# power absorb by a dc motor

#### aragon1971

Joined Apr 7, 2008
45
hi friends
i have a dc motor that the rpm control by an mosfet type n
in gate of mosfet applied a PWM signal
i would to ask you if it si possible to find a analytically the power that absorb by
the motor in a function oF τ (time that the mosfet is ON) ?

THANK YOU
George

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#### nsaspook

Joined Aug 27, 2009
7,179

#### KeithWalker

Joined Jul 10, 2017
794
DC motors have quite different characteristics, which are documented and can be used for speed, load and power calculations but when you are using PWM, the power is applied for very short periods of time. As each pulse is applied, Initially, the inductance of the armature would be high impedance as it charged up. Then it would become very low because of the high current caused by the starting torque. As the motor ramps up to a steady speed, the back EMF increases, reducing the current drawn. These will be very different for different motors and will depend on things like the number of windings on the armature, The size and type of armature core, the efficiency of the motor, the load on the motor, the strength of the magnets in the stator, etc.
You would have to take all of those factors into account, along with the frequency of the PWM signal and the results would only be for a specific type of motor under specific circumstances. You could make a generalized prediction but it would not be very accurate. At 50% duty cycle the motor will act as though the voltage across it is approximately half the source voltage but this will vary with the frequency of the PWM signal because of the factors mentioned above.

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#### aragon1971

Joined Apr 7, 2008
45
DC motors have quite different characteristics, which are documented and can be used for speed, load and power calculations but when you are using PWM, the power is applied for very short periods of time. As each pulse is applied, Initially, the inductance of the armature would be high impedance as it charged up. Then it would become very low because of the high current caused by the starting torque. As the motor ramps up to a steady speed, the back EMF increases, reducing the current drawn. These will be very different for different motors and will depend on things like the number of windings on the armature, The size and type of armature core, the efficiency of the motor, the load on the motor, the strength of the magnets in the stator, etc.
You would have to take all of those factors into account, along with the frequency of the PWM signal and the results would only be for a specific type of motor under specific circumstances. You could make a generalized prediction but it would not be very accurate. At 50% duty cycle the motor will act as though the voltage across it is approximately half the source voltage but this will vary with the frequency of the PWM signal because of the factors mentioned above.
thank you i have installed a exprement with a real dc motor and i find expirementally thta the rpm of motor increase when the the the time DT where D=τ/Τ (DUTY CYCLE) .. i would like to fins a theoritical explaination of this

#### aragon1971

Joined Apr 7, 2008
45

Joined Feb 20, 2016
3,425
Did you add a reverse connected high speed diode across the motor? That is recommended, and will alter things quite a bit. You may find it interesting to run your tests with and without the diode.
Make the current rating of the diode equal to the stall current of the motor.

#### aragon1971

Joined Apr 7, 2008
45
Did you add a reverse connected high speed diode across the motor? That is recommended, and will alter things quite a bit. You may find it interesting to run your tests with and without the diode.
Make the current rating of the diode equal to the stall current of the motor.
yes i have installed a real expiremnet with a rela motor and a diode in the motor

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