Electric Motors

Motors convert electrical energy into mechanical energy by the interaction between the magnetic fields set up in the stator and rotor windings.

There are a number of different types of electric motor:

Design factors to consider when choosing an electric motor:

Application
Environment
Commutation method
Duty cycle
No-load speed
Weight
Stall torque
Lifetime
Load (operating) point
Torque ripple
Power source
Controllability
Envelope (volume)
Heat dissipation

Armature

In an electric motor the armature is the rotating part.

Design Factors:

Larger wire gauge - Lower stator winding loss
Longer rotor and stator - Lower core loss
Lower rotor bar resistance - Lower rotor loss
Lower speed - lower rotor windage loss
Smaller fan - Lower windage loss
Optimized air gap size - Lower stray load loss
Better steel with thinner laminations - Lower core loss
Optimum bearing seal/shield - Lower friction loss

Identification

In most instances, the following information will help identify a motor:

Frame designation (actual frame size in which the motor is built).
Horsepower, speed, design and enclosure.
Voltage, frequency and number of phases of power supply.
Class of insulation and time rating.
Application

Locked Rotor Current

Steady state current taken from the line with the rotor at standstill, at rated voltage and frequency. This is the current seen when starting the motor and load.

Locked Rotor Torque

The minimum torque that a motor will develop at rest for all angular positions of the rotor, with rated voltage applied at rated frequency.

Losses:

A motor converts electrical energy into a mechanical energy and in so doing, encounters losses. These losses are all the energy that is put into a motor and not transformed to usable power but are converted into heat causing the temperature of the windings and other motor parts to rise.

Friction and Windage: this is primarily bearing friction and aerodynamic drag on rotor (and can include fan loss where motor is force air cooled). Independent of load.
Core Loss: primarily hysteresis losses in rotor and stator iron caused by fluctuating magnetic field. This is independent of load.
Stray Load Loss: occurs in rotor and stator iron and is roughly proportional to current squared, is induced by leakage fluxes caused by load currents.
I²R Losses: heating losses in rotor and stator conductors caused by current flowing through the conductor resistance. As it is the square of current it is generally small at no load and large at high load.

Lubrication

In order to reduce wear and avoid overheating certain motor components require lubricating. The bearings are the major motor component requiring lubrication.

Excess greasing can however damage the windings and internal switches, etc.

Stator

The stationary part of a rotating electrical machine.

Torque versus Speed

Torque versus Speed curve.

The no-load speed, stall torque, and the load point are used to establish the motor torque loadline. Knowing the no load speed and available voltage, you can then establish an initial back EMF constant and the motor torque constant. The stall torque combined with the loadpoint torque helps establish motor size. The duty cycle, temperature, and expected heat sinking are used with the motor size to determine the temperature rise of the motor.