Greenworks CSB412

I received a pair of “underperforming” Greenworks CSB412 chainsaws for evaluation.  The CSB412 has a 20" bar and is rated at 3.4 kW (4.5 horsepower).  Greenworks claims is equal to or better than a 60cc gas chainsaw.  The unloaded chain speed is 82 ft/s (25 m/s).

I did a fairly extensive write-up on the CSB410, so in this section I'll just highlight some differences between it and the CSB412.

Quantifying “Underperforming”

Both CSB412 chainsaws under evaluation here are essentially new.  A friend was sent a warranty replacement after complaining the one he purchased did not perform well.  I disassembled the original saw for analysis and tested the replacement.  Although the CSB412 is intended to be used with a 4.0 Ah battery, I tried a 5.0 Ah battery hoping that it would improve performance.  Sadly, this not only significantly increased the saw's weight, it became even less useful.

Surprisingly, a 2.0 Ah battery actually worked better (although it depleted very quickly).  The flashing red <!> never came on, but it was still not useful for jobs that would require a 20" saw.  I think the 5.0 Ah battery could supply enough current that the controller went into a current-limiting mode and “protected” the saw.  It most definitely did not run long enough for a thermal sensor to shut it down.

Theory

I used an optical tachometer and measured the motor speed at 12,670 RPM unloaded (no chain) @ 78.6 volts.  So it's probably rated to run at 13k RPM. 

It has a 7-tooth sprocket and uses a chain with a 0.325" (8.255 mm) pitch.  That calculates to the rated 25 meters per second at 13k RPM. 

The saw comes with a 4.0 Ah battery which has an energy capacity of 288 watts.  Assuming a full 80-volt charge, the current draw would be 42.5 amps (prior to accounting for any losses in the controller itself).  The high-capacity battery is not just to increase run-time, it also provides the higher current required for increased power capability. 

Using a clamp-on DC ammeter, I measured a draw of about 4 amps to just run the motor (no chain installed).  At the time, the battery was at 76 volts.  So the saw requires about 300 watts to “do nothing”.  Put another way, we could expect a new 4.0 Ah battery to “idle” a chainless saw for less than an hour.

Theoretically, operating at the rated power of 3.4 kW, this battery would be depleted in less than 0.085 hours (5 minutes).  And probably much sooner in practice. 

Teardown Video

Below is a 30-minute teardown video of a CSB412 saw I found on YouTube.  Honesty, it's pretty tedious.  I'd advise you to run at 2x speed and skip around.  I'll describe the greatest impediment to disassembly directly below the video.

Motor

The motor is marked “Globe 71405 T030101B-00”.   It is not part of Allied Motion's Globe Line of BLDC motors.

Physically, it uses an outer rotor that has an OD of 80 mm.  It exhibits 18 detent (cogging) positions.  There's an 83 mm ID shroud that surrounds the motor to keep it cleaner.  This motor required substantially more effort to rotate it by hand than a CSB410 motor.  At first, I thought the chain brake was partially on, but it was not. 

I counted 9 slots (winding coils) through the ventilation openings.  This number must be evenly divisible by three for any 3-phase motor.  With 9 slots, the permissible number of poles (magnets) could be 2, 4, 6, 8, 10, 12 or 14.  The number would likely be at the lower end of the range due to space constraints.  I have not disassembled the motor to count them. 

Electrically, the phase-to-phase resistance is 66 milliohms.  Inductance when measured at 1 kHz was 115 uH in the quadrature axis (rotor at a detent position) and 87 uH in the direct axis (rotor in-between detent positions).

The motor uses three Hall-effect sensors for position feedback to the controller.  

The NTC thermistor measures 40.3k ohms at 65 degrees F.

Controller

The CSB410's wiring is typical of an inexpensive consumer product.  The wiring for this motor controller (CSB412) looks more like something from an industrial application.

The controller itself (black blob at bottom) sits on a massive aluminum heatsink located on the underside of the saw.   This large chunk of metal adds considerable mechanical strength (and unfortunately weight) to the saw.

The low-power (interface) wiring is near the front of the saw.   It connects to the operator's control panel, trigger/throttle, chain-brake safety switch and Hall-effect sensors for motor position.

At the rear of the controller is the high-power wiring.  The 3-phase motor leads are red/yellow/black.  The battery negative lead is blue, and battery positive is brown.  The smaller white wire is for serial communications between the battery's BMS and the controller.

The black cube at the front of the controller is an electromechanical power relay.  Using a stethoscope, I could hear it engage an instant before the motor runs.