servo / wheels

cizzi
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servo / wheels

Would the 1064 phidget be suitable for 2 DC motors with the following specs?

• Motor Voltage: 7.2 V
• Stall Current: 12 A
• Single Gear head Motor Weight: 180 g
• Wheel Dimensions: 40mm wide x 64 mm diameter

Robert

Re: servo / wheels

Absolutely. You would need a 7.2v power supply to feed into the 1064 though.

cizzi
Phidgeteer!
Posts: 51
Joined: Mon Apr 16, 2007 10:33 am
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Re: servo / wheels

however the manual states "A 9 to 15V DC Power Supply", the motor I need to use is rated for 7.2 volts, does this mean I cannot complete my project using this device?

Robert

Re: servo / wheels

right, I forgot about that. I thought it was 6-15v.

What you would need to do is run it at 9v. The motor will work just fine. You just need to watch the current draw in high load, high speed situations to make sure you're not pushing the motor past what it's specified to run at current wise.

cizzi
Phidgeteer!
Posts: 51
Joined: Mon Apr 16, 2007 10:33 am
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Re: servo / wheels

how will I know it won't damage the motor running it at 9v? I understand about the amperage but the 9v in itself instead of 7.2v won't burn it instantly?

Robert

Re: servo / wheels

No it wont. There are two main factors involved here:

First is the speed control of the 1064. It varies the speed of the motor by PWMing the voltage going to the motor. The result being that the voltage being fed to the motor is actually less. For example, at a speed of 50, and an input voltage of 9v, the motor would only see around 4.5v. It would only be after you get the motor past a speed of 80 that the voltage across the motor would actually be above 9v, so in most circumstances, you are well within the safe voltage range of the motor.

The 2nd big factor is the power dissipation of the motor itself. It's not the voltage that hurts the motor, it's the current being pushed through the coils, which heat them up. Your motor specifies a stall current of 12A. This means that when the motor is so loaded down that it's not moving, there will be 12 amps of current flowing through the coils (assuming 7.2v). This equates to 12*7.2 = 86.4w of power. This is the important number. This would be considered the maximum safe amount of power going through the motor.

If you were to run the motor at 9v instead of 7.2, by going a little bit of math, you will find that the mximum safe current through the motor is 9.6A (86.4/9). You'll need to monitor the current consumption of the motor (which is built into the 1064), and reduce the motor speed if you ever saw the current go past 9.6A. That is how you would safely run.

If you WERE running at full power at 9v, stalled, you would pass about 15A of current through the motor, which CAN cause it to overheat, and fail, but if you watch it, this will never happen. I have a motor here that's rated at 12v, but I've run it up to 30v before with light load.. That rated voltage is simply a number saying that "at this voltage, you will never be able to push enough current through the motor to cause it to fail"

cizzi
Phidgeteer!
Posts: 51
Joined: Mon Apr 16, 2007 10:33 am
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Re: servo / wheels

I noticed that on the product specs page at http://www.phidgets.com/products.php?product_id=1064 it says input voltage 6V-15V but in the manual it says 9V-15V. I want to know what the real values are before I make a purchase.

Robert

Re: servo / wheels

Thanks for noticing that. The product manual IS wrong. The 1064 operates on 6-15v, so it should work just fine for what you want.