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:
- Commutation method
- Duty cycle
- No-load speed
- Stall torque
- Load (operating) point
- Torque ripple
- Power source
- Envelope (volume)
- Heat dissipation
In an electric motor the armature is the rotating part.
- 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
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.
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.
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.
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.
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.
See also: AC Induction Motors, Armature, Breakdown Torque, Brush DC Motors, Brushes, Brushless DC Motors, Capacitor Start Motor, Centrifugal Cutout Switch, Commutator, Compensation Windings, Compound Wound Motors and Generators, Drum Type Armature, Electric Motor Efficiency, Electric Motor Failure, Electric Motor Noise, Electric Motor Windings, Forbes, Prof George, Gamme Ring Armature, Generator, Induction Motor, Interpoles, Laminated Core, Lap Winding, Linear Motor, Motor, Nameplate Rating, Series Wound Motor and Generator, Shunt Wound Motor and Generator, Squirrel Cage Windings, Stall Torque, Stator, Stepper Motors, Synchronous Motor, Synchronous Speed, Windage Loss.