Single-phase motor control

[diegoterc3]

Hi! I'm new to the forum and have seen Dave's videos. Reaaaally helpfull.
I'm a Mechatronics student from Perú (or Peru whatever) and trying to make my own POV Globe.

(inspiration video)
(Own video _ spanish but it shows my work)

Now. I have done 2 prototypes. My first one used only 16 RGB LEDs and worked fine. The second one uses 112 RGB LEDs and they worked! Saddly the mechanics where off and it destroyed because of balancing. Well leaving that apart I am trying to control the velocity of a single phased motor. I am making an IGBT (IRG4BC20W) bridge and have some problems with it.
To control the IGBT's I am using the driver IR2110 and have some trouble understanding how the bridge would work.
I understand that the Low-Output of the driver will drive the IGBT with 15V. But the problems start when I try to use the High-Output.
Here's my thinking. (I've attach a picture to show what I mean / I know it shows mosfets but think of them as IGBT's please)

When I activate the low input of the driver the low output gives 15V to the IGBT's gate and since the emitter of it is connected to ground the voltage differece between the gate and the emitter is +15V so it closes the circuit and lets the load to have a reference. Up to this part there is no problem.

Now here is the problem. When I activate the high input, the high output is supposed to give the high IGBT the difference voltage between Vb and Vs but the emitter of the high IGBT is connected to the LOAD which has no voltage or reference!. So what voltage would HO give to the gate of the IGBT? and then when it closes the circuit it is driving up to 500V which will go directly to the Vs pin of the IR2110. This is what is giving me troubles. Can someone please explain if I am getting something wrong? I am using a PIC18F2550 to control the IR2110.

Oh! I almost forget. How can I calculate the resistence and capacitor values from the typical connection circuit?

I really appreciate your help in advance.
Thank you

[Kremmen]

OK, some basics:
-You don't need anything like an IGBT for this size application, whatever the exact type of motor you plan to use. That is total overkill and will only make your project more difficult. Regular MOSFETs are perfectly adequate and far more user friendly.
-You write that your motor is "single phase". Do you mean an actual single phase induction motor? That would definitely be far from my #1 choice for this application, and the half-bridge is not at all suitable for controlling one. So lets assume you meant a regular brush DC motor.
- Again, using a bridge is probably more than you need and only complicates the electronics. If i understand correctly, the idea is to speed the led ring up to a constant speed and then just maintain that speed? In that case a single pwm controlled FET is all you need. No need for a bridge driver, you can take the control output directly from a pwm pin of the MCU.

If any of these assumptions were wrong then we need to think again, but this is how i would do it.

P.S. A nice and fun application. I wish you luck in completing it

[diegoterc3]

Thanks for your reply Kremmen!
I will answer in order:

-I planned on using MOSFETs and read that the base of them couldn't switch at the rate I am planning to do it. Since I am using a PIC18F2550 that works at 48MHz CPU speed and the PWM I use to switch the IGBT gate is at 100us to emulate the sine wave.
-Yes, it is a single-phase induction motor. I got it from a LG washer machine great state. I got it for free and here in Perú DC brushless motors are really expensive and can't even talk about getting one from another country. I am designing the full brige it is just that the IR2110's Datasheet just shows the half bridge and I am trying to understand how it works so that I can design the full bridge.
-Yes, I just plan to speed the led ring up to a constant speed.
I will try to find one DC brushless motor but in that case how can I calculate the HP that it will require? Sorry I know this is a mechanics question and will ask somewhere else also but if anyone has any idea it would be great.

I really aprecciate your help.

[Kremmen]

Thanks for your reply Kremmen!
I will answer in order:

-I planned on using MOSFETs and read that the base of them couldn't switch at the rate I am planning to do it. Since I am using a PIC18F2550 that works at 48MHz CPU speed and the PWM I use to switch the IGBT gate is at 100us to emulate the sine wave.

