speed control of an induction motor

Thread Starter

CarltonDranko

Joined Dec 30, 2020
22
i want t o use the following circuit for speed control of SPIM the circuit below when tested results in the explosion of capacitors,, what might be the problem ?
if any improvements or modification do tell or send any working circuit. thank you in advance.
 

Attachments

LowQCab

Joined Nov 6, 2012
1,919
There's a very good reason why VFDs (Variable-Frequency-Drives) are so popular.

Triacs are a total pain-in the-butt, they're extremely noisy, and like to burn-up Motors for fun.
.
.
.
 

shortbus

Joined Sep 30, 2009
9,324
Like LowQ said, induction motors are speed controlled by frequency not voltage. To better understand watch this video, he uses real instruments to show that voltage has no effect on motor speed -
 

du00000001

Joined Nov 10, 2020
99
A "universal motor" (e.g. in a drill) can easily be controlled (open loop) with such a circuit.
As the OP wrote about "exploding caps" (not about a ruined motor): what were the ratings of the caps?
 

shortbus

Joined Sep 30, 2009
9,324
A "universal motor" (e.g. in a drill) can easily be controlled (open loop) with such a circuit.
As the OP wrote about "exploding caps" (not about a ruined motor): what were the ratings of the caps?

He also clearly said his motor was a "SPIM" -Single Phase Induction Motor. And his circuit won't work for them, no matter what caps are used. Also a "universal motor" needs no caps or other circuit to run and be speed controlled, a simple light dimmer can do that.
 

MaxHeadRoom

Joined Jul 18, 2013
24,993
Triacs will control applications such as ceiling fans etc, which are either shaded pole or PSC induction , 1/4hp max.
Also air exchangers with the same motors.
 

MisterBill2

Joined Jan 23, 2018
11,585
I successfully used a similar packaged speed control for a half HP "whole house" vent fan a few years ago. But it was a package from Graingers marketed for motor speed control and so I am not sure how it worked. But it did allow a significant speed reduction with nothing exploding. So there is some sort of circuit that will work. Probably it simply reduced the torque enough to make the fan speed drop.
 

OldTech

Joined Jul 24, 2009
4
i want t o use the following circuit for speed control of SPIM the circuit below when tested results in the explosion of capacitors,, what might be the problem ?
if any improvements or modification do tell or send any working circuit. thank you in advance.
What kind of capacitors are you using? If you're using DC electrolytics, then it's no wonder they're exploding. The capacitors have to be non-polarized in an AC circiut such as this.
 

jeffl_2

Joined Sep 17, 2013
64
What you have to keep in mind with a single-phase induction motor is there's a "shorted turn" of copper on the stator winding in order to generate the field in the rotor, hence the motor is at most maybe 35% efficient and has lousy torque so you aren't effectively "controlling" speed very well (also cannot be run in reverse). I had a friend a long time ago who got a patent (I'm sure it's expired) about putting a simple once-around "tach sensor" (opto or Hall effect, whatever) on the shaft, then you sense the speed, you have a pot to set demand speed, you integrate the difference and use the result to drive the circuit that generates the voltage that determines the phase angle (he did it in analog, nowadays in sufficient volume you might use an MCU). You still don't HAVE much torque but you're less likely to notice that you don't! In contrast for a three-phase induction motor the efficiency is MUCH higher and you would be better off using a VFD to control speed, there's app notes and dev kits and charts all over for that. In a larger motor it's more of a contest between a 3PIM and a BLDC, with the most obvious difference being the BLDC can also be position controlled (you still need to incorporate a sensor for that which can get pricey), the 3PIM really doesn't do position control but (a la Tesla) it has the advantage that it doesn't need rare-earth magnets either.
 

MaxHeadRoom

Joined Jul 18, 2013
24,993
What you have to keep in mind with a single-phase induction motor is there's a "shorted turn" of copper on the stator winding in order to generate the field in the rotor, hence the motor is at most maybe 35% efficient and has lousy torque so you aren't effectively "controlling" speed very well (also cannot be run in reverse).
That is a shaded pole induction motor only!
They can be reversed by swapping the rotor and end plates, end-for-end. ;)
The 3PIM, 3ph Ind Motor, can position using a VFD and encoder.
 

jeffl_2

Joined Sep 17, 2013
64
The 3PIM, 3ph Ind Motor, can position using a VFD and encoder.
In order to successfully "hold" a position you need to have the capacity to maintain a constant current (equivalent to torque or force) through the motor shaft to the load. Even if the range of the VFD output includes DC (which most don't from what I understand) you can't deliver any quantifiable amount of "DC torque" from an induction motor to a mechanical load regardless of how many drive phases it has, the torque function of the motor has a zero at DC (zero frequency). You can prove this to yourself because the amount of drive you can apply to a motor goes down substantially as you drop the shaft speed and drive frequency. I'm really not "trying to start an argument" here, maybe I'm going to wind up learning something, but in an early part of my career I spent a lot of time working on fairly sophisticated electromechanical servos, now we didn't use ALL motor technologies in ALL their available drive configurations, but I think I walked away with a pretty clear notion of what works and what doesn't. (We didn't work with polyphase motors and VFDs but that was largely because they weren't suitable for precision position control.)
 

MisterBill2

Joined Jan 23, 2018
11,585
That is a shaded pole induction motor only!
They can be reversed by swapping the rotor and end plates, end-for-end. ;)
The 3PIM, 3ph Ind Motor, can position using a VFD and encoder.
I have a reversible shaded pole motor, but it is certainly not common. The shading poles are each wound copper wire and the direction selection either short circuits or energizes the shading coils to change direction. It works well for the application but it is not very efficient.
And the statement that"induction motors can't be reversed" should have been "shaded pole motors can't be revered." I am guessing that all shaded pole motors would be induction motors, except for the synchronous motors, of course.
 

