Does a variable freq drive act like a class D amp... why so long to adopt them? - Page 1
Forum section: electronics / general

author: Beamin
author_rank: Super Contributor
timestamp: August 03, 2019, 09:42:50 pm
content: It seems that everything now has the much superior variable frequency ac motors where three stationary windings spin a loop of magnets. This to me seems like an ideal motor yet brushed motors were still popular until recently at least in things like electric cars and rc planes. Does the driver act like three class d amplifiers with a pwm/signal generator connected to them? Is it because there wernt cheap or reliable power transistors to power them and that's why they are just now starting to be used in more things? This is how they make the rc plane sized motors "beep"; by putting high frequencies much too fast to spin them? I remember when I first heard noises coming from the motor of an rc plane, I was quite surprised but now seems common sense.

author: james_s
author_rank: Super Contributor
timestamp: August 03, 2019, 10:19:47 pm
content: The motors you are referring to are brushless DC, commonly called BLDC motors. They are common today but they still cost more than a brushed motor which has had 100+ years of refinement. Brushed motors are cheap, robust, tolerant of abuse and work well enough for the applications for which they are used. BLDC motors cost at least twice as much, often several times more by the time you factor in the drive electronics so they are only used when the benefits outweigh the costs.

author: Whales
author_rank: Super Contributor
timestamp: August 03, 2019, 11:56:44 pm
content: Noises from brushless motors are indeed due to the PWM frequency used. This can be anything from hidden (good), filtered away (best), ear-screetching (bad) or intentional (music).My understanding is that brushless + (3-halfbridge) driver is sometimes cheaper than brushed + (2-halfbridge) driver. It depends on a lot of things, but brushless can typically get away with a lot less metal than brushed.Sidenote: RC planes don't use brushless over brushed for cost, they're in it for the better power to weight ratio.

author: Whales
author_rank: Super Contributor
timestamp: August 03, 2019, 11:59:40 pm
content: Quote from: james_s on August 03, 2019, 10:19:47 pmThis is actually really interesting to read into: brushed motors have been investigated for eons. I borrowed a "brushed machines" book from my library and discovered it's a world of dark art magic. A properly designed brush can last almost forever without being changed and there is all sorts of complex air flows (including suction onto the commutator) going on. By reading the visual patterns left by the brushes on the commutator you can diagnose all sorts of conditions.

author: langwadt
author_rank: Super Contributor
timestamp: August 04, 2019, 01:54:15 am
content: Quote from: Whales on August 03, 2019, 11:56:44 pmBLDC doesn't necessarily use pwm, it can just be just the commutator replaced with a "electronified" version

author: Whales
author_rank: Super Contributor
timestamp: August 04, 2019, 05:00:20 am
content: Quote from: langwadt on August 04, 2019, 01:54:15 amIf you are only simulating a commutator then you will only get "full on" and "full off" motor behaviour. That will work, but most brushless usage scenarios also want speed control.For speed control you need to be PWMing large amounts of power somewhere in your motor driver. Granted there are ways around this (resistive driving, variacs) but this is typically the most efficient & commonly used solution, at least in the scale of stuff I've seen. There's probably stuff out there that doesn't need itBig stuff?(N.B. even with commutator-emulation-only switching, you still get a switching frequency and therefore a noise. Albiet you no longer have direct control over the frequency, so playing music isn't as easy of an option)

author: Beamin
author_rank: Super Contributor
timestamp: August 04, 2019, 06:46:02 am
content: So why three phase and not 2 or 4 since magnets are bi polar? Does it have to do with the concept how three phase power (lines) are basically on all the time and can handle double the power that a two phase/conductor system can? Are these motors more expensive then brushed because they are new/less common? Seems like a lot more tooling and parts would be needed to make a brush and commutator then basically almost a solid state motor where only the part you want to move, moves. I noticed they are used as generators/motors/starters in the chevey volt which is quite a simplification especially since you can turn one into the other instantly. Is there a reason why the starter isn't used as the alternator on cars assuming you don't have a solenoid or belt set up that limits the two from trading uses.

