am
content: Thanks, i am not sure what the use will be.Though i wish there was a "digikey" for BLDC motors so we can see what voltages of motors are off-the-shelf available for an 12.5kw tethered drone which gets fed 800V up a cable.As kindly discussed here......the post PFC mains is 400V, so there must be a load of off-thje-shelf BLDC motors available for that voltage........?...but as for 400V+ then its going to get rare.I am sure that using 48V BLDC's would be a mistake...unless of course 13 x 48V, 10A motors were used to power 13 helicopter "drone-propellers"? (or rotators, being the more correct term)

author: mzzj
author_rank: Super Contributor
timestamp: February 11, 2020, 02:22:31 pm
content: Quote from: treez on February 02, 2020, 12:49:13 amYou are probably pretty much in "custom everything" category to get enough lightweight motors and drives.Closest to off the self solution would be a bunch of Vicor DC-DC converters, "hobby" drone motors and drives/bldc controllers. Largest hobby BLDC from hobbyking is 52V input and 10kW peak.