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

Question

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.

Answer 1

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).

Answer 2

The rotor of an induction motor experiences a torque proportional to the rotational speed difference between the rotor and the magnetic field, while the magnetic field rotates at a rate determined by the supply frequency and motor pole count. Theoretically if the rotor spins at the same speed as the field, the rotor experiences no torque, but as soon as the motor slows down even a tiny amount the torque (and current) rapidly build up.

Before the advent of variable frequency drives (VFD), such motors were generally restricted to fixed speed service. This is because if you try to reduce the speed by just reducing the effective voltage with a dimmer, you will find that the motor will still spin at approximately the same rate irrespective of how much dimming is applied while drawing greater current to compensate for the lower voltage (likely overheating the motor in the process).

VFDs allow the motor frequency and voltage to be scaled at will, allowing the speed to be varied. Induction motors mostly come in two flavours, single and three phase. While the three phase motors are well suited for being driven by a VFD, the motor you linked is a single phase induction motor. Single phase motors do not lend themselves well for variable frequency operation. This is because they really are two phase motors with a capacitor creating a phase shift in the current of the second phase. That capacitor was chosen for your mains frequency and becomes ineffective at lower frequencies.

You need a blower with a three phase motor and a VFD, or you need to use a different motor type. Only universal motors can be controlled with just a triac dimmer, but they spew out electrical noise and have brushes that wear out making them unsuitable for fans.

Answer 3

Use the phase angle control circuit, like SCR dimmer, possibly use shaded pole motor. A very suitable circuit for phase angle control is Atmel U2008B, it is also possible to make an interface with opto coupler and send setpoint speed with PWM output from MCU.

Answer 4

I was able to get what I wanted working by using a Proportional control solid state relay I found here It costed me around 18$ (USD). It relieved me of the pain to run a VFD and all complexities associated with it. A more famous supplier can be found here. But they were costly almost the price of VFD.