Comments on: Kitmotter 0002: It’s made of wood!

By: Andrew

Andrew — Sat, 04 Aug 2012 21:11:15 +0000

oooh I know what you can do about the scope showing double! you can make a poor man’s differential probe. I saw this on the EEVBlog.

Basically you use two probes, connect the two grounds together but to nothing else, and connect the two probe tips to the two points you want to measure. Then you set the prob to math mode, and display CH1-CH2 (or vice versa).

By: FrankTheCat

FrankTheCat — Tue, 17 Jul 2012 03:52:10 +0000

You’re entering truly Soviet engineering here. Should swap the ball bearings for bronze ones, since such extravagances are reserved for dirty capitalist pigs.

By: Roy

Roy — Tue, 17 Jul 2012 03:15:00 +0000

Music to my ears. :’-)

By: the chuxxor

the chuxxor — Sun, 15 Jul 2012 21:22:25 +0000

The scope always treats its channel ground as “0″. It took me a LONG time to realize what was happening.

Say you had a positive voltage (e.g. 15v) on phase A and negative (e.g. -15v) on phase B. A voltmeter on AC mode will tell you that the delta-V is 30v, which is correct.

But the scope will assume its ground probe to be 0v, so what it will display is “+30v” in that state. The motor is a fully floating measurement to it.

Now say the motor enters a state where there is positive voltage on B and negative on A. The voltmeter will still say delta-V of 30 volts, but the scope will show -30v. The resultant waveform on the scope LOOKS like 60Vpp.

The only way to get the scope to read 30Vpp is if I grounded it not to the phases, but to the star point (midpoint) of the windings.

By: Andrew

Andrew — Sun, 15 Jul 2012 20:40:41 +0000

Quick question: Am I reading your scope wrong? That screenshot says there’s a delta t between the two cursors of 200Hz, since that’s half the period that screenshot is at 100Hz. It also says there’s a delta V between the two cursors of 33.6V, so Vppk would be that. Cursor 1 is at 17v, and cursor 2 is at -16.6v.
Maybe you meant Vamplitude is ~16.8v average?