An electric motor is capable of transforming electric current into mechanical movement. They are used everywhere, from home to industries and factories. Some electric motors are preferred in certain areas than others for multiple factors. Everyone of them has multiple considerations:
- A voltage input specification
- Electrical and mechanical losses
- Power consumption
- Torque and speed output
Among other factors. This blog post refers to the most important and used electric motors available in the market and some of their differences. Futhermore, electric motors have an interesting feature: some of the energy given can be restored back to the grid or battery as it is being stopped. It’s called Regenerative Braking.
Types of Electric Motors
The most important criteria for classification of electric motors are the input source of energy. Thus motors are classified in two big groups.
Alternating Current Motors
All motors that uses electricity from the power grid, uses AC current and depends on a alternating voltage input, it can be considered as alternating current motor. Most of this motors can be plugged directly into a power outlet and work out-of-the-box. Some of them have efficiencies above 85% which makes them very attractive for applications that consumes lots of power.
All of them present more difficulties to control the speed and torque output than the direct current counterparts. It is possible, no doubt, but it is not a straight forward solution in most applications. In recent years however, the use of semiconductors have shifted into a simpler control of AC machines such as Variable Frequency Drives.
Direct Current Motors
Since all power outlets transfers AC current, a converter must transform the alternating current into a suitable direct current first. DC motors are generally less efficient and require more maintenance. Some of them need a controller to work.
DC motors are easier to control because of the simpler operation, thus they are more versatile. If speed variation is demanded for an application, DC motos are usually the way to go. Some have configurable windings (series, parallel, compound, etc) that can be changed in response to a better torque-speed performance, dictated by the application itself.
There is one kind that stands out from the rest which is the Brushless DC motor or BLDC for short. They usually have efficiencies above 90%, they are easy to control and have low maintenance or none. More on that later.
Popular Electric Motors
In this motors, the AC current is delivered to the stator directly and it is then passed into the rotor by a transformation act. The number of poles in the stator (i.e. the number of ‘norths’ and ‘souths’) determines the rotation speed of the rotor. In the market, you will usually find two types:
- Wound type, which is less common and used only in specialized circumstances.
- Squirrel cage, which offers build simplicity and robustness.
Single-phase Induction Motor
They use 2 wires, usually phase and neuter, and they are popular in domestic applications where 3 phase connections are unavailable.
To start, an auxiliary winding is required. They are recognizable for the ‘hump’, which is the capacitor. These kind of motors have less efficiency than the three-phase engines, from 50 to 60% generally.Single-phase motor. Respectfully borrowed from ATO.
Three-phase Induction Motor
Generally they are used in industrial or professional environments, where three-phase connections are available. Adding these extra phases make this machine very efficient, reaching about 85% and above.
An important characteristic is the rotation of the rotor because it spins at a lower frequency than the frequency of the stator. This concept is called ‘slip’ and it is measured of 0 to 1, as it would be a percentage.AC Induction motors. Respectfully borrowed from Wikipedia.
Using semiconductors for controlling the motor has greatly improved its operability. Variable frequency drives solves some drawbacks and issues that were present in this motor such as high peak current starts, torque-speed ratio and many more. Check it out here, in this post.
It means, on the contrary of the typical induction motor, that the rotor spins at the speed of the frequency of the stator. These machines are generally used as generators and they work marvelously in parallel (i.e. various generators interconnected working together. Most of the have a three-phase stator.
This AC machine also needs DC current to work! The rotor receives DC current in order to produce the movement by interaction of the magnetic field of the stator. Synchronous machines could also work as synchronous condenser, for power factor compensation. If the rotor doesn’t get enough DC current, it produces a lagging or positive power factor (as it were reactive inductive power). On the other hand, if it gets excessive DC current, then it produces the contrary effect, a leading or negative power factor (as it were reactive capacitive power).
They have important downside. They don’t produce to starting torque and they lose the torque if they are no synchronized with the stator anymore. One method to convert it into a self-starting machine is to add a damper winding.Synchronous Motor. Respectfully borrowed from OME Motors.
Brushless DC Motor
It shares many similarities with the synchronous. It is built with permanent magnets, which replaces the need for DC current in the rotor. The efficiency is greater than the three-phase induction motor and has higher power per weight ratio. The voltage input, as the name suggests, is direct current which works better with batteries (also a DC power supply).
It requires a controller to work, which increases the price but increases the speed control maneuverability. Some models has sensors to detect the position of rotor, especially of slower motors. On the other hand, higher speed motors, such as those used for drones, are sensorless because they are able to detect zero crossing among the coils.Brushless DC electric motor BLDC. Respectfully borrowed from Stepperonline.
Brushed DC Motor
They are simple motors and speed and torque control is specially easy. It was achieved by the variations of voltage in the armature and the field coil. Before semiconductors applications and tools, these motors were used wherever a speed variation was needed. They are not limited to small handheld motors, they could also be present in industries with several KW of power.
Their maintenance is high, since the brushed must be changed periodically, among other checks. The efficiency is one of the lowest among other electric motors, around 60%.
Brushed DC Motor. Respectfully borrowed from Wikipedia.
Until now, all the motors in this posts works with the same purpose: spin with force to do a certain job. It is unimportant to know the rotation position of the rotor and they are difficult to command if it is required to spin just a revolution and a half, for example. If the application requires advancing steps. the stepper motors works better.
It advances in equal steps and doesn’t unless it commanded to do so. It doesn’t need a feedback system, if there is a counter registers involved. There are motors that are built to have 50 to 200 steps per revolution. The coils receive electric pulses to move one step at a time and it moves forward as the sequence coils advances.
Stepper Motor. Respectfully borrowed from Wikipedia.
Servo or servomotors have a feedback mechanism to work. In contrast to stepper motors, they are used to determine the position of the rotor. The main purpose is not rotate quickly in thousands of revolutions per minute as motors, but to provide a quick angular movement, usually with less than 180º degrees with high torque. It requires a PWM signal to work, generally given by a microcontroller at around 50Hz.
Servomotor. Respectfully borrowed from Wikipedia.
There is no one-motor-fits-all-applications invented yet. In every circumstance, a certain motor works better than others since each of them has their advantages and fallbacks. It is therefore important to use the criteria to understand where it will perform better.
- A.E. Fitzgerald. Charles Kingsley, Jr. Stepehn D Umans. Electric Machinery 6th Edition. McGraw Hill. 2005.