1. ## dc motors, volts instead of Amps ???

ratings in motors is usually in watt
and w=V*I
so can we use higher voltage than rated with less amperage than rated to componste the diff, in the rated wattage ??

2. The more voltage causes to draw more amp. So the coils can be burned out if the rated voltage is exceed. You should use serial resistor.

3. well. sorry if i didnt made it clear .
i can control the motor amp and limit it to a certain number.
and increase the voltage afterwards
will this work ?

4. Size the series (serial) resistor so that it drops the voltage to the rated motor voltage at the rated current (usually locked rotor).

Thus, if your motor was rated at 10v and you apply 20v and the peak current of the motor is 10 amps, using E=IR,

10=10*R
or
R=1 ohm

Watts=I*I*R
or
Watts(resistor)=10*10*1

Resistor wattage= 100 watts

Be sure to heat sink it as this is a LOT of power to dissipate

• Originally Posted by max_imum2000
ratings in motors is usually in watt
and w=V*I
so can we use higher voltage than rated with less amperage than rated to componste the diff, in the rated wattage ??
If you are using a PWM amplifier then it is common to size the power supply voltage higher than the rated motor voltage, AMC reccomend 10% to 50% higher for their models.
The current can be limited by the drive. And high voltage of itself is generally not a problem only in situations where it results in a high current.
In a PWM amp where the voltage is high, the mean current is dependent on the pulse width.
Placing a resistor in series with any motor is generally not a good idea, due to the energy waste etc, if braking is required, it is much better to use at least dynamic braking.
Al.

• With a bit of creativity, you can probably take a current mode pulse width modulating IC and create a high speed switcher. Although these were designed for switching power supply controls, they can be used to limit motor current on a pulse by pulse basis. These include the SG3529 (LM3529????).

Another way is to use a switch mode brush DC motor drive IC. A real handy piece is the UC3637 that was/is supplied by TI/Unitrode is such an IC.

The application sheet for the IC shows a relatively easy to construct control and bidirectional power amplification circuit. The device uses mosfets (I>5 amps uses IRF9531's and UFN533's) and an IC package (Unitrode PIC900) for 12<I<5 amps. A bit of perusal of the International Rectifier catalog will surely find fets that will handle higher currents.

The 3637 offers current limiting, shutdown, undervoltage lockout and velocity feedback, 30Khz operation etc - it is essentially the brains of a servo driver. Simply add your own brush type DC motor, a tach feedback (info is listed on where to get one) and an analog input of +/-10 vdc and and the chip will drive the motor CW or CCW.

THe sense resistor in the motor ground leg of the H bridge driver shoud take care of your current control and the high freq fet switching should maintain efficiency.