For a particular car windscreen wiper mechanism, an electric motor is required t

Question

For a particular car windscreen wiper mechanism, an electric motor is required that can provide 5 N m of torque at about 35 rpm. A small permanent magnet d.c. motor is suggested as being suitable for this purpose. The manufacturer’s data is as follows:

Stall torque: 0.15 N m

Stall current: 15 A

No-load speed: 6000 rpm

No-load current: 0.5 A

Supply voltage: 12 V

Show that this motor has the power to do the job, and determine an approximate gear ratio with which it should be used in order to supply the necessary combination of torque and speed.

Explanation / Answer

Solution:-
Power required can be calculated as follows,
Power = Torque*Angular velocity
= 5*35*2*pi*
=18.31Watts (Motor required to drive the windshield.)
Now if look at the manufacturers design and data sheet
Given stall torque = 0.15N-m
Stall Current = 15A
No load current = 0.5A
No load Angular speed = 6000rpm
Now the relation between stalled torque and No load angular velocity is ,
motor = stall - stall */stall
5 = 0.15 – 0.15*/6000
4.85 = -0.15*/6000
So its angular velocity will be,
= 194000 rpm (Available)
While if we check the angular velocity of the motor required,
motor = ( stall - )*no load / stall
35 = (0.15 – )*6000/0.15
= 0.149N-m

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