low frequency PWM

[kenobe]

HI
I need to generate a PWM of duty cycle 20% to 80% at 5 to 15Hz.
I can not use the PWM function provided by the T100MD 888 because the frequency is too low . I need this frequency because my mechanical relay can not response fast enough.
I try to use timer but could not make it work.
Can some one pls enlighten me.
Thanks

[support]

Using PWM on mechanical relay is not a good idea. Relay has both limited contact life in mechanical  (typically 100K cycles) and electrical (typically 1 million to 10 million cycles). You should use the PWM on solid state relays only. It wouldn't take long before your mechanical relay is out of commission if you really run a 15Hz on/off cycles on it.

[kenobe]

Hi
I fully understand the limitations of the mechanical relays, however, I have no choice because the entire hardware system is given to me and the management decided to change software rather then hardware.
I hope you understand my problem and help me if you could.
Thanks

[support]

One project we did many years ago involved using ladder logic to program the PLC to control 15 heaters using technique similar to PWM. However, the period chosen was 20 seconds and resolution of the ON/OFF duty cycle is 1/100. You can download from:

   http://www.tri-plc.com/trilogi/15heaters/zip

You may want to follow the project and change the clock pulse use from 0.2s to 0.01s. That could drive the PWM at about 1 second period with resolution of 1/100. Another way is to change Fn #10 to non-differentiated custom function {CusFn} and use a Norm.ON contact to drive it. That will be the shortest period this techique can achieve and the period is entirely dependent on the scan time of the program and not easily definable.

[bongofury]

Management is likely to reconsider if numbers are used.  In support of what Support is saying:
15Hz = 15 cycles per second
15Hz X 60 seconds = 900 cycles per minute
900 X 1441 minutes (One Day) = 1,296,826

At 10,000,000 cycles of expected relay life in a 24/7 environment, this equals 7.7 days (10,000,000/1,296,826)  before failure of the relay, either mechanically or electrically.

Another factor to keep in mind is Relay Harmonic Chatter.  This is caused by the reed/contact assembly that all electro/mechanical relays share.  At the speeds you intend to run these you will end up with spurious operation due to RHC and Magnetic Flux Collapse of the relay coil, (satuation) at best.  Contact failure due to micro-pitting of the contact surfaces will probably occur well before the 10 million cycle life of a "normally" used relay.

It should be readily apparent that an electro/mech relay cannot be used in the application described with any confidence.
Best of luck......Dave

[kenobe]

Hi
Thank you very much for your help.
I have downloaed the 15heater.zip and studying it now, hopefully I can pickup something from there.
Dave
Thank you very much for your support. I totally agree with you. I have made a copy of your message and send up to my boss, lets wait for his reply.