# shunt motor question



## DavidDymaxion (Dec 1, 2008)

How big is your battery pack?

If you have a 144 V battery pack you'll need (this is a bad idea) to add about 4 ohms of resistance to get down to 30 Amps (4.7 Ohms with the field resistance). That resistor will have to dissipate I^2*R heat, or 30 A * 30 A * 4 = 3.6 kW! That could destroy your range, that could cut your range in half at low speeds. Additionally, 3.6 kW is a lot of heat to get rid of. Making it even worse, that is going to be an expensive resistor that can handle that kind of heat.

You'll either need a low voltage car to get that to work (72 V would be 1.5 kW (still ouch), and 36 Volts would be just 500 Watts of waste heat (more tolerable)), or you could use a small separate pack to power the field, or you could use a controller for the field.

Yes, your resistor needs to be sized for max field current -- you could conceivable be idling along in traffic for an extended period, it would need to be able to handle the heat.

Were you going to use a controller on the armature? Or are you trying to go old school?


billhac said:


> hello, I have a shunt motor in my car with only one field terminal and right now I have the field jumpered with a wire to the motor + . my question is can i install a resistor between the motor + and the field terminal to limit the current to the field. my field resistance is .7 ohms and i would like to add a 1k3 resistor to that to limit the current to 30 amps. now if that is possible then does my resistor need to be sized for the maximun field current, in other words does the resistor need to handle 30 amps of current.here is a picture of the actual motor, except the field wire is a bigger gauge on my car.


----------



## 9852 (Jan 17, 2010)

I use a curtis 1221 48-72v and i run my car at 60 v it works well for me, using a resistor for the field was just an idea to try to keep down the motor heat, but you answered my question, I all ready knew that but I just wanted to confirm it.


----------



## 9852 (Jan 17, 2010)

so if i used a seperate pack to power the field, could that pack be at a lower voltage?


----------



## jackbauer (Jan 12, 2008)

0.7r seems very low for a shunt wound motor. It seems more like a sepex to me. Are you sure its shunt wound? My motor has a shunt field resistance of 18r and draws 4 amps at 72v.


----------



## Guest (Jun 7, 2010)

It is a sepex but with one field terminal common with the armature.


----------



## 9852 (Jan 17, 2010)

correct, thanks gottdi, can i feed the field with a lower voltage, like 48v and run the armature at 72?


----------



## DavidDymaxion (Dec 1, 2008)

You can power the field with a lower voltage pack. Here are some caution points:


Now your field is running at fixed voltage all the time, so it's consuming power all the time it's on.
You have to be sure to switch the field on first, then the armature, and turn the field off last.
How are you going to weaken the field? You won't be able to get to max rpm, or get max start up torque, with a fixed field voltage. An old school way to do that is via a welder rheostat.
The field pack and armature pack will discharge at different rates, so they'll need to be charged separately.



billhac said:


> correct, thanks gottdi, can i feed the field with a lower voltage, like 48v and run the armature at 72?


----------



## 9852 (Jan 17, 2010)

iv heard of using a rheostat, hard to find one rated for the wattage needed.


----------

