Torque of DC Motor (1 Viewer)

[Arithela]

I don't get how the torque on a dc motor is maximum when the sides of the coil are perpendicular to the magnetic field and zero when parallel to magnetic field.

For exmaple, this question: Explain why the torque is maximum when the coil is at the position shown in the diagram (perpendicular to magnetic field).

 

[davidbarnes]

From HSC online - http://hsc.csu.edu.au/physics/core/motors/2578/phy931.html

The force on each long side produces a torque about the axis. As the forces are in opposite directions, and their lines of action are on opposite sides of the axis, they produce a torque in the same direction. Thus, their effect is to rotate the coil about its axis. The net torque is at its maximum when the plane of the coil is parallel to the field, as the perpendicular distance, d, to the line of action is maximum, and reduces to zero as the plane of the coil rotates to be perpendicular to the field, as the line of action of each force is then through the axis (d = zero). The direction of the torque alternates through a complete rotation of the coil: its direction is always to rotate the coil to be perpendicular to the field.

 

[YannY]

I don't get how the torque on a dc motor is maximum when the sides of the coil are perpendicular to the magnetic field and zero when parallel to magnetic field.

For exmaple, this question: Explain why the torque is maximum when the coil is at the position shown in the diagram (perpendicular to magnetic field).

I dont know what the other guy said lawls but this is how you should answer your question:

T=Fdsinx

now you should agree with me that perpendicular means 90 degrees.

so if we sub that 90degrees into sin x we'd get a maximum which is 1. lolz.

Cont to read if you are still confused.

Okay, look at the sin graph, you'd find it to be at the highest when x=pi/2 then it means when the angle is pi/2 i.e 90 degrees. you would get a maxium value if the angle is 90degrees.

Now from the torque view, if F and d remains the same then at perpendicular it is the highest.

 

[Steth0scope]

I think you're having trouble with knowing what theta actually is.

For Torque = Fdsin(theta), theta is defined as the angle between
(i) the direction of the force and
(i) the line joining the axle to the point of application of the force.

http://members.optusnet.com.au/markyz/Save0282.PDF
I've highlighted what theta is in the scan.

Don't confuse it with the theta for F=BILsin(theta) or Torque=BIAcos(theta) as they are defined differently.

I hope it helps. If you have any other questions just PM me.

EDIT: If you understand what we all said, you should get it ... hopefully. And read DavidBarnes reply. It does make perfect sense and is a correct explanation for the question you posed.

 

[Arithela]

Thanks everyone and especially the diagram, made it a lot more easier to understand

 

[shais]

http://members.optusnet.com.au/markyz/Save0282.PDF
.

so the first diagram is parallel to the maganetic field and the bottom one is prependicular to magnetic field ....

 

[Steth0scope]

the diagram isnt perpendicular / parallel ... the plane of the coil is though