driving a 1phase induction motor?

[Jordy101091]

Hi all,

I have done some research on this item on how to drive a single phase induction motor or an 3 phase motor.
How I would like to do this is as follow.

For driving a 1 phase induction motor I want to build something like this that controll de sinus wave from.



The first problem is that I need to shift phases 90 degrees. I need to do this because of the condensator. This condensator cannot take High frequenties it will short out.

This is only to start the motor. What I like to do is to control the speed by frequentie not by changing the voltage. I'm not sure how to do this.
I have downloaded some files form that explain how to make a lookup table for sinus waves I have attached that file.
Maybe you can help me to figure this out.

Here are some pics of the motor that I want to control.

[medelec35]

Hi Jordy101091 I don't believe that circuit will work. Would you mind posting the link to the circuit?
You have got an AC supply, whereas it must be a DC supply for several reasons.
1)Transistors are designed for DC.

2) The diode connected between the collector and emitter is for snubbing back emf from a DC motor that's connected to a DC supply.
If you connected to an AC supply, then on the negative half cycle all the diodes will be forwarding conducting!
This will have the same effect of short-circuiting the mains. So either blown diodes and/or blown fuse.

3)For the more basic circuits, PWM is used for controlling motors on DC and timing from zero-crossing is used for AC.

Controlling the frequency as you mentioned in your post! is VFD which is variable frequency drive.
This is far more complex than controlling the voltage method as shown below.
I have only controlled AC motors by the methods below and not by frequency.

There are two simple ways of controlling an AC motor using a microcontroller.
1) with a triac
and
2)with a FET short circuiting +ve & -ve of a bridge rectifier.
FETs are designed for DC. Since its shorting the +ve & -ve then there is only DC going to the FET. the Load is connected to the AC side. When the FET shorts the +ve & -ve connections, then the load is connected to the mains. When the FET is off, the load is disconnected from the mains.
In my opinion 2) is by far the best method. The load humming coming from AC motor associated with method 1, is eliminated with method 2.
But you must connect a suitable run/start capacitor connected in parallel with the motor or else the back EMF will cause too much current voltage across FET. If cap too high then too much current will flow though FET. You can use a low value resistor connected in series with source of FET to monitor current flow and shut off dive to fet if too high.
How the microcontroller can control the motor is:
A) Wait for zero crossing.
B) When zero crossing is detected, then switch FET on and start a timer. when suitable time has elapsed (9 ms for approx full speed, 5 ms for about half speed etc.) then shut FET off and go back to step A.

Hope is is a bit of a help.

Martin

[Jordy101091]

Can you explain in more detail how to control my motor with option (2) -> FET
I'm very interesting.

I dont know what you exactly mean, hope you can explain with some schematics.

Thanks Jordy

[medelec35]

IMPORTANT: MAINS CAN (AND DOES) KILL!
You should only attempt this if you have experience with live AC mains.
If not then you should get someone else who has experience dealing with mains to help.
Here is the principle. The Motor is AC not DC, so only AC will be powering it.
The FET I mentioned will replace the switch and is driven via an opto-coupler.
Obviously the correct FET and diodes must be chosen. E.g. ones that can handle motor current, and peak mains voltage. For the FET I would recommend at least 450 – 500V DC.
Since start up current is large for motors, I would also recommend a low value resistance in series with FET Source. I used a 1 Ohm 7Watts
Then if required the microcontroller can monitor the voltage drop across resistor, and cut drive to FET if excessive.

Motor Off.jpg (23.07 KiB) Viewed 6357 times

Motor On.jpg (25.2 KiB) Viewed 6357 times

[Jordy101091]

Hi medelec,

Your idea would indeed work, but the problem is that I want to control the speed of my motor.
But I think I have an idea.

Maybe I can build something like this:



I think I can drive the two mosfets by an PWM single, where the dutycycle is precalculated in my lookup table.
I can change the frequentie of this output signal by changing the peroid time of my PWM.

The only thing I need to figure out is how to let the PWM module communicate with my lookup table.

Medelec,

What do you think?

Regards Jordy

[JohnCrow]

Hi Jordy
What program did you use to do produce the circuit diagram in your last post?
They look really good

[medelec35]

Hi medelec,

Your idea would indeed work, but the problem is that I want to control the speed of my motor.