100us is actually quite OK for a MOSFET, no problem at all. You just need to pick a suitable MOSFET. It is actually the IGBT that is slow; their internal structure has built in features that cause a slow turn-off. That's why you don't find IGBTs in high frequency switching circuits, whereas you do find MOSFETs.
The speed of your MCU is irrelevant if you are using a gate driver such as the IR2110 to control the actual gate. IR has plenty of app notes for the IR2xxx family, just google and you wil find them.

-Yes, it is a single-phase induction motor. I got it from a LG washer machine great state. I got it for free and here in Perú DC brushless motors are really expensive and can't even talk about getting one from another country. I am designing the full brige it is just that the IR2110's Datasheet just shows the half bridge and I am trying to understand how it works so that I can design the full bridge.

Please make very sure what you actually have. As far as i understand, modern washers do _not_ use induction motors. They are either brush universal motors or brushless DC motors that are actually 3 phase AC motors. If you can provide a photo, it will clarify.
In any case a washer motor is... well. Like using a cannon to shoot a fly. It has orders of magnitude more power than you need to spin that little LED contraption. Also potentially dangerous because they are direct off line motors and you maybe would be better off not having that.

-Yes, I just plan to speed the led ring up to a constant speed.

Then you will get by with a very, very much smaller motor.

I will try to find one DC brushless motor but in that case how can I calculate the HP that it will require? Sorry I know this is a mechanics question and will ask somewhere else also but if anyone has any idea it would be great.

Its OK, i do a lot of that
In this case the power you need is only that needed to overcome bearing and air friction. There is no traction load at all. If the ring is well balanced and you have good bearings (like say the shaft ends in a needlepoint resting on a hard bearing cup), the power will be almost nothing, a few watts at most. So even a small toy motor could probably pull this off. The motor does not by any means have to be a _brushless_ motor. That is an unnecessary complication because brushless motors are very complex to control compared to brush motors.

I really aprecciate your help.

De nada. Happy if i can help.

[diegoterc3]

Its OK, i do a lot of that
In this case the power you need is only that needed to overcome bearing and air friction. There is no traction load at all. If the ring is well balanced and you have good bearings (like say the shaft ends in a needlepoint resting on a hard bearing cup), the power will be almost nothing, a few watts at most. So even a small toy motor could probably pull this off. The motor does not by any means have to be a _brushless_ motor. That is an unnecessary complication because brushless motors are very complex to control compared to brush motors.

I think I didn't explain myself well. The one that is really small is using a 12VDC motor. I am trying to make a bigger one which has 112 RGB LEDs. I don't think I can use a toy motor with it.

There is another video I have showing it working and the motor. Watch from minute 5 to 7 please. Also I've attach some pictures too.



I will search for a suitable MOSFET for my application.

I will also inform if there is any news on the motor selection.

Please let me know if you have any other ideas. Thank you

[Kremmen]

I think I didn't explain myself well. The one that is really small is using a 12VDC motor. I am trying to make a bigger one which has 112 RGB LEDs. I don't think I can use a toy motor with it.

There is another video I have showing it working and the motor. Watch from minute 5 to 7 please. Also I've attach some pictures too.



I will search for a suitable MOSFET for my application.

I will also inform if there is any news on the motor selection.

Please let me know if you have any other ideas. Thank you

Ok the photo and video made things clearer. The motor you have really does appear to be a single phase induction motor. It is good in the sense that the motor is simple and reliable, but bad in the sense that speed control of an induction motor is not so simple. To do it "properly" you would need to vary both supply voltage and frequency. Usually that is done with a ready made VFD drive unit because of the complexity.
Do you actually need to vary the speed in real time or is it a question of just accelerating to some speed and then keeping it constant? In the latter case my suggestion would be to 1: dimension the transmission so that the device turns at the desired speed when the motor is connected directly to the AC supply. 2: design a simple circuit using a bidirectional optoisolator and triac to ramp up the motor voltage from 0 to full AC over a reasonable acceleration time. For this you need to detect AC line zero crossings and implement a simple variable delay in the MCU to trigger the triac at decreasing delays until in the end you trigger the triac continuously (or bypass it using a relay or whatever you consider best). This will not give you an exactly controlled acceleration ramp but maybe that is not so important. I assume the shaft angle is measured by an encoder or similar for LED synchronization anyway?
Examples of the triac circuit i mentioned  can be found by googling.

