Chinese concrete mixer performance

The problem

I have purchased a cheap Chinese concrete mixer rated for 90l and with a 650W motor which overheats. The motor stopped after about 2 hours use, most of which was running with an empty bowl, and when loaded, with only small loads (<30% rated capacity). After several hours cooling down, the motor operated again, suggesting a polyswitch embedded in the windings to protect the machine.

Motor specifications:

230VAC 50Hz;
Power output: 650W;
Speed 1370RPM, slip=8.6%.

The motor current, power and speed were measured with no load, no belt, absolutely no external load.

No load measurement:

V=240, I=1.4, f=50.1Hz;
Pa=336VA, Pr=200W;
Speed=1495RPM, slip=0.33%.

So, no load power is relatively high. One expects that the no-load power of a good 650W motor should be much less than 100W. Where is the power consumed?

The slip suggests , fan loss, rotor loss, windage and friction loss to be 0.33/8.6*200=7.6W. Therefore almost 200W of Hysteresis and Eddy Current losses… classic signs of too little iron / poor grade iron, too few turns.

The solution

Whilst the motor is obviously poorly designed and suffers from huge Hysteresis and Eddy Current losses, it would be usable if the machine temperature can be kept at safe levels.

The fan in this fan cooled motor is quite inadequate, there is very little air flow from the fan cowl directed along the side of the motor body. To exacerbate the problem, the motor is housed in an un-vented weather proof cover over itself and the drive belt / pulley system, so the motor operates in a pocket of hot air. The weatherproof cover reaches a temperature of 40° above ambient after prolonged operation.

Initially a vent hole was cut in the top part of the weatherproof cover to encourage natural convection and draw cooler air in around the motor. It helped significantly, but the motor body still reached a temperature rise of 80° after two hours of mixer no load operation (belt on).

A supplementary muffin fan was added to the weatherproof cover to exhaust the hot air. The fan was wired to operate even when the mixing motor was off to assist in cool down if intermittent operation was necessary on hot days. This measure keeps motor external temperature rise to less than 60°, and the machine temperature rise for a normal working load is acceptable.

VK1OD on the 'net. I appreciate feedback, click on the ... in webm...@vk1od.net for my email address.