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However, instead of a rotating field on the stator, the phases are subsequently abruptly energised to pull the rotor magnets forward. Similarly, to the PMSM the BLDC also has permanent magnets attached to its rotor. Sensitive and expensive encoders or resolvers are used to sense the exact rotor position in order to recreate the right phase of the reference sinusoidal supply voltages. In order to achieve near-sinusoidal current in all machine phases, all three inverter legs are Pulse Width Modulated (PWM) modulated with sinusoidal reference signals. Naturally, the magnet of the rotor would tend to align with the stator magnet, causing it to rotate along, at the same speed as the stator field, somewhat as it is shown in Fig. This magnetic field is the equivalent to having a magnet rotating along the internal stator circumference. Thanks to the sinusoidal shape of the stator currents and a near-sinusoidal distribution of the stator winding, this field has a constant magnitude but its spatial angle is changed so that it rotates with a uniform speed around the stator circumference, somewhat like the arms of a clock. The stator creates a Tesla’s rotating magnetic field. As the name suggests, the PMSM has a permanent magnet (or an array of them) mounted on its rotor. This section will explain in simple terms the main principles of PMSM drive. Therefore, a 1 kW, 48 V, 3 phase system for the two machine types will be investigated here. In the area around a kilowatt of power, there is prospect for both the BLDC and PMSM applications to be designed. In fact, even the machine structure is similar, with the difference being in the shape of the produced Back Electro-Motive Force (EMF) having trapezoidal shape with the BLDCs and near-sinusoidal with the PMSM machines. 3) the PMSMs have completely different modulation and control methods. While the higher efficiency, higher power and torque density makes investment in the more expensive PMSMs justified for these applications.ĭespite having the same drive structure as the BLDCs (shown in Fig. The noisier and higher torque ripple operation of the BLDC makes their employment in these applications undesirable. Some of these applications are Electric Power Steering, Starter/Alternator and Transmission pre-charge pumps. Both of the afore mentioned methods allow for better distribution of losses across the six MOSFETs, in turn enabling higher power margin up to the maximum die temperature.Īpplications that require even higher powers such as the drives involved in the propulsion of the vehicle are mostly realised with PMSM drives. Also, the roles of the switching MOSFET in one phase and the conducting MOSFET (not switching) in the other active phase can be swapped. As it will be shown later on, the switches operate in sequence, so that one of the three switching pairs does not operate at any instant, allowing the devices to cool. This enables current sharing across more devices, inherently increasing the power that can be delivered. 3 there are more MOSFETs employed in the drive compared to the Brushed DC motor. Besides, as it can be seen from the drive configuration in Fig. This enables greater robustness, less maintenance, higher power and speed operations. These motors, unlike the Brushed DC motors, do not need a physical connection to the rotor. Examples of such applications are water-pump, engine-cooling, anti-locking brake system, fuel-pump and electric steering. Higher power and control complexity drives in today’s vehicles are mostly realised with BLDC motors.