[IanB]

In the (distant) past I found that simple induction motors were found in things like tumble driers that ran at a constant speed, whereas brushed universal motors were found in washing machines that needed speed control for agitation, spinning, etc.

An induction motor I got from a clothes drier just needed connecting to the mains and it would start and run at its design speed. It didn't need any special speed control. With such a motor I would just connect it to the POV globe using suitable gearing and control it with a switch.

My induction motor was configured to run in one direction, but it could be reversed by swapping the order of some wires inside leading to the coils. I was able to fit a DPDT switch to make the motor reversible (but it had to come to a halt before flipping the reversing switch or it would just jolt and keep running in the same direction). If you had a similar motor and you needed to reverse it on demand you could probably do the same trick with a relay.

[johansen]

one way to get fixed 60hz synchronous operation from an induction motor is to take the rotor out and grind flats in it.

as far as changing the frequency you don't need to bother changing the voltage in real time as well, provided you run the motor faster than its nominal rating.

most induction motors are designed to run completely saturated as well and it might make sense to run them below nominal voltage at the expense of much lower power. (70% efficiency is pretty good for anything less than 1/2 hp)

[diegoterc3]

Ok the photo and video made things clearer. The motor you have really does appear to be a single phase induction motor. It is good in the sense that the motor is simple and reliable, but bad in the sense that speed control of an induction motor is not so simple. To do it "properly" you would need to vary both supply voltage and frequency. Usually that is done with a ready made VFD drive unit because of the complexity.
Do you actually need to vary the speed in real time or is it a question of just accelerating to some speed and then keeping it constant? In the latter case my suggestion would be to 1: dimension the transmission so that the device turns at the desired speed when the motor is connected directly to the AC supply. 2: design a simple circuit using a bidirectional optoisolator and triac to ramp up the motor voltage from 0 to full AC over a reasonable acceleration time. For this you need to detect AC line zero crossings and implement a simple variable delay in the MCU to trigger the triac at decreasing delays until in the end you trigger the triac continuously (or bypass it using a relay or whatever you consider best). This will not give you an exactly controlled acceleration ramp but maybe that is not so important. I assume the shaft angle is measured by an encoder or similar for LED synchronization anyway?
Examples of the triac circuit i mentioned  can be found by googling.

Yes, I am accelerating to half the rpm of the motor and keep it constant. The thing is that for the persistence of vision to occur I have to show at least 15 to 20 frames per second and through some calculations I found out that the minimun speed of the shaft had to be 500 rpm. Now I am not sure how good the quality of the animation would be but it would be nice if I can control the velocity of the motor because if for some reason I don't like the quality I can just change the velocity by software in the other case I would have to change the transmission again and again. I've been thinking of changing the transmission with belts instead of gears because they are easier to change and safer because if someone decides to trip over the functioning mechanism it would just leave a bruise instead of cutting everything off.
Now I have found some circuits using SCR but I don't know how reliable these circuit can be, because as you said to control this kind of motor properly I need to vary the frecuency and the voltage. What do you think?

[diegoterc3]

In the (distant) past I found that simple induction motors were found in things like tumble driers that ran at a constant speed, whereas brushed universal motors were found in washing machines that needed speed control for agitation, spinning, etc.

An induction motor I got from a clothes drier just needed connecting to the mains and it would start and run at its design speed. It didn't need any special speed control. With such a motor I would just connect it to the POV globe using suitable gearing and control it with a switch.

