high power DC/DC converters - Page 1
Forum section: electronics / general

author: Simon
author_rank: Unknown
timestamp: February 10, 2011, 06:20:06 pm
content: With the advance of technology in small off road vehiles for specific things (like air craft maneuvering vehilcles) and the move to electric discussions have started at work on how to cope with the different voltages the vehicle manufacturers use which we would have to run air conditioning equipment from. The trickiest part being to run the compressor off a suitable motor and powering it from the varying voltages required by different vehicles.The power involved could range from 3-7 KW so a DC to DC converter of this power is no small task to find. Are their off the shelf converters that have fairly wide input voltages capable of handling up to 7 KW or will thei have to have something made externally "ad hoc" ?

author: Zero999
author_rank: Super Contributor
timestamp: February 10, 2011, 09:33:35 pm
content: A compressor would typically have an AC motor so an inverter is required. If the peak AC voltage is the same or a little lower (not much less than 75% otherwise efficiency will suffer) than the battery voltage, there's no need for a DC-DC converter, the motor can run from a MOSFET or IGBT bridge switched by low power electronics. If the voltage is different, a DC-DC converter is required before the inverter.An electric vehicle will normally use AC motors to move which require inverter(s). I imagine the compressor's speed would need to be varied independently of the vehicle's speed so would be run from a separate inverter.If all the parts were designed for one vehicle in mind, it would make sense to match the peak motor voltage to that of the batteries to maximise the efficiency.

author: Simon
author_rank: Unknown
timestamp: February 10, 2011, 09:40:31 pm
content: yes I guess that in actual fact a brushless motor may be used ? the main issue is that they would probably settle for a motor and compressor combination that would become their standard and then need to run it from different vehicle voltages to satisfy each customer. speed control would be a requirement and pwm was mentioned. if a large DC motor were used and the voltage drop not a lot then PWM power could solve the whole issue but i'm guessing that supplying short high voltage puses to a motor designed for less than half that voltage in the first place may lead to issues

author: sonicj
author_rank: Frequent Contributor
timestamp: February 11, 2011, 03:47:22 am
content: electric forklifts might make a good point of reference. they usually consist of a large traction motor for the drive system and smaller traction motor for powering a hydraulic pump which operates the forks/lift. both motors will likely be controlled via pwm.traction motors are nice for drive systems since they produce enormous amounts of torque from a standstill. the downside is that they use brushes which wear, wear on the coms and are often difficult to service. for something like a compressor, i would think a brushless setup would be ideal. very little maintenance, efficient, cheaper to manufacture, etc.voltage ratings of a motor are rarely if ever set in stone and will vary depending on load/mechanical limits.the prius might be another good example of how to deal with large dc/dc conversions. it uses a 200v main pack, 500v boost converter for the motor and 12v for normal electronics and instrumentation.-sj

author: Simon
author_rank: Unknown
timestamp: February 11, 2011, 06:47:23 am
content: Well electric fork lifts tend to work off 24-48V, The vehicles we may be asked to fit work off 200-400+ volts as they have to be very powerful. For example one customer makes vehicles used to shunt aeroplanes around in air ports. some vehicles may be 12 or 24 volts so a simple inverter might work there as they are considering mains motors but I'm sure they will soon find themselves in deeper water than they imagined when trying to make a choice.it did occur that being a motor we want to power regulation, ripple and exact output are not critical and a very basic SMPS could do the trick. Just used a pixed PWM signal comand the "switch" and feed it though the LC filter with flyback diode and you will have roughly the output you want. Multiple stages could easily be parralleled making life easier in breaking down the current carrying requirements, but that's just a wild idea of mine

author: scrat
author_rank: Frequent Contributor
timestamp: February 11, 2011, 08:36:03 am
content: I won't be really scared about high-to-low voltage conversion, since the ways could be a two-stage conversion (step-down + inverter) or, much simplier, an inverter designed for high voltage.The opposite (low-to-high) is much more a pain, if you need high efficiency, reliability and possibly small size.Running an inverter from a lower DC bus than what designed or a low-voltage motor from a high-voltage bus isn't really an issue. We usually do it in our lab, since we use the same inverter (designed for the mains) to feed the various motors, and the inverter is fed from a variac, so usually (if not needed) I use a fraction of the whole mains voltage, especially for the first tests on the control.The only concern is you must ensure the current limit is met (taking 1kW from 100V requires a DC current 10 times less than the same 1kW at 10V!), and the inverter "voltage low alarm" must be adjusted for each situation. The PWM high voltage pulses to a low-voltage motor can be a problem only if the motor insulation is not rated for high voltage, but this shouldn't be the case. Phase inductance acts as the "filter", and you typically don't need the current ripple to be so low.

