20 | Electric Motor

The principle of an electric motor is based on the current carrying conductor which produces magnetic field around it. A current carrying conductor is placed perpendicular to the magnetic field so that it experiences a force.

History

The first electric motors were simple electrostatic devices described in experiments by Scottish monk Andrew Gordon and American experimenter Benjamin Franklin in the 1740s. The theoretical principle behind them, Coulomb's law, was discovered but not published, by Henry Cavendish in 1771. This law was discovered independently by Charles-Augustin de Coulomb in 1785, who published it so that it is now known with his name.

source: https://en.wikipedia.org/wiki/Electric_motor

Fleming's Hand Rule

Fleming Left Hand Rule

(for Motor)

When current flows through a conducting wire, and an external magnetic field is applied across that flow, the conducting wire experiences a force perpendicular both to that field and to the direction of the current flow.

Figure 1 Fleming left hand rule.

Fleming Right Hand Rule

(for Generator/Dynamo)

Fleming's right-hand rule shows the direction of induced current when a conductor attached to a circuit moves in a magnetic field.

Fleming's right-hand rule states that when thumb, forefinger and center finger of right hand are held mutually perpendicular to one another, thumb, forefinger and center finger represent directions of motion, field and current induced in the conductor, respectively.

Figure 2 Fleming right hand rule.

Electric Motor Component

A typical electric motor consisting of two part/section.

non-moving part known as stator winding/coil
moving part known as rotor winding/coil
centrifugal switch (for older motor)
rotor shaft with winding known as armature
motor nameplate (technical information)
motor terminal
capacitor (single phase motor only)

Electrically, a motor consists of two components which move relative to each other and which together form a magnetic circuit.

Stator winding - The stator is the stationary part of the motor's electromagnetic circuit surrounding the rotor, and usually consists of the field magnets, which are either electromagnets consisting of wire windings around a ferromagnetic iron core or permanent magnets. It creates a magnetic field which passes through the rotor armature, exerting force on the windings.
Windings are 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.
Rotor winding - In an electric motor, the moving part is the rotor, which turns the shaft to deliver the mechanical power. The rotor usually has conductors laid into it that carry currents, which the magnetic field of the stator exerts force on to turn the shaft. Alternatively, some rotors carry permanent magnets, and the stator holds the conductors. There must be an air gap between the stator and rotor so it can turn.
Field magnets - This part creates a magnetic field which passes through the armature. It is usually a set of electromagnets surrounding the rotor, consisting of wire windings on a ferromagnetic iron core which guides the magnetic field. Alternatively it can be one or more permanent magnets.

Armature - This is the part through which the electric current flows which develops the force. Like field coils, it consists of wire windings on a ferromagnetic core. When electric current passes through the wire the magnetic field from the field magnet exerts a force on it, called the Lorentz force, turning the rotor.
Commutator - This can be found on rotor shaft and it is also known a rotary electrical switch in some motors that supplies current to the rotor. It consists of a cylinder composed of multiple metal contact segments on the rotating armature of the machine. Two or more electrical contacts called "brushes" made of a soft conductive material like carbon press against the commutator, making sliding contact with successive segments of the commutator as it rotates, supplying the current to the rotor. The windings on the rotor are connected to the commutator segments.

Figure above is stator and rotor section

Nameplate (Motor Information)

Brand and model
capacity in Power,P or horsepower, HP
number of pole winding
full load ampere (FLA)
class of insulation
service factor (s.f)
speed of motor
supply voltage and frequency

Category of electric motor

AC motor
DC motor
Universal motor

Operation principle term

Induction - the motor rotates by electromagnetic field force to drive torque on rotor shaft.
Synchronous - the motor speed is in line with frequency supply. This producing stable speed and may changes when supply frequency is different.

