Please help, i'm stuck AC Motor Controller

[DaveCat]

After searching the web for days and weeks i've finally plucked up the courage to ask you guys. I have a reasonable knowledge of Electronics but this one is a baffler.

I have a 16V AC Synchronous hobby motor that I want to vary the speed of.

How do you do it?

Thanks for your time

 

[picbits]

You have a couple of ways to do this.

Either vary the input frequency or bang an encoder on it and chop the ac input with a triac with a variable trigger angle. The slippage of the motor at lower power enables the speed of the motor to be varied.

I've used the triac/encoder as a closed loop system before on my washing machine controller. It was good for around 60rpm minimum up to 3000 rpm maximum.

 

[dknguyen]

After searching the web for days and weeks i've finally plucked up the courage to ask you guys. I have a reasonable knowledge of Electronics but this one is a baffler.

I have a 16V AC Synchronous hobby motor that I want to vary the speed of.

How do you do it?

Thanks for your time

16V AC Synchronous hobby motor = brushless motor? Because at that low, voltage and the fact that it is a hobby motor probably means it is a brushless motor (which is a special kind of synchronous AC motor designed to run off of low voltage DC, but the DC has to be turned into AC via a special motor controller).

So is it a DC brushless motor you mean?

A regular brushed motor controller is an H-bridge which consists of two half-bridges (it's a a bit like a two-phase inverter). A brushed motor, however, has brushes to automatically switch the polarity of the voltage,currents and magnetic fields inside the motor in the proper order to let the motor turn.

IN a brushless motor you must switch the polarity of the magnetic fields yourself. So you need circuitry to sense the rotor position- an absolute encoder, hall sensors, or back EMF sensing circuitry (for sensorless operation). Also, since a BLDC almost always has 3 phases, you need to use a 3-phase inverter (3 half-bridges). So the circuit is considerably more complicated than a regular DC brushed motor driver.

It is not a trivial circuit, and if you are starting out you really should build H-bridges first for brushed motors.

https://en.wikipedia.org/wiki/Image:H-bridge_inverter_cjc.png
https://en.wikipedia.org/wiki/Image:3-phase_inverter_cjc.png

 

[DaveCat]

Hi picbits,

Thanks for the reply. So far, i've figured out that I need to vary the frequency but am stuck on how to proceed. I looked at a Triac circuit last night but was'nt sure if it went as low as I need it.

The motor runs at a maximum of 15 rpm. I want to be able to vary it lower.
If it helps, the motor is used in a 1:87th scale model.



EDIT:

dknguyen.

It is this motor here:

**broken link removed**

 

[dknguyen]

How many wires are coming out of the motor? Just two? If it's just two...it really is a tiny, low voltage synchronous motor and not a BLDC motor! Weird!

In that case you have to control the motor speed by varying the frequency (not the voltage). So you need to use a transformer, bridge rectifier, and large smoothing capacitor (or just a battery) to get some DC voltage somehow.

THen you need to build a two-phase inverter circuit (an H-bridge). You then switch it on and off to make an AC-like waveform that varies in frequency. You can use MOSFETs rather than triacs since you are switching a DC voltage, and not an AC one. THe waveform you make will be more like a square wave unless you greatly increase the complexity of your circuit.

The next step-up is to add in an inductor/capacitive filter on the output of your inverter and vary the PWM input to the inverter sinusoidally. THe filter will filter out the high frqeuencies only letting the fundamental through producing a sine-wave.

 

[picbits]

At that price I'd seriously consider using a small stepper motor.

Whats its intended use ? A stepper will give you almost infinitely variable low speed RPM.

By the link you gave I guess you're in the UK so if you want an cheap steppers give me a yell as I've got a box of brand new ones which will run off 12-24v.

 

[Pommie]

It could also be a brushed motor with wound stator coils, also weird. If this is the case then varying the frequency would have no effect.

As picbits says, I'd use a stepper for the job.

Mike.

 

[stevez]

I've seen but never used, small motors described as synchronous - probably made to drive mechanisms that required relatively accurate timing - clocks come to mind but there are certainly other applications. My guess is that you'd treat them as you would a larger AC motor and vary the frequency. Note there are particular voltage/current relationships that are to be maintained as you vary the frequency. Took a class on the subject - can tell you I remember that part - enough so that I'd consult a proper manual if I planned to actually do it.

 

[DaveCat]

Guys,

Thanks for the replies. I've thought about changing to a stepper but it may prove difficult, since the motors are used to drive scale models. They are fit to dimensions and are moulded into the models. The likelihood of getting an exact fit is remote. I've already looked around for some small DC motors that match with no joy.

I could make it fit with some jiggery pokery but this is not exactly a need, more of a want. It may be such that I live with it for now.

Thanks anyway, the research i've done into H Bridges and inverters has proven to be a valuable learning experience in itself, so it has'nt been an exactly fruitless journey.

 

[picbits]

I would double check with the supplier to make sure its not a misprint - it might be a DC motor and you could have been chasing your tail for nothing.

 

[Hero999]

According to the picture, there are just two wires.

**broken link removed**

The wierd thing is that the specification does say that it requires 12V-16VAC and doesn't specify the frequency. Do they sell a separate controller module?

It seems hard to imagine that they'd sell a motor that looks like a child's toy that requires you to build a complicated driver for it to work.