The Definitive Guide To Electric Motors – PART 2
In the second part of our guide to Electric Motors, we take a quick look at how they work and some applications for the most common types of motor.
An electric motor is a machine which is powered by electricity and converts it into a mechanical force. There are a number of devices which do this such as a loudspeaker or industrial fan for example, but the electric motor specifically generates ROTARY motion. This rotary motion can drive a shaft on which other mechanical elements can be driven. This motion can then be applied to drive the machinery behind things as small as a wristwatch to gigantic ship propulsion propellers. Electric motors are quickly replacing the more traditional hydraulic cylinders in airplanes and also in some military equipment.
Since the late 1800’s we have seen the emergence of a variety of electric motors that are now categorised by the way they are manufactured and purpose. We will cover this in more detail further down this article but essentially an electric motor can be categorized by the way it is powered, by either a battery (Direct Current) or Alternating Current, respectively referred to as DC and AC. Let’s, for now, just consider that these 2 types allow us to segregate our electric motors into simple categories and we will cover them in more detail shortly.
How does an electric motor work?
Electricity describes the flow of electrons within a material which allows for this movement to happen, this type of material is called a “conductor”, materials which do not allow for the free flow of electrons are called “insulators”.
One of the best and widely available types of conductor is copper. Within an electric motor strands of copper wire are tightly wrapped around magnets. The magnets have different poles which have the effect of attracting and repelling the electrons within the copper wire.
The effect of introducing a power source to the copper wire excites the electron flow and causes the electro-magnetic force to increase.
If you were to build or take apart an electric motor, here are the internal components that you will find.
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 which carry currents that interact with the magnetic field of the stator to generate the forces that turn the shaft. However, some rotors carry permanent magnets, and the stator holds the conductors. Devices such as magnetic solenoids and loudspeakers that convert electricity into motion but do not generate usable mechanical power are respectively referred to as actuators and transducers. Electric motors are used to produce linear force or torque (rotary).
In the centre of an electric motor you will find what is known as the Stator. This is a stationary part of the electromagnetic circuit and is usually constructed of permanent magnets or magnetic windings. The core of the Stator is made up of many thing metallic sheets which we call laminations, the purpose of which are to reduce the energy lost compared to using a solid core; using these laminations in this way results in a more efficient motor.
There is an air gap between the rotor and the stator which is very important. This gap is made as small as possible to improve the overall performance.
Windings are the visible wires wrapped in coils around the magnetic core. Once a current is applied to these windings the different magnetic poles become energised.
There are two basic magnetic pole configurations that we usually find within an electric motor and we call these “salient-pole” and “non-salient pole”.
Within the salient-pole motor the windings are wrapped around below the pole face itself whereas in a non-salient pole motor (also known as a distributed field or round-rotor), the winding is distributed instead the pole face slots.
As shown above, the commutator is made up a slip ring which is used to insulate the segments from each other and also from the electric motor’s shaft. It has the effect of switching the input of AC and DC currents.
The commutator revolves with the segments but when stationary and supplied with an armature current, then causes the current in the circuit to be reversed and for rotation to begin as the rotation is repelled by each magnetic pole.
Without he commutator the motor would stop as the current is reversed in an optimal manner. Modern motors have other ways of braking such as via electronic controllers, sensor controls and permanent magnet motor fields. Electro-mechanically commutated motors are now increasingly replaced by more modern externally commutated induction and also by permanent magnet electric motors.
Categories of electric motor
Generally speaking electric motors operate on three different physical principles: magnetic, electrostatic and piezoelectric however, the most common being electromagnetic.
Magnetic fields are created both in the rotor and the stator; a current flows in one direction and then another. This gives rise to a torque force and a resultant rotation motion.
This is achieved through switching the poles on and off in a manner which is timed, or by varying the strength of each pole, again in a manner which is timed.
Asynchronous or Synchronous
AC electric motors are either asynchronous or synchronous.
A synchronous motor is synced with the moving magnetic field speed for all normal torque conditions. The magnetic field must be provided by a method other than induction such as separately stimulated windings or from permanent magnets.
It is worth noting that within all of the principles laid out above, none actually require that the rotor actually is required to rotate. Torque can be exerted upon the windings of the electromagnets and one way of taking advantage of this is with the “coreless (ironless DC motor). This is a form of DC motor which is built for very quick acceleration. The rotor itself can be constructed as a winding-filled cylinder, or perhaps a self-supporting structure which comprises purely of a magnet wire and bonding material.
The rotor itself can then fit inside the stator magnets. A different type of construction sees the rotor winding wrapped around the stator magnets where the rotor is able to fit inside a magnetically soft cylinder which is able to serve as the motor housing.
The reason this type of motor can experience fast acceleration is due to the rotor being formed from copper or aluminium resulting in a much lighter mass weight.
Pancake / ServoDisc / Axial rotor motor
Known by other names such as the ServoDisc, the Pancake motor has a very flat profile and has windings shaped as a disc running between many high flux magnets.
The unique advantage of this type of motor is that torque variations caused by changing attraction between the iron and the magnets greatly improves efficiency, but variable-speed controllers must use a higher switching rate because of the decrease in electromagnetic induction.
These motors were originally invented to drive the capstan(s) of magnetic tape drives in the burgeoning computer industry, where minimal time to reach operating speed and minimal stopping distance were critical. Pancake motors are still widely used in high-performance servo-controlled systems but where initially designed to drive the capstan within magnetic tape drives. Thanks to the variety of construction techniques now available they tend be is used in applications from high temperature military to low cost basic servos.
There are a number of other types of motor which are used for different circumstances. These include (but not limited to):
Servomotors are used in areas like machine tools and pen plotters as well as other process systems and as such are designed for speed, torque, and power. Dynamic response characteristics such as winding inductance and rotor inertia are also important.
A servo system differs from some stepper motor applications in that the position feedback is continuous while the motor is running; a stepper system relies on the motor not to “miss steps” for short term accuracy, although a stepper system may include a “home” switch or other element to provide long-term stability of control. For instance, when a typical dot matrix computer printer starts up, its controller makes the print head stepper motor drive to its left-hand limit, where a position sensor defines home position and stops stepping. As long as power is on, a bidirectional counter in the printer’s microprocessor keeps track of print-head position.
Stepper motors are a type of motor frequently used when precise rotations are required. In a stepper motor salient poles are controlled by a set of external magnets that are switched electronically. The stepper motor may not rotate continuously; instead, it “steps” from one position to the next as field windings are energised and de-energised in sequence. Depending upon the sequence these rotors may turn backwards or forwards, may change direction, stop, speed up or slow down arbitrarily at any time.
Stepper motors are often used in computer printers, optical scanners, and digital photocopiers to move the optical scanning element, the print head carriage (of dot matrix and inkjet printers), and the platen or feed rollers. Also quartz analog wristwatches contain tiny stepping motors.