# Transistor instead of Contactor?



## gerd1022 (Jun 9, 2008)

This might be completely off base, but could you use a large power transistor (mosfet or IGBT) instead of a main contactor? It seems to me you would get more switches and it would be cheaper on ebay. 

This might be wrong, but they pretty much do the same thing right?


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## frodus (Apr 12, 2008)

one reason is voltage drop across the device.

Another is cost, same amount of current capability of the contactor you're replacing is likely going to cost just about the same (including driver circuitry). 

Another is, failure mode.... will it fail on? Contactors won't unless they are closed under load and the contacts weld. Most controllers out there (Not the Zilla) Have a "fail on" mode. Thats not good for something you want to use as a safety device.

You get TONS of switches with a contactor (If you use a precharge resisitor). If you don't use one, and close under load, you'll hurt the life cycle expectancy of the contactor.

Another thing I thing to consider.... is that alot of times a proven mechanical device can better suit the application than a digital device. Lots of power stations/Smelters/etc use relay logic still because its almost bullet proof unless a relay fails or a wire comes loose. If either happens, the failure mode is NOT on due to the spring pulling the relay back to "off". Same thing with a contactor.... the contacts are spring loaded. if you lose power, it SHOULD disconnect the load. Some have interrupt currents of 2000A.... This is invalid if your contacts are welded shut (should not happen if you use a precharge).

Just some ideas, not saying you can't do it, but they're things to consider.


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## gerd1022 (Jun 9, 2008)

all good reasons... thanks


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## ww321q (Mar 28, 2008)

gerd1022 said:


> This might be completely off base, but could you use a large power transistor (mosfet or IGBT) instead of a main contactor? It seems to me you would get more switches and it would be cheaper on ebay.
> 
> This might be wrong, but they pretty much do the same thing right?


I was wondering the same thing but not as a main contactor but for switching batteries from series to parallel J.W.


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## major (Apr 4, 2008)

gerd1022 said:


> This might be completely off base, but could you use a large power transistor (mosfet or IGBT) instead of a main contactor? It seems to me you would get more switches and it would be cheaper on ebay.
> 
> This might be wrong, but they pretty much do the same thing right?


Hi gerd,

Yeah, they're both used as switches. But by the time you properly implement the transistor switch, you'd basically have another motor controller. What's the sense of having two motor controllers in series? And that wouldn't cost less. Contactors are a pretty good value for what they do. Stick with that.

Regards,

major


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## ww321q (Mar 28, 2008)

Why do you need a complex controller to turn on an IGBT ? J.W


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## ga2500ev (Apr 20, 2008)

Someone must have been reading my mind with this thread. I have been thinking about each of these issues over the last few days.

I had some thoughts about building a contactor controller. Lee Hart posted a pretty reasonable 5 speed design on the EVDL list a while back. You can find a schematic here:

http://www.mail-archive.com/[email protected]/msg07669.html

```
_________________________________||__________||___
        |          _|_       __|__+   |   ||   |  |   ||   |
        |       D1 /_\  S4a   ___ B1  |   S1   |  |   S2   |
       _|_          |____||____|  -   |        |  |_      _|
    D5 /_\       +__|__  ||   _|_     |___|/___|    \/\/\/ |
        |       B2 ___        /_\ D2     +|\-         | R1 |_
        |        -  |__________|          C1         _|_    _| MOTOR
        |      S3         |                       S5 ___    _| FIELD
        |______||_________|                           |     _|
        |      ||         |                           |    |
    ____|_____            |                           |____|
  _|_       __|__+       _|_                               |
D3/_\  S4b   ___ B3      /_\ D6                           _|_
   |____||____|  -        |                      MOTOR   /   \
+__|__  ||   _|_          |                    ARMATURE |     |
B4 ___       /_\ D4       |                              \___/
   |__________|___________|________________________________|
```
With a series motor you wouldn't need the field weakening contactor S5. 

B1-B4 are 4 equal battery strings. For a 144V pack they would each be 36V while for a 120V pack it would be 30V each..

S1 is the on/off main contactor.
S2 is the low power acceleration bypass for the resistor
S3 is the voltage doubler doubling the pack to 60V/72V.
S4 is a DP(SD)T contactor that quads the voltage to 120V/144V.

4 contactors, 6 diodes, 1 resistor (along with precharging resistors) and one cap is all that's needed for a simple controller.

So I got to thinking could IGBTs be substituted for the contactors? The voltage drop (2-3V) and the heat generated would be a killer.

