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Comparison between Brushless DC Permanent Magnet Motor and Brushed DC Permanent Magnet Motor

                          Comparison between Brushless DC Permanent Magnet Motor 

                                           and Brushed DC Permanent Magnet Motor

   Similarities and differences between brushless DC permanent magnet motor and brushed DC permanent magnet motor 

   The typical inner rotor structure of the brushless DC permanent magnet motor (BLDCM) is compared with its corresponding conventional brushed DC permanent magnet motor (PMDCM), since the mechanical contact of the electric/commutator is eliminated, the original rotation inside The armature becomes the outer stationary armature, and the static permanent magnet pole that was originally outside becomes the rotating permanent magnet rotor inside.

   After such internal and external alternation, the motor has a relatively high torque/inertia ratio, which can provide relatively large output power, relatively long life, relatively high reliability, relatively high operating speed, and the like. Therefore, from the viewpoint of considering the performance of the motor, the brushless DC permanent magnet motor will be the best choice. From the perspective of cost considerations, brushless DC permanent magnet motors are sometimes the best choice when evaluated according to the total cost of the application, ie on the total life of the product.

Comparative analysis of cost and life

   A typical brushed DC permanent magnet motor with a selling price of 40 yuan to 45 yuan can be equivalent to a brushless DC permanent magnet motor with a selling price of about 150 yuan in terms of main force index. Obviously, the price of a brushed DC permanent magnet motor of about 40 to 45 yuan is lower than that of an equivalent brushless DC permanent magnet motor. However, this is the initial cost of the motor, not the total cost. If the brush DC permanent magnet motor can only maintain 2000h, and the life of the final product using the brushless DC permanent magnet motor is 20000h, the brush DC permanent magnet motor must be replaced 10 times, the total cost is 400 yuan to 450 yuan ( The cost of downtime and maintenance has not been taken into account). Obviously, a brushless DC permanent magnet motor of around 150 yuan is actually a more cost-effective solution because it will last until the entire life of the product.

Performance comparison analysis

   The general comparison between brushless DC permanent magnet motor (hereinafter referred to as BLDCM) and brushed DC permanent magnet motor (hereinafter referred to as PMDCM), mainly from the aspects of commutation, maintenance, mechanical characteristics, efficiency, volume power, etc. .

● commutation

BLDCM realizes electronic commutation by means of rotor position sensor;

The PMDCM is mechanically commutated by brushes and commutators.


BLDCM requires little maintenance due to the absence of brushes and commutators

PMDCM requires periodic maintenance

●Mechanical (speed / torque characteristics

BLDCM is flat (hard) and can run at all speeds under load conditions;

PMDCM is medium (medium hard), when running at higher speeds, the brush friction increases and the useful torque decreases.

● Efficiency

BLDCM has high efficiency because there is no brush pressure drop;

PMDCM is medium.

●Output power / form factor ratio

The BLDCM is easy to dissipate heat because the armature winding is placed on the stator connected to the casing. This excellent heat transfer characteristic allows the size of the motor to be reduced, so that the ratio of the output power dimensions is high;

PMDCM is medium/low. The heat generated by the armature is dissipated in the air gap, which increases the air gap temperature, thereby limiting the ratio of the output power form factor.

●Moment of inertia

BLDCM has a low moment of inertia. Since the permanent magnet is placed on the rotor, the dynamic response is improved;

PMDCM has a high moment of inertia and limits dynamic characteristics.

●Speed range

BLDCM is wider. No mechanical restrictions imposed by the electric/commutator;

PMDCM is relatively narrow and there are mechanical limitations imposed by the brush.

●Electrical noise

BLDCM is low; the arc of the PMDCM brush will cause electromagnetic interference to nearby equipment.

●Manufacture price

BLDCM is relatively high and PMDCM is relatively low.


BLDCM control is complex and expensive; PMDCM is simple to control and affordable.

●Control requirements

BLDCM: There must be a controller in order to operate the motor, but the same controller can be used for variable speed control;

PMDCM: For a fixed speed, no controller is required; the controller is required when there is a shifting requirement.

●Comprehensive comparison

Of course, in those applications that are short-lived or have a short working life, whether it is civilian or military products, or even in some aerospace technology fields, brushless DC permanent magnet motors without controllers, with the help of modern technology And advanced manufacturing technology can fully solve the mechanical contact problem of the brush commutator, it will have higher operational reliability than the brushless DC permanent magnet motor, and become the user's preferred product.

DC motor fault check and analysis

   A direct current motor is an electric motor that converts direct current electrical energy into mechanical energy. Because DC motor has good speed regulation performance, it is widely used in electric power drag, common faults of DC motor and its inspection and determination methods.

Armature winding ground fault armature winding ground fault

   The armature winding ground fault is the most common fault in DC motor windings. The grounding fault of the armature winding generally occurs at the slot and at the bottom of the slot. The insulation resistance meter method or the calibration lamp method can be used for the determination.

Armature winding short circuit fault

   When the insulation resistance of the armature winding is measured by the insulation resistance meter, if the resistance value is zero, the armature winding is grounded. When the millivoltmeter method is used for determination, if there is a certain relationship between the commutator piece and the motor shaft The voltage value indicates that the winding element connected to the commutator segment is not grounded; on the contrary, if the reading is zero, the winding component connected to the commutator segment is grounded.

Armature winding short circuit fault

   If the armature winding is severely short-circuited, the motor will burn out. If only a single coil is short-circuited, the motor can still operate, only the spark on the commutator surface becomes large, and the armature winding heats up seriously. If it is not found and eliminated in time, it will eventually cause the motor to burn. Therefore, when a short-circuit fault occurs in the armature winding, it must be eliminated in time.


   The armature winding short-circuit fault mainly occurs in the inter-turn short circuit of the same slot winding component and the short circuit between the upper and lower winding components. Common methods for finding the short circuit are:

1.way tester method. After the short-circuit tester is connected to the AC power source, it is placed on a slot of the armature core, and the saw blade is moved parallel to the other slots. When a large amplitude vibration occurs, the winding components in the slot exist. Short circuit fault.


2 millivolts meter method. Add 6.3V AC voltage (using DC voltage) to the K/2 or K/4 two commutating segments (K is the number of commutator segments), and contact the commutator with two test leads of the millivoltmeter. On the adjacent two commutating segments, the inter-chip voltage of the commutator is detected.

   During the detection process, if the reading of the millivoltmeter suddenly becomes smaller, it means that the armature winding components connected to the two commutator segments have a turn-to-turn short circuit. If the voltages of the commutating segments are equal during the detection process, there is no short circuit fault.


   Armature winding open circuit fault armature winding open circuit failure

   This is also one of the common faults of DC motors. Practical experience has shown that the armature winding breaking point generally occurs at the welding of the winding component lead wire and the commutator segment. The reasons are as follows: First, the welding quality is not good, and second, the motor is overloaded and the current is too large to cause the welding. This kind of breaking point is generally easier to find. As long as you carefully observe the solder joints at the riser of the commutator, you can find it by using a screw driver or tweezers to move the solder joints.

   If the trip point occurs inside the armature core slot or is not easy to find, you can use a millivoltmeter to connect the 6~12V DC power supply to the two commutator segments on the commutator that are separated by K/2 or K/4. The millivolt meter is used to measure the voltage between adjacent two commutating segments, and the measurement is performed step by step. When the two commutator segments connected by the broken winding are connected by the millivoltmeter, there is a reading indication, and the pointer is struck. If the millivoltmeter is connected across the two commutator segments to which the intact winding is connected, the pointer will have no reading indication.