The motors I am currently using do not have enough torque for what I need. I have some 250 watt, 24V motors that are already geared right. I was wondering how much current you think I could run thru the motor controller with my current setup and how much current I could run thru them if I attached a heat sink or something similar to the bottom of the board.
Nice picture, it is great to see Pololu products being used. I also see a Baby-O in the background there!
What is the current draw like on your motors? Do you know what the free-run and stall current draws are like?
Are you planning to leave your 10 A fuses in place?
The high-power motor driver 36v9 is rated at 9A continuous with an ambient temperature close to “room temperature.” We estimate, but have not tested, that the motor drivers will work at up to 12A with a big enough heat sink. I don’t think you should draw current as high as 15A but if you do, you should keep in mind that that is the limit of the terminal blocks that you have soldered into your setup.
It looks to me like your motor drivers may be going into some enclosure that may have an ambient temperature higher than room temperature. If this is the case it will be harder for the MOSFETs to dissipate heat and they will not be able to do 9A continuous. If the ambient temperature of your your enclosure is around room temperature and your motors are usually drawing much less than 10 A each, I think your setup should be fine without additional heat sinks. However, please keep in mind that current spikes also contribute to the temperature of the MOSFETs. If the motors draw a lot of current for a short time it can overheat the MOSFETs.
The datasheet says the MOSFETs cannot be operated above 150 degrees C. I wouldn’t want to even approach that limit as temperatures in electronics can fluctuate quite rapidly. When testing these motor drivers we used an infrared thermometer to get a rough idea of the temperature MOSFETs. What you might do for your application is slowly increase the current draw while measuring the temperature on the MOSFETs. If the temperature does not reach a steady equilibrium and instead is increasing rapidly, you are drawing too much current and should stop powering the motor driver. It may be tough, though, to get accurate readings of the temperature and you should be very careful if you try this. In general you want to have a large margin of safety; you don’t want to be melting valuable motor drivers!
Yep that is is a baby orangutan you see. I am using all but 2 of the ports on it. This is for a senior design project with 4 switches, 3 motors, 2 wheel speed sensors, 3 LED’s, a voltage sensor for the batteries, a 2 axis joystick, and maybe a 2 axis accelerometer.
The motors I am using right now are 140 watt, but I still managed to blow one of the 10 amp fuses today when I was holding one of the motors stopped. These are cheap motors from some unknown brand purchased by the previous team so I do not have many specs on them. The 250 watt motors have a max current of 14 amps printed on them. I do not know what the free run current is. I was looking for 12.5 amp fuses to use, but they don’t make blade fuses in that current so I will just leave them at 10 amps right now. I think I will just limit the PWM to 50% see how that works. I don’t have any way to really measure the temperature of the mosfets, but they should be kept at room temperature. There are 3 fans in the case right now, the 2 above the motor drivers are blowing air in and the third one is on the other end sucking the air out.
I may consider stepping up the PWM speed after we present everything and see if I can find some sort of temperature sensor and watch the motors. If I do this I will tell you what happens. Maybe I will get something like this to watch the current: sparkfun.com/commerce/produc … ts_id=9028 and see if I can place a small temperature sensor near the mosfets.