What about MOSFETs? Well you'd have to parallel them and instead of having one point of failure or runaway, you'd have 5.

Still wondering if it would be worth getting this setup to test out. I've pretty much convinced myself to homebrew a controller. I'm just trying to figure out if a contactor controller is more feasible than doing a PWM using a IGBT and a microcontroller.

ga2500ev


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## Dennis (Feb 25, 2008)

> Another is, failure mode.... will it fail on? Contactors won't unless they are closed under load and the contacts weld. Most controllers out there (Not the Zilla) Have a "fail on" mode. Thats not good for something you want to use as a safety device.
> 
> You get TONS of switches with a contactor (If you use a precharge resisitor). If you don't use one, and close under load, you'll hurt the life cycle expectancy of the contactor.
> 
> ...


Took the words right out! LOL I was going to say the same. You know even the Navy ships and submarines use contactors I believe. So you know if they use it then it's definitely proven technology.


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## Thalass (Dec 28, 2007)

You can get solid-state contactors which are, I believe, more or less a very large transistor operating as a switch (cutoff, then straight to saturation). But I'm pretty sure they're crazy expensive, when you can just get a regular contactor and buy a new one when the points start to wear.


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## gerd1022 (Jun 9, 2008)

I actually made a relay speed controller with a DC drill motor for an inverted pendulum controls project. It worked, but was pretty loud (lots of clicking), and im not sure if i'd want a similiar setup controlling my car. Plus, arent contactors lives greatly reduced if operated under load?

I am also building my own speed controller for my conversion. I should have a prototype in the next week or two.


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## ww321q (Mar 28, 2008)

The reason I wanted to use IGBTs as contactors (not for the main contactor) to be able to do battery switching . I need several and used IGBTs are cheaper . As a mater of fact I was getting ready to order my main contactor from Newark tonight . 
http://www.newark.com/jsp/content/printCatalog.jsp?display=single&cat=cat125&page=1562
J.W.


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## VDubber (Jun 2, 2008)

ga2500ev said:


> ...I had some thoughts about building a contactor controller. Lee Hart posted a pretty reasonable 5 speed design on the EVDL list a while back. You can find a schematic here:
> 
> http://www.mail-archive.com/[email protected]/msg07669.html
> 
> ...


Yup, I love his *Rectactor* circuit. Very creative and simplistic. Each contactor (or switch) only sees a sub-module voltage, not the full pack voltage. 

The downside is that you would have no current limiting. And without the field weakening, you would only have four power levels. Seems that four IGBT modules or arrays and there driving circuits would cost a bit more then one, as well. It also uses quite a bit more (3 times?) heavy gauge wire then PWM controller.

You could micro-control a Rectactor power stage, but once you go through the trouble of a micro and an IGBT module (and it's driving circuit) - why not reap the benefits of a PWM controller?


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## rctous (Jun 18, 2008)

ww321q said:


> The reason I wanted to use IGBTs as contactors (not for the main contactor) to be able to do battery switching . I need several and used IGBTs are cheaper . As a mater of fact I was getting ready to order my main contactor from Newark tonight .
> http://www.newark.com/jsp/content/printCatalog.jsp?display=single&cat=cat125&page=1562
> J.W.


which contactor exactly were yo looking at? These seem very cheap I have seen them for EV and they want like 150 bucks for them. Is the quality of these good enough to use for EV?


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## ga2500ev (Apr 20, 2008)

VDubber said:


> Yup, I love his *Rectactor* circuit. Very creative and simplistic. Each contactor (or switch) only sees a sub-module voltage, not the full pack voltage.


It actually got me to thinking would it be possible to use MOSFETs for switches instead of contactors. Digikey has some IXYS MOSFET modules that are 70V 200A for $32 each when you buy 10 of them. Use a pair for each switch (except for the main and resistor contactor) and you'd have a winner.



> The downside is that you would have no current limiting. And without the field weakening, you would only have four power levels.


The lack of current limiting is of some concern. The 4 power levels I don't think will be.

I'm really looking for simplicity and costs. Since it's going to be a homebrew, it needs to be simple and reliable.



> Seems that four IGBT modules or arrays and there driving circuits would cost a bit more then one, as well. It also uses quite a bit more (3 times?) heavy gauge wire then PWM controller.


IGBTs can't work well here because of the voltage drop. The MOSFETS only have an on resistance of 6 milliohms, so the voltage drop at 150A would be less than 1W. With the IGBTs a 2V voltage drop at the same 150A would be 300W! Ouch!



