Electronic – How to control a blower using the analog output from a PID controller

induction motormotor controllerpid controllersolid-state-relayvfd

I built a biomass burner which is powered by a blower. For precise control of temperature I am using a PID controller PID500 from a company called selec . I passed the PID analog outuput to a VFD which in turn controlled the speed of a blower powered by a 3 phase motor. Everything is working as expected.

Now I am progressing with building a smaller version of this burner. In this case I am going for a centrifugal blower powered by a single phase motor. The specifications of this blower motor are

  • Voltage 230 volts
  • Frequency 50hz
  • input power 100 watts
  • current 0.4 amps
  • speed 2400 rpm
  • capacitor 2 mfd

I think using a VFD will be an overkill for this small project. As of now I am controlling the blower speed manually using a voltage regulator but we overshoot or undershoot the required temperature.

I want to control this blower motor with analog PID output as in the previous case. I don't want a on/off kind of solution-Please correct me if my understanding is wrong. A proportional modulating control is required as it is good for precise temperature control.

I did my search and was suggested to use a electric digital dimmer control.

Would like suggestions and pointers which will let me get the right solution.

Best Answer

Induction motors are called "asynchronous" because they don't go an exact speed based on the ac frequency, not because they can be easily speed controlled using voltage/current limiting instead.

  • For a thorough explanation of the motor type, the Wikipedia Article is a great place to start.
  • In general terms, induction motors can run with fair efficiency/control between 90-100% of "synchronous speed" (at 50hz, that's 3000rpm divided by the number of coil 'poles' in your motor, which is likely 1), and act as generators between 100-110% of "synchronous speed."
    • The further from 100% of synchronous speed the motor is turning/being turned, the more current it will draw/generate (without external current limiting) to produce torque towards recovering to its synchronous speed.

Thus, for best speed control of an induction motor, you'll need some form of VFD. Luckily, however, being single-pole & only 100W, you could use a rectifier+capacitor to store the power you'll need, then a timing circuit & "Logic-Level Gate" Power MOSFET to generate the needed frequency quite easily (could use as few as 4 total components added to your circuit).