Is it harmful to control three phase motor with relays to drive with PWM? - Page 1
Forum section: electronics / projects

author: tlhsglm
author_rank: Contributor
timestamp: April 01, 2022, 09:56:47 am
content: Hi dear friends,Can we drive three phase induction motor with relays to give a PWM signal without VFD? Can i safely control speed of motor?Mechanical relays can't do the job of course but if we use SSR's ?I have searched it but i didnt trusted is it will be harmful to the motor?I thinking to drive 5.5kw motor. It is not so much important motor for me and i havent vfd but i would like to try it.My motor is now directly connected to the main's R S T cables. I will connect R S and T inside 3 seperate SSR's and then to the motor. I will open SSR's at the same time with arduino when PWM HIGH and close them when PWM pulse LOW.PWM's min and max range will be 50% and 100% for keep motor cool.Do you think is it work or is it will be harmful to motor?

author: Kleinstein
author_rank: Super Contributor
timestamp: April 01, 2022, 10:07:17 am
content: When running at a lower speed the motor will have significant loss. The power drawn is still essentially the same as with the full speed, just a possibly lower torque (depends on the load).Many motors don't like this and get too hot. Motors with a large slip (and thus poor effiency) and thus a speed significant lower then the ideal value from the mains frequency (e.g. 3000 rpm with 50Hz) can work this way. This are mainly lower power motors, like < 300 W.The main question is if the motor gets to hot. There are 2 effects: additional loss and the fan gettting less efficient.

author: Berni
author_rank: Super Contributor
timestamp: April 01, 2022, 10:16:20 am
content: You are NOT supposed to run a induction motor this way.When an induction motor is made to run at a speed significantly different than what the power frequency dictates this means that the rotor must run with a lot of magnetic slip. This is the equivalent of driving slower with your car by holding the clutch half way so that it slips with the speed difference you want to archive. A lot of power is getting turned into heat by that slippage.It does work for slowing down the motor, but it is only really practical on small motors that are also under a even smaller load. That way the motor can absorb and dissipate the waste heat while they are so small that you don't really care for the wasted power.If you want to use a dimmer style speed control then you must get yourself a brushed universal motor. Those work great at reduced voltage and don't care about mains frequency.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: April 01, 2022, 02:05:07 pm
content: what you are describing is a "Soft Starter", see:https://www.eaton.com/us/en-us/products/controls-drives-automation-sensors/soft-starters/how-to-choose-between-a-soft-starter-and-a-variable-frequency-fo.htmlthey behave, essentially, like a large light dimmer, reducing the inrush current to the motor during starting to help prolong motor service life. i spent a year or so doing firmware testing on them for a local manufacturer, so have some familiarity.a Soft Starter is intended to ONLY be active when starting up (or stopping) the motor, and once the motor is up to speed may even be bypassed with mechanical relays. as others have pointed out, trying to run a motor continuously with 'throttled back' power will end in smoke and tears.if you want to run the motor continuously at areduced speed, you need a VFD. but be aware that there are still limitations, as a motor will run sub-optimally at a frequency substantially different to that for which it was designed. as i recall, a 50Hz motor started to get unhappy below around 30Hz when under load.btw, a very approximate rule of thumb is that the DOL starting current (when not using a Soft Starter) can be up to 8x the running current, and any configuration making use of a Soft Starter should always be designed to still function if the Soft Starter is removed. ie,neveruse a Soft Starter as a way of getting around inadequate supply or wiring. ignoring this, will end in smoke and tears.cheers,rob :-)

author: CaptDon
author_rank: Super Contributor
timestamp: April 01, 2022, 02:15:02 pm
content: Most of the SSR stuff in your power catagory will be triac/scr devices and will only turn off during zero crossing and will be a nightmare on 3 phase. Trying to create a VFD by using off-the-shelf SSR's will be a total waste of time and burned up hardware. I can't even imagine the harmonics and imbalances that would be placed on the 3 phase power source?? Likely to blow up other stuff on the same power source not even related to your project. Real VFD's for the most part rectify the incoming power to produce a 'link voltage' on a capacitor storage bank and then use IGBT units in an H-Bridge configuration to synthesize the desired voltage/frequency/amperage for the desired motor speed/torque. Your idea does none of that!!!

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: April 01, 2022, 06:15:12 pm
content: You will need 3 accurate sin proportioned PWM signals 120 degrees out of phase with each other switching a fairly high DC voltage. The typical switching device is an IGBT. Consideration of any flavor relay should be discarded.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 01, 2022, 06:54:18 pm
content: The PWM like (pulse packet) type control is (was) used with low power (e.g. 50-100 W) motor, for FANs and small pumps e.g. for the heating system or solar warm water. Often they are 2 phase with a capacitor, though not exactly 3 phase, but close and equally bad to regulate.These are motor that start with a lot of slip (e.g. 1800 RPM instead of the theoretical 3000 for 50 Hz mains) and can than be controlled down to some 600 RPM. As pumps the cooling is from the water anyway, so non problem with a fan. Because of the poor (e.g. < 50%) efficiency such motor are increasingly replaces by BLDC or similator motors.