Question about Allen-Bradley PWM instructions

I have come across a system using an AB shoebox PLC, which drives a
motor speed controller via 1 output and 2 PWM instructions, to achieve 2 different speeds.

I have no experience with PWM so please excuse my ignorance I assume the PWM instruction works by setting the state of the output to "x" amount of milli/microseconds on, and "y" amount of milli/microseconds off, which the motor controller interprets as an analog signal of sorts (I assume it is in the milli or micro range of seconds as the AB's output LED appears solidly on to my eye). As the PWM instructions have 100 and 750 as their source constants, and the program's PWM comments mention "speed control 0-20000Hz" I am baffled. The way I interpret this is the instruction is operating the output relay at least 100 times per second, which seems a little hard to believe.

Thanks for any help.

Well the AB microLogix 1500 PLC comes with 2 High speed FET outputs which according to the liturature can operate at upto 20K Hz. Perfect for feeding Step into a stepper drive. The problem I see is the the PWM function is turned on/off with the lader logic, so you may have a hard time getting an exact amout of pulses sent to the device you wich to control. Something I will be playing with in the near future. Hope that helps

Your basic idea of the PWM signal is correct. Using your 'x' and 'y' notation, the signal is:

   x

_______

(x + y)

and is defined by the duty cycle parameter.

   1

_______

(x + y)

is your output frequency.

The output points used for the PWM output are solid state. They can survive just fine at the 20kHz max switching frequency.
You state that you two values (100, 750) used in your plc program. Are these moved into PWM:0.DC or PWM:0.OF (assuming you are using output point 0)?
DC is duty cycle and is in terms of percent * 10. So if this is where the number is going, 100 is 10% duty cycle, 750 is 75% duty cycle.
OF is output frequency in Hertz. It basically defines the period of one PWM cycle.
In a typical PWM implementation you would keep your output frequency (OF) fixed and vary your duty cycle (DC) to change your desired output. This varies the percent of a cycle that then output is on, which the drive inerprets as a speed reference.

If you really are varying the OF value, the PTO (pulse train output) function is probably closer to what you want to use.

Keith

Thanks for the help guys.