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FrSky SP-RPM - Smart Port RPM and Dual Temperature Sensor

saml45

New member
I posted this on RCGroups, but thought I would try hear also.
I installed a FrSky SP-RPM - Smart Port RPM and Dual Temperature Sensor in my plane and find the RPM's to be off by over a thousand. I have the poles set correctly for my motor and the blade numbers in the set up correct. My motor has 12 poles, so I set the LED's for 6 pair and the blade count to one for a 2 blade prop. When I run the motor up to half stick, my read out on the transmitter reads 7600. When I check it with an optical tach known to be accurate, it reads around 6500. That is a big difference. At full throttle it is almost 1550 RPM difference. Can these things be calibrated or is this as accurate as they are?
This a Taranis Plus tx and a S8R rx. Any thoughts. Sam
 

Konrad

Well-known member
Commutation pulse measuring is a very accurate way to measure rpm.
Per the manual, put on a set of JST pigtails on an of the three phases of your motor. (That is hook up the JST connector to any two of the three power leads coming out of the brushless motor. Hook this JST pigtail to the sensor. Go into pole setting mode (solid Red LED).
Looking at "The Display of Brushless Motor Pole Pairs" block (3,2,1,0), for your 12 pole motor you want the LED for 3 & 2 lit up. (I assume you have LED 1 & 2 lit, but by the math it might be a 14 pole motor. What brand and model number are you using)

Now in the Taranis Telemetry Setting Page you want to use "1" as the blade setting.

This is because the sensor has no idea as to the number of blades that are on the motor's output shaft. It could have a single blade racing prop or a 14 blade EDF fan. The sensor is not measuring blades rather electrical pulses from the commutation cycle.

This messes up a lot of folks, the RPM Telemetry page on the Taranis was originally written for an optical tach so you did need to know the blade number. But with a commutation counters, like what you have with this sensor, this value needs to be set at 1

https://www.frsky-rc.com/wp-content/uploads/2017/07/Manual/RPM.pdf

All the best,
Konrad
 
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Konrad

Well-known member
I see that Fr Sky use the term "Pole Pairs" verses the motor industry term "Pole" for number of magnets. You may need to divide the number of poles by 2 that manufacters says to get the correct pole pairing for the sensor. So if it is a 12 pole motor you might need to use "6" . If it has 14 magnets you might need to use "7".
 
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saml45

New member
Commutation pulse measuring is a very accurate way to measure rpm.
Per the manual, put on a set of JST pigtails on an of the three phases of your motor. (That is hook up the JST connector to any two of the three power leads coming out of the brushless motor. Hook this JST pigtail to the sensor. Go into pole setting mode (solid Red LED).
Looking at "The Display of Brushless Motor Pole Pairs" block (3,2,1,0), for your 12 pole motor you want the LED for 3 & 2 lit up. (I assume you have LED 1 & 2 lit, but by the math it might be a 14 pole motor. What brand and model number are you using)



I counted the poles numerous times, but could never manage to get more than 12. So i used the settings for 6 pairs and 1 blade and got the results posted. So I went ahead and set it for 14 pole motor and it worked correctly.
It is a E-Flite G160 245kv motor.
 
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