Hand Tool Motors (240v)

[Hysteresis]

Why do Manufacturers invariably use Commutator ac motors rather than Single Phase capacitor start induction motors. Clearly the induction motor does not need a Commutator or brushes therefore should be cheaper and better to use.

 

[Jackrae]

So many reasons but for starters get hold of an induction motor of similar power rating of a commutator motor and compare it for size and weight.

 

[conny]

Commutator motors can more easily be speed controlled, even with electronic speed modulation.

 

[Jackrae]

The 'new' way is to avoid the use of commutator motors and go for 3-phase motors, especially for battery powered units. With the use of rare earth stators and high speed rotors, tools can be built using extremely small, high torque, variable speed motors.

 

[Hysteresis]

Interesting few responses.
I Started to think a little more about the two machines. The induction motor Torque/Slip characteristic makes it almost, but clearly not quite, a synchronous motor so speed control becomes a matter of frequency control as conny suggested. With modern electric circuitry, this shoulder not be a problem these days. With respect to bulk as Jackrae commented, I don't have the associated data to make the comparison but I would have thought they would be comparable in size.
Jackrae suggest that the 'new' way was is to go with three phase motors. Clearly this would be difficult with small hand held machine but again with modern electronic design a three phase system for a small machine seems to be not unreasonale. This begs the question, "Has anyone tried this approach for hand held machines". The generation of a small three phase system could incorporate Frequency variation also thereby giving reasonable speed control. If any company has not developed such a system, maybe it would be worth some research. The adantages of an induction motor over the commutator alternative is of course the elimination of the damn commutator and the carbon brushes associated with it.
I am sure some company somewhere has investigated the generation of a small electronically derived variable frequency three phase unit and develop it for a hand held machine. The induction motor will be a lot cheaper production option and longer life expectancy due to there being no commutation to contend with.
I look forward to any comments.

 

[JohnW2]

I am sure some company somewhere has investigated the generation of a small electronically derived variable frequency three phase unit and develop it for a hand held machine. The induction motor will be a lot cheaper production option and longer life expectancy due to there being no commutation to contend with.

To throw in a rather cynical comment/question/suggestion, is expending time and effort in developing a product which will have a 'longer life expectancy' necessarily going to be regarded by a company as being in their commercial interest?

Kind Regards, John

 

[plugwash]

I understand the high-end cordless tools world is moving towards "brushless" designs which I understand are essentially 3 phase motors driven electronically.

 

[Jackrae]

There are oodles of manufacturers, Bosch, Makita and Dyson to name but three, producing brushless battery operated hand-tools. The motors are multi-phase and variable speed. When you consider the power of the brushless electric motors used in model aircraft it wasn't a big leap to using the same motor concept in hand tools.

 

[ericmark]

Torque with a simple brushed motor can be high at low revs. I remember using a dynamo as a motor for a pipe line trolley, by altering field strength you can control speed and torque. We have seen powerful motors like in the Dyson vacuum cleaner, but to get that power it runs at a very high speed.

To use an induction motor at variable speeds and torque output the normal method is first turn AC to DC, store it in a capacitor, then turn it to AC at the frequency required. Likely using a three phase motor, although simple computer fans often more than 3 phases, so there is a balance between room and weight of motor and room and weight of controller, typical e-bike with motor in the wheel has a large box nearly as big as motor on the frame to control that motor, the ones built into pedals however the control box is often in the motor case.

The static motor inverter

is really speaking a large box, fitting that into the hand tool is not easy.