On Fri 24/05/2019 18:06, Johnny B Good wrote:
On Thu, 23 May 2019 08:52:29 +0100, The Natural Philosopher wrote:

On 23/05/2019 08:01, Andy Burns wrote:
Woody wrote:

most modern machines use a stepping motor which makes it much easier
to control direction and speed of rotation.

The engineer said my hotpoint washer/dryer (post indesit/ariston
takeover) uses a 3-ph motor driven by a VFD.

almost the same. Its a very moot point as to when a multipole 3 phase
motors ceases to be a 3 phase motor and becomes a stepper motor.

There's a very strong distinction between those two motor types. I
believe what you may have had in mind was the very fine distinction
between permanent magnet BLDC and three phase synchronous motors where
the distinction *is* very much finer.

A three phase synchronous motor (regardless of pole pair number) when
carefully designed, should ideally exhibit no 'cogging effect' as it is
manually rotated. Perfection in this regard is hard to achieve so some
cogging effect is still likely to be observed with such a test.

The endearing characteristic of a true three (or poly-phase) motor
(whether of induction or synchronous type) is the constant torque output
throughout each revolution once up to speed (no torque ripple).

Three phase motors designed to be driven directly from a 50 or 60 Hz
supply run at just the one fixed speed which is fine for most industrial
processes and those domestic applications where variable speed is not
required (fan motors or pumps).

Of course, this leaves the issue of getting them up to speed on
application of mains power which is solved by star to delta switching of
the windings in large industrial machines along with current limiting
resistors (smaller domestic sized machines forego the complexity of star
to delta switching, relying on current limiting resistors alone).

Such starter gear complexity only has to deal with the relatively
infrequent nought to 3000rpm speed changes, allowing the current limiting
resistors used to be rated for a short term, non repetitive duty cycle.

Until semiconductors became available that could cost effectively use
high frequency switching of the voltages and currents required to drive a
high voltage DC to three phase variable voltage and frequency inverter,
it hadn't been practical to use three phase motors for variable speed
applications such as domestic washing machine drum drives.

Today, such VFD control modules and associated sub HP rated three phase
multi-pole pair drum motors are becoming an ever more common feature of
the modern domestic washing machine.

Whilst even today, after a decade or more since they first started to
appear in high end white goods, they are still an expensive option.
However, they're beginning to become more commonly featured in high end
fridge freezers as ultra quiet, high efficiency surge free starting
compressor motors (which is useful for anyone looking to provide power
from a small emergency genset during a power blackout - what's not to
like about a VFD motor, other than their rather high price premium?).

Getting back to the VFD three phase motor's close cousin, the permanent
magnet (is there any other type?) BLDC motor driven by an ESC (electronic
speed controller) from a DC supply, torque delivery in this class lacks
the smoothness of a true three phase motor since only two of the three
phase windings are actually being driven at any one time by the
controller.

Obviously, this is no great detriment in their typical usage or else
they wouldn't be so widely deployed as high efficiency alternatives to
the classic DC brushed motor used in battery powered cordless drills,
screwdrivers, chainsaws and drone prop motors.

In the latter case, this allows each of the three windings to provide a
back emf zero crossing signal in turn to drive the commutation switching
circuit in the 'sensorless' ESCs used with drone propeller motors.

This 'sensorless' technique whilst fine with propeller or fan loads,
isn't suited to BLDC motors driving gross mechanical loads such as drills
and starter motors since it relies on the motor to actually be spinning
to generate the commutation sensing signal, the timing of which is
critical to the smooth and efficient operation of such motors.

To achieve the starting reliability and smooth running of a traditional
DC brushed motor, a BLDC motor's controller requires an accurate
mechanically synchronised signal independent of speed which works even
when at a standstill (startup or stalled by the load).

With the more traditional design of BLDC motor, this is achieved by the
extra complication of a trio of Hall Effect sensors, precisely aligned
for maximum efficiency during their manufacture, making them more
expensive.

However of late, AMS have produced a neat alternative solution[1] which
can trivially be retrofitted to any three phase alternator whether
permanent magnet type such as used by emergency inverter gensets or
separately excited as in an IC powered vehicle's alternator, neatly
converting them into a BLDC motor (a lightweight solution to upgrading a
pull cord only start emergency inverter genset to electric start for one
instance).

Although the BLDC motor has a 'cogging' characteristic more akin to that
of a stepper motor, the difference in this case is that it's an
undesirable side effect best minimised by design whilst in the stepper
motor case it's a desirable effect best maximised by design. Also, the
major difference between the two motor types is that stepper motors are
normally a two phase design and the BLDC motor is always a three phase
one, more akin to that of a traditional three phase AC motor.

[1] For the more curious amongst you, there's an in depth article
covering the subject of BLDC control at this web address:-

https://www.electronicproducts.com/A...l_ICs/Sensors/
Absolute_position_sensing_the_key_to_better_brushl ess_DC_motor_control.aspx

https://tinyurl.com/y2u5n9r9

HTH & HAND! :-)

Hmmm.
In that case why does the motor of our Miele washer do 15000rpm when
spinning?

--
Woody

harrogate three at ntlworld dot com