Types of electric motor

AC asynchronous motors

AC polyphase - An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. Example of polyphase motor are squirrel-cage or wound-rotor induction motor.
split-phase capacitor-start and split-phase capacitor-run
shaded-pole motor

AC synchronous motors

Wound-rotor synchronous motor -
Hysteresis motor -
Synchronous reluctance motor -

Self-commutated motors

Brushed DC - A brushed DC electric motor is an internally commutated electric motor designed to be run from a direct current power source.
Brushless DC motor (BLDC) - A brushless DC electric motor, also known as an electronically commutated motor or synchronous DC motor, is a synchronous motor using a direct current (DC) electric power supply. It uses an electronic closed loop controller to switch DC currents to the motor windings producing magnetic fields.
Universal motor - The universal motor is a type of electric motor that can operate on either AC or DC power and uses an electromagnet as its stator to create its magnetic field. It is a commutated series-wound motor where the stator's field coils are connected in series with the rotor windings through a communtator.
Switched reluctance motor (SRM) - The switched reluctance motor (SRM) is an electric motor that runs by reluctance torque. Unlike common brushed DC motor types, power is delivered to windings in the stator (case) rather than the rotor. This greatly simplifies mechanical design as power does not have to be delivered to a moving part, but it complicates the electrical design as some sort of switching system needs to be used to deliver power to the different windings.

Safety & PPE

Personal Protective Equipment (PPE) required when handling an energized running electric motor.

wear rubber gloves
wear safety shoe when standing near an electric motor

Precautions

do not wear any metal object on body when comes near to an energized electric motor.
use proper insulated tools when working on an electric motor.
use testpen to check if metalframe of motor body is earth properly.

Motor Starter Circuit

Basically there are :

direct-online
star-delta starter (voltage reduction starter)
auto-transformer (voltage reduction method)
primary resistance starter (using resistors to limit current during starting)
inverter drive (known as soft starter)

Technology changes in motor starters

Resistor are used to control and limits the current flowing to motor.
Induction device to control and limits the current flowing to motor.
ACVV - Alternating Current Variable Voltage - using electronics and power diodes to control current flow operating via power diode or silicon control rectifier methods.
VVVF - Variable Voltage Variable Frequency - using electronics PCB to control over supply frequency and creating a smooth operation flow and speed can be effectively controlled by using frequency manipulation.

Protection for motor

Electric Motor must be protected from the following fault.

overload - using overload device
short circuit - using mold case circuit breaker for three phase motor.
earth fault - add on earth leakage relay

Reminder:

The body of an electric motor are made of metal, when motor is operating, the magnetic field exist and may cause electric static charge to happen when any metal object is near to this magnetic field in operation.

Formula electric motor

Load current (1ph motor), Ib = P ÷ ( 240v x Eff x pf ) this formula applies for single phase motor.
Load current (3ph motor), Ib = P ÷ ( √3 x 415v x Eff x pf ) this formula applies for three phase motor.
Motor speed, (RPM) RPM = 120 x Freq ÷ nos. of pole
Overload setting, OL = FLA x s.f
Calculating the % slip speed, % slip = (( Ns - Nr ) ÷ Ns ) x 100
How to convert from power to horse power, HP = P ÷ 746
How to convert from horse power to power, P = HP x 746
How to calculate rotor frequency, f' = s x f where speed differences is; s = (Ns - Nr)÷Ns

Example motor calculations

Change the value of 12 HP into watts ?

Answer: 12 HP x 746 = 8,952 watts (power)

Given an electric motor is 5200 Watt/ 415volts, power factor 0.85 and efficiency of motor is 85%, calculate the current ?

Formula: Ib = P ÷ ( √3 x 415v x Eff x pf )

Answer:

Current design, Ib = 5200W ÷ ( √3 x 415v x 0.85x 0.85 )

= 10.01 A (ampere)

A three phase motor nameplate indicates that the operating supply frequency is 50Hz and total stator winding is 6, calculate the speed of the motor capable to generates?

Formula: RPM = 120 x Freq ÷ nos. of pole

Answer: Speed motor (synch speed) = 120 x 50Hz ÷ 6 = 1000 RPM

Given Ns (synch speed) 1500 RPM and Nr (reading speed) is 1390 RPM, calculate the percentage slip ?

Formula: % slip = (( Ns - Nr ) ÷ Ns ) x 100

Answer: % slip = (1500-1390)÷1500 x 100 = 7.3 % slip

A three phase motor FLA stated 35 A, the motor is given s.f value of 1.2, calculate the motor tripping current setting?

Formula: OL = FLA x s.f

Answer: Overload setting current = 35A x 1.2 = 42 A (or ampere)

Given Ns = 1000 RPM, Nr = 960 RPM, standard supply frequency is 50 hertz, calculate the rotor frequency ?

Formula: speed, s = (Ns - Nr)÷Ns

frequency rotor, f' = s x freq

Answer: s = (1000-960)÷1000 = 0.04

rotor frequency, f' = s x f = 0.04 x 50 Hz = 2 hertz