Solid State relay kills induction motor fans

Have you had any other fans of the same make/model running on just

115VAC to see if they run any longer? They could be just cheap fans whose oil dries up.

Hard to see where a SS AC Relay could hurt an induction motor if the motor isn't overheating.

Does the fan still spin? Is there continuity in the winding?

John :-#)#

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Do you do TIG/MIG welding in this garage?

The motor still spins easily. There is no DC continuity from the power p lug but I beleive this is normal for induction motors? Perhaps the overheat sensor has opened permenently. Home Depot gave me a replacement so I will try it again. These fans typically last for many years when plugged directl y into a wall socket. This is a real mystery to me. Perhaps SS relays gener ate some large spikes on their output? I should check it with a scope. Thanks for the comments.

ends a 6 VDC battery voltage to control a 120 VAC solid state relay which p owers up the vent fan. The fans always quit within 2 months to a year with no obvious defects. These are inexpensive 20 inch box fans with induction m otors. I have tried several brands and styles of fans. The latest was a Hom e Depot unit that only lasted 2 months.

om a SS relay? What is killing the fans? I have taken two apart and there i s no smoke smell, no visible damage and if there is a capacitor it tests OK .

No. I don't own a TIG/MIG welder.

Thanks for the detailed treatise on mechanical relays. But I am using a solid state relay (SSR).

You have an interesting problem. I hope you figure it out. I have never used SS relays, but I can't image they cause spikes and other anomolies that would kill a simple AC motor. Is the relay rated for inductive loads?

By the way, I have a similar fan I purchased years ago. I just measured its DC resistance and found it to be about 18 ohms on its high setting. Clearly, that number is no indication of its impedance at 60 Hz while in operation, but it definitely conducts DC. My best guess is something is causing the motor windings to overheat and eventually open. Unfortunately, I can't think of any way to explain why that would happen.

Pat

I think but don`t know! If the SSR is bad (or not designed for inductive load) it may have different half wave signals, so a DC current may flow thru the motor and heats the coil (too much). A light bulb didn`t show any defects, because it converts all current to heat and a little light :)

Just my 2 cent.

I will recommand a true mechanical relay for this job.

Wolfgang

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I can't be bothered to study SSRs so I just speculate... Could it be that the ones you use just deliver one polarity of AC instead of both, like a rectifier? Resulting in a DC component which the fans do not like...

Mike.

That's an interesting thought. Even if most SSRs handle full AC, maybe this particular one is faulty and only passes one polarity. That would certainly make the fan motor unhappy.

Pat

The motor would run at half speed at best.

The motor won't overheat though unless it stalls/is jammed and can't self cool.

It is more likely that the motors are just of a cheap quality construction (for example are the UL or CSA rated?) and fail far too easily. A SS AC modular relay is simply a high current TRIAC and these are used on many fans around the world - what do you think a motor speed

control is?

A moments research pulled up this PDF:

(quote)

The following will demonstrate the process to determine the effective ratings of non-motor control rated Solid State Relays for use in motor control applications: Example question: ?Can I use a standard Solid State Relay (which is not included in the Crydom Motion Control Brochure and therefore without HP or KW Motor rating) to control the start/stop of a motor?? The answer:

Yes, you need only to consider the motor nominal current value (FLA), inrush current value (LRA), motor power factor (typically 0.1 to 0.9) to

select the appropriate turn-on switching type (zero-crossing or random) and possible need for SSR transient protection to select an appropriate Solid State Relay.

(end quote)

John :-#(#

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ds a 6 VDC battery voltage to control a 120 VAC solid state relay which pow ers up the vent fan. The fans always quit within 2 months to a year with no obvious defects.,,, there is no smoke smell, no visible damage and if ther e is a capacitor it tests OK.

Three possibilities: your thermostat could be chattering (sending a bouncy- switch signal to the SSR) and that is causing rectification. Your SSR could be a zero-c rossing type, which is exactly the most stressful motor-start situation (and an inexpensive mot or might take a magnetization at turn-off, then saturate at turn-on, and melt its fu sible protector). Third, the SSR might be susceptible to some other signal than your thermost at (RF or maybe even input/output feedback oscillation).

If you use an AC relay, it cannot operate fast enough to rectify; if you feed that AC relay with a triac-output optoisolator, with a smallish capacitor shunti ng the input terminals (1 uF?), there ought to be no drive-side problems.

Some induction motors will have a thermal fuse incorporated onto the main winding. In the absense of a thermal fuse, this is my amateur speculation: The switching of the SSR creates voltage spikes. Normally these are easily absorbed by the induction motor. With enough time and heat, the lacquer insulation on the motor windings gets brittle, thermal cycling compromises its ability to insulate, then the voltage spikes begin jumping across high potential areas (between winding ends or where wire leads are tied to windings). If there is a capacitor involved, the arc is that much more spectacular, and the motor's demise quicker. Scott

You need a non inductive load on the same circuit, along with a capacitor across it..

Your motor windings are getting killed by the drop out of the TRIAC at each phase angle. You need to suppress it somehow so that both the HV pulse that shouldn't be there and lack of lower Z at the base of the sine wave, for the motor and cap of the motor to properly operate..

These are cheap motors and they run warm to start with, minimum copper and coating on the wire is used.

Try getting your hands on a motor run capacitor, somewhere around

3..5uf at the proper rated voltage and put that across the load side of your SSR.

ALso, putting a non-inductive load on the output helps, too... If you can get a low wattage incandescient bulb that would be fine or maybe a 5 watt R at around 400 ohms or so across the SSR load, along with the run cap.

Also, I suspect when you connect these components, you'll also hear a slight difference of motor noise, which indicates this issue..

SSR switches don't always have a proper snubber in them, so you may also want to use a 100R and .1 Cap in series around the SSR, back to the soure so that the load will cycle back to the line on spikes.

Have a good day Jamie

diode the relay

use the resistor

use 2 Tyco/Bosch mechanical relays with diodes for relays and inputs

r plug but I beleive this is normal for induction motors? Perhaps the overh eat sensor has opened permenently. Home Depot gave me a replacement so I wi ll try it again. These fans typically last for many years when plugged dire ctly into a wall socket. This is a real mystery to me. Perhaps SS relays ge nerate some large spikes on their output? I should check it with a scope.

the thermometer rapidly operates relay on and off circuitry passing someth ing of this to the fans who are confused square one without dioding separat ing the systems unbalanced properties.

AC relay with a triac-output optoisolator, with a smallish capacitor shunting

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they're mating

You got it. As they age the bearing dry and they pull more current. Eventually they open the Microtemp protecter within....

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