My induction motor was configured to run in one direction, but it could be reversed by swapping the order of some wires inside leading to the coils. I was able to fit a DPDT switch to make the motor reversible (but it had to come to a halt before flipping the reversing switch or it would just jolt and keep running in the same direction). If you had a similar motor and you needed to reverse it on demand you could probably do the same trick with a relay.

Yes, I tried to run it as it was on my second prototype and there were two things I didn't like.
1. The mechanical noise was anoying!
2. Since the motor didn't have any control, it would just try to accelerate at maximum speed as soon as I turned it on and this would cause A LOT of vibration and it would unbalance the mechanism. That's how it got destroyed 

That is the reason I am trying to control the speed. I am also changing the mechanical design just to make sure.

[diegoterc3]

one way to get fixed 60hz synchronous operation from an induction motor is to take the rotor out and grind flats in it.

as far as changing the frequency you don't need to bother changing the voltage in real time as well, provided you run the motor faster than its nominal rating.

most induction motors are designed to run completely saturated as well and it might make sense to run them below nominal voltage at the expense of much lower power. (70% efficiency is pretty good for anything less than 1/2 hp)

Hi Johansen, I am trying to run the motor at a lower RPM and have heard that if I don't control the voltage the inner coil would heat up, melt the insulation and cause a short circuit. I would like to confirm this from another source too since I haven't experienced it myself.

[IanB]

Yes, I tried to run it as it was on my second prototype and there were two things I didn't like.
1. The mechanical noise was anoying!
2. Since the motor didn't have any control, it would just try to accelerate at maximum speed as soon as I turned it on and this would cause A LOT of vibration and it would unbalance the mechanism. That's how it got destroyed 

Then I'm not sure it is an induction motor. An induction motor will run at a fixed moderate speed and the speed will not change very much whether the motor is unloaded or loaded. An induction motor usually is very quiet. It will make a humming noise rather than a screaming noise.

On the other hand a universal motor will try to run as fast as it can especially if there is no load on it. It will scream with a lot of noise and vibration unless speed control is used to slow it down. (The motor in an electric hand drill is usually of this type.)

I suspect that you may therefore have a universal motor. If you got it from a washing machine it is almost certainly of this kind.

On the other hand, speed control should not be too hard. You just need a regular triac dimmer circuit with some protection against the back-emf spikes from the motor to stop it damaging the triac.

[Kremmen]

In a "normal" induction motor there is no inner coil as such. Not one wound from copper wire at least, that is. Instead what they have is a so called squirrel cage that is just cast aluminium in the shape of 2 rings at each end of the rotor, connected by rods of the same cast in longitudinal grooves on the rotor surface. Such a rotor needs to be real hot before damaging the cast aluminium...

It is your project of course and you do as you please. Still please heed my warning: speed control of an induction motor is a can of worms best avoided unless you do a "professional" job. If it was me, i would definitely spend some time calculating / prototyping until you know what speed you want the loop to spin. Then build the transmission to get that speed. Controlling acceleration roughly is just a question of reducing the starting torque to avoid yanking the mechanics. That doesn't need to be precise or complicated - or even theoretically correct - because it is a momentary thing.
This way you will get a very reliable and simple system- simplicity is one of the main virtues of an induction motor.
The second alternative that i would still consider myself is using the simple acceleration controller as speed controller as well. It is not correct in that it doesn't change the supply frequency, but it will work for a simple constant torque load like yours.
You could get one of those from eBay for just a few dollars - less than the components separately... That could be the easiest way forward - you just switch the power on/off form the MCU program and let the AC speed control take care of the motor. You can set the speed with a simple pot - no need to fiddle with program. All you need to control is a relay to energize the motor.

Edit: If IanB is right and you do have a universal motor after all, then this kind of controller will work for that also.

[Zero999]

Universal motors are series wound and are not designed to run unloaded as they will reach an unsafe speed and possibly self destruct. Universal motors are used in applications which never run unloaded such as vacuum cleaners and electric vehicles.

