Electric Motor Rotation

I believe that some single phase motors incorperate a capacitor to in fact convert them to two phase, if this capacitor has deteriated by internal corrosion it may be going open circuit until it is restored by a switching surge.

I have noticed this behaviour in capacitors in TV's etc.

Are you sure that it's actually turning backwards, and not just an optical illusion? I mean, is it sucking instead of blowing?

If it is a squirrel cage forward curve fan, it will blow regardless of the direction of the rotation, it just won't blow very well when going backward.

It sure sounds like the capacitor could be the problem. They are used to direct the start up direction by providing a kick in the right direction without it the rotor can spin in either direction.

Thanks for the input all--I guess I'm not going to know until I dissassemble the unit and try replacing the capacitor if it does in fact use a capacitor. Some motors use a second set of coils and contacts that disengage as the motor comes up to rpm. Even if that fixes it, the question still remains--what could be different about the startup sequence when turned on by an ordinary thermostat compared to the correct operation when turned on by using the circuit breaker. This is a commercial FM broadcast facility and so there may be a significant amount of induced EMF and I have, in fact, had problems over the years with commercial grade thermostats failing in these kinds of environments--don't know why--when the possible doesn't answer our needs, we must look to the impossible I suppose. I'm still very curious as to the mechanism involved here. Thanks all.

I have come across this problem and agree that the starting cap or something associated with it is the most probable suspect. As somebody else mentioned the starting cap may be switched in at startup by some mechanism and it may be this that is failing. I remember a problem that used a thermistor to switch the cap. When the motor was cold it would connect the cap starting the motor in the correct direction, once running the motor would heat up and the thermistor would disconnect the cap. The moron that designed the circuit however never though about what would happen if the motor was hot when it started. The result was that if the motor was hot it would start in the wrong direction 50% of the time. I would suspect if you tried tripping and resetting the circuit breaker several times then it may replicate the fault and start up backward

I believe the issue is in the starting circuit. In some single phase motors there is a run circuit and a starting circuit. The run circuit provides torque but also relies on inertia, for continuous rotation. The start circuit applies torque in the desired rotation. Disabling the starting circuit allows you to bump start the motor in either rotation (watch your fingers).

My experience has been that a contact in the starting circuit fails causing this problem. The contact is operated by a centrifugal device. When the motor reaches a given speed, the centrifugal device opens the starting circuit.

Cleaning the contacts buys you a little time, but replacing them is the longger term fix.

ALV

Good Morning Chaps/Chapettes

It could be that the thermostat is making ang breaking in short time, i.e. the stat opens and the current is removed from the motor which then allows the fan to idle, as the pressure within the room exceeds the exterior pressure the air will flow from the room via the fan vunturi, this will cause the fan to change rotation if the stat then closes during this period and the motor is shaded pole type it will run in reverse albeit at lower rpm.

Solution is to increase the differential setting on the stat i.e. the temperature difference between cut in and cut out. This will allow the room pressure to balance prior to start up

yes ive had this happen, with the air flow, inside to out,, is enough to cause the fan to rotate backwards when its off,, no power,, then when the power returns,, the motor will start in that direction, it cant stop and restart in the correct direction,, some motors will,, it can be fixed only by 3 ways,, one replace motor with one that forces correct direction,, 2nd,, put a electric selinoid brake on current motor to stop it when at rest,, connect this to the input of motor and when the power is applied, the brake releases and correct direction is started,, 3rd,, stop the air flow from inside to out,, put a set of louvers in front? seal the leaks? not the best, but dont know what your dealing with, i take it that this fan/blower is a blade? most cage blowers will work in both directions,, just not as well in the incorect one,, have a good one!

You may have bleed over from lightning. I've done tower installations and have seen similar instances occur, ussually after storms.

Very good input--I now know a good deal more than I did before and have several possible failure mechanisms to investigate. First time I've used this forum to specifically address a problem and I am impressed with the real-world experience I have been able to tap into. Many thanks to all of you who took time to share your knowledge with me.

Lonnie

I've had a little more "thought" time. If your problem is having to travel to the transmiting station and reset the motor, you could install a pressure differential switch on the building, to a 1 or two minute delay switch, to a contactor controlling the fan. This control switch could also trigger an alert/alarm if needed. This would resset the motor automatically for you and send you a report.

