This is a discussion on the method of developing a permanent magnet, 4 Pole 2 brushes DC blower motor for use in a vehicle HVAC system. By modifying the magnet orientation in terms of pole alignment, magnetic field intensity, armature winding technique, and brush alignment, the motor mass and volume is reduced 15% to 30%. These improvements are achieved without reducing motor performance, increasing motor cost or increasing motor brush noise.
Keywords: 4 Poles 2 Brushes DC motor; Armature Winding; Improved Lap Winding; Weight; Performance;
There is an increasing need for compact cars in urban settings without compromising passenger interior space. And vehicle emissions and fuel economy legislation requirements continue to become stricter. So, all OEMs seek reductions in system and subsystem/component weight of at least 15%. DC blower motors in vehicle HVAC systems are included in this trend.
For HVAC systems in vehicle, there are studies to reduce the size and weight of the DC blower motor by using 4 magnets and 4 brushes with conventional Lap Winding, or using 4 magnets and 2 brushes with armature odd slots and conventional Wave winding . However, these methods lead to blower motor noise problem which is the critical criteria of blower motors for all OEMs .
There is also a study on replacing the conventional segment type sintered ferrite PM in DC motor by a ring-type anisotropic bonded NdFeB PM . This method will reduce motor weight and size by 50% and achieve a high power-to-volume ratio of the motor. However, like the another studies, this method has some drawbacks that rare earth magnets are too expensive and as their name implies are rare, so they are not practical for use in automotive blower motors.
This study will show how we can overcome the blower motor noise problem and reduce the motor weight and size, without using rare earth magnet.
Improved Lap Armature winding for 4 Poles 2 Brushes Motor with Armature 14 slots
A. Conventional Lap Winding with Armature 14 slots
With 14 slot Armature:
- The total number of slots: S = 14;
- The total number of poles: P = 4;
- Total number of coils = S = 14;
Winding pitch/coil span (the spacing between the two coil sides of a coil, expressed in terms of number of slots between the sides) is S/P = 14/4 ≈ 3 .
Commutator pitch (a separation of coil sides of a coil in terms of number of commutator segment) =1 because it is Lap winding, so ends of a coil are terminated on two consecutive segments  Figure 1.
Since the coil span is 3, the first coil has sides 1’ and 12; the coil can be expressed as (1’-12). Coil side 1’ is terminated on commutator segment 14, because the commutator pitch yc is +1 for progressive lap winding, 12 to be terminated on commutator segment 13. So naturally next coil (14’ - 11) should start from commutator segment 13 and the coil side 11 terminated on segment 12 as shown in Figure 1b because all coils are connected in series in DC Motor. Keep continuing, we will have the complete progressively Lap Winding [2,3,5].
To determine the carbon brush location, assume the slots 14, 1, 2 are under the influence of the north pole which means slots 4, 5, 6 are under south pole, slots 7, 8, 9 are under north pole and slots 11, 12, 13 under south pole. Since S/P (14/4) is not an integer, slot 3 and slot 10 are assumed to be in the neutral zone (for Motor with 14 slots) . And direction of
Figure 1: Conventional Lap Winding Diagram
Figure 2: Magnetic direction of 4 Poles DC Motor
rotation of the cylindrical armature is clockwise direction.
The magnetic force direction of 4 poles as the Figure 2 can be determined by using the numerical analysis method on Maxwell software (or equivalent software, like JMAG).
The simple process of this numerical method is as following:
- Loading CAD data of motor into Maxwell software or JMAG.
- Assign material for motor casing, magnet, Rotor Core and copper wire.
- Input boundary condition.
- Mesh Generation.
- Run the analysis.
With this direction of Magnetic force from Figure 2 together with armature rotation direction, we can apply Right Hand Rule to show the directions of EMF in each coil side by arrows as shown in the Figure 1b, whereas :
: going into page
: coming out of page
Base on the EMf direction in the Figure 1b, we can see:
- In commutator segment 5, 12 arrows converge and go in these commutator segments. So the Brush (+) will be placed at commutator segment 5 and 12 in order to obtain maximum voltage.
- In commutator segment 1, 8 arrows converge and go out of these commutator segments. So the Brush (-) will be placed at commutator segment 1 and 8 in order to obtain maximum voltage.
