In the broadest sense, the major difference is in the rotor itself. In a squirrel cage induction motor, current is induced into the rotor from the field (stator) through the air gap, and conducted through aluminum (or other material) bars, which are most often die cast in the slots of the rotor laminations. In the case of a DEC Star® motor, the rotor itself contains permanent magnet material, which is either surface-mounted to the rotor lamination stack or embedded within the rotor laminations. In either topology, electrical power is supplied through the stator windings.
It doesn't have to, but we chose this topology to reduce cogging torque.
The most basic source of cogging torque is the interaction or attraction of the permanent magnets and the steel structure of the stator as the motor rotates. These attractions and overcoming them prevent the rotor from turning smoothly. Another source is the interaction of the rotor magnets and the stator winding when it is energized, due to harmonics. Cogging is often an undesirable feature, causing noise, vibration and non-uniform rotation, so during product development, minimizing this effect was a design “CTQ” (Critical To Quality). As a result, DEC Star® motors have extremely low cogging torque, resulting in smoother operation at all speeds, virtually eliminating torque and speed “ripple”.
High current or high operating temperatures can cause magnets to lose their magnetic properties. The drive reduces the risk of high current “demag”, as these devices are equipped with over-current protection. The motor design minimizes the possibility of excessive temperatures causing magnet failure, due to the selection of high temperature magnets, incorporation of thermostats and low operating temperature of the motor. Permanent magnets, once demagnetized, cannot recover, even if the current and/or temperatures return to normal levels.
Power density is simply the ratio of output power or horsepower to physical size or volume of the motor. There are many factors such as material characteristics and temperature constraints that limit how much power a machine can deliver of a certain size. Different topologies and machine configurations address these limitations in various ways. For example, rare earth permanent magnets produce more flux for their physical size than the magnetic energy (and resultant torque) produced by an induction motor’s “squirrel cage rotor”. As such, a DEC Star® motor can have higher power density than an equivalent rated IM.
Form factor is a different way of looking at power density. In this context, form factor refers to the physical (primarily dimensional) properties of the motor, which may be defined as the active materials or the overall envelope size. The simplest view of this is the frame size reduction potential of the DEC Star® versus IM…assuming the same power rating. Another way of looking at this is the ability to “down-frame ” the same power rating into a physically smaller package (frame size).
The term commutation comes from the world of DC motors. It refers to how current is routed to the right coils, in the DC rotor, at the right time to generate the torque needed by using brushes and a commutator. A commutator is round, mounted on the rotor shaft and has conductive pads on the O.D. on which the brushes rest. Current is conducted from the brushes to the commutator and then to the connected coils as the rotor spins. This term has been carried over to brushless DC motors where the brushes and commutator have been replaced by electronics and a sensor on the rotor shaft. In this case current is still “switched” but by electronics. The term loses meaning in the world of AC motors but still sometimes refers incorrectly to how the AC voltage is generated in the drive.
The Evergreen IM is designed to replace 1075 RPM, direct drive PSC (Permanent Split Capacitor) indoor blower motors in any residential or light commercial HVAC system, 5 ton or less of cooling and 150k Btu or less of heating capacity. That includes all fossil fuels, electric heat, A/C, heat pump, dual fuel and geothermal systems. The Evergreen IM is built in two multi-horsepower sizes to replace 1/5 – 1 horsepower PSC motors. Both models are dual voltage (115/230) and dual rotation (CW/CCW). This reduces motor selection and inventory to just two for almost all applications.