Lawn tractor electromagnetic PTO clutch gap vs. field winding current draw - Page 1
Forum section: electronics / general

author: Skimask
author_rank: Super Contributor
timestamp: June 17, 2015, 02:25:00 am
content: Got an issue that's got me a bit perplexed...Have a lawn tractor with an electromagnetic clutch on the front of the engine which engages the mower deck. Standard type clutch set up, sort of like an air conditioner compressor clutch in your car.Clutch wiring has a 10 amp self-resetting circuit breaker inline with the field windings. Problem started a few weeks ago where the breaker would trip after about 1/2 hour of running. The gap between the "rotor" and "stator" was set at about .045". The specified gap is .018".I put an ammeter on there. Clutch draws about 6 amps continuous when the gap is set to .018". Set the gap to about .030" and the field windings draw about 15 amps.Why is this? What am I forgetting?(Side note...I replaced the circuit breaker for grins. Same results.)

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 17, 2015, 02:40:32 am
content: If it's an AC coil, it will draw more current if it doesn't pull in.

author: Skimask
author_rank: Super Contributor
timestamp: June 17, 2015, 02:44:43 am
content: DC coil.Well, DC fed from the battery/alternator anyways, roughly ~13VDC with about 3 volts of ripple with the engine running.The question is...Why do the windings draw more current if they don't fully pull in the disc?

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 17, 2015, 03:07:54 am
content: Is it a "pull/hold" setup as is often the case for large solenoids? The only other thing I can think of is a partial short.

author: amyk
author_rank: Super Contributor
timestamp: June 17, 2015, 04:28:38 am
content: The inductance changes with the gap. 3V(!) of ripple could be enough "AC" for the inductance change to cause a noticable change in current. What's the DC resistance of the coil?

author: Skimask
author_rank: Super Contributor
timestamp: June 17, 2015, 05:11:42 am
content: Yes, "pull/hold", but no electrical connection between the field windings and the clutch "plate".Turn on the juice, clutch engages, blades spin. Turn off the juice, clutch disengages, blades no spin.Inductance change makes sense now that it's mentioned. Of course a chunk of metal moving a few thousandths of an inch would change everything when everything is in such close proximity in the first place.The charging system of the lawn tractor is nothing more than a full wave rectifier and a transistor based regulator providing current for the alternator field coils. Ugly waveforms at best, good enough to keep a Pb battery charged up and run the ignition/headlights/etc.DC resistance of the field windings of the clutch? No idea...haven't measured it.Seems to me I saw a spec of something like 6 ohms awhile back. That doesn't add up as the draw is about the same 6 amps on 2 identical lawn tractors.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 17, 2015, 05:38:30 am
content: Does it run from an auxiliary rectifier?"Pull/hold" refers to some way to run the coil full power for pulling in but then reduce power to keep it held in. It could be done with a ballast resistor, tapped coil, or PWM and the "pull in" phase controlled by a sensing switch or thermal or electronic timer. Or in the case of AC, the coil inductance does it automatically in a very clever way.

author: tautech
author_rank: Super Contributor
timestamp: June 17, 2015, 05:54:35 am
content: Here's the real guide to troubleshooting:http://www.milfordpower.com/v/vspfiles/assets/images/Garden_Tractor_Clutch_Troubleshooting_and_Installation_Guide.pdf

author: G7PSK
author_rank: Super Contributor
timestamp: June 17, 2015, 07:42:09 am
content: Two winding solenoid. One to pull in the other to hold the first is switched off by a contact operated after it has pulled the clutch, there can be several things wrong here. The contacts could be fused or the pull in distance is critical and the clutch needs adjusting or replacing or the operating pin is worn or broken.