> You could micro-control a Rectactor power stage, but once you go through the trouble of a micro and an IGBT module (and it's driving circuit) - why not reap the benefits of a PWM controller?


No disagreement. In homebrewing you need something that's simple and reliable. I'm really looking for a cost effective failsafe controller while I work on a PWM controller.

ga2500ev


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## VDubber (Jun 2, 2008)

ga2500ev said:


> It actually got me to thinking would it be possible to use MOSFETs for switches instead of contactors. Digikey has some IXYS MOSFET modules that are 70V 200A for $32 each when you buy 10 of them. Use a pair for each switch (except for the main and resistor contactor) and you'd have a winner.


MOSFETs might work.

Some now have a current limiter built in - you go over current or temperature, and the FET disables the gate, turning itself off automagically. They re-enable the gate as soon as they cool down a little (0.1 - 2 secs)

It would be a jerky ride up a hill though. 

Most also will happily be switched with 10-15V. You could switch them with a 'rotary encoder' - a insulated disc with copper traces on it in a specific pattern and a set of contacts. The throttle cable would rotate it.

Or you could just grab a Alltrax for $300-$550. That might be an easier path and just as cheap.


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## ga2500ev (Apr 20, 2008)

I actually think that I have a workable, inexpensive design that may do the trick. The objective is to build the rectator that I listed in a previous post. It has two major components: power diodes and contactors. There are some surplus IXYS 1200V @ 450A diode modules here for $40:

http://theelectrostore.com/shopsite_sc/store/html/diode-meo450-12da-ixys-meo-450-12-da-fred.html

6 of these @ $40 each is $250 or so.

Instead of contactors use MOSFETs to switch the batteries. Fortunately because the Vds will be a quarter of the total pack voltage a lower voltage MOSFET can be used. Mouser is carrying a STMicro 100V @ 120A power MOSFET for $1.87 each when bought in packs of 10. 

http://www.mouser.com/Search/ProductDetail.aspx?qs=7b1TReoOb8dgUNxCmojx9Q%3d%3d

You can use these for the serial switches and distribute 5 for each switch for less than $10 each for 3 switches. That'll derate the voltage by 66% and the current by 33% at least with 400A current limiting.

Current limiting is a concern. Why can't a timed cutout of the main contactor (which needs to remain in the circuit) be done in an overcurrent condition?

It's starting to look feasible. The main contactor would be required in any circuit. So its costs doesn't really count against the controller costs. An IGBT could be used to bypass the resistor, though as I have stated throughout this discussion that the 2 volt drop would be painful. So really a second contator (or a higher voltage set of MOSFETs) would be needed for S2.


The driver would consists of microswitches that power the gates, It would need to be little more than a resistor/zener circuit switched in an out by the microswitches.

BTW I searched around for a 144V @ 400-500A Alltrax. Do they exist? Even the golf cart controllers for them ranged upwards of $600.

ga2500ev


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## ww321q (Mar 28, 2008)

Lee Hart also mentioned a controller using a 48 volt controller . Using parallel / series switching it could be used at 96 volts because the controller only sees 48 volts . The bad part is before your controller goes in to current limit you would need to disengage the series set . I was thinking at top speed , switch from my 72 volt controller to bypass and switch batteries from parallel to series 144 volt or 60v to 120v . If you start to go to fast let off and coast back down . And don't even use the controller at high speed . J.W.


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## VDubber (Jun 2, 2008)

ga2500ev said:


> You can use these for the serial switches and distribute 5 for each switch for less than $10 each for 3 switches. That'll derate the voltage by 66% and the current by 33% at least with 400A current limiting.
> 
> Current limiting is a concern. Why can't a timed cutout of the main contactor (which needs to remain in the circuit) be done in an overcurrent condition


That what I meant by "a bumpy ride up hills". The motor will gobble up current at lower voltages (the batts are in parallel, remember?). If you cut the current to limit it, things will get a little jerky. And make sure to check the 'make and break' ratings on the main contactor. Even the best ones have dramatically shortened lifespans when switching large currents. 

PWM controllers do this so fast that you do not notice it. They use the motor's inductance and resistance along with some big honkin' capacitors to smooth out the ON/OFF transitions.

You will also have to deal with flyback - when you switch off the motor current, it wants to keep flowing due to the inductance of the motor. The voltage in the motor circuit goes up quite a bit IIRC. Perhaps placing a flyback diode and a large cap parallel to the motor will help...


