Review of mower starter motor current using Owon HDS242S and Hantek CC-650

This article documents capture of starter motor current on a Toro MX4250 mower with the Toro V-twin engine. The test is part of diagnosis of possible starter motor problems.

The current was captured with a Owon HDS242S hand held DSO and Hantek CC-650 current probe.

The motor is a permanent magnet DC motor with bendix gear.

Above is a screenshot, vertical scale is 50A/div, measurement is of battery current.

First phase is where the motor shaft spins up with only its own inertia as load, current is initially high and quickly reduces to 250A. During this time the bendix gear is accelerated upwards to engage the flywheel.

Next phase is when the bendix gear is at the end of the helix and starts turning the flywheel, current increases to around 350A with the initial cranking load.

At about 130ms, the engine passes through a compression and current peaks locally (not enough time for induction here so has not fired), falling then until the next compression peak at about 330ms. Note that this engine has compression relief (like most larger ‘small engines’).

The engine starts on this compression and accelerates, reducing the load on the starter motor and current falls. Around 400ms the bendix is driven downwards by the flywheel and current falls back to a no-load value around 100A (with some charging current pulses).

The start key is released at about 740ms and current ceases.

The negative current pulses from about 740ms is the charge current flowing to the battery.

One can deduce the approximate cranking speed from the timing of the compression peaks (approximate because it is a V twin)… around 300RPM.