Measuring AC or DC motor Speed,

[PrescottDan]

What piece of equipment can a tech use to measure the speed of an AC or DC motor?

I have seen in schematics of parallel capacitors and diodes across a AC or DC motor. What does a parallel capacitor or diode do to the motor? is this setting a phase shift to the motor

What kind of a circuit will help prevent a motor from over-heating?

 

[zeyad.kassaby]

Check the plate on the motor
for AC there is the AC volts ,number of phases ,internal connection type,current for fixed ac motor speed,and optimum RPM for it
if using AltiVar(freq converter) the actual speed is displayed on the alivar it self
the issue is with the DC motor you have to use external tool like TacoMeter and for current don't exceed the voltage over the plate

 

[D.A.(Tony)Stewart]

A tech can make a tach using a 1-shot for each pulse using various signals such as filtered commutation noise with hysteresis, Hall sensor to sense rotating magnetic field,or monitor current using a current transformer or small shunt resistor again using a bandpass filter to detect commutation pulses. Optical shaft tachs work well too using Vishay/Sharp IR reflective sensor pairs with black ink stripe on rotor shaft end.

Over heat requires understanding of rated load and applied load power. A Fan helps. Current limiters can help ( e.g. Windshield and power windows use Metal Oxide Inrush Current Limiters or ICLs and cost two bits from https://www.Digikey.com come in all sizes like resistors from mA to many Amps and may be put in parallel

 

[vivek.roy2991]

What piece of equipment can a tech use to measure the speed of an AC or DC motor?

Optical enocders, tacho-generators are two easy examples. Tacho generators give noisy data. Optical encoders give better and more reliable results. Of course you would need a microcontroller with a quadrature encoder interface(QEI) module like the dsPIC30F4011. The TIVA C series launchpad from TI has a Arm Cortex M4 microcontroller that has the QEI module. It is cheap.

 

[chuckey]

Strobe? with a white dot on the motor shaft.
Frank