When we encounter electromagnetic compatibility issues with DC motors, it is usually because DC motors with commutation air (such as rotor excitation speed-regulating motors) generate certain electromagnetic interference during motor operation.
When a DC motor or generator is running, the current and electromotive force in the circuit change direction when the armature winding components move from one branch to another. This change in current direction is accomplished by a mechanical device consisting of the commutator and brushes. Poor commutation conditions will cause electric sparks from the brushes. The electromagnetic interference caused by these spark discharges has a wide frequency spectrum, and its intensity varies with the magnitude of the spark discharge. Note: The sparks generated between the commutator and brushes during commutation in a DC motor are the most direct cause of electromagnetic interference!
A. First, a simple internal structural analysis of the following commutator motors: DC drive motor & DC drive fan.
As shown in the diagram: the contact surface between the commutator and the winding carbon brush is a point of alternating current change. This results in conducted reflection and radiated emission!
During the commutation process of a DC motor, when the current in the armature winding components changes direction, there are reactive electromotive forces and armature reaction electromotive forces. Their directions are consistent with the direction of the armature current, which always hinder the current change in the commutation components, delaying the commutation process and thus generating electromagnetic sparks!
As shown in the diagram, the internal structure of a DC fan also exhibits current sparking points at the contact surfaces between the commutator and the carbon brushes of the windings. This results in conducted reflections and radiated emissions!
As shown in the figure: the typical operation of brushes and commutators.
The bottom surface of the brush is made into a bevel. When the brush starts running, one edge of the bevel contacts the commutator surface. After running for a period of time, this edge becomes an arc-shaped bottom surface and then a cylindrical surface. This cylindrical surface and the cylindrical surface of the commutator are very well matched, and the sparks and noise of the brush will be very small!
B. The electromagnetic field distribution of a DC motor commutator is symmetrical.
Commutator electromagnetic sparks can be classified into the following levels:
Level 1.1 has no sparks.
2.1 grade: There are small areas with weak, point-like sparks under the brush;
3.1 grade exhibits weak sparking under most brushes;
4.2 grade has large sparks at all brush edges, and this level of sparking is only allowed under short-term overload or short-term impact load.
5.3 level exhibits significant sparking at the edges of all brushes, with sparks flying outwards. This level of sparking is only permitted during rheostat-free starting and motor reversal, provided that the commutator and brushes are functioning correctly!
C. Suppression of interference sources in DC motors
Generally speaking, sparking under the brushes indicates poor commutation. Sparking exceeding a certain limit can affect the normal operation of the motor. However, it's not necessary to eliminate sparks completely; a motor can still operate normally with only weak sparks. For a normally functioning motor, the spark level should not reach level 2. From the perspective of electromagnetic interference suppression, when the sparking under the motor brushes is greater than or equal to level 2, we should first try to suppress electromagnetic interference by improving the motor's commutation.
We only consider using an interference suppressor to suppress electromagnetic interference from the motor when the sparking from the motor brushes is less than level 1.5. This is the design I will discuss below, focusing on optimizing the motor's electromagnetic interference based on circuit principles!
D. EMI Design of DC Motors
Based on the above design mechanism, an interference suppressor, specifically an absorptive low-pass filter, is used at the power input terminal close to the motor. This provides a low-impedance path for interference sources from the motor while absorbing and limiting the radiation of interference sources to the outside via the power cable, thereby achieving EMI optimization. Therefore, connecting a filter in series on the power input line is the most direct method.
Noise sources (interference sources) of DC motors: changes in the motor's commutation current.
Noise characteristics: Internal structure - commutator - carbon brush mechanism - power supply leads - electromagnetic field - stator - casing (metal) - grounding terminal!
Transmission path: motor connection cable, ground connection wire, etc.!
The recommended suppression principle diagram is as follows:
In the diagram, terminals 1 and 2 are the input terminals of the filter; terminals 3 and 4 are the connection terminals of the commutator motor.
The typical EMI optimization circuit structure for DC motors is as follows:
The above is a general EMI design circuit for a DC motor. The capacitor between the two differential-mode wires in the diagram is an X capacitor, with recommended parameters of 104-474. 104 can be used when suppressing interference power, and 474 when suppressing interference voltage. For the feedthrough capacitor (three-terminal capacitor) in the diagram, 1nF-10nF is recommended, mainly for suppressing interference power. For some systems considering cost and compatibility, a Y capacitor can be used, with a recommended design value of 471-472. The stabilizing resistor R in the circuit adjusts the Q value of the filter and also serves as the discharge resistor for the X capacitor. The inductors L3 and L4, closest to the DC motor, effectively suppress electromagnetic sparks from the motor commutator!
Note that L3 and L4 should be connected as close as possible to the carbon brush end; in small capacity motors, this inductor L3 = L4 = 60uH - 200uH is generally designed; L3 and L4 mainly play the role of compensating for delayed commutation, improving the electrical balance performance of the motor commutation device, and suppressing electromagnetic sparks of the commutator.
1. In portable or handheld products without a grounding wire, the grounding terminal of the interference suppression filter can be connected to the metal casing of the DC motor;
2. In some applications where motors have grounding wires, the high-frequency grounding effect may become poor if the grounding wire is too long! Attention needs to be paid to the motor's grounding connection.
3. If using a grounded capacitor, the high-frequency impedance of the outer casing should be as low as possible, and the core lead wire should be as short as possible to reduce electromagnetic coupling;
4. The common-mode inductor in the circuit also has a significant effect on EMI. When using it, the inductance is related to the capacity of the motor for different purposes. The leakage inductance of the common-mode inductor helps to reduce interference power. In practical applications, if the capacity of the series-wound motor is around 500W, the recommended common-mode inductor is 1.8mH-5mH (1KHZ). At the same time, the leakage inductance Lr is about 2% of the inductance Lm, which has the best suppression effect!
E. The following is a reference design for EMI optimization of a practical DC fan;
The following products failed the EMI test.
Through my basic theory and design optimization methods, the test was passed.
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