A parallel wound brushed motor is what you need for this application because it will run at a constant speed, regardless of the load. Appliances such as drills, food mixers use parallel wound motors because they're often operated with no load.

I would suggest a parallel wound brushed motor for this application because it can be run unloaded and it can easily be controlled with a phaser controller circuit, similar to a lamp dimmer.

[Kremmen]

Universal motors are series wound and are not designed to run unloaded as they will reach an unsafe speed and possibly self destruct. Universal motors are used in applications which never run unloaded such as vacuum cleaners and electric vehicles.

Not necessarily. A universal motor can be series, parallel or compound, completely depending on the application. As it is perfectly possible that a washer or dryer can run empty (whether it makes sense or not) it is highly unlikely to have a series motor. It is not impossible but i would consider it a serious design error. You can find series wound motors in applications such as you mention, and certain kinds of traction systems where a no-load situation is not possible.

A parallel wound brushed motor is what you need for this application because it will run at a constant speed, regardless of the load. Appliances such as drills, food mixers use parallel wound motors because they're often operated with no load.

Yes. And i think a washer would have one of those for the very reasons you also mention.

I would suggest a parallel wound brushed motor for this application because it can be run unloaded and it can easily be controlled with a phaser controller circuit, similar to a lamp dimmer.

That is what we have been recommending all along here

[Zero999]

Not necessarily. A universal motor can be series, parallel or compound, completely depending on the application.

Can't a universal motor be powered from either AC or DC? I'd suspect a parallel wound motor designed for AC operation would fail if it were powered from the same DC voltage as the stater winding would overheat as it will have no inductive component at DC. A series wound motor should be able to run off DC, it'll just go a little faster and as long as it's not too fast it shouldn't be a problem.

[Kremmen]

Not necessarily, and in this case your suspicion is moot anyway because the AC motor would still be used as an AC motor. The only thing to watch out is if the motor happens to have a separate field coil connection. That could well be the case in order to achieve a constant field independent of the armature voltage. Lacking detailed photos of the motor we cannot say...

[diegoterc3]

This way you will get a very reliable and simple system- simplicity is one of the main virtues of an induction motor.
The second alternative that i would still consider myself is using the simple acceleration controller as speed controller as well. It is not correct in that it doesn't change the supply frequency, but it will work for a simple constant torque load like yours.
You could get one of those from eBay for just a few dollars - less than the components separately... That could be the easiest way forward - you just switch the power on/off form the MCU program and let the AC speed control take care of the motor. You can set the speed with a simple pot - no need to fiddle with program. All you need to control is a relay to energize the motor.

Edit: If IanB is right and you do have a universal motor after all, then this kind of controller will work for that also.

So in other words, I could buy an AC motor accelleration controller and give the signals from my MCU and it will work?
I am planning to have it run for at least 12 hours (it can be more).

On the other hand what are the cons of controlling the voltage but not the frecuency in an AC motor? Will it be able to work safely on a 12 hour basis'?

[Kremmen]

The great benefit of an induction motor is that it will effortlessly and reliably run indefinitely, basically forever, provided you don't grossly overload it and that cooling is adequate. The acceleration controller is called a "soft start" device. Those are meant to smoothly accelerate an induction motor from standstill to full speed, with adjustable acceleration time. Usually the acceleration time is set with a simple potentiometer but it depends on how sophisticated device you have.
Additionally, you may need a power relay to switch the mains power on and off.
You can find basic information about soft starting from this booklet from my former employer ABB: http://www05.abb.com/global/scot/scot209.nsf/veritydisplay/2985284834bcff7fc1256f3a00274038/$file/1sfc132002m0201.pdf
It may be a bit heavy for your application, but the general info should be good.

[diegoterc3]

Thank you so much for your replies! I will do the soft start for my induction motor. It is easier and I just have to calculate the gear relation for it. When I finish this I will post a video and a circuit