Phase control Shaded pole motor - Page 1
Forum section: electronics / beginners

author: Vindhyachal.takniki
author_rank: Frequent Contributor
timestamp: March 13, 2015, 11:57:03 am
content: 1. I have to control the speed of shaded pole motor. For this i have selected phase control method using TRIAC.I have a connected a 10K pot at adc input of MCU, as pot is rotated MCU sense the reading & accordingly control the phase. (voltage = 220vac)2. Now if I connect a 100W bulb at the output & then rotate the pot, I am successfully able to control the intensity of bulb like in gradually goes dimmer or brighter.Also I have connected a ac voltmeter at load, I can see gradual increase/decrease in voltage of bulb as phase control is done.3. Problem arises when I connect motor;Case1: When there is no parallel capacitor at motor connected (ceramic 450V):Motor start to show on/off behaviour. When volateg of pot at MCU adc pin goes high, motor gets fully on & below that it goes fully off.However if I connect bulb instead of motor, it works fine.Case2: parallel capacitor at motor connected (ceramic 450V):Motor shows random behaviour. Sometimes it picks up the load & starts roating & sometimes it do not even run. At some point full speed is on, at some point only by half speedWhat could be problem?

author: Andy Watson
author_rank: Super Contributor
timestamp: March 13, 2015, 12:03:53 pm
content: Induction motors are essentially controlled by the frequency of the AC supply.

author: Zero999
author_rank: Super Contributor
timestamp: March 13, 2015, 02:52:15 pm
content: Yes induction motors are ideally controlled by varying the frequency.However it's possible to control the speed of a very lightly loaded shaded pole motor using a phase controller.What is the motor driving? It's only suitable for fans. Don't expect any torque.Another thing you could try is connecting an incandescent light bulb in parallel with the motor.

author: Vindhyachal.takniki
author_rank: Frequent Contributor
timestamp: April 17, 2015, 08:47:13 am
content: 1. I have tried & recorded some results.2. Snubber circuit is added.3. I have attached a pdf to show some results:4. Page 1 shows how circuit is setup i.e how drop across sense resistor is measured by CRO. (circuit says current probe but its where drop is measnured across 5.4ohm resistor.)5. Page 2 shows voltage & current relationship.6. Page 3 shows a circuit where I have placed a capacitor in parallel to motor.7. Page 4 shows its relationship voltage & current relationship.8. Page 5 shows the capacitor which I had used. I don't know what its exact value. But by checking it at LCR meter, value shows around 2.2uF.9. Page 6 shows sense resistor (5 resistors in parallel of 27 each) , & how drop across it is measured by CRO. I placed ground wire close to sense tip of probe, so as to have minimum distance.10. On doing phase control as soon as I try to do it suddenly entire voltage across the motor goes down.11. What I am planning to do now, is to place a zero crossing detector across sense resistor & will do phase control whenever current goes zero. Will see what happens then.12. Should I proceed this way or there is any other method?13.File is available athttp://www.docdroid.net/xn32/design123.pdf.html

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 17, 2015, 09:27:22 am
content: There are triacs specifically designed for inductive loads - usually called "snubberless" - you should be using one of those, and firing it with a burst of negative-going pulses.

author: ConKbot
author_rank: Super Contributor
timestamp: April 17, 2015, 01:07:23 pm
content: Phase angle control on little shaded pole (or other induction motors, but it wouldnt even be something to consider for much over ~1/3 HP) motors works when the load torque varies directly with the speed. I.e. fans and pumps.If you try and use it on something where the torque load isnt linear, then results will be non-linear also. As said, frequency controls the speed of induction motors. Non synchronous ones develop torque by 'slipping' i.e. the motor runs slower than the synchronous frequency. I.e. in 60 hz land you may get 3550 rpm at no load, 3500 at light loading 3400 rpm at heavy loading. The voltage across it determines how much current it can draw and therefore how much torque it can put out. iirc more torque = more current flowing in the rotor, and more back-emf at the same RPM.

author: Vindhyachal.takniki
author_rank: Frequent Contributor
timestamp: May 05, 2015, 10:22:23 am
content: 1. Attach is the ckt diagram showing snubber circuit & triac connections. Ckt again don't work.2. I had bought a low cost ceiling fan regulator & strangely it works. I had checked its circuit, the additional feature is it has diac to control firing angle rather than MCU pin for controlling it via opto-triac.They are using 4q triac.s3. I think my snubber circuit values are wrong?