Electric motors convert electrical into mechanical energy. Achieved by the interaction of the magnetic fields set up in the stator and rotor windings.
There are a number of different types of electric motor:
- AC Induction Motors
- Brush Direct Current Motors
- Brushless Direct Current Motors
- Stepper Motors
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 electric motors the armature is the rotating part.
The design of the rotor is dependent on the type of motor.
Breakdown Torque
The maximum torque a motor will develop at rated voltage without a relatively abrupt drop or loss in speed.
Brushes
Sliding contacts, usually carbon, that make electrical connection to the armature (rotating part) of a motor or generator.
In the image the brushes can just be seen pushing through the plastic holders.
Brushes wear down over time until contact is lost with the commutator and the motor stops working. Brushes are often a replaceable item, replacing brushes in the Unimat 3 motor.
Brushes can also generate a lot of carbon dust and this can lead to electrical breakdowns in the windings.
Centrifugal Cutout Switch
A centrifugally operated automatic mechanism used in conjunction with split phase and other types of single-phase induction motors.
The switch disconnects the starting winding when the rotor has reached a predetermined speed and reconnects it when the motor speed falls below it. Without this the starting winding would be susceptible to rapid overheating and subsequent burnout.
Commutator
An electrical switch that periodically reverses the current direction in an electric motor or electrical generator.
The commutator consists of a ring of the copper wire endings from the surface of the armature. When the armature turns, the commutator is contacted by brushes which slide over it and transmit the generated electricity on to be used
Compensation Windings
Windings embedded in slots in pole pieces, connected in series with the armature, whose magnetic field opposes the armature field and cancels armature reaction.
Compound Wound
Machines that have a series field in addition to a shunt field.
Such machines have characteristics of both series- and shunt-wound machines.
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
Drum Type Armature
An efficient, popular type of armature designed so that the entire length of the winding is cutting the field at all times. Most wound armatures are of this type.
Efficiency
The efficiency of a motor is the ratio of electrical power input to mechanical power output.
Failure Mechanisms
Most motor failures stem from damaged bearings or stator windings.
Gamme Ring Armature
An inefficient type of armature winding in which many of the turns are shielded from the field by its own iron ring.
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
Interpoles
Small auxiliary poles, placed between main field poles, whose magnetic field opposes the armature field and cancels armature reaction. Interpoles accomplish the same thing as compensating windings.
Laminated Core
A core built up from thin sheets of metal insulated from each other. The insulation normally consists of a thin coat of varnish.
Localized currents are induced in an iron core by the alternating magnetic flux. These currents translate into losses which end up as heat and their minimization is an important factor in lamination design.
Lap Winding
An armature winding in which opposite ends of each coil are connected to adjoining segments of the commutator so that the windings overlap.
Linear Motor
A motor consisting of two parts, typically a moving coil and stationary magnet track.
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 electric motor components require lubricating. The bearings are the major motor component requiring lubrication.
Excess greasing can however damage the windings and internal switches, etc.
Nameplate Rating
The full-load continuous rating of a generator or other electrical equipment under specified conditions as designated by the manufacturer, and written on the nameplate.
Noise
There are numerous applications of electric motors and the type of noise produced by each may be very specific to the installation and type of motor. There can be a number of reasons why electric motors are perceived as being noisy.
Series Wound
Electric motor in which the armature and field windings are connected in series with each other.
Shunt Wound
Machines in which the armature and field windings are connected in parallel with each other.
Squirrel Cage Windings
A type of rotor winding in which heavy conductors are embedded in the rotor body. The conductors are shorted together at the ends by continuous rings. It is widely applied in ac induction motors. Physically, it appears as a rotating squirrel-cage, thus the name.
Stall Torque
The maximum torque without burning out the motor.
Stator
The stationary part of a rotating electrical machine.
Synchronous Speed
The speed at which the rotating field in an ac motor revolves. This speed is a function of the number of poles in the field and the frequency of the applied voltage.
Torque versus Speed
The no-load speed, stall torque, and the load point are used to establish the motor torque load-line.
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 load-point 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.
Windage Loss
The power absorbed by the fluid surrounding a rotating body.
Windings
Wires that are laid in coils, usually wrapped around a laminated soft iron magnetic core so as to form magnetic poles when energized with current.