How do fewer volts / more amps effect brushes / heat?

[Peter VanDerWal]

> I've got a 48vdc motorcycle that's running a PM motor. I'm
> changing it from a 48vdc to a 36vdc system. (Because
> I'm switching battery chemistries.)
>
> Given that as the voltage goes down amperage goes up, how
> will that potentially effect:
>
> 1. Brush wear?
>
> 2. Operating temperature of controller and motor?

Assuming it's a fixed ratio, changing pack voltage will not effect the
motor.  The motor voltage at 10 mph remains the same regardless of pack
voltage.  Ditto 15mph, 20mph, etc.

The only effect lowering your pack voltage will have is a reduced top speed.

Unless you are planning on changing the drive ratio.  If you change the
drive ratio in order to maintain the same top speed, then you will end up
running higher current through the motor will run hotter and wear it out
somewhat sooner.
However, given the limited range of ev-cycles, I doubt the bike will
outlive the motor under either situation.  Assuming, of course, you stay
within the safe operational limits of the motor.



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[Peter VanDerWal]

> For example, my motor L91-4003 is rated for 120 volts, 12 kw continuous,
> 46.5 kw peak or, 130 amps continuous, 150 amps for 30 minutes, or 500 amps
> peak. Well I'm running over 128 volts. I need to make sure that my duty
> cycle is SHORTER than the spec sheet provided.
>
> Either that, or I have to run less than 130 amps at 128 volts in order to
> meet the "continuous" temperature point, less than 150 @ 128 volts for the
> 30 minute point, and I must peak at less than 500 amps at 128 volts or I
> could damage something.
>
> If I'm wrong, hopefully someone will speak up and correct me.

The rtings are motor loop current, not battery loop current. As long as
you don't exceed the motor loop current, battery voltage is not that
important.  Assuming of course, that you limit battery voltage to a level
that won't cause flash over.

So with your 128V pack, you can still run 130 amps of continuous motor
current (as long as the motor is spinning fast enough).
This will likely result in less than 130 amps from the battery.

You can't, however, judge your limits by battery current.

If you are climbing a long hill at 130 battery amps, your motor amps could
easily be over 500 amps (regardless of whether your pack is 120V or 128V).
Under this situation you can NOT run the motor continuously.  In fact you
probably shouldn't run it at this level for more than a couple minutes.

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[Bob Tregilus]

Hi -

I've got a 48vdc motorcycle that's running a PM motor. I'm
changing it from a 48vdc to a 36vdc system. (Because I'm
switching battery chemistries.)

Given that as the voltage goes down amperage goes up, how
will that potentially effect:

1. Brush wear?

2. Operating temperature of controller and motor?

Thanks!

Bob


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[Jeff Major]

--- On Thu, 7/3/08, Bob Tregilus <atc@...> wrote:

>> Hi -
>
> I've got a 48vdc motorcycle that's running a PM
> motor. I'm
> changing it from a 48vdc to a 36vdc system. (Because
> I'm
> switching battery chemistries.)
>
> Given that as the voltage goes down amperage goes up, how
> will that potentially effect:
>
> 1. Brush wear?
>
> 2. Operating temperature of controller and motor?
>
> Thanks!
>
> Bob

Hey Bob,

Many DC motors are actually rated as 36/48 volt motors, meaning they can run either 36 or 48 volts.  But they will have different HP ratings on 36 vs 48V.  Depending on ventilation, the motor may or may not have the same current ratings, 36 vs 48V.  Obviously, going from 48 to 36 volts will decrease the speed proportionally.  Are you O.K. going slower?

Or are you going to change the ratio to get the same speed?  In that case, you may well overload the motor, and possibly controller.  Brush wear will probably be the least of your problems, IMO.  If the motor doesn't overheat, the brush wear will likely be about the same as before.

Regards,

Jeff M


     

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[Richard Acuti]

I think the best way to look at it is "How many watts can my motor safely handle/produce?"

So, if your motor is rated for V Volts at I Amps, V x I= Watts.

So, if you lower the voltage, you need to find out how many amps the motor will see from the controller and determine if the new V x I = the same amount of watts as your previous configuration. If it does, then you're golden.

