Hi Sir

dear sir,
i would like to know if it is possible to use a PLC to control any induction motor less thank 1hp with the PLC having a PWM ?

i just want to try a test thru this principle ;

PLC---> PWM----> induction motor 3phase less than 1hp

no feedback required. if the PWM could be used to control the motor, would it be feasible to control the start stop of the motor too?


please advice thank u

I think overall it is best not to try it.

regards.....casey

Is it physically possible? Probably. Most PLCs are limited to 230 VAC on relay outputs, so if it was a 230 VAC motor you could make it work a few times. Some PLC oputputs are rated to 5 or 10 amps, so you wouldn't immediately fry the outputs. You would probably burn out the PLC outputs in a short time, though, and you might get some peculiar PLC operation from the EMI (ElctroMagnetic Interference) and RFI (Radio Frequency Interference) on the motor power leads.

Is it legal? In the US only if the motor has internal thermal protection.

Is it good practice? Definitely not. You should use a motor starter, which is a three phase contactor combined with thermal overload protection, and control the coil of the starter with your PLC outputs.

My answer would be simple.

crying_baby said:

if the PWM could be used to control the motor, would it be feasible to control the start stop of the motor too?

Click to expand...

NO! Buy a drive unit for induction motors unless you want to get an education from the school of hard knocks.

kc9ih said:

I think overall it is best not to try it.

Click to expand...

OK, but it doesn't look like you are sure of your answer. Why not?

What does a induction motor drive unit do that a PLC doesn't or can't? Why is the problem more difficult than just sending out a PWM signal?

I wouldn't do it myself.

Some people just want to do it.

Ho matter what, unless it is TOTALLY impossible.

Sometimes, you let them try.

One area farmer bought and rebuilt a chevy 327, and got a transmission and driveline to run his auger and grain dryer fan, when he had 3 phase power at the grain bin already.

Another uses an Allen Bradley rack to control the chicken house lights. One or two intermatic timers would have done fine.

regards.....casey

I'm not sure I get the question, or some of the answers. We commonly use PWM to control 1/4 to 1/2 hp. door operators. The PWM controller is a seperate device which can be setup with presets, or you could use an analog signal from the PLC as a reference. The presets are open low, med, high; close slow, med, & high. quite simple really. I've never provided a PWM signal directly from the PLC.

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.

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.

Take a look at what PWM provides...

  +-+ +--+ +---+ +----+ +-----+ +----+ +---+ +--+ +-+    <-- ??? What Voltage

  |1| |2 | |3  | |4   | |5    | |4   | |3  | |2 | |1|

--+ +-+  +-+   +-+    +-+     +-+    +-+   +-+  +-+ +--  <-- 0 Volts 

The off-duty portion is constant and the on-duty varies.

While the pulse is ON, what is the angle of the voltage? Is it Positive? Is it Negative?

The pulse would have to be exactly synchronized with the frequency of the line voltage in order to produce a reasonable sine-wave at 60 Hz.

If the intent is to produce a "frequency drive" then the concept is fatally flawed because you can not control the frequency of the line voltage.

Frequency Drives convert AC to DC then produces a set of positive pulses (in DC) and a set of Negative pulses (in DC) to produce a psuedo-AC-Signal.

This scheme can not do so.

There are far too many problems to describe without designing an actual frequency drive.

Using PWM to control a motor is fine if you send the PWM signal to a driver which uses the PWM signal to drive a Voltage (effective voltage) to Frequency converter. That frequency then becomes the operating (reference) frequency of the motor driver. I doubt very much that you send a PMW signal directly to a motor, Mike.

Ron, SSR's won't help.

Do you like smoke?

I would say that the post itself is original. This shows the Original Poster is thinking outside the box.

Perhaps, though, we should consider latching the box once we get him back inside.

reply

Mike Williams said:

Do you like smoke?

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no.

but i knwo this is a raw topic . in the manual it explains that it can be configured for PWM or pulse oputput, IC693UDR005 PLC is used.

there's not going to be any feedback from the motor. would it be feasibble to manipulate in such way that it works?

i gonna test the PLC's output current tomorrow and hopefully get the PWM to test teh output waveforms. hm.. the current is also a problem now.. d

how about high speed counters> ? would it be more feasible to obtain such a raw (naked froma any other external devices , just plainly, PLC, n motor, or a inverter) control over a 3phase acmotor?


wait, i came across 0 part in the manual that states the PWM could be used to control a polyphase acmotor. only can control steppers n dc motor. does this mean it cant use to control a AC 3phase mtor w/o any external starters or.. drives.

there must be somway.

Thank you

dear sir

Peter Nachtwey said:

NO! Buy a drive unit for induction motors unless you want to get an education from the school of hard knocks.

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sori i no mean offence to any1, but...

using a drive woiuld be totally out a topic.

this has to do with controlling the PLC to drive the acmotor using PWM.

Thank u

crying_baby said:

sori i no mean offence to any1, but...

using a drive woiuld be totally out a topic.

this has to do with controlling the PLC to drive the acmotor using PWM.

Thank u

Click to expand...

Hi fren

Yeah I think some school would like their student to learn the hard way which we'll not implement in real world, since drive is already invented. But this is education
True also, if the school allows to use drive then setting the parameters in the drive will only takes half an hour to learn, while the hard way u may need to do a lot of research like what he's doing now
Sorry can't help much, I'm consider newbie in PLC world compare the the gurus here

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 ?