The method I posted does indeed control the speed of the AC motor.
As explained previously. the chip will have to wait for zero crossing. Then FET is switched on for a time period of a time period between 4ms and say 9.4 ms. Not 10 ms as you don't want to miss zero crossing point.
You will find that the voltage across your motor will be between 80V and 230V (assuming a 230V supply)
so 7ms could be 120V etc.

As for your latest circuit. If using 1 PWM, to drive a H bridge driver, you will need complementary transistors to make sure only one transistor (one connected to +ve and one connected to -ve) from each side is conduction at a time. Also you don't need lut if you don't want to. PWM can be derived form a linear formula, which in turn can be determined by a pot etc..

[Jordy101091]

@ JhonCrow

I'm using National Instruments Multisim v11.0

@ Medelec

I think I want to use a lookup table. because I would like drive this with an Microcontroller and digital input not analoge.
Instead of an pot. meter I want to use up-down buttons to change the frequentie. the data of the sine wave wil always be the same, the only thing I have to change
is the period time of my PWM to change the frequentie of my AC output signal.



With the values of Complete Array Declaration String, I can use these to control my PWM duty cycle. And finnaly create this pattern.



With my digital control input (up-Down) I can could change the peroid time of my output signal and by that the frequentie.

[Steve001]

Hi,

FIRSTLEY I HAVE TO AGREE WITH MEDELEC MAINS VOLTAGES CAN AND DO KILL !!!!!!!


for speed control of a AC motor you need to alter the frequancy to it.As medelec stated your circuit will have the same effect as shorting out the mains supply.

You need also to provide interlocks so that if (or when) a transistor / mosfet fails the other half transitor cannot be switched on.
looking at your motor it is hard to tell if it is a capacitor run or capacitor start motor .
If your motor is a capaictor run motor it is more than likely a 3 phase item.

Transistor invertors are very widely used, feedback diodes are still required and are an essential paert to the invertor operation.the bipolor transistor can be switched on much faster but does require a large amount of base current during the on period, the base current has to be carefully controlled in magitude to keep the transistor just in saturation. Too high a base current extends the turn of time and delays the removal of base current which is necessary when a fault occurs.

Darlinton arrangements considerably reduce base drive currents at the expense of increasing the switching times.
Care must be taken to allow a delay of sevral micro seconds after turn off of the transistor before it's complementary transistor is turned on, otherwise ther is a risk of reconduction in the outgoing transistor , giving a short circuit across the dc source with the two transistors.

By far the fastest switching device is the power mosfet, used in invertors at the lower power ratings is proportional with the lower power rating.Being a voltage controlled device the gate firing controls are far less troublesome and it is possable to implement the controls directly from a micro circuit.

Using commutation networks limits the invertor frequency to about 10kHz with bipolor transistors and about 25Khz with a power mosfet these figurs are a guide only because it is necessary to ballance conduction and switching losses together with the load losses.A limiting factor is the reverse recovery of the feedback diodes.

The object of using faster switching devices is not only to enable the invertor to operate at higher frequanceys but to enable the inverter to be controlled in pulse width moduation mode. The more switchings that can be made within each cycle the more low order harmonics can be eleminiated. by eleminiating the low order harmonics a much smaller filter can be employed to convert the stepped wave form to a sine wave


Protection of inverter devices is very important in the event of a load short circuit or any other fault condition.
Carfull fuse selection is a must as extra collector current will take the transistor out of saturation, a high collector emitter voltage been developed, leeding to high losses internal to the transistor and desruction without the current being raised to a level to operate a fuse. Likewise the current in a power mosfet is self limiting with a drain - source voltage developing which will over heat the device.For both the transistor and the power mosfet it is neccessary to detect the fault and turn off the device by removing the base current or gate
voltage or removing the dc input voltage to the inverter.

care MUST also be observed when connecting and where you are connecting oscioscopes as the effect of a connected scope slows down the turn off time of the device and can also prevent a device from fully turning off giving a short circuit across the dc source with the two transistors.

Not to put a down on things but i would replace the motor for a DC Unit and a speed controller (A lot easier)

Afterall this is mains voltage and saftey is a must !!


sorry to waffle on hope this helps

steve

[medelec35]

I'm incline to agree with Steve.
It's fine experimenting with low voltage & current, but mains power is a different matter altogether.
With the huge surges that can occur, and great chance of death, Its probably better sticking to a ready made bought version just for the job of controlling high voltage/current motors. Last thing you would want is a fatal shock or something blowing up in your face.
Sorry for sounding so negative, but you can't put a price on life.