Mark VII motor not starting

[Ash1562]

Thanks all for your comments and time.

This is now fixed once I fixed the relay by bolting it back on the motor housing

The comment "this is a motor not a generator"is too mislead for me to even know where to begin. Please look up the fundamentals of electro motor force, how Faraday discovered EMF by moving a magnet in a coil, then rethink please.

Happy Sunday all!

[jsburger]

Thanks all for your comments and time.

This is now fixed once I fixed the relay by bolting it back on the motor housing

The comment "this is a motor not a generator"is too mislead for me to even know where to begin. Please look up the fundamentals of electro motor force, how Faraday discovered EMF by moving a magnet in a coil, then rethink please.

Happy Sunday all!

I understand all that. I overhauled aircraft generators for the USAF for 12 years. All those generators had permanent magnets in them to create a small output voltage (AC not DC) when they were spun up by the aircraft engine on start up. That small AC voltage went to a generator control unit (GCU) that rectified it to DC fed it back to the exciter stator creating a magnetic field that induced an AC voltage in the exciter rotor that was fed to a rectifier that is in the main rotor. That DC magnetizes the main rotor. That magnetic field is rotating inside the main stator windings and produces the AC output of the generator.

Shopsmith motors are induction motors. There are no magnets in your motor. You can rotate it mechanically and there will be no voltage produce since there is no magnetic field until an AC voltage is applied to the stator windings.

[Ash1562]

then why did I see voltage as I spun it up by hand? Thanks in advance for teaching me something here!

[jsburger]

then why did I see voltage as I spun it up by hand? Thanks in advance for teaching me something here!

How much voltage?

Probably a little residual magnetism in the iron of the rotor from the last time it was run. But, that would produce an AC output from the main stator. You said you had the meter on DC.

[JPG]

To give you an accurate answer will require total knowledge of the meter you were using.

I speculate, that the 'spikes' were the result of random noise including lead contact variance.

I am assuming you were using a 'digital readout meter'.

SO, does this mean the motor is now behaving normally?

As for dc indicating on an ac meter there is little internally that prevents any non zero emf from invoking a response(again what is in de little box?) . Especially 'spikes'.

[Ash1562]

JPG - yes, operating normally. Thank you.

I was using a digital multimeter with DC volts to see if there was anything, even spikes in the millivolts, to indicate the wiring hadn't melted.

Even set at DC, a meter will read AC voltage at its root mean square value. So a 110V AC will read at 110 divided by square root of two. Again, magnitude wasn't important here - just to show there was some response.

[JPG]

If you had the meter set to milivolt range no wonder there were 'spikes'.

Your comment re dc/ac indications was sorta accurate.

In the old days(analog meter) the 'detector' was typically an average responding d'arsonval moving needle movement. The ac was rectified and the scale differed to account for the average response to be calibrated to take in account the average of a rectified ac voltage and used rms as an assumption of the sinusoidal shape of the voltage waveform. That average was .636 of the peak voltage where rms is .707. Thus the scales differed by that ratio.

With a digital meter all this is merely interesting(?) history. They are a different animal entirely.

[RFGuy]

JPG - yes, operating normally. Thank you.

I was using a digital multimeter with DC volts to see if there was anything, even spikes in the millivolts, to indicate the wiring hadn't melted.

Even set at DC, a meter will read AC voltage at its root mean square value. So a 110V AC will read at 110 divided by square root of two. Again, magnitude wasn't important here - just to show there was some response.

This is not correct. A DMM (Digital Multi-Meter) set to DC mode but connected to an AC source will show either 0V or it will show an out of range condition. You will never measure a pure AC voltage source and get the RMS reading of an AC signal in the DC mode of a DMM. The only caveat to this is if the source has a DC component to it, then you are measuring an AC + DC signal - however it is still likely to still result in an out of range result unless the AC voltage is quite low.

It has been a few decades since I studied motor design in college, but my understanding is spinning the motor shaft by hand on an AC induction motor and measuring voltage is pointless, i.e. no discernible information can be gleaned about the motor by doing so. If you see a small AC or DC voltage in doing so, I don't believe it will have any meaning in troubleshooting the motor. It's a different story if the motor design is a DC motor.