Variable speed motor ?

[SillyOldDufferModerator @sillyoldduffer]

Posted by Joseph Noci 1 on 23/10/2022 06:31:25:

Posted by Andrew Johnston on 22/10/2022 23:03:25:

Brushless DC motors generally have three sets of coils which are switched in sequence, but it isn't 3-phase in the generally understood meaning of the term.

Andrew

…

And Dave (SOD) – they are not DC motors by any stretch…the 'DC pulses they are fed' is misleading. Only the controller's source for the PWM generation is DC. The magnetic field within the motor is a 3 phase sinusoid, or close to , thanks to Di/Dt assimilating those PWM DC pulses into a current in the motor windings, the value of which varies as it would in an 'ordinary' 3 phase motor connected to 3 phase mains supply.

The controller can be regarded an electronic commutator because it's action in switching DC to the windings is similar to the way a mechanical commutator works. Like brushed DC motors, brushless motors contain permanent magnets.

Not sure how to interpret this at all…Many brushed DC motors do NOT contain permanent magnets – series type DC motors have none..There is no similarity between commutator action in a brushed motor and and induced motion from a rotating 3phase generated magnetic field –

…

Drifting badly from the OP's questions , but misinfo does not help other readers of the topic….

Oh dear, I'm in trouble again! It may be semantics in that I (and Andrew) draw a distinction between AC 3-phase motors where the rotor field is created by Induction and Brushless DC motors where the rotor field is created by permanent magnets, whereas Joe doesn't. Fair enough in the sense that both types rotate a magnetic field generated by three stator windings, but I submit the use of permanent magnets rather than induced magnetism in the rotor makes a big difference. BLDCs and 3-phase motors have different operating characteristics, with BLDC's offering several advantages.

In my defence, Wikipedia say:

A brushless DC electric motor (BLDC motor or BL motor), also known as an electronically commutated motor (ECM or EC motor) or synchronous DC motor, is a synchronous motor using a direct current (DC) electric power supply. It uses an electronic controller to switch DC currents to the motor windings producing magnetic fields which effectively rotate in space and which the permanent magnet rotor follows. The controller adjusts the phase and amplitude of the DC current pulses to control the speed and torque of the motor. This control system is an alternative to the mechanical commutator (brushes) used in many conventional electric motors.

Wikipedia's description is compatible with other web descriptions of BLDC motors .

So I say although BLDC motors and 3-phase induction motors have star or delta windings and other similarities, it's a step too far to claim a BLDC is a three-phase motor. For example, spinning a BLDC generates electricity whereas spinning a 3-phase induction motor doesn't. As far as I know, all electric cars use BLDC motors because they can do regenerative braking. A conventional 3-phase induction motors like the one on my lathe can't.

Poor Tim! Could be we're all confused…

Dave

[Clive SteerParticipant @clivesteer55943]

I think some electric car manufactures are looking at induction motor technology because production of modern rare earth magnets is mainly entered in China leaving Western industries vulnerable to trade restrictions.

Motors can be poly phase and servo motors for machines are often have 5 "phases".

CS

[Michael GilliganParticipant @michaelgilligan61133]

In the words of the prophet:

.

When I use a word,’ Humpty Dumpty said in rather a scornful tone, ‘it means just what I choose it to mean — neither more nor less.’

‘The question is,’ said Alice, ‘whether you can make words mean so many different things.’

‘The question is,’ said Humpty Dumpty, ‘which is to be master — that’s all.”

[ ref. Alice in Wonderland ]

.

MichaelG.

[Anonymous]

Posted by SillyOldDuffer on 23/10/2022 10:27:45:

…use BLDC motors because they can do regenerative braking. A conventional 3-phase induction motors like the one on my lathe can't.

Yes they can! All the electric vehicle drives I have worked on used regen braking irrespective of whether an induction motor or a BLDC motor was used. A bigger problem is charge acceptance by the battery without damaging it, normally by over-voltage.

The last big power system I designed was a 3-phase boost converter that took the output from a 3-phase generator, driven by a diesel engine, and created a 600V DC link. Conversely the 600V DC link could be used by the power electronics to generate 3-phase PWM signals (just like a VFD) to drive the generator, as an induction motor, to start the diesel engine.

Andrew

[Clive SteerParticipant @clivesteer55943]

As Andrew says induction motors certainly can regenerate and most VFD's provide an active braking capability which may need a load resistor if high level of regen energy has to be absorbed. Care must be used though as too harsh braking can lead to chucks unscrewing on Boxford, Myford and similar lathes.

CS

[Tim StevensParticipant @timstevens64731]

It's a funny old world. My car has a starter which runs on DC which is turned on and off by a commutator, and the current through each of the windings alternates. It relies on Induction, but it is not an induction motor. It also has brushes which are not in any sense of the word brushes. The car has points which are not pointed but must be kept flat. The power cycle has four phases, one of which is called induction, but it is not an induction motor or a four phase motor. I mend it with a soldering iron which is made of copper. The generator has three brushes, and this system is called 'Constant Current Regulation'. Its main characheristic is that the current is not constant or regulated.

And now I am further confused having read your responses – for which I am grateful, as they show that my concerns about the meanings of words are justified.

And those of you with an interest in typography will note that this response is not justified at all.

Regards, Tim

Edited By Tim Stevens on 23/10/2022 13:52:48

[Michael GilliganParticipant @michaelgilligan61133]

[Rich2502Participant @rich2502]

Does the motor need to be any specification for VFD use or will any three phase motor work ?

This would allow me to sources a 1 hp used motor and save some money.

[not done it yetParticipant @notdoneityet]

A motor rated for 240V, of the correct power rating, would be preferable – 240V VFDs tend to be cheaper.

Alternatively you could run a 415V motor, rated at 1 3/4HP, and run it in star configuration from a 240V output VFD.

[Author]

Posts