author: IanB
author_rank: Super Contributor
timestamp: August 04, 2019, 06:52:02 am
content: Quote from: Beamin on August 04, 2019, 06:46:02 amIt's nothing to do with poles on a magnet. Three phases is the minimum to create a rotating field (you do want your motor to rotate, don't you?)This is why shaded pole motors have "shading" and why othertwo phasesingle phase induction motors have capacitors. If you don't have three real phases you need some way to emulate a rotating element to the magnetic field to get the motor to start turning.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 04, 2019, 12:41:17 pm
content: Some small BLDC motors are 2 phase. That makes it easier to make the stator more compact, but requires 4 push pull outputs (no big deal at those low power levels - just integrated into the driver chip) rather than 3 for 3 phase.Quote from: Beamin on August 04, 2019, 06:46:02 amThat's becoming more common with the adoption of start/stop technology.

author: langwadt
author_rank: Super Contributor
timestamp: August 04, 2019, 06:53:56 pm
content: Quote from: IanB on August 04, 2019, 06:52:02 amonly need 2 phases, most stepper motors are two phasethe typical US split phase is not two phase

author: langwadt
author_rank: Super Contributor
timestamp: August 04, 2019, 06:59:45 pm
content: Quote from: Whales on August 04, 2019, 05:00:20 ammost cooling fans like in a computer I think does itI've seen a driver for a 24V refrigerant compressor that had variable switchmode supply in front of the on/off commutatorQuote from: Whales on August 04, 2019, 05:00:20 ama mechanical commutator has the same noise plus all the arcing etc.

author: james_s
author_rank: Super Contributor
timestamp: August 04, 2019, 08:04:26 pm
content: My grandmother had a 1960s garden tractor that had a combination starter-generator. Once automotive electrical systems grew to the point where alternators had significant advantages over DC generators (load not carried through brushes) the combination units were not practical until modern power electronics made powerful brushless motors and efficient voltage regulators possible.

author: Beamin
author_rank: Super Contributor
timestamp: August 16, 2019, 08:36:51 pm
content: Quote from: IanB on August 04, 2019, 06:52:02 amWhy is three the minimum because you can make a triangle with three sides but no shape from two sides (with straight lines anyways)? So then how does (in terms of your 3 phase statement not how does it work in general I understand that) a two polarity brush motor work if you need three?

author: Zero999
author_rank: Super Contributor
timestamp: August 16, 2019, 08:59:57 pm
content: Quote from: Beamin on August 16, 2019, 08:36:51 pmYes, three magnets are required because it forms a triangle. In a shaded pole motor, the shading rings create an extra pair of magnets with another phase shift, to ensure the motor starts properly.

author: soldar
author_rank: Super Contributor
timestamp: August 16, 2019, 09:16:35 pm
content: Quote from: IanB on August 04, 2019, 06:52:02 amWhen you say "two phase" you really mean "single phase". Two phases 90º apart would be the same as 4 phases.

author: IanB
author_rank: Super Contributor
timestamp: August 16, 2019, 10:25:26 pm
content: Quote from: soldar on August 16, 2019, 09:16:35 pmYes, I meant single phase in terms of the electrical supply. I have corrected the post.What I really meant was that if the N and S poles were exactly opposite and alternating there would be no turning moment on the rotor to help it start going in the right direction.

author: Zero999
author_rank: Super Contributor
timestamp: August 17, 2019, 01:53:13 pm
content: Quote from: soldar on August 16, 2019, 09:16:35 pmBut a 90º phase shift is commonly referred to as two phase.https://en.wikipedia.org/wiki/Two-phase_electric_power

author: Beamin
author_rank: Super Contributor
timestamp: August 19, 2019, 06:17:41 pm
content: This is a really good one too. Would love to have some of that equipment, never again will that stuff be made. So sad.