3 phase supplies - UK Vintage Radio Repair and Restoration Discussion Forum

Boom · 20th Sep 2010, 10:15 am

Sorry if it is too far off topic for the forum just sling it off.

It's ages since I was anywhere near 3 phase stuff so can anyone tell me should I read an output from any two of the three windings with a digital meter? (No center connection).

My motorbike seems to be putting out a slightly low charge rate. I'm measuring 3VAC(ish) from the windings but the battery voltage itself does fluctuate with engine speed from 12V to 14V indicating there must be an output from them.

Is the problem something to do with trying to measure the 3 phase supply with a digital meter?

Thanks

ppppenguin · 20th Sep 2010, 10:20 am

Phase to phase voltage will be SQRT(3) (approx 1.7) times the phase to neutral voltage. This is true for any 3 phase system.

Station X · 20th Sep 2010, 10:55 am

I could be that the high frequency of the alternator's output is fooling the meter.

threeseven · 20th Sep 2010, 11:16 am

Is the DC output low or fluctuating ? Obviously on a nominal 12 VDC system the voltage across the battery with engine running should be stable in the region of 14.4 VDC. If not there is a rectifier or regulator problem, although these two units are often combined on bike systems.
Actually I just read that the voltage is fluctuating so I reckon it will probably be the rec/reg unit/units.

20th Sep 2010, 12:24 pm

Bike alternators (usually) use phase chop (short circuit) regulation before the rectifier, this confuses the heck out of any meter connected to the input. As long as the output is about 14V or so when running at 1/3 maximum RPM and above it's OK. The 'cure' for this is a new battery, old knakered ones need more volts to charge. The sure way to check is to measure the charge current, kick start 10A approx dropping to 1/2 amp quickly, electric start 20A+ then 1A or so after a few minutes. OK a new battery is expensive, but what would you pay to get home on a dark winters night?

Sean Williams · 20th Sep 2010, 4:32 pm

Ok, from my dim and distant memories of being a RAC patrolman....

Three phase charging ring diagnostics.

Unplug multiplug that feeds rectifier/regulator pack.

Run bike up, and measure phase to phase, and phase to ground - should expect to see around 18-35v, possibly more if you increase engine speed.

Plug reg unit in again, should see no less than 20v on load per phase.

A normal DMM is fine for this sort of thing.

Usual faults are shorted regulator/rectifier pack, or in rare cases an O/C stator ring - normal resistance tests will show these up.

HTH
Sean

happytiger · 11th Nov 2010, 7:38 pm

Quote:

Originally Posted by ppppenguin

Phase to phase voltage will be SQRT(3) (approx 1.7) times the phase to neutral voltage. This is true for any 3 phase system.

NO...
This only applies to a sine wave. Most small alternators for battery charging do not give a true sine wave, this is not important as its being rectified anyway, also the frequency varies between wide limits.

McMurdo · 11th Nov 2010, 8:37 pm

I'd think the old test of putting the headlamp on and revving up is as good a test as any

20th Sep 2010, 10:15 am	#1
Boom Retired Dormant Member Join Date: Jan 2007 Location: Westbury, Wiltshire, UK. Posts: 2,451	3 phase supplies Sorry if it is too far off topic for the forum just sling it off. It's ages since I was anywhere near 3 phase stuff so can anyone tell me should I read an output from any two of the three windings with a digital meter? (No center connection). My motorbike seems to be putting out a slightly low charge rate. I'm measuring 3VAC(ish) from the windings but the battery voltage itself does fluctuate with engine speed from 12V to 14V indicating there must be an output from them. Is the problem something to do with trying to measure the 3 phase supply with a digital meter? Thanks

20th Sep 2010, 10:20 am	#2
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: 3 phase supplies Phase to phase voltage will be SQRT(3) (approx 1.7) times the phase to neutral voltage. This is true for any 3 phase system.

20th Sep 2010, 10:55 am	#3
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: 3 phase supplies I could be that the high frequency of the alternator's output is fooling the meter. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

20th Sep 2010, 11:16 am	#4
threeseven Octode Join Date: May 2010 Location: Worcestershire, UK. Posts: 1,880	Re: 3 phase supplies Is the DC output low or fluctuating ? Obviously on a nominal 12 VDC system the voltage across the battery with engine running should be stable in the region of 14.4 VDC. If not there is a rectifier or regulator problem, although these two units are often combined on bike systems. Actually I just read that the voltage is fluctuating so I reckon it will probably be the rec/reg unit/units. Last edited by threeseven; 20th Sep 2010 at 11:22 am.

20th Sep 2010, 12:24 pm	#5
Guest Guest Posts: n/a	Re: 3 phase supplies Bike alternators (usually) use phase chop (short circuit) regulation before the rectifier, this confuses the heck out of any meter connected to the input. As long as the output is about 14V or so when running at 1/3 maximum RPM and above it's OK. The 'cure' for this is a new battery, old knakered ones need more volts to charge. The sure way to check is to measure the charge current, kick start 10A approx dropping to 1/2 amp quickly, electric start 20A+ then 1A or so after a few minutes. OK a new battery is expensive, but what would you pay to get home on a dark winters night?

20th Sep 2010, 4:32 pm	#6
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,517	Re: 3 phase supplies Ok, from my dim and distant memories of being a RAC patrolman.... Three phase charging ring diagnostics. Unplug multiplug that feeds rectifier/regulator pack. Run bike up, and measure phase to phase, and phase to ground - should expect to see around 18-35v, possibly more if you increase engine speed. Plug reg unit in again, should see no less than 20v on load per phase. A normal DMM is fine for this sort of thing. Usual faults are shorted regulator/rectifier pack, or in rare cases an O/C stator ring - normal resistance tests will show these up. HTH Sean __________________ Engineers make things work and have spare bits when finished

11th Nov 2010, 8:37 pm	#8
McMurdo Dekatron Join Date: Jun 2009 Location: Staffordshire Moorlands, UK. Posts: 5,271	Re: 3 phase supplies I'd think the old test of putting the headlamp on and revving up is as good a test as any __________________ Kevin