RPM Calculation

Hi,

I have got a 1 hp motor which switches on 24 v supply.

Now i was supposed to calculate the rpm of the motor.My idea was to use a sensor that can read the pulses & reading the pulse inputs in plc & by means of using HSC in the s7200 plc.


Now i dont know what type of sensor to select... Once the hardware side was done then it will be easy for me to execute in software..

So pls tell me whether my theory was ok & if ok what type of sensor has to be used

Regards,

Nelliyan.G

the actual Max RPM of the motor along with the switching capacity of the HSC and the timing of the Pulses are needed.
Example
if the RPM is 1 /min
if you sense the edge of the fan blade for example and there are 6 blades then there will be a pulse every 10 seconds - sounds easy
But - if you run the motor at 1800 RPM there will be 1800 x 6 = 10Khz
all still sounds good for the HSC
the issue that you need to look at is the actual Pulse length
the blade is maybe 3mm or 1/8th so the actual on Pulse is so much faster that the HSC will not see the actual pulse

Nelliyan said:

Now i dont know what type of sensor to select...

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An inductive proximity sensor is often used for these applications, looking at a metal target on the rotating component. You have to make sure that the pulse rate of the target does not exceed the switching frequency of the sensor (usually around 2 kHz for the typical prox).

How accurate do you need to be??
A DC tachometer connected to an analog PLC input is the simple way to do that. It will give you a wide speed range but it is not extremely accurate.

I have got a 1 hp motor which switches on 24 v supply.

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Do you mean that your 1 Horsepower motor runs on 24 Volts DC? If so, does your Power Supply provide a variable DC voltage, so that the motor can be run at any speed from 0 to 24 volts?

...by means of using HSC in the S7-200 plc

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Which model of Siemens S7-200 PLC do you have? Some S7-200 models have (normal, not HSC) DC Inputs with 0.3 milisecond response times, while other models (6ES7 212-1CA01-0XB0, 6ES7 212-1GA01-0XB0, have 120 Volt AC Inputs with 10 to 15 milisecond response times. Then there are the 214 models with VAC and DC Input models (6ES7 214-1CC01-0XB0, 6ES7 214-1DC01-0XB0, 6ES7 214-1GC01-0XB0 ) that have input switching times of 0.2 to 8.7 miliseconds.

The High-Speed Counter Input speeds range from 50 Hz, 2000 Hz, 7000 Hz, up to 20,000 Hz (cycles per second), again depending on the model of S7-200. The S7-212 has a 50 or 2000 Hz HSC, the S7-214 have 50, 2000, or 7000, while the S7-215 and S7-216 models have the fastest 20,000 Hz input speeds.

What sensor you can use (and whether you will need to use a High Speed Counter Input) will also depend on your PLC model, as well as the maximum speed of your motor, and the size of your target. If your taget has a wider surface, your sensor ON time will be longer, so you may be able to use different sensors (with a wide target instead of a narrow target).

I know that as a beginner, you thought that all PLCs were about the same and have the same capabilities! I think this belief is caused by the teachers not ever mentioning the differences in class.

In the past I have used a frequency to analog converter. It receives a pulsed input and outputs a 4-20mA analog signal, which you set up the scaling on the device.

Companies like Pheonix do them, an example

Frequency to Analog Converter

PeterW said:

Companies like Pheonix do them

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I have used these before http://www.pepperl-fuchs.us/usa/en/classid_247.htm

They were used more as a safety style, to verify that the fan was up to a min speed before the rest of the machine would start

Only certain models of the Siemens S7-200 will accept the EM231 or EM235 Expansion Modules with Analog Inputs.

I bet that Nelli is working on a student problem for a certain digital-only model of the S7-200, the one in the school lab that all students just know will do all things for all people, and that all other PLCs in the whole world are exactly like it.