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Old 03-20-2011, 02:17 PM   #4501 (permalink)
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The voltage decision isn't trivial, but there is an easy to understand breakpoint.

You can get really good MOSFETs rated up to about 200V. Above that they get expensive and inefficient, and IGBTs are a better solution.

A 200V MOSFET will support a pack voltage up to about 160V, perhaps 170V if you are really careful with the design and capacitors. And they are easily paralleled for high current or better efficiency.

Once you move to IGBTs, you might as well really increase the voltage. 600V ones are not much more expensive or less efficient than 300V ones. And since they don't parallel very well -- you can only use one or two devices instead of a dozen MOSFETs -- you'll want higher voltage/lower current.

Understand the specs of the devices you are getting. Cheap used IGBTs are almost always hard to drive and slow to switch.

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Old 03-20-2011, 08:28 PM   #4502 (permalink)
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Hey Paul,
Sheesh...wen u put me on2 this forum/thread who would guess that id b takin up half my life slowly trawlin thru it...but very very impressed with watchin ur well deserved,hard earned labours blossom in2 what it has become!&u said i dont giv up!!Think u&all the other people who hav contributed 2get this where its at deserve a MASSIVE medal!Anyhow-i asked u once if 2motors could run off this controller(the144v500A)&u told me yes!&seein as u&everyone else on here developed the Contrlr ill take that 1500% as a given.Yet when i posted a question on another forum B4 getting put onto this by u i was told definately NOT!..whilst gettin thrown aload of super"2 hi brow im smarter than u" kind of talk!I dnt get angry oft but this guy did it!GRrr!As i(think)i told u im buildin the EV body from scratch,weldin the chassis,the complete suspension-F&R,fabricatin all the parts needed 4 the runnin gear(excludin the axle),the bodywork wiv fibreglass!&u know,i just thought put a welder in this guys hand&watch him blow himself up!TW*T!Gimme' bricks n mortar&ill build u a wall,giv me wood&ill build u a door/window wiv nice dovetails,giv a welder&metal&ill build u a chassis...giv anythin electronic(engin-ear-y)&I WILL lick or chew it,then try 2ask what flavour it is!This is 2long now so sorry(i ramble alot-u wnt get used 2it)!CAN 2 motors run off this controller is my question?
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Old 03-20-2011, 08:39 PM   #4503 (permalink)
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Add on....

Sorry...also forgot to ask on here regards components!wen i had a look few hours back at the wiki most of the parts are from DigiKey...they do supply the UK as well,but i dont want 100 of something when 6 will do&there stock is slightly different thats if they carry it at all etc etc...got any ideas anyone!!!Im cannibalising some of the bits as well.The power capacitors i can get but there slightly higher than the ones stated...50mm high as opposed to 40mm.Think i need to sit and do some measurements maybe....is there complete schematics of the 144v500a?Thanx....
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Old 03-20-2011, 09:44 PM   #4504 (permalink)
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Well, mine has been running two motors ever since day 1. First in parallel, then in series (more torque, less speed).
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Old 03-20-2011, 09:51 PM   #4505 (permalink)
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Quote:
Originally Posted by harlequin2 View Post
two motors
I think many of us would love to hear more about Harlequin2's dual motor setup.

Could you tell us more about it? (In a new thread please! )
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Old 03-20-2011, 10:42 PM   #4506 (permalink)
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Quote:
Originally Posted by bennelson View Post
I think many of us would love to hear more about Harlequin2's dual motor setup.

Could you tell us more about it? (In a new thread please! )
Not sure if it warrants a new thread! Maybe I'll give it a go and stick some photos up as well, but basically the space for motors in the BMW was long and thin so a single, fat motor wouldn't fit whereas two long skinny ones, end to end, just went in nicely. And give about the same total power and torque.

There does seem to be a lot of ignorance about controllers here, so let me see if I can shed some light:
The controller is merely a high speed switch with a variable on-to-off ratio. At zero speed, the controller is on for a very short time, applying full battery voltage and the current limiter switches it off as soon as the current reaches the limit, 500A in this case. So you get a series of short pulses of high current. This means the average voltage across the motor is quite low and it slowly increases as the motor speed comes up. At full speed, the controller is on 100% and the motor back emf limits the current. But the point is, the full battery voltage is applied to the motor every time the controller switches on. You can sort of think of volts being proportional to power and amps being proportional to torque.
A series wound dc motor will absorb whatever you force through it until it melts or flys to bits!
The faster a motor turns, the higher the "back emf" it produces and so the more volts you have to apply to force current through it and make it turn faster. This means that your top speed is limited by how many volts you have available and at full speed the controller is switched on all the time, connecting the motor directly to the battery. In my case, two motors in series across a 144V battery "see" 72V each and this gives me a top speed of 110 kph. When I had them in parallel, each saw the full battery voltage and I got 130 kph.
Next, capacitors. You don't need much capacitance to absorb the inductive spikes generated when the controller switches off, what you do need is the lowest effective series resistance ESR that you can get because the current - which at the instant of turn off is the same as the motor current - flows through this ESR and hence generates a voltage. If you have 500A flowing, you only need an ohm to generate 500 volts!
Also, the ripple current capability of the caps is important. If you have only 10 caps, then each must carry 50A which is asking quite a lot. So, to choose capacitors, don't worry so much about the capacitance but look for low ESR and high ripple current capability. Of course, high C values generally give lower ESR and higher ripple current capabilty.

