Older DC fractional HP motors

Some had field rheostats for speed control. As for the 90 volts, likely the output voltage of a rectified line voltage using the rectifier technology and line voltage of the time.

90V probably became standard at some time in the dim past for reasons that haven't applied since 1920, but has been carried forward because it's a standard.

Just a guess, but you stumble across that sort of thing all the time.

This describes 90VDC motors as suitable for thyristor-regulated rectified 120VAC.

The mercury vapor tubes used as high power rectifiers before comparable silicon diodes became available dropped around 15V.

-jsw

Hmm. That sounds about right if you used the field coil as the inductor in a choke-input "filter".

With modern rectifiers (silicon ones) that would work out to about 64 VRMS. But vacuum tube rectifiers would have a lot more series resistance, so that might work out with 110 VAC (the old line voltage, which then jumped to 115 VAC, then 117 VAC, and now 120 VAC.) (And the frequency used to be specified as Cycles-Per_Second (CPS) or '~' instead of Hz. :-)

Also -- the rectifier was likely not used with filter capacitors, so the RMS value of a half-wave rectified 110 VAC might be pretty close to that.

Enjoy, DoN.

Depends on the tube. I have vacuum tubes that pass 1000 amps to the plate. About the size of your for-arm. Mil grade.

Then the real nasty ones for plating and welding - Ignatrons (sp) that are mercury filled in a pool to splatter 10's of thousands of amps.

The big tubes I have are gas filled and were motor controls. Took two on each motor and there were 3 motors to spin a very large triangle antenna.

Mart> >>

Voltages might have started out (in the Dark Ages?) at 110vac, but they were 115v in my teens and are 120/240vac nowadays. It has been that way for a long time.

I was thinking that rectification caused the drop to warrant building

90v and 180v DC motors.

120VAC*.707=84.84V Average, unfilterd DC

That sounds wrong. The 0.707 multiplier is applied to the peak voltage (170V) to get to the 120 VAC rating. Applying it again to the result just feels wrong. :-)

However, if you are half-wave rectifying, with no filtration capacitors, the RMS (Root-Mean-Square) value of that half cycle, followed by another half-cycle of time, could be pretty close to the

90V rating for the field.

O.K. -- Looking in this site:

gives this for a half-wave rectified value:

====================================================================== Half rectified wave Vrms= Vpk/2 ======================================================================

so -- for the 170 V Peak, we get 85V RMS

so -- 90 V field allows for a surge of 127 VRMS ac -- so a worst-case value.

Enjoy, DoN.

See? It's safe for a 90vdc motor. ;)

Is that the constant for a gas rectifier tube, or a selenium stack?

Actually, it is the result of a pure rectifier -- no voltage drop, so any real rectifier would have more drop. All of these are larger drops than typical silicon rectifiers of today -- at least until you get to seriously high voltages, which would have a number of silicon junctions in series (at about 0.7V each) with resistors in parallel to keep the reverse voltage evenly divided.

But in essence, any half-wave rectifier would produce voltages safe for the motor. Stay clear of full-wave bridges, however, or filter capacitors. :-)

Enjoy, DoN.