question concerning multicomp dc motors

Hi Dudu,

While I am not going to be able to answer your specific question I will begin the discussion which may get others involved and perhaps some useful information will come from it. You are dealing with a multiple variable problem. Voltage and current are on one side of the equation and since we can measure them quite accurately we should have a good idea of the energy going into the motor. For any given input of Voltage the Current draw of the motor will be a value such that the voltage times the current will equal the heat being dissipated in the motor plus the torque (caused by internal friction of the motor) times the angular velocity. If we could count on the torque, caused by internal friction, remaining constant over a range of angular velocities our analysis would be much easier. Unfortunately this will not be the case as the frictional torque will certainly increase as the angular velocity increases. Perhaps the frictional torque will not even increase in a linear manner but to simplify the problem I would begin by assuming that it does. I would also assume that the heat generated is constant and part of the friction factor. This will simplify our equation to Voltage times Current equals Torque times Angular Velocity plus a factor that will account for the increase in torques as the angular velocity increases. We can now take measurements at a low angular velocity. The angular velocity of the shaft is measured with a tachometer, the voltage is measured with a voltmeter and the current with an ammeter. We can now calculate the Torque for this low RPM. Let's call this specific Torque value "TL". Next we repeat the process for a higher angular velocity. Next, take the measurements as before and calculate the new and greater torque that is present at this higher angular velocity. Let's call it "TH". Note: It will help if the low angular velocity and the high angular velocity that you choose bracket the test range where you intend to take your experimental measurements. Now by assuming a linear relation between angular velocity of the motor and the resultant frictional torque we can come up with a coefficient that we can use in our future calculations. Over the range of the low to high angular velocity, the change in Torque divided by the change in Angular Velocity will equal this coefficient, let's call this "k". We will use this factor so that we can calculate the frictional torque of any angular velocity in the range of our experiment from the angular velocity in terms of the original torque "TL". Our formula becomes Torque( at specific angular velocity) = TL + k times (same specific angular velocity). Now you are ready to start adding an external load to the motor in the form of a dynamometer. We still have our power input to the motor (V*A) and we have the angular velocity as measured by our tachometer. From this we can calculate our total Net resultant torque. Now we use our formula for torque of friction at this angular velocity to calculate the frictional torque and subtract it from the total net resultant torque and the answer is the torque that is being added by the dynamometer. Compare this figure with the dynamometer to see how well it tracks with predictions in the Data Sheet.

John

Hi Dudu,

While I am not going to be able to answer your specific question I will begin the discussion which may get others involved and perhaps some useful information will come from it. You are dealing with a multiple variable problem. Voltage and current are on one side of the equation and since we can measure them quite accurately we should have a good idea of the energy going into the motor. For any given input of Voltage the Current draw of the motor will be a value such that the voltage times the current will equal the heat being dissipated in the motor plus the torque (caused by internal friction of the motor) times the angular velocity. If we could count on the torque, caused by internal friction, remaining constant over a range of angular velocities our analysis would be much easier. Unfortunately this will not be the case as the frictional torque will certainly increase as the angular velocity increases. Perhaps the frictional torque will not even increase in a linear manner but to simplify the problem I would begin by assuming that it does. I would also assume that the heat generated is constant and part of the friction factor. This will simplify our equation to Voltage times Current equals Torque times Angular Velocity plus a factor that will account for the increase in torques as the angular velocity increases. We can now take measurements at a low angular velocity. The angular velocity of the shaft is measured with a tachometer, the voltage is measured with a voltmeter and the current with an ammeter. We can now calculate the Torque for this low RPM. Let's call this specific Torque value "TL". Next we repeat the process for a higher angular velocity. Next, take the measurements as before and calculate the new and greater torque that is present at this higher angular velocity. Let's call it "TH". Note: It will help if the low angular velocity and the high angular velocity that you choose bracket the test range where you intend to take your experimental measurements. Now by assuming a linear relation between angular velocity of the motor and the resultant frictional torque we can come up with a coefficient that we can use in our future calculations. Over the range of the low to high angular velocity, the change in Torque divided by the change in Angular Velocity will equal this coefficient, let's call this "k". We will use this factor so that we can calculate the frictional torque of any angular velocity in the range of our experiment from the angular velocity in terms of the original torque "TL". Our formula becomes Torque( at specific angular velocity) = TL + k times (same specific angular velocity). Now you are ready to start adding an external load to the motor in the form of a dynamometer. We still have our power input to the motor (V*A) and we have the angular velocity as measured by our tachometer. From this we can calculate our total Net resultant torque. Now we use our formula for torque of friction at this angular velocity to calculate the frictional torque and subtract it from the total net resultant torque and the answer is the torque that is being added by the dynamometer. Compare this figure with the dynamometer to see how well it tracks with predictions in the Data Sheet.

John