Fundamental of Electrical Motors:
Electrical motor converts electrical energy into mechanical energy. Main parameters of electrical systems are voltage, current and resistance. While parameters of mechanical system are speed, torque (force) and friction. Electrical Power is product of voltage and current. That means if we consider voltage source is constant, power is proportional to current and thus current demand increases with power. Whereas mechanical power is product of speed and torque. That means for the same power output requirement, heavier load can be driven at low speed, or higher speed can be achieved at lighter load.
Fig. 1 Energy Conversion
If consider basic electric circuit having voltage source and constant resistance, then according to Ohm’s law (V = IR), current becomes directly proportional to the voltage. More voltage results more current. Like in mechanical system more torque results more speed (angular velocity or rotation) considering constant friction.
Fig. 2 Bearing of electric motor
Electric Motor is a device where shaft rotates when motor is supplied by voltage (AC or DC). It is assumed here that everyone knows about the fact that bearing attached with shaft allows mechanical structure to rotate freely (least friction) and so there is no discussion about bearing in further discussion. Thus, understanding working of electric motor means to understand why shaft rotates when motor is supplied by the voltage. Same thing is represented as block diagram view in Fig.3. As already discussed, when voltage produces current, and torque develops speed, electric motor can be further streamlined as shown in Fig.4. Here supplying voltage results in current, current results in development of torque and torque results in speed. Thus, understanding how an electric motor works, is to understand how electrical current develops mechanical torque. It’s required to understand few basic principles of electromagnetic and mechanical arrangements which explained in next section.
Fig. 3 Representation of electric motor as one block
Fig. 4 Representation of electric motor in electrical & mechanical parameters