Hi Sir

[crying_baby]

reply

Crying Baby.

Your experiment will fail.
A small VFD will do the job reliably.
Why do you insist on doing it with PWM from a regular PLC ?

its a proposal to do a simple , accurate and fast respond of PLC in inverter fed induction drives. its not convern of accuracy but its just need a simple putcome of speed, position and fast brake control for the induction drive.

 

[JesperMP]

Crying Baby.

Do you know about 3-phase AC induction motors ?
It is not enough with just one relatively constant PWM output.
There are 3 phases, and positive and negative periods, all in all 6 PWM signals.
These signals must vary quickly to form something that approaches 3 sine-modulated PWM pulse trains.

[my lecturer] is just simply propose the student to do a simple , accurate and fast respond of PLC in inverter fed induction drives

Reading the above, I am not even sure that your lecturer is asking you to drive an AC motor directly off a PLC's PWM outputs.
It could be that you have to determine the motor response over the time when the PLC changes the control of the motor (via a VFD possibly).
And it could be something completely different.

 

[crying_baby]

Crying Baby.

Do you know about 3-phase AC induction motors ?
It is not enough with just one relatively constant PWM output.
There are 3 phases, and positive and negative periods, all in all 6 PWM signals.
These signals must vary quickly to form something that approaches 3 sine-modulated PWM pulse trains.

Reading the above, I am not even sure that your lecturer is asking you to drive an AC motor directly off a PLC's PWM outputs.
It could be that you have to determine the motor response over the time when the PLC changes the control of the motor (via a VFD possibly).
And it could be something completely different.

could you suggest a way a hint that this could be done in another way but still, using a PLC>? maby control a stepper motor or dc motor? (less den 1hp).

if so, could i use the PWM outputs directly?

 

[elevmike]

Crying_Baby,

1. PWM = Pulse Width Modulation, is a power control signal used to drive a DC motor at variable speeds. Instead of varing the armature voltage, the Drive pulses out short full voltage pulses for low speeds and longer full voltage pulses for higher speeds. This is not intended for controlling AC motors, or stepper motors. The PLC does not provide a PWM signal, but maybe a reference analog signal to the PWM motor controller, that in turn provides the PWM power signal to the motor.

So to make it short. the use of a PWM signal to drive an AC motor is not a proper application.

 

[JesperMP]

Crying Baby.

For stepper motors, I think that it should be possible to drive them directly off a PWM output. It must be a small size motor, otherwise extra hardware has to be added.
That said, I have zero experience with stepper motors.

You are changing what you want to do quite drastically.
3-phase AC motors, stepper motors, DC motors are so different that I cannot imagine your lecturer not specifying what you should examine. Now you have gotten a lot of people started on the 3-phase AC motor, and then you just dump it.
First thing, before trying to figure out a solution, is to understand the question.

 

[Steve Bailey]

You can't control a 3-phase AC motor by connecting DC outputs to the motor leads. To vary the speed of the motor, you will need a VFD (inverter).

The PLC PWM output is a low-voltage DC signal. A 3-phase AC motor requires an AC signal.

The rated current of the PLC output at 24 VDC is 0.75 amp.

The speed of a 3-phase AC is a function of the frequency of the applied voltage. Even if you argue that the PWM waveform looks like an AC waveform, the PWM output does nothing about frequency.

How were you planning to wire this up? A 3-phase AC motor needs three wires to it. There is only one solid-state output on your PLC (IC200UDR005).

Do you need any more reasons?

You might be able to do crude motion control with a PWM output fed to a VFD's analog speed reference input. Make sure the RMS voltage from the PWM output doesn't exceed the maximum value that the VFD can accept. The basic strategy would be to have the PLC create a trajectory, and assuming that the motor follows the reference. The area under the plot of velocity vs time is the distance traveled.

Without feedback, the emphasis is on the word crude. Your accuracy and repeatability will be lousy.

 

[crying_baby]

reply

Most AC motor drives have built-in PLC-type functionality. Admittedly, this functionality is limited, but it's a lot less limited than a fried PLC.

sori , i wasnt mean to offend any1, but
imma talkin about common ones w/o build in PLC

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This is an interesting concept, never thought about it before. I have used plcs PWM for a variety of applications including stepper and dc motors.
I wouldnt attempt to drive the motor directly from the PLC outputs. I would look at using solid state relays to power the motors then use PWM to control the SSR's.
I dont see a cost savings though. The SSR's would probably cost $100 or thereabouts plus development and experiment time. An AC drive for 1HP can be obtained between $100 and $200 (US)depending on needs. As was mentioned a drive has some plc type capabilities so a plc may not be needed in some small applications.

whisper u

-----------------------

Take a look at what PWM provides...

+-+ +--+ +---+ +----+ +-----+ +----+ +---+ +--+ +-+ <-- ??? What Voltage
|1| |2 | |3 | |4 | |5 | |4 | |3 | |2 | |1|
--+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-- <-- 0 Volts

thanks terry, reli thorough advice u gave.
--------------
thank u elevmike

Crying Baby.

For stepper motors, I think that it should be possible to drive them directly off a PWM output. It must be a small size motor, otherwise extra hardware has to be added.
That said, I have zero experience with stepper motors.

