ECN Forum
Posted By: Cinner Electric motor rotation - 10/31/07 01:13 AM
Is the correct rotation of a motor always marked on the exterior of the motor?
Posted By: NORCAL Re: Electric motor rotation - 10/31/07 01:24 AM
IMO, the rotation should be marked on the equipment powered by the motor.
Posted By: Trumpy Re: Electric motor rotation - 10/31/07 03:32 AM
In most cases yes.
Provided no-one has modified the connections in the terminal box for you.
Posted By: leland Re: Electric motor rotation - 10/31/07 03:35 AM
Originally Posted by NORCAL
IMO, the rotation should be marked on the equipment powered by the motor.


Agreed. Motors power equipment.
Posted By: JValdes Re: Electric motor rotation - 11/01/07 05:20 PM
Three phase motors are NEVER marked for rotation. They are designed to go either direction. You choose the direction you need.
Pumps/fans/ect... have an arrow or some marking to allow it to operated properly.

Some applications I have worked on, call for the motor to use both directions on the same piece of equipment.

Posted By: jraef Re: Electric motor rotation - 11/04/07 01:37 AM
What good would it do to mark the motor? The motor mfgr. has no idea how you are going to hook it up or what the incoming line sequence is at your facility. ABC is relative. For instance, PG&E here in California is A-C-B with relation to other facilities in the US, but that means nothing to everyone local here. It just is what it is.

CBS: Connect it, Bump it, Swap leads if needed
Posted By: togol Re: Electric motor rotation - 11/04/07 02:25 PM
yep,

just make sure the load is disconnected before bumping

some things don't like going backwards
Posted By: JValdes Re: Electric motor rotation - 11/04/07 03:04 PM
jraef,
Correct my friend. When I supervised a motor shop some years ago, some customers would call and ask if we could mark the leads, so the motor would turn in the direction they wanted without "bumping". Of course we could not garranty rotation on three phase.
There are some phase detectors on the market that claim they can do this. But how can you know the complete facility is phased exact at every panel, disco or motor starter. I have even seen some companies try to do this. I have never found any of them to be successful.

Now, DC motors can be wired for a particular direction, but it must be a "shunt wound, compound wound or series wound motor.
Posted By: electure Re: Electric motor rotation - 11/04/07 03:38 PM
In my experience, 3Ø motors will rotate the way you don't want them to about 95% of the time.

They'll rotate the proper direction the other 5%. laugh

Tom has a good point....some machinery doesn't take well to being rotated backwards.
Posted By: JoeTestingEngr Re: Electric motor rotation - 11/04/07 04:54 PM
On the other hand, some motors work too well backwards. Our bus garages use 5 HP Red Jacket STPs in our underground storage tanks. (USTs) I always tell our electricians to check them both ways after replacement. I ask them to check static pressure and to squeeze off 10 seconds and check the flow rate. More than once, they've told me they didn't bother because it looked fine. I jumped on them about why I would waste my time asking them if I didn't feel it to be important. At least half the times, they've called back telling me that they got higher pressure and flow after switching the leads.
Joe
Posted By: gfretwell Re: Electric motor rotation - 11/04/07 05:21 PM
Phasing was such a problem to get right that IBM just started putting a phasing jumper in the machines that cared (3p motors) and the ones that might self destruct had phase rotation detectors so they wouldn't even power up if it was wrong. (disk drives that didn't "unload" the heads)
Prior to that we were swapping the phasing in our disconnect box (or some other convenient place). It got so out of hand that there probably wasn't one machine in the country that had all the wires on the lugs they shipped on.
Sometimes we had machines that had it swapped a couple different places in a single machine.

Part of the problem was, do you look for clockwise rotation on the face of the plug or the face of the receptacle? IBM couldn't make up their mind about the from one plant to another so a brand new tape drive from Boulder would be phased differently than a disk drive from San Jose frown

We always had one particular motor we would look at to see if it was going the right way.
Posted By: togol Re: Electric motor rotation - 11/04/07 06:57 PM
there was a particular Heidelberg sheet press that I had pulled a motor from....I was hoping someone else would be lucky enough to get to re-install it,
but I drew the short straw again .anyway I asked the operator to tell me if it was running right ...he was wrong, and the damage estimate was around $30,000.

luckily his co-workers told the plant manager that I was told the rotation was fine and I was off the hook.

ironically the rotation was on the flywheel behind a panel on the bottom of the press so it was my fault after all...
Posted By: JValdes Re: Electric motor rotation - 11/05/07 03:57 PM
gfretwell,
In circumstances as you mentioned is it important to acheive the correct rotation the first time?

