What is an Electric Motor?
An Electric Motor is a device that turns electric current flow into mechanical rotation of a spindle, or rotor. The rotation is turned into linear motion in many applications.
What is an Electric Motor?
An electric motor is a device that turns electric current flow into mechanical rotation of a spindle, or rotor. The rotation is turned into linear motion in many applications. How does an electric motor work?
There are many variants and options of Electric Motors; for example DC motors – brushed or brushless and AC motors – induction (or asynchronous) and synchronous. The motors can operate at a variety of voltages to suit an application and the supply available.
The operation of a motor relies on two properties of electric current. The first is that an electric current, flowing in a wire or coil, will create a magnetic field. The second is that a changing current in a conductor, for example from an ac supply, will induce a voltage in the conductor (self-inductance), or in a secondary conductor (mutual-inductance). A current flow in the circuit of a secondary conductor will also produce a magnetic field, as outlined above.
For a magnet, like poles repel and unlike poles attract. In all motors, the construction uses this property to ensure continuous rotation of the rotor.
The diagram below shows a three-phase AC wavform; each phase is separated by a phase angle of 120° as shown in the vector diagram in the middle.
At a particular phase angle, there will be a resultant direction for the field which can be calculated by vector addition; the permanent magnet(s) in the rotor will look to align with the field direction and, as the AC waveform ‘progresses’ in time, the rotor will turn, as illustrated.
And so on through one complete cycle (3600) where the rotor will effectively return to its start position and repeat the process again.
How do I select an electric motor?
Not all applications lend themselves to the use of a three-phase synchronous motor; whilst size efficient for its power, the motor above would be far too large, for example, to drive a DVD player. Further, three phase supply would not be ideal for a domestic (or most commercial) situations; so, application is an important consideration in determining size and voltage supply.
The power (via torque) required from the motor is a vital consideration; what are the dynamic considerations of the application – the load, acceleration/deceleration and distances to be moved radially or laterally?
Also important is the stability of rotational speed; is the motor required to run at a constant speed, even at low revolutions?
Finally, environmental conditions will be a consideration – what is the operating temperature and is water or dust likely to be a problem? Will the motor be operating in an explosive environment and require an ATEX rating? Types of electric motor
As stated above there are numerous options for motors; powered DC or AC and various voltages, dependent on application.
A relevant consideration when selecting motors, is the difference between servo and stepper motors. A servo motor has a feedback mechanism – the feedback signal being compared to a set point until there is zero difference, when the motor has reached a desired position
A stepper motor also offers control but can be considered a digitised version of a motor with a special construction. Multiple independent stator coils (the stator is the stationary part of the motor) and a specially designed rotor allow the motor to step to a specified position, or angle, against a command.
Stepper motors are ideally suited to low power, low cost applications, such as a CD drive. Conversely, servo motors are better suited to higher power, high acceleration and high accuracy applications. Typical applications of electric motors
Electric motors are found in a wide variety of applications – domestic, such as CD’s, DVD’s washing machines, etc and commercial, such as medical and offices and industrial applications. When coupled with a linear actuator mechanism, typical applications are in automotive, materials handling, robotics, food and beverage and packaging, amongst others.
Do I need anything else to make electric motors work?
A suitable electrical supply and the associated cabling to the equipment is essential. In any application, the motor will need to be connected to its driven components, either directly, via gears or belts and this may require vibration damping. Temperature sensors are a sensible addition and, in the event of possible overheating, a fan with suitable ventilation will be required.
Cabling is required for power and control signals between the motor and drive.
Visit our Electric Actuator section to find out more.