You are changing what you want to do quite drastically.
3-phase AC motors, stepper motors, DC motors are so different that I cannot imagine your lecturer not specifying what you should examine. Now you have gotten a lot of people started on the 3-phase AC motor, and then you just dump it.
First thing, before trying to figure out a solution, is to understand the question.

well, its written a polyphase induciotn motor. (inverter fed induction drives). the motor is the 1 i m going to decide , the PLC is provided. its so difficult to decide now. T___T :Cry:

 

[crying_baby]

a vfd

You can't control a 3-phase AC motor by connecting DC outputs to the motor leads. To vary the speed of the motor, you will need a VFD (inverter).

The PLC PWM output is a low-voltage DC signal. A 3-phase AC motor requires an AC signal.

The rated current of the PLC output at 24 VDC is 0.75 amp.

The speed of a 3-phase AC is a function of the frequency of the applied voltage. Even if you argue that the PWM waveform looks like an AC waveform, the PWM output does nothing about frequency.

How were you planning to wire this up? A 3-phase AC motor needs three wires to it. There is only one solid-state output on your PLC (IC200UDR005).

Do you need any more reasons?

You might be able to do crude motion control with a PWM output fed to a VFD's analog speed reference input. Make sure the RMS voltage from the PWM output doesn't exceed the maximum value that the VFD can accept. The basic strategy would be to have the PLC create a trajectory, and assuming that the motor follows the reference. The area under the plot of velocity vs time is the distance traveled.

Without feedback, the emphasis is on the word crude. Your accuracy and repeatability will be lousy.

thank u steve, that means i need a vfd to achieve this control scheme? Yes, it will be the word Crude to help me achieve it.

apart from what i just said.. there isnt any hope to achieve it.

How were you planning to wire this up? A 3-phase AC motor needs three wires to it. There is only one solid-state output on your PLC (IC200UDR005).

omg. this is absurd to control a 3phase acmotor...

help..

 

[crying_baby]

does any1 can hint on how this proposal could be modified.. without going too far from the main point. . i mean more chances for sucess than this current topic..

thank u

 

[Pierre]

Then again. Could we use a PLC to measure the internal temperature of an oven?

Knowing at which temperature the plastic casing would melt, and the melting points of the other components of the PLC we could easaly observe and have a semi-precise chart of the oven temperatur curve.

Why not?

 

[elevmike]

Crying Baby.

For stepper motors, I think that it should be possible to drive them directly off a PWM output. It must be a small size motor, otherwise extra hardware has to be added.
That said, I have zero experience with stepper motors.

Not...A PWM power signal cannot be used to drive a stepper motor. As pointed out earlier the off cycles in a PWM signal are constant. The pulse train to drive a stepper motor is varied depending on the speed. These are two very different applications. Apples & Oranges.

 

[crying_baby]

Then again. Could we use a PLC to measure the internal temperature of an oven?

Knowing at which temperature the plastic casing would melt, and the melting points of the other components of the PLC we could easaly observe and have a semi-precise chart of the oven temperatur curve.

Why not?

offtopic

 

[JesperMP]

Mike,
you may be right. As said I have no experience with stepper motors.
I may have confused PWM with PTO that I have briefly examined in the MicroLogix. With PTO (Pulse Train Output) it seems to be possible to create a simple control for a stepper motor (without feedback).

 

[elevmike]

Crying baby,

PWM is used to drive a conventional DC motor.

VVVF Drive is used to drive an AC motor.

Steppers/Pulse trains are used to drive stepper motors. Stepper Motors are used in smaller applications for positioning & speed control. A stepper indexes the motor to a preset position.

All three are very different from each other. You cannot use a PWM controller to drive a Stepper, or AC motor. You cannot use a stepper drive to control an AC or Conventional DC motor. YOU cannot use a VVVF Drive to control a Stepper motor or a Conventional DC motor.

Your AC induction motor will be controlled by and AC (VVVF) Drive. The drive will take either an analog, or digital input from the PLC and output a varied frequency & voltage to the motor. The drive can be setup for predetermined speeds (fast/med/slow/fwd/rev) depending on the digital inputs, or an analog signal.

You need a primer on AC drives... Click here: http://www.patchn.com/mtrwhtpaper.htm

 

[crying_baby]

PWM is used to drive a conventional DC motor.

VVVF Drive is used to drive an AC motor.

Steppers/Pulse trains are used to drive stepper motors. Stepper Motors are used in smaller applications for positioning & speed control. A stepper indexes the motor to a preset position.

All three are very different from each other. You cannot use a PWM controller to drive a Stepper, or AC motor. You cannot use a stepper drive to control an AC or Conventional DC motor. YOU cannot use a VVVF Drive to control a Stepper motor or a Conventional DC motor.

Your AC induction motor will be controlled by and AC (VVVF) Drive. The drive will take either an analog, or digital input from the PLC and output a varied frequency & voltage to the motor. The drive can be setup for predetermined speeds (fast/med/slow/fwd/rev) depending on the digital inputs, or an analog signal.

elevmike,
do u mean , i can still achieve what proposed IFF i use a VFD?maby i should 've reduce to how i m going to control a ac machine instead of the proposed way