T-Pain Is On A Boat: Re: [Electric Boats] Why gear reduction?

Wednesday, February 24, 2016

Re: [Electric Boats] Why gear reduction?

Yes .. and no.

There is a huge variety of motors.
Slow turning high power motors exist.

But, in general, its very cheap, and easy, to make powerful high power
motors with high rpm and gear them down.
So the gear reductions in electric motors are mostly (90%) to do with
economics, ie $$$.
And partly to do with engineering.

2 examples.
Washing machine motors today use a VFD and a very high speed, about
12.000-20.000 rpm 3 phase motor.
This is very, very cheap to do in quantity.

A one-off VFD costs about 50-90$, in the 1-2 Hp sizes, like a washing
machine motor.
The motor probably costs about 30$ to make- and if it were well made for
5$ more (they are not) would last essentially forever.
The VFD probably costs about 10-20$ to make, in washing machine
quantities, ie 1M/unit model.

A high power traction motor, such as in Tesla cars, uses a 9:11 gear
reduction.
This gives great torque, fairly cheaply, and fairly easily.
The 500 kW / 700 Hp motor is about 40 kg in mass, and thus costs about
8x40 == 320 $ to manufacture in parts and labour.
You can see the actual power, continuous, of a Tesla car is approx 90
kW, its on the registration docs.
For short term (maybe half an hour, iirc) the motor can output 500 kW ..
but will overheat over time.

The motors are/were originally made in japan, and tesla also makes some
(there are bills of lading online).
There is also a video of them being made on youtube, in the Tesla factory.

Old-tech motors, like in treadmills, use permanent magnets and a
different type of controller.

They work fine, but the newer 3-phase motors are;
-cheaper to make
-last longer
-are much more efficient - and this translates to less heat.

The biggest development in the last 10 years is that the VFD
manufacturing costs have dropped by over 90%.
Power electronics have gotten really really cheap.

The tech itself is not new.
My old Bridgeport milling machine, M-head, is made about 1940-1950.
It has a 3-phase motor, that still works perfectly well, near silent at
all speeds.
I use a 2.5 kW VFD to control it.
The motor is 18 kg in mass, 1/2 hp output (continuous).
Its probably had 5000 - 10.000 - 30.000 hours of use (no way to tell).

The next, better, version of electronically controlled motors is a
brushless ac servo motor.
Its similar to the VFD/3phase, just technically different inside.
Again, its more efficient, less heat, less noise, lasts for a very long
time.

A new 2.5 kW servo, such as I use on the lathe, is about 10 kg in mass.
== 3.3 Hp => 7x more powerful than the 3-phase bridgeport motor.
And, about 14x "better" per mass.

Its equally silent, accelerates 10x faster, 3000 rpm and positions.
A servo like this will accelerate to 3000 rpm in 20 ms, or 0.02 secs (no
load).
Yes 2 hundreths.

This type of servo motor can run at 100% output indefinitely (motor
lifetime 10.000 hours or so).
The servo (== similar to a VFD) electronics have a fan on the high power
versions, and the fan will die eventually (just like PC processor fans).
The servo will output 300% torque for upto 3 secs.
Very useful in machine tool applications.

So gear reduction in electric motors is still done, but mostly because
this makes the whole system cheaper and easier to do.
A simple 1:n gear reduction is not necessarily expensive, and can be
quite efficient.
Single-gear reduction tens to run at about 95% efficiency, I think
(maybe a bit more).

A (inboard traction engine) marine gearbox, otoh, runs in an oil bath,
needs an oil cooler because it generates lots of heat, meaning its
inefficient (and noisy, somewhat).
The marine gearbox is big and heavy, in traction applications ie big boats.
There is about 10-15% loss, as the heat energy comes from somewhere.

All above needs to be taken into context, as well.
The small motor/VFD washing machine is not meant to run 24x7 - and thus
is not heavy and robust.
Lifetime is low (on costs basis. More is not needed).
Equally, like an outboard, they are not meant to run 24x7 at 100% and
would fairly quickly fail.

The marine gearbox can run at 100% forever, lifetime 3000-5000-10.000
hours (when properly rated).
Thats also part of the reason they are big and heavy and robust.

On 23/02/2016 21:06, Chris Hudson clh5_98@yahoo.com [electricboats] wrote:
> The simplest answer is heat. With the motor fully loaded at 1657 RPM
> the motor current is much much less, so less heat. The motor would
> likely burn up turning 650 RPM fully loaded.

--
-hanermo (cnc designs)