DC Electric motor TACH

[mazur50]

A buddy and i race toy cars and he has a motor analizer that is a power supple and tach

basicly all you do is hook the positive and negative to the motor and it tell you volts amps and rpm.

do have any clue how they are measuing rpm with no sensor.

 

[Nigel Goodwin]

It measures the ripple caused by the commutator switching.

 

[mazur50]

do you know a way i can make that circuit.

 

[Nigel Goodwin]

No, I've never done it, but for a start I'd use a scope to check what you actually get on the supply rails, so you know what sort of levels you need to amplify.

 

[Hero999]

But you need to know how many poles there are on tha amature; a 5 pole motor will give a totally different reading to a 3 pole motor.

 

[Nigel Goodwin]

But you need to know how many poles there are on tha amature; a 5 pole motor will give a totally different reading to a 3 pole motor.

Obviously, but most model motors are similar.

 

[Sceadwian]

Most 5 pole motors used in the RC hobby are for servo motors, and there aren't even many of those. The motors that actually produce the power for moving are almost exclusivly 3 pole motors, or DC brushless, but those are mostly used in aircraft.

 

[mazur50]

The motor is a three pole motor.

 

[mazur50]

Any more idea on how to read rpm. Like can you use a multimeter or somthing.

 

[Sceadwian]

You could do it if the meter supported a frequency measurement mode. Place a magnet on the device to be measured (or two or more to maintain ballance) and make a coil to feed into the meter. It really depends on the voltage required to trigger the meters frequency measurement.

 

[mazur50]

You could do it if the meter supported a frequency measurement mode. Place a magnet on the device to be measured (or two or more to maintain ballance) and make a coil to feed into the meter. It really depends on the voltage required to trigger the meters frequency measurement.

But is there a way to read RPM with no Pickup. There is a power supply in out hoppy that will show you Voltage Amperage and RPM for a motor and all there is two wires hooked up to the motor.

what about this circuit.

 

[Sceadwian]

Sure, but I have no idea how to implement it practically though, or of any meters that have the feature. Basically it reads the back EMF during commutor switching to determine RPM. But it wouldn't work with a brushless motor.