What determines max rpm and amperage a motor can handle? - DIY Electric Car Forums

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#1
12-05-2008, 04:17 PM
 booksix Senior Member Join Date: Aug 2008 Location: San Diego Posts: 248
What determines max rpm and amperage a motor can handle?

Especially when dealing with a modified lift motor. How do you know how much amperage is safe and how high your can spin it?

#2
12-05-2008, 05:50 PM
 Georgia Tech Senior Member Join Date: Dec 2008 Posts: 406
Re: What determines max rpm and amperage a motor can handle?

OK good discussion. When dealing with RPM let’s just for a moment separate the Voltage issue from Mechanical restraints issue. First we all know that RPM is directly affected by the amount of voltage supplied to it. General rule, if you double the voltage for a given torque or amperage, if it’s not stalled, you double the RPM, this is general rule that we all can safely follow. Now given that, there is a Max Voltage that a DC motor can take and it has to do with a number of factors, commutator design, like the number of bars, Turns in the field poles or stator, How well the motor is insulated, although I can't see any motor being any less the class H insulation. Motor size can dispel a lot about the voltage rang of a motor generally the larger diameter is going to mean a armature with more slots and big diameter commutator thus higher voltage capable. These are all the major factors that play maximum voltage.
Maximum RPM is really restrained by the Mechanical properties of the armature. If you have notice some of these electric Hot Rod shops will trick out an Armature by winding bands at the commutator connection and at the turn end and even have a groove in the middle of the armature with bands as well. If an Armature spins too fast it will mechanically burst or fly apart and ruin the whole motor typically speaking. So these bands are preventive ways to make the Armature able to with stand higher RPM. Another issue to High RPM is brush and commutator ware. The higher the RPM the more friction is generated and shorter the life of you brushes and commutator. This is another reason why AC motors have an advantage among other things as well.

Torque is direct function of how much current is forced through the motor, more current makes more torque. The amount of torque per amp is depended on how the motor is designed in general the bigger the motor the bigger the torque per amp again "in general" but there are other factors as well, I can give you one example of where one motor was smaller than another but made bigger torque per amp. The limiting factor to the amount of current you can for through these things is strictly depending on two things alone wire cross sectional size, and Brush Commutator size, and those two ALONE.

hope this helps
#3
12-05-2008, 05:59 PM
 Georgia Tech Senior Member Join Date: Dec 2008 Posts: 406
Re: What determines max rpm and amperage a motor can handle?

Looking at a forklift motor here are some things to look at:

1) size and weight of the thing: Try to get something over 100lbs
2)Take a look at the brush rig and note the amounts of copper used and the holder.
Also look at the pig tails on the brushes, are they nice and think?
3) Are the brushes double wide? tripple wide? or single wide? double wide is preferable.
4) Is the shaft user friendly? or is some inverted spline shaft?
5) Note the voltage rang of the thing. Is it 48V? 72V is even better?!! or is 24V?

also note is it a drive motor? or pump motor? If its a Drive motor might have 4 terminals for reversing

hope this helps..

Last edited by Georgia Tech; 12-05-2008 at 06:02 PM.
#4
12-05-2008, 06:02 PM
 booksix Senior Member Join Date: Aug 2008 Location: San Diego Posts: 248
Re: What determines max rpm and amperage a motor can handle?

Very much so! Thank you.

Ok, so, when using a forklift motor, it is quite easy for builders to up voltage (because it doesn't change the amperage the motor pulls) and amperage usually won't be exceeded in the typical EV....?

I just ask because I found some GE motors (I've been looking for cost effective, matching motors; i need two in the 9-11" range). The rebuilder claims they are good up to 144v (although I might want to run 156) and 450 amps. The amps is the issue... I'd like to have the option of upping this in the future as I'm building a performance car. So, this would take rewinding the armature and/or the field coils?
#5
12-05-2008, 06:04 PM
 Georgia Tech Senior Member Join Date: Dec 2008 Posts: 406
Re: What determines max rpm and amperage a motor can handle?

Quote:
 Originally Posted by booksix Very much so! Thank you. Ok, so, when using a forklift motor, it is quite easy for builders to up voltage (because it doesn't change the amperage the motor pulls) and amperage usually won't be exceeded in the typical EV....? I just ask because I found some GE motors (I've been looking for cost effective, matching motors; i need two in the 9-11" range). The rebuilder claims they are good up to 144v (although I might want to run 156) and 450 amps. The amps is the issue... I'd like to have the option of upping this in the future as I'm building a performance car. So, this would take rewinding the armature and/or the field coils?
can I ask a SERIOUS question if I may.....which GE motor is it that you are looking at?
#6
12-05-2008, 06:12 PM
 booksix Senior Member Join Date: Aug 2008 Location: San Diego Posts: 248
Re: What determines max rpm and amperage a motor can handle?

#7
12-05-2008, 06:15 PM
 Georgia Tech Senior Member Join Date: Dec 2008 Posts: 406
Re: What determines max rpm and amperage a motor can handle?

Quote:
 Originally Posted by booksix http://cgi.ebay.com/ws/eBayISAPI.dll...tab%3DWatching

shoooo...for a moment I thought you was looking at the one I was trying to get.....

Keep in mind from the looks of it your going to have lots of Torque per amp, and less RPM per volt!! If you can safely run it up to about 240 volts you should make some good power at around 4500 to 5500 RPM. I would not trun this big motor any faster than that...

Last edited by Georgia Tech; 12-05-2008 at 06:18 PM.
#8
12-05-2008, 06:16 PM
 booksix Senior Member Join Date: Aug 2008 Location: San Diego Posts: 248
Re: What determines max rpm and amperage a motor can handle?

ahhahahaha!! That's funny! that is a serious note!
#9
12-07-2008, 09:07 PM
 Voltswagen Senior Member Join Date: Nov 2008 Location: Freehold, NJ Posts: 778
Re: What determines max rpm and amperage a motor can handle?

Georgia Tech
I need to ask a dumb question:
If the Motor that Booksix is considering is rated at 48v....how can you safely run it at 5 times the voltage? (240v)
My ES-31B is rated at 72 - 120v could I safely run this at 600v?
I'm not being argumentative.
I'm just wondering what determines a DC Motor voltage rating and why the rating can be exceeded.
#10
12-07-2008, 09:15 PM
 booksix Senior Member Join Date: Aug 2008 Location: San Diego Posts: 248
Re: What determines max rpm and amperage a motor can handle?

Most of the DC motors can be run at high voltages by simply advancing the brushes to prevent arcing. Now, 240v may be extreme and the gauge of the windings may not hold up but overvolting to the 144-156 range is quite common. As for your motor, it could quite possibly be overvolted... how high depends on the insides, that's all I know anyway...

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