author: Zero999
author_rank: Super Contributor
timestamp: February 11, 2011, 07:37:25 pm
content: Quote from: Simon on February 11, 2011, 06:47:23 amAt that voltage range I'd recommend buying standard AC motors and inverters as it will work out much cheaper than designing something and having it custom made.Inverters are used to control the speed of AC motors powered off the mains. A rectifier converts the mains to DC bus voltage (this is often slightly higher than the mains peak voltage) and an IGBT bridge converts the DC to a three phase PWM waveform. There usually is no need for any smoothing as the motor's inductance is normally high enough to smooth the current through it. There's normally an option to connect the inverter directly to a DC voltage, rather than to the mains and some even require a separate rectifier module. Often regenerative breaking is performed, not only to save power but to obviate the need for a huge breaking resistor.Here's a list of companies who do variable frequency drives which could directly be run from 200V to 600V DC to power standard three phase AC motors. I recommend talking to them before buying as you may need a custom wound motor or DC-DC, if the battery voltage differs widely to a standard peak AC voltage.http://www2.nord.com/cms/us/product_catalogue/frequency_inverters/frequency-inverters.jsphttp://www.lenze.co.uk/http://www.danfoss.com/BusinessAreas/DrivesSolutions/Products/Frequency+Convertershttp://www.sea.siemens.com/us/Products/Drives/Pages/drives-home.aspx

author: Simon
author_rank: Unknown
timestamp: February 12, 2011, 08:02:12 am
content: Well, I'm not sure how this will pan out. They said not to worry too much about it yet but well I'm curious just the same and one day they will want answers. I'm actually not sure why they would need speed control in a motor driving a compressor, at the moment it's not used, the compressor is run off the main engine or a dedicated one and it engaged by a clutch only when needed. In the case of dedicated electric drive they can just cut the power.I think they are hoping to find a one size fits all solution which is quite unlikely due to the variation of vehicles, some that do work off 24V but others that require up to 400V. For example road sweepers aren't hugely powerful so if you go and dump a 5KW load on it your going to kill the range of the vehicle. Something the customers may not have grasped yet, they don't under stand just how much power is needed to drive a compressor. Speed control may address the power issue if a reasonable weight unit that suits all can be found.I have suggested going for a decent voltage motor as a 5KW 24V motor is not going to be easy to setup.and no I'm not in a hurry to start designing something for them. Knowing very well how this sort of power can get out of hand in a poorly made product I'm very tentative as i know damn well they can mess up the best of things with their choice of cheap crap suppliers and subcontractors

author: scrat
author_rank: Frequent Contributor
timestamp: February 12, 2011, 10:11:04 am
content: I don't think you will find a "one fits all" solution off the shelf, for such a range of input voltage - output power.Even a custom solution of that kind could not be worth, since the voltage-current range (apparent power, usually the rated power) will be very large to accomodate all configurations, and apparent power is obviously one of the metrics for cost.Maybe one could go for a custom solution where most of the hardware is the same, while the power input and output hardware modules change between the various sizes.Maybe they will need variable speed for efficiency reasons, like it has become a must to have variable speed control on home air conditioning.Quote@Hero: DC bus voltage is a little bit lower than the peak mains voltage, since it comes from the mains rectified. Regenerative drives cost a little more, since they need a controlled rectifier (thyristors or IGBT/MOSFET).

