Hi im been wondering about this for some time.

It seems that the DC motor torque is proportional to the current drawn by the motor. However, i find that whenever i increased the voltage applied to the motor , the motor tends to rotate faster.

Therefore, isnt the voltage controlling the motor torque, rather than the current.
Or is the motor speed and motor torque not related to each other... ahhh im quite confuse here

 

I'm not exactly getting what your trying to say, but think of the motor a load with a resistance. If you know the current draw and the voltage applied to the motor then you can work out motor resistance. I'm not sure whether it would be a linear increase or log. Electromagnetics isn't one of my strong points.Sry. :x

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For a DC motor, the speed (RPM) is proportional to the applied voltage.

The current that the motor will take from the supply is proportional to the load.
A motor with no load will take very little current, a motor which is loaded and doing some work will take more current.

The load on the motor appears as a torque on the output shaft, the more load, the more torque.

So to try and summarise:

Set the motor speed by setting the supply voltage, as you increase the load (torque) on the motor, the current taken from the supply will increase. The motor will also slow down a bit as you increase the load, if you need a constant speed, turn up the supply voltage to compensate.

I hope this helps you to understand a bit better.

JimB

Edit, the relationship between speed and torque will depend on what the motor is driving. for example for a fan, I think the torque is proportional to the cube of the speed.

 

In general, the RPM of a DC motor will settle out when the rotating torque developed by the motor equals the resistance to the rotation that comes from a load on the shaft, friction losses from shaft bearings or brush contact and air friction from the rotating motor assembly. A DC motor without a load on the shaft would like to spin faster and faster however bearing friction, brush friction and air friction (might be called windage for the 'wind' that's created) increase exponentially with RPM.

If you increase the load on the DC motor it will slow down unless some steps are taken to increase the power delivered to the motor. I am not sure what happens to the current if you keep the voltage constant- maybe I'll check it on the bench tonight.

An AC motor, that is not a univeral type (anything larger than 1/4 hp is probably not universal) will rotate in step with the line frequency. With minimal load the current draw is low. As load is increased the RPM changes only a small amount due to slip but otherwise the current increases. A truly synchronous motor does not change RPM at all but these are very expensive motors.

I hope this helps.

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stevez

 

DC motors are also DC generators. As you apply a voltage a current will start to flow which is proportional to the resistance of the motor (this is the stall current). As the motor starts to spin it will generate a voltage that opposes the applied voltage - I.E. it tries to send current backwards (into the battery etc) as it is now a generator. At a particular speed the 2 voltages will almost cancel out, the difference being the power the motor is producing. This may be friction, windage, I^2R losses (Heat) or output torque (With no output torque, this is the no-load current). As the output torque increases, the difference between the voltage applied to the motor and the voltage generated by the generator increases and therefore the current increases. This increased current is the output torque (power) of the motor.


I just read that back and it makes no sense to me. Maybe someone else can put it in better words. It is midnight here! :roll:

Mike.

 

yes its right the back emf of the electronagnets is pusing thecurent back.

an motor can only use up 0.5 mA whith no load but whith an load it can draw up to 50mA (i have such an motor and it wery silent and eficent)
or i have one big that needs 3A whith no load and 6 A stoped

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Il give you shocking experience.

 

Rpm x torque = Power


Power= Volt x Ampere

You increse volts you decrease ampere result = power = same of the object.

If you increase rpm you where increasing Volts...

if you try to stop the engine it needs more amps soow voltage will drop like rpm does...

and amps increase result power = same..

Hope this is right.

TKS

 

prey good explanion of the power to me.

and here is an explanaion of why dosoe it draw more curent wen at lower rpm:
an motor is aculy on rotating electromagnet betwen two magnets.On evry thurn the polatery of the electromagnet is swaped twice.Wen the electromagnet thurns on it will build up an magnetic field around it and wen it swithed off that field colapses and produces electricety the oposite polatery on the contacts of the electromagnet (its an high voltage spike that can shock you).So the more times it rotates the more of these backvards spikes will be made and thse hold back the curent.So the motor spind faster and faster until the inputer electrical power is the same as the power geting lost as heat

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Il give you shocking experience.