Single phase power has no direction of rotation. That is why it is necessary to use a starting capacitor to initiate that rotation one way or the other as mentioned earlier. After that a running capacitor is usually used to just maintain the rotation.

The starting capacitor is controlled by a centrifugal switch, on the end of the motor, so that it is not in the circuit for more than a few seconds at most. If however the fan is ALREADY rotating the wrong direction, say from what is called "windmilling", the speed swicth may already be operated by the windmilling rotation, so the starting capacitor never comes on line, then the running capacitor just keeps the motor spinning in whatever direction is was already spinning.

What you need are some dampers that close when the blower is not running to prevent the windmilling in the first place, or at least have it not be so fast as to prematurely operate the centrifugal switch inside the motor.

If it is a single phase motor,then look for connections to the capacitor ends. 2 wires comming to capacitor are from Motor windings, one on each end of cap. One end of cap will have 1 more wire. It is from motor on/off switch. Change this wire to the other end of capacitor. It will change the direction of rotation of motor.

If it is 3 phase motor then change 'R' phase to 'S' phase & vice versa. Action is: to interchange 2 of 3 phases connections

wish you best of luck

Is the motor PSC type? If so, this can happen when the capacitor is weak. A single phase motor is not self starting. It requires a start assist. In PSC motor, the capacitor acts as the start assist as well as that remains peremenantly in the circuit improving the power factor. A PSC motor with weak capacitor will run in both direction depending on the initial external push it gets. In the case explained, there can be a back flow of air from inside the room to outside when the blower is off as the inside pressure is more than the outside pressure. So, at this period if the the motor takes a start with weak capacitor, that can run in reverse direction. A PSC motor with healthy capacitor will not run in reverse direction in this circumstance also. So, pls check the capacitor condition, if it is a PSC motor.

NOTE:

No start-up can be attained in Single-phase. At least 2 phases are required to run a motor. In a single-phase motors another phase [ a SPLIT-PHASE Leading or lagging to the phase] is generated by 1. SHADING POLES, 2. adding a 2nd Winding at a phase-angle.

In Single-pole motors only may change direction while in operation or on Start-up.

3Phase motor can't unless 2 of the 3 phases are interchanged. This is normally done in systems where you have to FWD or REV in process.

Changing of direction can NEVER happen intermittently.

1. It could not be a Shaded-pole motor as it is normally in small instruments like Electric-Watches, controls etc. Their Stator core is cut slightly on both pole; on one-side of each pole or the other-side. This is todetermine the direction of START. This type of motor can NEVER change direction.

The largest I ever met is used in water-pumps of desert coolers [almost less than 1/10 HP].

2. The other type is Capacitor-Start motors. They range to almost 2 HP.

Have 2 windings; Start & Run. One end of both the windings tied togeher & makes the COMMON [GND] end. A capacitor is connected across both left-over ends of windings. Switched end it becomes. Now The Live of power is connected to one of 2 terminal of capacitor to determine the direction of flow.

In this type of motors capacitors dry-up & decrease capacity. But this only causes decrease in speed or STALL if almost cap is zero.

In this case push the fan in any direction & it starts running, but at a slow speed. No air drift.

In situation described motor starts in a direction to which it isforced by differential-pressure, BUT in this case it has no power to run against a force. It will run till the differential in pressure remains or no pressure difference.

This cannot suck or force air.

2a. In another type a Centrifugal-swith [cut-off type] is used to supply current to Aux-phase. On attaining speed centrifugal-cut-off switch cuts off power to aux-phase. Motor runs on RUN winding.

In this type of motor, if the switch is bad, motor will remain stalled until pusg in any direction, manually or by air flow in any direction. Once started it will run with full

power.

2b. Another arrangement in which "Gravitycontrolled" switch is used in series with Aux-Winding. Used normally in Refridges. The coil of "Gravitycontrolled" switch is connected in series with main [RUN] winding,. On switch on heavy current is drawn by motor to start so "Gravitycontrolled" switch's plunger is pulled up to make contact to Aux[Start] winding.

Motor starts & current is decreased & "Gravitycontrolled" switch is released to drop down & disconnect Aux-windings. This arrangement is never used for fans.

