Is there any difference between a speed control unit of a 3phase bldc and 3phase induction motor? (Considering the complexity in design, efficiency, weight, cost etc)
Supposing the wattage of the motor is 1.5kw -2.5kw
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There are smaller, less complex, and less expensive control units for small BLDC motors. Control units equivalent to induction motor control units may provide more control features, better performance in some respects and a higher ratio of motor torque per unit mass. The design specifics of the motors must also be considered. A lot of the research is being applied to motor and controller design for electric vehicles. In that area, the brushless motors are generally considered to be permanent-magnet synchronous motors (PMSM). In their essentials, BLDC and PMSM machines are the same. However, there appear to be differences in the details of the design, mathematical modeling and control approaches.
The power side of the controller is essentially the same, whether it's used for BLDC or an induction motor. A DC bus is used to power three half-H phase drivers, one to each phase.
The control side of the controller is quite different, whether the motor simply has to spin (like for propellors or an eBike) or has to adopt a specific position while turning (so for servoes or hover-boards). This is because an induction motor needs slip between the drive field and the rotor, and a BLDC motor is synchronous.
BLDC is rather simple to control if you use a six step commutation with help of Hall sensors.
Things get complicated when you have an PMSM which is very similar to BLDC, but you use three phase sine wave output. What you need is an encoder for rotor position feedback.
If you swap the PMSM with induction motor, then you have two distinct choices: V/f characteristics, which is simpler method or FOC. FOC is same as PMSM control method but the calculation complexity is even bigger, since you have to estimate rotor flux position by means of mathematical model and encoder feedback.
Lastly, you have a sensorless FOC control for both PMSM and induction motor. Again, the induction motor is more complex for the extra rotor flux estimator.
Cost of control: The FOC for both is almost equal price and mostly you are able to connect both motors on the same control.
Cost if the motor: the induction motor is lot cheaper than PMSM.
Efficiency: The PMSM has higher efficiency, as the PM rotor doesn't heat. It is also suitable to hold a dead weight at standstill.
Overall: The induction motor drive is a winner in cost, meanwhile the PMSM dive is the winner in terms of efficiency and high dynamics.
The real difference between a three phase induction motors and a BLDC is that one is open loop while the other has feedback.
Typically an induction motor is driven with a fixed frequency or a variable frequency and there is some delay between application of power and synchronism depending on the load torque. During that acceleration the torque varies rapidly as the excitation angle varies relative to the rotor position until it finally reaches the synchronous offset position.
A BLDC on the other hand uses feedback to apply the switching currents at appropriate times in the rotary cycle to keep the torque relatively constant during acceleration. This provides smoother and greater overall acceleration.
One is frequency driven the other is torque drive. It's the difference between driving a stepper motor open loop and attaching an encoder.
Of course you could hook up an encoder to a three phase induction motor to do the same thing, but then, when do you stop calling it an induction motor.
Which motor is better depends on a great extent on what it has to drive. If the load is fairly constant, like driving a fan for example, an induction motor is fine. If however the load can vary greatly, for example in a vehicle, then BLDC is more appropriate.