September 2021

BY Damien Coleman, Product Manager/ EBI Specialist at Snap-on T he rise in demand for high-tech vehicles has challenged engineers to rethink and create more sophisticated internal systems which promote efficiency. The humble motor is a great example of this, with many vehicle manufacturers now using brushless direct current (DC) motors in automotive applications. The brushless motor has many advantages when compared to conventional brushed motors. For one thing, it produces less noise since interaction between the brushes and commutator is no longer a factor. At the same time, no electrical arcing takes place, and there also a greater service life. In theory, this motor should last the lifetime of the vehicle. Efficiency vs complexity While more efficient, the commutation control of the motor is more complex. A conventional brushed motor uses two brushes, one positive and one negative, to allow current to flow to the armature of the motor. The reason for this is a magnetic field must be set up within the armature winding which opposes the magnetic field generated by the permanent magnets attached to the motor casing (yoke). The opposing magnetic fields will cause the armature to rotate due to the repulsion/attraction of the magnetic field. The polarity of the current flow through the armature is ‘switched’ by the commutator segments. These segments provide the electrical link between the brushes and the armature. This allows for constant and smooth rotation of the armature while the current is flowing through the motor. Fig.1 shows the current flow through a brushed fuel pump from a Ford Focus. The commutator has eight segments, and each segment has a slightly different current profile. By identifying one segment, we can use the cursors to calculate the time for one revolution of the pump. In this example the pump speed is 8.61ms for one revolution. This equates to almost 7,000 RPM. The calculation for this is, there are 60,000ms in one minute, so 60,000/8.61=6,969 RPM. A blockage in the fuel system will result in lower speed and higher current flow, whereas a leak within the fuel system will result in higher speed operation and lower current flow. Occasionally a vehicle may present with low fuel pressure with no visible leak. An area that should be investigated is the pipe from the fuel pump to the top of the tank unit, a leak here will result in fuel leakage within the fuel tank. Brushless DC motor operation A brushless DC motor requires the polarity of the current flow through each winding to be monitored to ensure the same operation under all conditions. This commutation of 32 AFTERMARKET SEPTEMBER 2021 TECHNICAL/SNAP-ON www.aftermarketonline.net FUEL PUMP PROGRESS: BRUSHLESS DIRECT CURRENT (BLDC) MOTOR the motor is managed by an electronic control module (see Fig.2), and for accurate operation the module must know the position of the motor so the correct ‘phase’ is energised for uniform operation. Previously this was achieved by using digital hall-effect sensors on the rotating member of the motor. This resulted in additional hardware and cost. Another method is for the control module to motor the back electro-motive force (EMF) in the un-energised winding. One phase (winding) will be controlled by a positive pulse width modulated (PWM) output from the module while an opposing phase will be connected to a hard ground. This leaves one phase unconnected from the control module at a given time. Understanding the waveforms It must be noted that whichever winding is un-energised will have half the applied voltage present during operation. This is due to the connection point being between windings. Kirchhoff’s law states that the sum of the volt drops in a series circuit is equal to the applied voltage. See Fig.3. This table demonstrates the module control of each phase during operation. At any one time a phase is on (PWM), connected to ground or uncoupled from the circuit. Also please refer to Fig.4. The oscilloscope waveforms in Fig.5 to Fig.10 are from a vehicle and can be used to identify the control of each phase during normal operation. These images match the table above. diagnostics.snapon.co.uk Moving his attention to elsewhere in the vehicle, Damien looks at how technological change is leading to new developments in the engine Yellow trace Green trace Blue trace Off PWM PWM Off Ground PWM Ground Off Ground Ground Off PWM PWM Off PWM Off Ground Ground Off PWM