Surprisingly, a brushed DC and brushless DC motor can act as operators. Many engineers are always fascinated by the idea, however, there are some factors to take into consideration when designing the drive.
Brushless DC motors are ideal for AC voltage applications, while brushed DC motors are suitable for generator applications that need a DC voltage output. You can use a brushless DC motor for DC voltage output, however, you’ll need a voltage rectification circuit.
On that same note, if you are using a brushed DC motor for AC applications, then DC to AC electronics are required. To first understand how a DC motor can serve as a generator, we need to first understand how a DC motor works.
What is a DC Generator?
A DC generator is an electrical machine that converts mechanical energy into electricity. When the conductor comes in contact with the magnetic flux, the EMF will be formed based on Faraday’s law of electromagnetic induction.
The electromotive force will result in a flow of current when the conductor circuit is closed off. You can also learn about What are DC Motor Drives ?
Parts of the DC Generator
The parts of a DC generator are very similar to that of a DC motor, hence why it can be used interchangeably. Without changing the construction of a DC generator, it can be used as a DC motor, and that’s why it’s sometimes called a DC machine.
The important parts of a DC generator include;
- Stator. The work of the stator of a DC generator is to form magnetic fields where the coil spins. A stator usually has two magnets with repelling sides, and they are located in the region of the rotor.
- Rotor. The rotor features slotted iron laminations with slots that are arranged to achieve an oval armature core. The main job of the lamination is to reduce the loss from the eddy current.
- Poles. The work of the poles in a DC generator is to support and hold the field windings in place. These windings are usually connected in series/parallel by the armature windings.
- Coil . The coil of a DC generator is a copper wire that will be wound to the armature and each side of the coil is either linked to two opposite bars of split ring commutator or two slip rings.
- Armature Windings. The armature windings are usually in a closed-circuit form and connected in series or parallel to boost the produced current.
- Commutator. The function of the commutator is to change the AC voltage to the DC voltage within the armature winding. It’s made with a copper segment and these segments are protected from each other by mica sheets.
- Brushes . The brushes in the DC generator help form the connections between the commutator and the external load circuit.
How Does a DC Generator Work?
An electric motor is an electrical machine that converts electrical energy to mechanical energy using electromagnetic phenomena.For a DC motor, a mechanical force is generated whenever a current-carrying conductor is put in a magnetic field.
When armature windings are linked to a DC supply, an electric current is usually established in the windings. The fielding winding or permanent magnets may further provide the magnetic field. In cases like this, the current-carrying armature conductor will experience a force due to the magnetic field.
The commutator is segmented to achieve a unidirectional torque. If it’s not segmented, the direction of the force will reverse each time the direction of the conductor’s movement is revered in the magnetic field.
Is it hard to understand?
I’ll explain it to you briefly again. When a current-infused conductor is put in a varying magnetic field, an emf is created in the conductor.Following Fleming’s right-hand rule, the direction of the induced current switches when there’s a change in the direction of the conductor’s movement.
For instance, imagine an armature spinning clockwise and a conductor moving upward on the left side. If the armature finishes a half rotation, the conductor’s motion will be reversed downwards.
Now, this is how a DC motor works.
Click to learn: Small DC Motors: What It Is & Working Principle & Types
Rules for Using DC Motors As Generators
DC & AC Voltage
Use brushless motors with a voltage rectifier, and for DC voltage, use a brushed DC motor. For AC voltage, using a brushless EC motor with 2 phases is recommended. When it comes to brushless motors, hall sensors are not needed.
Ideally, generators operate at a speed of 1000 rpm, sometimes less. When compared to DC motors, that’s a really low speed. So, when generating 10 v or more at 1000 rpm, a speed constant of 100rpm/v is needed.
However, it’s a bit complex to find such windings as there are only a few high-resistance windings on large motors that meet the requirements. With smaller motors what you get is a higher speed constant.
Without factoring in the load, the winding should have a constant speed, and as an alternative, the motor speed can be accelerated using a gearhead.
When choosing a motor for your generator, don’t base your choice on only power consideration. To balance the torque requirements, you’ll need a motor with a significantly higher power rating than the generated power.
This is especially true if the generator speed is really low in comparison to normal motor speeds.
Current and Voltage Limitations
To get the best winding of any DC motor, you need to consider the generated voltage and current requirement. To be on the safe side, choose a winding that can supply the required voltage, regardless of the load it’s carrying,
For instance, for a fixed generator speed, you’ll need a generated voltage of the winding that is larger than the required voltage.
Generators require motors with high generator constant, but these windings also have a high resistance as well. With high resistance, the output voltage decreases when under load, and the output voltage becomes sensitive to the load current.
If you plan on achieving a stable output voltage over a certain load range, then go for a larger motor with resistance lower on motors with high generator constant.
Torque and Speed Limitations
The torque on a generator is the deciding factor for the size and type of motor. When choosing a motor type, go for the variant with a continuous torque higher than that of the generator.So, to calculate the amount of torque, you’ll need to factor in the type of operation it’ll be used for.
A few questions to answer include; how long the generator will run for, will it be in intermittent cycles, short intervals, or long periods? The answer to this will help you choose a motor type with sufficient continuous torque. Plus, don’t forget the maximum speed of the DC motor, but that’s rarely an issue because generators usually require low speeds.
The work of a gearhead is to increase low speeds. Some gearheads like spur gearheads and planetary gearheads can be back driven, and those are better alternatives. The essence of using a gear motor is to supplement and assist the slow driving mechanisms in generators.
When using the gear-motor combination there are certain recommendations;
- The gearhead has to be driven in reverse operations.
- Spur gearheads are better than planetary gearheads because they can be driven back easily and the back-driving efficiency is always higher.
- High reduction gearheads of 3 stages and higher are not back-driven, so, your best bet is to use planetary gearheads of stage 1 or two that can be controlled from the output.
On that note, there are two types of generators; AC generators and DC generators. However, our focus is on DC generators today.
Types of DC Generator
There are two types of DC generators, and they are classified based on which field excitation is given.
- Separately Excited Generators
- Self-Excited Field Generators
A DC motor is a ready-made generator that can generate electricity once you turn the axle of the generator using a mechanical source of energy.Contrary to public opinion, a brushed DC motor running in generating mode is quite efficient, although it may not be as efficient when functioning as a motor.
But you can still achieve high efficiency with the right selection of motor and operating speed. When choosing operating points, consider the mechanical and electrical factors involved.
If you have limited knowledge of the ideal motor to use for applications like tachometer generators and energy harvesters, then it’s in your best interest to consult professional engineers.
If you have any questions, you can click here to contact Donghui Motor, we have professional staff to answer your questions.