Hajee,

I have a couple of points that I disagree with you on. Firstly you stated;

"It could not be a Shaded-pole motor as it is normally in small instruments like Electric-Watches, controls etc.

I think you will find that most electronic analogue watches now use stepper motors rather than shaded-pole motors. Just watch the second hand, it jumps in one second or 6º increment. You will find that very hard to do with a shaded-pole motor.

Secondly you stated

"The other type is Capacitor-Start motors. They range to almost 2 HP.

Have 2 windings; Start & Run. One end of both the windings tied togeher & makes the COMMON [GND] end. A capacitor is connected across both left-over ends of windings. Switched end it becomes. Now The Live of power is connected to one of 2 terminal of capacitor to determine the direction of flow.

In this type of motors capacitors dry-up & decrease capacity. But this only causes decrease in speed or STALL if almost cap is zero."

I have seen these motors with both faulty capacitors and faulty start winding switching components start backwards and run at full speed.

My money is firstly on a faulty start winding switch fault followed by a dud capacitor.

My comments:

I think you will find that most electronic analogue watches now use stepper motors rather than shaded-pole motors. Just watch the second hand, it jumps in one second or 6º increment. You will find that very hard to do with a shaded-pole motor.

Of cource inlatest developements, I was taking for older models

I have seen these motors with both faulty capacitors and faulty start winding switching components start backwards and run at full speed.

I came to discussion to correct my statement when I read your comments.

I accept the error. Actully a single phase AC motor cannot self-start but runs without start circuit after it gets a kick for start in either direction @ full power. As I mentioned in case of Centrfugal-Swith types & motor for Refridges run after start-up circuit is cut-off. It is correct tothis extent.

[it is a chance that it stopped after last run at a point where it was somewhat offset of stator & rotor & get started]

But with a bad capacitor [ in motors where start circuit remains connected] it will run at a slower speed proportional to the loss of capacity.

This is due to the reason that the capacitor is not in start-circuit but it is in series with run windings.

This point clarified by my worker by practical demo & measurements of windings guage & resistance. [at least it is in my country's market & all Japanese imported fan motors]

Fans for house-hold, ie Ceiling, Table & Padestal fans all I got demo for their running

with good caps & bad ones. I have no doubt in it now.

Kind regards & thanks for your comments

Wow! Much good input--I appreciate it. When I get the opportunity to return the that site and actually work on that unit (and hopefully find the fault rather than just replacing the motor), I will post my findings. Thanks all.

Stepping back for a sec...spent a small period as an assistant station engineer for local TV...

Either:
1 - put a barometric/gravity damper over the opening, or if you don't mind servicing it when needed, a motorized one, so that airflow is stopped in the 'wrong' direction whenever fan is off...and make sure there is one on the "relief" louver as well, so wind blowing on that face of building doesn't force airflow backwards while fan is trying to start up.

- or - better - been there, did this -

2 - Recommend next replacement be with a multi-tap motor.
a) Wire the lowest-speed tap to be on 24/7 for this type of facility, so that a small positive pressure situation exists at ALL times, regardless of the thermostat. That is your insect-resistance, and at 1/4 speed uses tiny current, especially compared to the transmitter.

b) Wire the thermostat to a DP relay that will open the low-speed tap line and close the high-speed tap instead, whenever the additional airflow is needed.

Very simple wiring with a 1-phase motor, and anyone with the bucks to put up an FM transmitter is unlikely to scrimp on your relay/contactor enclosure to control the fan that keeps it all from overheating.

Good luck.

Useful comment--thanks much.

I've seen this occasionally with three phase as well as single phase motors. in the case of three phase: The source wiring sequence may have been changed somewhere up the line. In the case of a single phase motor, the capacitor may be leaking electrically -- enough to still function but not enough to provide the reactance necessary to create the phase shift to create a purely resistive circuit. (remember capacitive reactance is 180 degrees out of phase with inductive reactance) Voltage leads, current lags in an inductive circuit. If possible measure the actual value of the capacitor -- after you've discharged it of course. If that's not possible, simply change the capacitor, and try it. Most units are around $20.00 to $30.00so the cost isn't too prohibitive unless it's OEM.

It might be a bad set of start windings, in which case replacement is the only option.