Brush DC Working Principles
The electromechanical properties of motors with an ironless rotor :
Graphic express
“speed-torque”
with ironless rotors can be described by
means of the following equations:
U 0 =MxR M +k E x ω
(4) characteristic:
By calculating the constant k E and k T from the
1. The power supply voltage U 0 is equal to dimensions of the motor, the number of turns
n
n 0
the sum of the voltage drop produced by the per winding, the number of windings, the
U
0
I
current I in the ohmic resistance R M of the diameter of the rotor and the magnetic field
rotor winding, and the voltage U i induced in in the air gap, we find for the direct-current
the rotor :
U 0 =IxR M +U i
micromotor with an ironless rotor:
(1) M = U i = k
I ω
(5)
0
M L
M
L
I
M L
M
To overcome the friction torque M f due to
R
M
I
U
0
Which means that k = k E = k T the friction of the brushes and bearings, the
motor consumes a no-load current I 0 . This
The identity k E = k T is also apparent from the gives
following energetic considerations: M f =kxI 0
U
I
The electric power P e = U 0 x I which is supplied
and:
to the motor must be equal to the sum of the
2. The voltage U induced in the rotor is to Joule’s law) P = I x R :
i
proportional to the angular velocity ω of the P = U x I = M x ω + I x R
mechanical power P m = M x ω produced by
the rotor and the dissipated power (according
2
v M
2
e 0 M
U 0 = I 0 x R M + k x ω 0 where
ω 0 = 2π x n 0
60
hence:
rotor :
U i =k E x ω
(2)
=P m +P v
k=U 0 -I 0 xR M
ω 0
(8)
Moreover, by multiplying equation (1) by I, we
It should be noted that the following also obtain a formula for the electric power Is it therefore perfectly possible to calculate
velocity ω express in radians per second and P = U x I = I x R + U x I
relationship  exists  between  the  angular   P e :
e 0 M i
the speed of rotation n express in revolutions
2
the motor constant k with the no-load speed
n 0 , the no-load current I 0 and the rotor
resistance R M .
per minute:
The equivalence of the two equations gives
ω = 2π n
60
Mx ω =U i xI
or U i = M and k E = k T = k
The starting-current I d is calculated as
follows:
ω
I
I d =U 0
3. The rotor torque M is proportional to the Quod erat demonstrandum.
rotor current I:
R M
M=k T xI
(3) Using the above relationships, we may write
It must be remembered that the R M depends
the fundamental equations (1) and (2) as
It may be mentioned here that the rotor torque follows:
to a great extent on the temperature; in other
words, the resistance of the rotor increases
M is equal to the sum of the load torque M
L
U 0 =IxR M +kx ω
(6)
with the heating of the motor due to the
supplied by the motor and the friction torque and :
dissipated power (Joule’s law):
M f of the motor :
M=M L +M f
U 0 =MxR M +kx ω
k
(7)
R M = R M0 (1 + γ x ?T)
Where γ is the temperature coefficient of
By substituting the fundamental equations (2)
and (3) into (1), we obtain the characteristics
of torque/angular velocity for the dc motor
copper ( γ = 0.004/°C).
As the copper mass of the coils is
comparatively small, it heats very quickly
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