Now, let's say it turns out that you'll be exposing the motor to more power than it's rated for. Lee (I think) explained to me, that motor ratings are just points on a temperature graph. So if your motor is rated for 5 minutes at (I) Kw, 25 minutes at (Z) kw, and continuous duty at (B) Kw, then you just have to ensure that you don't exceed the duty cycle (running time) at your power level so that you don't melt down your motor.

For example, my motor L91-4003 is rated for 120 volts, 12 kw continuous, 46.5 kw peak or, 130 amps continuous, 150 amps for 30 minutes, or 500 amps peak. Well I'm running over 128 volts. I need to make sure that my duty cycle is SHORTER than the spec sheet provided.

Either that, or I have to run less than 130 amps at 128 volts in order to meet the "continuous" temperature point, less than 150 @ 128 volts for the 30 minute point, and I must peak at less than 500 amps at 128 volts or I could damage something.

If I'm wrong, hopefully someone will speak up and correct me.
_________________________________
Message: 9
Date: Thu, 03 Jul 2008 11:15:04 -0700
From: Bob Tregilus
Subject: [EVDL] How do fewer volts / more amps effect brushes / heat?
To: ev@...
Message-ID:
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Hi -

I've got a 48vdc motorcycle that's running a PM motor. I'm
changing it from a 48vdc to a 36vdc system. (Because I'm
switching battery chemistries.)

Given that as the voltage goes down amperage goes up, how
will that potentially effect:

1. Brush wear?

2. Operating temperature of controller and motor?

Thanks!

Bob


________________________________

Rich A.
Maryland
http://www.austinev.org/evalbum/371.html
http://patriotfuel.blogspot.com/




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[Bob Tregilus]

Oops, I forgot to add all other variables will remain equal with the exception that I'm re-gearing for speed (although, it may be a bit slower ~45mph instead of 50 and I never had any heat problems at 48vdc). This is an Etek motor on a converted 1976 Kawasaki KZ400. Follow this link for details:
http://www.evalbum.com/971

Bob

Bob Tregilus wrote:

Hi -

I've got a 48vdc motorcycle that's running a PM motor. I'm
changing it from a 48vdc to a 36vdc system. (Because I'm
switching battery chemistries.)

Given that as the voltage goes down amperage goes up, how
will that potentially effect:

1. Brush wear?

2. Operating temperature of controller and motor?

Thanks!

Bob


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[Jeff Major]

Hi Bob,

Oh yeah, the gear change.  I thought you might have that up your sleeve.

So for kicks and giggles, let's say you going to 37 volts from 50 volts.  That'd take you down to 37 mph from 50.  Not exactly, but in the range.  So to get it back up to 45 mph, you change the gear by 20 percent.  Doesn't sound like much.  But at current limit during accelerations, you get 20 percent (or is it 25) less wheel torque.  Meaning that it takes longer to accelerate.  More time at current limit.  More time in motor overload.  More heat in the motor.

Could be trouble.  I'd keep an eye on motor temperature.  A close eye.  From what I hear, and a few I've seen, those Eteks don't tolerate abuse well at all.  And another thing, at the slower motor rpm, it may not cool itself as well.

Regards,

Jeff M


--- On Thu, 7/3/08, Bob Tregilus <atc@...> wrote:

> Oops, I forgot to add all other variables will remain equal
> with the
> exception that I'm re-gearing for speed (although, it
> may be a bit slower
> ~45mph instead of 50 and I never had any heat problems at
> 48vdc). This is an
> Etek motor on a converted 1976 Kawasaki KZ400. Follow this
> link for details:
> http://www.evalbum.com/971
>
> Bob
>
>
> Bob Tregilus wrote:
> >
> > Hi -
> >
> > I've got a 48vdc motorcycle that's running a
> PM motor. I'm
> > changing it from a 48vdc to a 36vdc system. (Because
> I'm
> > switching battery chemistries.)
> >
> > Given that as the voltage goes down amperage goes up,
> how
> > will that potentially effect:
> >
> > 1. Brush wear?
> >
> > 2. Operating temperature of controller and motor?
> >
> > Thanks!
> >
> > Bob

.


     

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