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## ww321q (Mar 28, 2008)

rctous said:


> which contactor exactly were yo looking at? These seem very cheap I have seen them for EV and they want like 150 bucks for them. Is the quality of these good enough to use for EV?


Some EV suppliers actually sell these . I found Allied has them even cheaper . Now lets see ? I think I read some place that the ones with 12volt coils mean 12volt max ! not 13 v or what ever . The EV200 has a economizer on the coil to reduce current draw . Thats the one that I think I'll get . 
http://www.alliedelec.com/catalog/catalogpages/200607/1125.pdf?Catalog=&PageNum=1125

....................... J.W.


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## ww321q (Mar 28, 2008)

ga2500ev said:


> Current limiting is a concern. Why can't a timed cutout of the main contactor (which needs to remain in the circuit) be done in an overcurrent condition?
> ga2500ev


 Or use an amp gauge and keep an eye on it ..J.W.


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## ga2500ev (Apr 20, 2008)

VDubber said:


> That what I meant by "a bumpy ride up hills". The motor will gobble up current at lower voltages (the batts are in parallel, remember?). If you cut the current to limit it, things will get a little jerky.


But can't you simply step on the pedal and switch the pack to a higher voltage? That should immediately cut the current in half since the voltage doubled right? 

I have never driven a contact controller. However it just seems to me that between 4 speeds and 5 gears that it should be possible to get a setting to get up a hill without kicking in the current overlimiter.



> And make sure to check the 'make and break' ratings on the main contactor. Even the best ones have dramatically shortened lifespans when switching large currents.


For the EV200 the number of make/break cycles @ 144V and 500A is well into the 10000s. And since it's the main contactor, it'll only rarely have to break at load.



> PWM controllers do this so fast that you do not notice it. They use the motor's inductance and resistance along with some big honkin' capacitors to smooth out the ON/OFF transitions.


I'm a fan of PWM controllers. But I've also written enough firmware to know that having a no brainer backup controller is always a good idea.



> You will also have to deal with flyback - when you switch off the motor current, it wants to keep flowing due to the inductance of the motor. The voltage in the motor circuit goes up quite a bit IIRC. Perhaps placing a flyback diode and a large cap parallel to the motor will help...


You'd need that for any type of controller. Standard issue equipment.

ga2500ev


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## ga2500ev (Apr 20, 2008)

ww321q said:


> Or use an amp gauge and keep an eye on it ..J.W.


I'd rather have a failsafe. Just in case.

ga2500ev


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## VDubber (Jun 2, 2008)

ga2500ev said:


> But can't you simply step on the pedal and switch the pack to a higher voltage? That should immediately cut the current in half since the voltage doubled right?


Kinda. Given a constant load, doubling the voltage would lower the current draw. But it would be extremely counter intuitive, and the motor will want to speed up... increasing your speed and drawing more current again. You could get used to it I suppose. Just never let any one else drive it 



> For the EV200 the number of make/break cycles @ 144V and 500A is well into the 10000s. And since it's the main contactor, it'll only rarely have to break at load.


http://relays.tycoelectronics.com/datasheets/ev200.pdf

Sorta. The rating is actually 4000 cycles for 500A @ 120VDC. Based on the graphs, it would be about 3500 cycles for 500A @ 144VDC. And I was referring to the current limit suggestion someone mentioned. This would be a full current break then make.

And make sure you read note 4: "The maximum make current is 650A to avoid contact welding"

Just don't turn it back on at too high of amps. It will then be permanently on.

This whole issue basically points out the single main economic benefit of PWM DC motor control, and why it's so popular: it protects your expensive battery investment quite well. Spending a few hundred more to make your $1600 in batteries last twice as long (or more) seems a good investment.

Now as a backup controller, it might be a good idea. I would be interested in seeing how well the MOSFETS work. Of course it wouldn't be a 'hot swap' backup. You would need to rewire the pack (at a minimum) to switch from your main PWM controller to a Rectactor.

I was just thinking of carrying a cheap 48V Allltrax as a backup to the 96V TSE I was planning on using. Just a quick series of unplugs and replugs of my anderson connectors would switch from one to the other (imagine being on the side of the road in the rain with a dead controller).


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## ww321q (Mar 28, 2008)

ga2500ev said:


> I'd rather have a failsafe. Just in case.
> 
> ga2500ev


 I think thinking is better . On a hot day like it was here today 106deg F you may want to take it easy on your system . On a day that it's only 70deg F you may be able to push it harder . But then again if you have unlimited funds to build an EV then !You know?it don't really mater . J.W.


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