author: Zero999
author_rank: Super Contributor
timestamp: February 12, 2011, 10:48:07 am
content: Quote from: Simon on February 12, 2011, 08:02:12 amIf the drive has regenerative breaking, the motor could be used as an alternator to recharge the battery when the engine is driving the compressor.QuoteAnother option is to use a standard inverter to power the AC motor off the battery but that doesn't give any speed control or regenerative breaking.QuoteWhat's the largest load connected to the battery? If it's the air conditioning unit then just standardise the motor/battery voltage and use a DC-DC converter to power the lower power equipment.Don't forget that the same AC motor can be run at different voltages depending on whether it's wired as star, for example a motor designed to run off 230V delta will work off 230V*sqrt(3) = 400V in star configuration, same for a US motor designed for 208V start or 120V delta.You could standardise at 26 12V lead acid batteries connected in series which would give 377V hot off the charger (14.5V per battery) and 273V when flat (10.5V per battery) whichwill give between 193V and 267V RMS enabling you to power a motor designed for 230V. At the lower voltage range, you'll loose some torque and at the higher end of the range the inverter should be able to prevent the motor voltage from exceeding its maximum rating by limiting the duty cycle.Quote from: scrat on February 12, 2011, 10:11:04 amYes, it's more efficient to vary the speed of a compressor than to turn it on and off because the mechanical loading is non-linear - if I remember rightly doubling the speed requires four times the power.QuoteNo because there's normally an active power factor correction unit in there which is a boost converter.QuoteI don't think so, a few extra lightweight semiconductors work out cheaper than a large oil cooled breaking resistor.

author: scrat
author_rank: Frequent Contributor
timestamp: February 12, 2011, 01:01:40 pm
content: Quote from: Hero999 on February 12, 2011, 10:48:07 amThe phase voltage peak out of a 3-phase inverter (remaining in the linear PWM range) is sqrt(3)/3=0.577 of the DC bus. So about 0.41*Vdc rms.Quote from: Hero999 on February 12, 2011, 10:48:07 amTypical industrial drives (supplied by the mains) don't have a PFC, so no boosting.About regeneration... Pushing back power on a DC bus doesn't cost so much (it really comes for free with the inverter, as I think of it now!:)). Drives powered from the mains are another story, since an additional inverter and control algorithm must be implemented. Maybe regeneration isn't needed for a compressor, since braking shouldn't be necessary.

author: Zero999
author_rank: Super Contributor
timestamp: February 12, 2011, 03:35:36 pm
content: Quote from: scrat on February 12, 2011, 01:01:40 pmYes, you're right, didn't account for that.QuoteI've not seen a sinlge drive without PFC, maybe in the US but here in Europe all inverters need to have PFC.QuoteYes, you're right about the cost of the inverter and it being free for DC. For AC it depends on the size of the motor and the load, for small motors it's cheaper to have a chunky resistor but for large motors driving a load with lots of inertia I'm pretty sure it's cheaper to have inverter large resistors are expensive, not to mention the cost of electricity.QuoteProbably not, other than the possibility of using the motor to charge the battery when the engine's running.

author: Simon
author_rank: Unknown
timestamp: February 12, 2011, 08:04:23 pm
content: I think we are missing a few points here. the aircon is to be independant and fully electric, so will use electris all the time and it's own motor all the time.Yes they will not find a one fits all solution but development for small volumes is expensive, maybe we will end up with a high and low power model in the end.

author: Zero999
author_rank: Super Contributor
timestamp: February 12, 2011, 08:40:24 pm
content: Quote from: Simon on February 12, 2011, 08:04:23 pmThen I don't see the problem, it doesn't have to interface with anything so it can run from whatever voltage you want. The only reason to want lower voltages is for safety but the voltages we're talking about are considered to be low voltage and is only as dangerous as the mains. The only problem is the batteries are permanently live but it's no worse than a UPS system which uses similar voltages. Just only allow trained people to work with batteries >48V. In my opinion any battery pack under 96V is fairly safe, shock wise, because if it's first disconnected half way you have two 48V packs. Either way observe the usual rules when working with hazardous voltages and you'll be fine.

author: Simon
author_rank: Unknown
timestamp: February 12, 2011, 08:43:47 pm
content: The issue is finding a motor that is suitable and powering that motor from different DC battery pack voltages