So the Conventional Lap Winding is used in DC motor having more 4 poles, but 2 pair of brushes is required for each pair of poles [2,3,5].
B. Improved Lap Winding for Armature with 14 slots
With the conventional Lap winding, 4 poles of magnet must have 4 brushes, so because the friction of 4 brushes with commutator will be higher than 2 brushes with commutator, and the timings of contact of the brushes differ, the noise of blower motor cannot meet the customer requirement, the abnormal noise (“tick – tick” sound and electrical fluctuation sound) in the vehicle cabin will be excessive and annoying to drivers and passengers. So the need is that we have to reduce 1 pair of brushes .
With the Improved Lap Winding, we can achieve this.
Winding pitch (coil span): Like Conventional Lap Winding, if the total number of slots (S) is 14, and the total number of poles(S) is 4 then coil span is S/P = 14/4 ≈ 3 teeth . Because it is Improved Lap winding, so commutator pitch should be separated into 2 types in order to complete winding :
yc1: the separation of coil side of a coil in terms of commutator segment, yc1= +1 for progressive winding
yc2: the separation of end coil side of a coil with the start coil side of the next coil in terms of commutator segment, equal
Whereas: S: No of Slots, P: No of Poles, K: a number (integer or fractional) to make the yc2 an integer Figure 3.
Instead of using the + Ve Brush at commutator segment 12, and – Ve Brush at commutator segment 8, we connect the commutator segment 8 with segment 1 and commutator segment 12 with 5 by copper wire as shown in Figure 4 .
Since the coil span is 3, the first coil has sides 1’ and 12 and the identification of the coil can be expressed as (1’ -12). We terminate coil side 1’ on commutator segment 14, since it is still the lap winding, coil 12 to terminated on commutator segment 13 (yc1=1). Then from commutator segment 13, connect commutator segment 13 and 6 by copper wire (yc2= 7). And again, because in DC armature winding, all coils are connected in series. So naturally next coil (7’ - 4) should start from commutator segment 6 and the coil side 4 terminated on segment 5 as shown in Figure 3. Keep continuing, we will have the complete Improved Lap Winding.
Like the conventional Lap Winding, assume slots 14, 1, 2 are under the influence of the north pole which means slots 4, 5, 6 are under south pole, slots 7, 8, 9 are under north pole and slots 11, 12, 13 under south pole. Since S/P (14/4) is not an integer, slot 3 and slot 10 has been assumed to be in the neutral zone (for Motor with 14 slots). And direction of rotation of the cylindrical armature is clockwise direction. So we will have the location of 2 brushes as in Figure 3.
By using this technique, we can achieve DC Blower motor 14 slots with 4 Poles and 2 Brushes.
First, in order to verify the present approach that it will help to reduce Motor weight without reducing the motor performance, prototype sample of 4 Poles 2 Brushes DC Motor were built as Figure 5.
Then, performance of the samples will be tested on performance tester machine 1. The samples 2 will be put on the jig 3 of the tester machine, and connected to load creator 5 by coupling 4 as Figure 6. The performance test specification will follow the customer specification as the “Performance data” in Table 1.
Moreover, weight and dimension of the samples are also measured and recorded in Table 1 as well Figure 5&6.
According to the testing result, we can see that performance of 4 Poles 2 Brushes motor and 2 Poles 2 Brush are similar, however, 4 Poles 2 Brushes is lighter than 2 Poles 2 Brushes ~ 23%, and the size is smaller as well Table 1.
Note: Definition of Winding spec 0.9*12T(S)*25T*14 Slots:
- 0.9 is the copper wire diameter
- 12T is number of turns
- S is the single winding
- 25T is the thickness of Rotor Core
- 14 is the number of Rotor Core slots
In order to reduce blower motor weight, we have to reduce the size of magnet and the amount of copper wire. But in a conventional blower motor with 2 smaller magnets and 2 brushes, the magnetic force would not be strong enough to keep the same performance as requested. Hence with the Improved Armature winding, we can use 4 Sinter Ferrite Magnets and still keep 2 brushes. We can reduce the motor size and weight without compromising the performance and brush noise which is a very important criteria for blower motors.
We would like to thank Dong Jin Vietnam JSC together with all colleagues in RD department for the support.
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