MaxHeadRoom

Joined Jul 18, 2013
24,993
In order to successfully "hold" a position you need to have the capacity to maintain a constant current
I worked for some years retrofitting CNC M/C and mostly used Mitsubishi systems, their 3ph spindles were capable of positioning to a set point in order to allow tool change to occur for just one example.
 

pyroartist

Joined Oct 9, 2015
114
This thread interests me because for several years I ran a cheap 20 inch box fan in my garage roof vent all summer. I rigged an old mechanical thermostat with a mercury switch in it to control a 25 amp solid state AC relay which powered up the fan. It was either on or off. No speed control. This arrangement would work perfectly for a summer or two but eventually the fan would just stop working and I had to go buy a new one. I had to switch over to a good ol mechanical relay and have not bought a new fan for years now! Inspecting the dead fans never revealed anything. They don't seem to have a capacitor in them so I don't know how they start. I am guessing these SS relays have a triac in them. What was going on here?
 

MaxHeadRoom

Joined Jul 18, 2013
24,993
This thread interests me because for several years I ran a cheap 20 inch box fan in my garage roof vent all summer. I rigged an old mechanical thermostat with a mercury switch in it to control a 25 amp solid state AC relay which powered up the fan. It was either on or off. No speed control. This arrangement would work perfectly for a summer or two but eventually the fan would just stop working and I had to go buy a new one. I had to switch over to a good ol mechanical relay and have not bought a new fan for years now! Inspecting the dead fans never revealed anything. They don't seem to have a capacitor in them so I don't know how they start. I am guessing these SS relays have a triac in them. What was going on here?
If it had a cap, then most likely a PSC , Permanent Start Cap' motor.
These have a small capacitor in series with a run winding and common in fans.
 

MisterBill2

Joined Jan 23, 2018
11,585
Certainly electronic (triac ot SCR) switching can be very hard on motors and lights with electronics. A CFl in a socket with a solid state sun-switch, or a SSR motion sensor, is destroyed in seconds. I have not experimented with non-dimmable LED lights yet, they are not cheap enough to destroy painlessly.
 

jeffl_2

Joined Sep 17, 2013
64
I worked for some years retrofitting CNC M/C and mostly used Mitsubishi systems, their 3ph spindles were capable of positioning to a set point in order to allow tool change to occur for just one example.
Yes, and that's a spindle axis NOT a dimension axis! For most of us "position control" refers to HOLDING a position against a known maximum applied force, like a tool against the workpiece. You're just talking about rotating a spindle through a certain number of degrees to face a particular direction, once the endpoint is reached the force stops and it's just held by shaft friction, for most of us that's not quite "position control" but I can see where the issue can get a little ambivalent. Like I said I didn't mean to "start a flame war", and if I had thought of that "use case" I probably wouldn't have brought it up in the first place, sorry to bother you!
 

jeffl_2

Joined Sep 17, 2013
64
I have a reversible shaded pole motor, but it is certainly not common. The shading poles are each wound copper wire and the direction selection either short circuits or energizes the shading coils to change direction. It works well for the application but it is not very efficient.
And the statement that"induction motors can't be reversed" should have been "shaded pole motors can't be revered." I am guessing that all shaded pole motors would be induction motors, except for the synchronous motors, of course.
It's fairly common in current parlance to refer to shaded-pole motors as "single-phase induction motors", I don't think it's sufficiently ambiguous terminology to merit a ruler across the knuckles! As for "reversible shaded-pole motors" yes I have heard of same but I don't recall either seeing one "in the wild" or offered for sale as a general-purpose replacement part. I have the impression they're mostly made to order for appliance OEMs. I sort of imagine for most applications a reversible PSC is the same function for not a lot more money but again I have never worked in the appliance field and haven't studied the comparison so cannot be certain. (Motor terminology changes over time and is seldom taught nor are the choices between designs accurately documented.)
 

MisterBill2

Joined Jan 23, 2018
11,585
It's fairly common in current parlance to refer to shaded-pole motors as "single-phase induction motors", I don't think it's sufficiently ambiguous terminology to merit a ruler across the knuckles! As for "reversible shaded-pole motors" yes I have heard of same but I don't recall either seeing one "in the wild" or offered for sale as a general-purpose replacement part. I have the impression they're mostly made to order for appliance OEMs. I sort of imagine for most applications a reversible PSC is the same function for not a lot more money but again I have never worked in the appliance field and haven't studied the comparison so cannot be certain. (Motor terminology changes over time and is seldom taught nor are the choices between designs accurately documented.)
I did not "knuckle rap", and although I have designed in many induction motors, both single and 3 phase., I never refer to a shaded pole motor as anything other than a shaded pole motor. The induction is understood, shaded pole is seldom assumed.
 

jeffl_2

Joined Sep 17, 2013
64
Well maybe I'm just looking at this from the standpoint of "motors you design controls for", or maybe it's because the term "PSC" doesn't contain the word "induction", and technically it ISN'T "single-phase", and you don't typically use the term "induction" when doing a search to buy one. If you've been designing with three-phase induction motors (three or four wires) and you "downshift" to consider using single phase, most of the people I "hang with" assume you're talking about the everyday, low-torque', low-efficiency variety, it's the only "single-phase induction" motor most of us are aware of, if you want to insist on saying "shaded pole" along with that OK but it sort of implies there's another kind in common use, maybe there is and I'm not aware of it.
 
Top