Speed control of single phase induction motor - Page 1
Forum section: electronics / projects

author: Ralphy007
author_rank: Contributor
timestamp: December 05, 2020, 01:53:13 pm
content: I am hoping to use the V/f method without feedback, as a hobby. Just want to check my understanding is correct.I want to keep V/f constant at say 4.8 (Vrms = 240v @ 50Hz). Therefore if I want to run the motor at 30Hz the average voltage must be 144v.This is done using a PWM signal which variable duty cycle. In this case the duty cycle would be 30/50 = 60%, (50Hz).Because the deired speed is 30 Hz, then the fundamental frequency of the PWM is 30 Hz i.e. period T = 0.033333s.The duration of the PWM ON pulse is 0.6 x 0.03333 = 0.2s.The height of the PWN will be 144 / duty cycle = 144 / 0.6 = 240.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 05, 2020, 02:11:21 pm
content: Only for modified sine waves. For pure sine wave, you use a much higher PWM frequency, typically 2-20kHz.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 05, 2020, 03:13:10 pm
content: Ideally, you would recreate a sine wave by chopping at a significantly higher frequency than the fundamental frequency. Some 5 .. 10 kHz is fine for motors, the motor inductance itself provides filtering and with 10kHz the current ripple is already minuscule.What kind of motor do you have? Capacitor-run motors are internally two-phase (the capacitor is just a cheap and crappy way to provide the second phase; the shift would be ideally always 90 degrees, but with a capacitor, it's less and varies!), and if this is the case and you have access to the internal terminals (i.e., the capacitor is visible), you can ditch the cap and design an actual two-phase inverter instead. I have done that.

author: Zero999
author_rank: Super Contributor
timestamp: December 05, 2020, 07:14:07 pm
content: What sort of single phase motor is it? Shaded pole, capacitor start or run?What's the load?If it's a shaded pole motor driving a light load, such as a fan, you'll be able to get away with a phase control/lamp dimmer circuit.If it's capacitor start/run, then it's a big more tricky, since the phase shift will be frequency dependant. The usual 50Hz could be used to start the motor, then reduced, once it's running.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 06, 2020, 02:46:27 pm
content: Quote from: Zero999 on December 05, 2020, 07:14:07 pmIf you have the possibility to control frequency, you want to do the exact opposite, start at lower frequency so that f ~ rpm, and V/f = constant, roughly.

author: Zero999
author_rank: Super Contributor
timestamp: December 06, 2020, 05:29:41 pm
content: Quote from: Siwastaja on December 06, 2020, 02:46:27 pmYes, that's normally best practise for three phase motors. The problem is, a capacitor run motor might fail to start at too lower frequency, so it you might have to start it at a higher speed, to get it to run, before slowing it down.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 06, 2020, 06:15:09 pm
content: Quote from: Zero999 on December 06, 2020, 05:29:41 pmYeah, they are designed to run with so much slip, the optimum start frequency may be surprisingly high. The only way to know is by trial and error, this is optimization. With more control opportunities, the end result will be far better than running off the mains, in any case. Although, if you go through the trouble of designing and building a VFD for single phase induction motor, you likely want to run it as two-phase without the capacitor, removing the unoptimal and variable phase shift of the capacitor from the equation.The second phase likely requires lower voltage (so smaller V/f multiplier) than the "main" phase, because the impedance of the run cap is in series with it in the original design, be careful not to burn the windings.

author: james_s
author_rank: Super Contributor
timestamp: December 06, 2020, 06:50:00 pm
content: You need to make sure it spins up quickly at least to the point where the centrifugal starting switch activates and make sure the speed doesn't dip down to where that drops out. Overall a 3 phase motor makes a lot more sense if it's at all possible.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 06, 2020, 07:27:51 pm
content: The old Vector inverters with V/Hz ramp up did that ramp up in about a second. That got discontinued because of some patent dispute, which is a shame because that was one of the cheapest brand of inverters back in the day that would work well with motor loads.

author: WattsThat
author_rank: Frequent Contributor
timestamp: December 07, 2020, 02:32:15 am
content: Single phase motors are notoriously difficult to run using inverters. 99.9% of the “single phase vfds” you’ll find online are really single phase input, 3 phase output devices that are unsuited for single phase motors. The majority of said single phase motors will let their smoke out when slowed down due to various reasons, several have already been mentioned.Unless you’re well versed in power electronics and digital systems, you’ve got a lot of ground to cover before you’ll ever create a working VFD. I say that as someone that who makes a salary applying complex VFD systems, designing one that doesn’t just blow up requires significant experience in several fields of study.There is only one commercial brand that I am aware of in that business, Invertek out of the UK.https://www.invertekdrives.com/variable-frequency-drives/optidrive-e3-single-phase/features