天同師兄,有關dc.motor,請回答

2010-12-07 11:31 am

盡量希望由天同師兄解答!~
天同兄日前你回答過有關 back e.m.f. http://hk.knowledge.yahoo.com/question/question?qid=7010111301689 既問題,我對最後你的解釋有點不能理解
q:當coil既angular velocity等於V/BAN既時候, current會等於零.
我更加更加五明, 如果佢冇current又點保持住個angular velocity?
你既回答:In an ideal motor, the coil encounters no friction. When the angular speed of the coil w equals to V/BAN, the current reduces to zero (for reason, see answer above). There will not be any net force acting on the coil. But at this moment of time, the coil is rotating with an angular speed. By Newton's First Law, any body, including a body in rotation, will continue its state of rest or uniform motion (uniform rotation in this case). Hence, it is because of inertia that the coil continue rotating under a no net current and no net force condition.

因為我都係同發問果位用同一本書,我睇完你既ans都明你所解答,但如果照你咁講,
個motor冇current都可以郁,power=i V,咁iput power除左starting motor果陣之外咪唔洗再input power?

回答 (1)

天同

2010-12-07 5:27 pm

✔ 最佳答案

Q: 個motor冇current都可以郁,power=i V,咁iput power除左starting motor果陣之外咪唔洗再input power?

Yes, you are right. This is really the case for an ideal motor under a "no load" condition. At starting, the motor acquires energy from the power source (e.g. battery), it retains that energy in rotation, because there is no dissipative force (such as friction). At this stage, the power source no long supplies any additional energy for the rotation of the coil. The coil rotates merely because of inertia. This is just like you push an object on a frictionless surface. Once you give it a push, the object goes at a constant speed withou the need
of any additional force.

Of course, such situation cannot occur in reality. No motor is ideal. At least, there is some frictional force on the axis of the motor that would dissipate the rotational energy of the motor coil. Hence, for a practical motor under a "no load" condition, there is a small current flowing through the coil providing a torque to balance the frctional force. The power source therefore needs to supply energy to keep the coil in rotation at a constant speed.

When there is a load applied to the motor, the resistance force acting on the coil increases. Under such condition, the current through the coil has to increase further in order to provide a larger torque to do mechanical work. Consequently, the power source needs to supply a larger current, hence a higher power, to keep the motor running.

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