I read this post a bit late it seems.
rami wroteThe cover is in Chinese (literally) so my question is: how can I know the required input voltage and amps using a multimeter?
First, even if the cover is in Chinese, there's bound to be some numbers with the sign "V" on it. If not, then there would be a name or code number which you can use to track the motor via Google Images. Also, each motor has the logo of the company on it. So use a website like Alibaba.com or made-in-china.com to track the company logo.
If you want to use a multimeter, then I suggest you use a step-up variable power supply with a good current supply (higher current rating will not hurt the motor, the motor will consume the current it needs; lower current however will lead to your motor staying stationary). Now most engineers prefer to connect a resistor in series with the motor so as not to damage the commutators or armature windings:
Because the speed of rotation controls the flow of current in the armature, special devices must be used for starting DC motors. When the armature is at rest, it has virtually no resistance, and if the normal working voltage is applied, a large current will flow, which may damage the commutator or the armature windings. The usual means of preventing such damage is the use of a starting resistance in series with the armature to lower the current until the motor begins to develop an adequate back voltage.
As such, I prefer you connect a resistor of small resistance like 5 or 10 ohms (depending on the physical size of your motor, since the smaller it is the smaller the size of the armature and hence the smaller its voltage rating).
Starting with 0.5V (this is the smallest voltage rating I've ever seen on a motor), step up the voltage by half a volt each time. Once the motor starts running, use a multimeter to measure the voltage across the resistor (it should be very low). The voltage difference between the power supply's voltage and that of the resistor would be your voltage rating for the motor.
To find the current rating, some calculations and measurements need to be made. When the motor's shaft isn't connected to anything, the motor is said to run at no-load. In this case, the counter or back EMF (electromagnetic force) produced by the motor is slightly less than that supplied by the power source. This enables a small current to flow in the circuit, sufficient to produce the required torque for the motor. This current is said to be the no-load current which is the nominal current rating. To find this current, place a multimeter in series with the power source, series resistance, and motor (of course the multimeter should be used as an ammeter).
At first, you will notice that the current is high. As the motor picks up speed, the current drops until the no-load current is reached and the motor runs as fast as it can.
