Motor controller
[1] A motor controller might include a manual or automatic means for starting and stopping the motor, selecting forward or reverse rotation, selecting and regulating the speed, regulating or limiting the torque, and protecting against overloads and electrical faults.
A wall-mounted toggle switch with suitable ratings may be all that is needed for a household ventilation fan.
A direct on line starter can be used if the high inrush current of the motor does not cause excessive voltage drop in the supply circuit.
The maximum size of a motor allowed on a direct on line starter may be limited by the supply utility for this reason.
For example, a utility may require rural customers to use reduced-voltage starters for motors larger than 10 kW.
To reduce the inrush current, larger motors will have reduced-voltage starters or adjustable-speed drives in order to minimise voltage dips to the power supply.
More modern starters have built-in timers to switch from star to delta and are set by the electrical installer of the machine.
The machin's operator simply presses a green button once and the rest of the start procedure is automated.
The hotter the strip, the more it deflects to the point it pushes a trip bar which disconnects power to the contactor solenoid, turning everything off.
By using an autotransformer or a series inductance, a lower voltage is present at the motor terminals, reducing starting torque and inrush current.
Since the autotransformer or series reactor only carries the heavy motor starting current for a few seconds, the devices can be much smaller compared to continuously rated equipment.
By breaking the rotor into parts and electrically connecting these parts to external resistances via slip rings and brushes as well as control contactors, the magnetic power of the rotor can be varied - i.e. reduced, for starting or low power running.
Although a much more complex process, it means the currents (electrical loads) being switched are significantly lower than if reducing the power to the main feed of the motor.
A third way to achieve a very smooth progressive start is to dip resistance rods into a conductive liquid (e.g. mercury) which has a layer of insulative oil on the top.
When motor reaches about 80% of its full load speed, it will begin to run in a delta connected stator winding.
Now we shall see how a star delta starter reduces the starting current of a three-phase induction motor.
[7] The above function achieved by using a power contactor and timer in automatic star delta starter.
The automatic star delta starter is manufactured from three contactors, a timer and a thermal overload.
This is accomplished by reducing the voltage to the AC terminals and at the same time lowering current and kvar.
The overload relay has a normally closed contact which opens due to heat generated by excessive current flowing through the circuit.
Thermal overloads can be manually or automatically resettable depending on their application and have an adjuster that allows them to be accurately set to the motor run current.
A second type of thermal overload relay uses a eutectic alloy, like a solder, to retain a spring-loaded contact.
When too much current passes through the heating element for too long a time, the alloy melts and the spring releases the contact, opening the control circuit and shutting down the motor.
If a momentary loss of supply voltage occurs, the contactor will open and not close again until a new start command is given.
This connection also provides a small degree of protection against low power supply voltage and loss of a phase.
[12] Without a time sequenced schedule, any attempt to restart many motors simultaneously could lead to partial or total site wide power failure.
Other position feedback methods measure the back EMF in the undriven coils to infer the rotor position, or detect the Kick-Back voltage transient (spike) that is generated whenever the power to a coil is instantaneously switched off.
How long the pulse remains high (typically between 1 and 2 milliseconds) determines where the motor will try to position itself.
The usual setup is to have a positioning controller, known as an indexer, sending step and direction pulses to a separate higher voltage drive circuit which is responsible for commutation and current limiting.