Hope this was not too boring and maybe helpful!
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Old 03-21-2011, 06:21 AM   #4507 (permalink)
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Quote:
Originally Posted by DJBecker View Post
The voltage decision isn't trivial, but there is an easy to understand breakpoint.

You can get really good MOSFETs rated up to about 200V. Above that they get expensive and inefficient, and IGBTs are a better solution.

A 200V MOSFET will support a pack voltage up to about 160V, perhaps 170V if you are really careful with the design and capacitors. And they are easily paralleled for high current or better efficiency.

Once you move to IGBTs, you might as well really increase the voltage. 600V ones are not much more expensive or less efficient than 300V ones. And since they don't parallel very well -- you can only use one or two devices instead of a dozen MOSFETs -- you'll want higher voltage/lower current.

Understand the specs of the devices you are getting. Cheap used IGBTs are almost always hard to drive and slow to switch.
The reason to go with 400 volts on the pack is because of regulations. The competition we're entering in limits pack voltage to 400 volts. The previous motor controller we used was "rated" to 250 volts. but it was almost certainly based on mosfets and not igbts....(i say almost cause it was hard to tell from the ashes that were left over after the caps caught fire and the battery bar arced to the bottom of the box.)

I've looked at specs for 1200 volt igbt's and 600 volt igbt's. the 600 volt ones have only about half the forward voltage drop from collector to emitter under full current as the 1200 volt rated ones. this would mean only half the power dissipated in the device. What is the reason behind igbt's not being able to be run in parallel as well as mosfets? driving current to turn them on? space available in the controller box? inconsistencies between devices?

*edit*

actually i just found this:
http://www.irf.com/technical-info/appnotes/an-990.pdf

i'll read through it and see if i have any questions

Last edited by isaac_alaska; 03-21-2011 at 06:40 AM..
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Old 03-21-2011, 07:51 AM   #4508 (permalink)
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I think IGBTs, like diodes, have the property that the hotter a device gets, the lower the voltage drop. So, if you have 2 in parallel, the hotter one hogs more and more of the current, and can lead to thermal runaway. If you can keep both reasonably close in temperature, it's not so bad to run them in parallel.
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Old 03-21-2011, 08:56 AM   #4509 (permalink)
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Quote:
Originally Posted by MPaulHolmes View Post
I think IGBTs, like diodes, have the property that the hotter a device gets, the lower the voltage drop. So, if you have 2 in parallel, the hotter one hogs more and more of the current, and can lead to thermal runaway. If you can keep both reasonably close in temperature, it's not so bad to run them in parallel.

Yeah i just caught that in that paper that i linked in my last post. it stated that keeping the igbt's thermally "tightly linked" was very important in ensuring that they shared the load somewhat equally (within about 20 percent of each other.) Higher total current actually led to better load sharing, as did higher temperature (as they heat up they share the load more qually,) higher switching frequency and lower duty cycle.

At least according to what i read in that paper, it looks like 3 of the devices could reliably be run in parallel, as long as they could be well thermally linked (share the same heat sink.)

would anyone on here happen to know of a better place to buy new igbt's than ebay? DJ made a good point that pre-used ones probably aren't going to be very well matched.
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Old 03-21-2011, 08:50 PM   #4510 (permalink)
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Really, you shouldn't even consider IGBTs unless you are planning on driving an induction motor. Or perhaps a brushless dc motor.
Series wound dc motors such as are commonly used in EV conversions (Netgain, Advanced DC, etc) don't need voltages higher than 150 - 170 to achieve full speed and 200V FETs are quite happy at those levels. You do need the higher voltages for induction motors though as the higher inductances mean higher back emf and hence more volts are needed to push the current through the windings.
FETs are generally easier to drive than IGBTs and they don't mind being paralleled as much either. They also have lower losses - usually - because their on resistance is less. Switching times are much the same, so switching losses are similar.

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