Do variable frequency drives use the same basic design for big & small motors? - Page 1
Forum section: electronics / general

author: e100
author_rank: Frequent Contributor
timestamp: June 03, 2024, 09:46:03 am
content: An example of small would be a 50w motor for a quadcopter, and large would be a 3kw industrial motor powering some machine in a factory.Are they all just basically an oscillator feeding a class D amplifier?The class D audio amplifiers I've seen have large choke filters on the output whereas a quadcopter speed controller typically doesn't. Presumably the motor winding is used at the choke. Is that how it works?

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 03, 2024, 12:05:39 pm
content: Some large ones use multi level converters, but the vast majority just have simple 3 phase bridges.

author: Berni
author_rank: Super Contributor
timestamp: June 03, 2024, 12:44:10 pm
content: They are not the same.Industrial machines are typically run by asynchronous 3 phase motors. They are indeed essentially a sine wave signal generator feeding a big powerful 3 channel D class amplifier.Drones instead uses sensorless brushless motors (can also be called a permanent magnet synchronous 3 phase motor). These can't be run on a fixed frequency like asynchrnous ones can. So they have specialized control algorithms inside the controller to calculate the motors rotor position from the voltage and current, then use that information to apply the correct phase of the sine wave to pull the rotor forwards. They also have a startup sequence to get the motor spinning in open loop control mode, since the sensorless rotor sensing only works once the rotor gets up to enough RPM. The motor twitches erratically during this open loop startup, but that is okay for drones since they drive a propeller. In electric vehicles this is not acceptable, so those also have a positional sensor on the motor shaft that is used to get more accurate rotor position for better control down to 0 RPM.But yeah the power stage is the same because 3 phase is 3 phase. The filtering is not essential for operation, but bigger drives will not pass EMI regulations without it.

author: tom66
author_rank: Super Contributor
timestamp: June 03, 2024, 12:48:32 pm
content: Quote from: e100 on June 03, 2024, 09:46:03 amA choke is needed on the output of a class D amplifier because it typically connects to loudspeakers via long lengths of cable. The high frequency currents flowing along the cable would create a major CM emissions risk. The risk is far lower in a drone. Also remember EMC rules don't apply if you don't care about them (a simplification, but the import authority is far less likely to go after a small consumer drone toy manufacturer than a manufacturer of high-end consumer electronics.)

author: Psi
author_rank: Super Contributor
timestamp: June 03, 2024, 12:57:10 pm
content: Quote from: e100 on June 03, 2024, 09:46:03 amMaybe one of those hand sized micro-quad might have 50W motors.but a normal quadcopter motor would be in the order of 400-800W at max power maybe even over 1kW for a racing quad.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 03, 2024, 01:27:55 pm
content: Quote from: Berni on June 03, 2024, 12:44:10 pmNot so much causing EMI issues, but rather standing waves building up to the point where the motor insulation could break down. Industrial environments are going to have lots of EMI (especially if arc welding is used), a little VFD isn't going to add much more.

author: e100
author_rank: Frequent Contributor
timestamp: June 03, 2024, 02:35:01 pm
content: Can I add a follow up question in relation to electric scooter terminilogy, or perhaps marketing speak.When a motor controller is described as having a 'sine wave output' does that imply that internally it's class D at some frequency above the minimum 64Hz needed to turn an 8 pole motor at 8 revolutions per second (needed for 25kph) ?The corollary being that non-sine wave controller is outputting a plain 64 Hz square wave signal, or is it more complicated than that?

author: Infraviolet
author_rank: Super Contributor
timestamp: June 03, 2024, 04:53:56 pm
content: "When a motor controller is described as having a 'sine wave output'"I suspect it will usually mean sinusoidal driving rather than 6 step commutation. There won't really be a sine wave truly present, but rather the PWM frequency will be fairly high, compared to the frequency at which the rotor is moving (electrical rotations per second, not the same as mechanical rotations/s) and the PWM percentage will be set so that if you low pass filtered it across the time taken for an electrical rotation then you'd get a sine wave (or SVM modified sine-wave like shape where subtracting two phases gives a sine wave difference between them). The actual 3 phases of coils will, depending on the angle the rotor is supposed to be at at a given time, each spend a different proportion of their time connected to the positive voltage supply vs the proportion they spend connected to a shared ground which lets (at low speed when there's not so much back emf) them recirculate current like a buck converter.

author: WattsThat
author_rank: Frequent Contributor
timestamp: June 04, 2024, 01:14:33 am
content: Quote from: NiHaoMike on June 03, 2024, 12:05:39 pmMulti-level output VFD’s are almost exclusively medium voltage (>1KV) due to device ratings and to reduce dv/dt motor stresses. LV drives with outputs up to 3-4 MW can be achieved with simple, single level parallel bridges.