DC motor

[uaefame]

Hello there,

If a DC motor is rated 48V and i only supply half that voltage what will happen?
If a DC motor under load condition needs 6A and i only supply half the current because i don't have big current supply what will happen?
If i want to control position of a DC motor what you recommend?

Thanks in advance
A favour is requested "I know my reputation is so low, i just want to have a clean start if you don't mind rasing my reputation so i can start over " Ignore the lost few lines its just a request.

 

[dknguyen]

It will run half as slow under no load. UNder load, it will run (48V no load RPM) - (24V no load RPM) slower than if it was running at 48V.

So if it runs 1000RPM no load at 48V, then it will run 500RPM no load at 24V. THe difference between no load speeds is 500RPM. So for the some fixed load where it runs 800RPM at 48V, it would run at 800RPM-500RPM = 300RPM.

You can use incremental encoders to measure absolute position if you have a reference point you can calibrate to (like on start-up). THen you keep track of how far away from that reference you have travelled.

Otherwise you can use absolute encoders to keep track of position. Magnetic encoders like the MAE3 or MA3 from US Digital seem to be the cheapest ones infinite rotation ones. Potentiometers are cheaper but only work for slow, less than 360 degree rotations.

EDIT: THis is all assuming that the applied load is LESS than the motor's stall torque (which varies with voltage. Half the voltage, half the stall torque.) Otherwise, like blueroom says, it won't spin.

 

[blueroomelectronics]

Do something helpful and your reputation will usually go up.
Who can say what a motor will do with half the voltage, might run, might not, might get hot, might do nothing at all. Most likely if it does spin it'll have no torque. Doesn't your school teach you anything?

 

[electronic spark]

You also want to have stall current control, so if it it stalls you don't want it to burn the brushes or motor

 

[uaefame]

I am trying to control a dc motor in one direction therefore i am using a pic and transistor.

Q> If i move the motor to a define position using high logic from pic, then i send low logic from pic, how can i keep the position of motor hanging in air or not moving during low logic pic?

I mean when i send low logic to a dc motor that is connect to a arm, the arm move downward with gravity how can i prevent it.

Sorry for my poor explaination

 

[erosennin]

Use a worm gear???
It will only allow rotation in one direction. Is this acceptable?

 

[Ubergeek63]

generally it will go half speed.

BEMF is directly proportional to speed and is the primary limitation to DC motor current.

When PWMed the motor inductance will limit current as well, but only on an instantaneous basis.

For example a 110V 3HP motor I work with often has a 2mH inductance and a 1 ohm armature with a 30A full load rating. At no load full speed it might take 5A@110V. If I were to put 220V on it, it would take 180uS for the current to increase 10A limited by the inductance.

To answer your last question, you can not prevent it from falling conveniently using that configuration. you need a bidirectional drive and an advanced feedback loop. You should look at stepper motors.

 

[audioguru]

If the motor needs 6A but your power supply has a max allowed current of only 3A then the power supply will be overloaded when the motor works hard.

 

[Hero999]

You'll also get half the torque too.

Just use more batteries to get 48V.

Or you could use a 50V transformer and bridge rectifier with no filter capacitor. Be careful as 50VAC is considerably more dangerous than 48VDC, the peak voltage is 70V and alternating current inteferes with your heart's rythem more than DC.

 

[Styx]

DC motors are (from a theory P.O.V.) the best (in practice the need for brushes and all make them limited in power applications)

Speed is proportional to the voltage applied
Torque is proportional to the current it draws

Power at the shaft is equal to the Torque at the shaft x angular velocity of the rotor


With those 3 facts


1) IF You were to drop the terminal voltage applied to the DCmachine the rotor speed would drop
2) IF you were to put a current-limit on the supply feeding the DCmachine AND it was drawing higher then that, then the speed of the rotor would drop (power in == power out and all that).

IF the current limit was too low and as such even at stall the DCmachine could not oppose the load then well... it would the act as a generator