If the answer is yes, there is a way to do it. Install an encoder on the motor. You will need a sensing device to ensure it runs the direction you chose.
An encoder outputs a square wave relative to the input voltage. Thus while looking at an encoders output with a scope, one direction would be a positive wave and the other a negative wave. I use the word wave loosely as it is not a true wave. It is digital. But they do work, with an encoder and a device to accept the signal and report to the starting mechanism.
Posted By: JoeTestingEngr Re: Electric motor rotation - 11/05/07 11:47 PM
JValdes,
Most encoders I know of will output a square wave based on their supply voltage and loading. Dual, bi-phased outputs, usually 90 degrees offset, are required to sense direction. Please provide a link to some that act as you describe.
Joe
Posted By: gfretwell Re: Electric motor rotation - 11/06/07 01:42 AM
J, back in the olden days nothing was really hurt if the motors were running the wrong way. Things just didn't work and at a certain point you might start running hot because the blowers were not moving enough air but usually you couldn't get it doing enough to warm up.
That was when one of the things running was the hydraulic pump that made things go. It got serious when disk drives stopped "unloading" the heads. If you spin up a drive backward the heads will not "fly".
They put in a phase rotation detector that interrupted the power on relay circuit so you just got a red light. You would pull a plug, swap it 180 degrees and plug it back in. That swapped 2 phases.
Posted By: JValdes Re: Electric motor rotation - 11/06/07 05:02 PM
www.baldor.com
www.wegelectric.com
www.bei.com ?
www.automationdirect.com

Joe...Check within these web sites for the encoder manuals.
Or just Google "encoders" on the web.
There is much info to examin....John

ps....all encoders produce a square wave directly proportional to the input voltage. So if 5 vdc is the source the output will be 5 volts. You see, an encoder does nothing but count revolutions. And is capable of direction selection as the encoder can output a negative or positve square wave depending on the shaft rotation.

Resolvers output a sign wave that also can produce voltage either negative or positive, but are analog, not digital.
I hope the web sites I have provided can be more specific and answer any questions you may have......John

gfretwell.... I am not familiar with your application. I was referring to electric motors and equipment. I am not literate in the inner workings of computers....John
Posted By: gfretwell Re: Electric motor rotation - 11/06/07 07:12 PM
The phase detection circuit is fairly simple. It just makes square waves from the phases and looks at phase A to make sure phase B gets there before Phase C with a simple digital trigger circuit. That way you know if it is safe long before you get the motors moving.

Back in the olden days "computers" (at least the I/O boxes)were really more akin to what you would see in industrial machines with hydraulics moving paper in printers and moving the heads in disk drives. That is where you had the 3 phase motors. Once electronics allowed stepper motors and voice coil head movement we didn't see as many big 3 phase motors but they were still used for disk spindles and such until they got smaller.
The encoders you speak of are used a lot. In the olden days they were magnetic pickups driving a square wave generator, later it was slotted wheels and photo detectors or hall effect devices. In boxes that moved cards or checks they used air sensors to see the card once they got too fast for switches. Optical sensors get dirty too fast.
I really miss the days when things dripped oil on the floor tho. That was when computers were interesting to work on.
The stuff was still pretty fast. A 1403 would print 1100 lines a minute (132 char per line) and that was 1960 technology.
Now it is just "cut open the box and plug in a new one"
Posted By: JoeTestingEngr Re: Electric motor rotation - 11/07/07 01:58 AM
OK John, I'll stick with my statement but explain it a little more. An encoder doesn't count anything or send out anything that varies other than the frequency of the output square wave. If you want to sense direction, you have to buy one with a second, quadrature output. This output, viewed individually, doesn't provide any additional information. However, if you were to positive edge trigger a scope on "A" output, and observe the "B" level at trigger,it would be one logic level for CW and the other for CCW rotation. It's a huge waste of an encoder to use one for just direction sensing. Feeding "A" to an edge triggered flip-flop and "B" to a D input is enough for that. But then it would still indicate the last direction of rotation when there is no rotation. So the circuit grows and the application is worthless for anything more than Science Fair. Now if you want to set up one-shots or other means of over/under frequency detection or feed a charge pump or PLL as part of a speed or phase locking servo, there are some apps there. More commonly, they function as part of a position servo system. Here the "A" would be the edge triggered clock of a ? bit counter and "B" would feed the count UP/DOWN input. The counter output reflects a position on a motion axis. On power up these systems tend to have to index to figure out where they're starting out. These circuits can be realized in TTL or older logic but are more likely to be part of a PLC program scanning digital inputs these days.
Joe
Posted By: gfretwell Re: Electric motor rotation - 11/07/07 04:06 AM
IBM managed to do phase detection on one chip. I don't know if they made it or bought it.
Posted By: JoeTestingEngr Re: Electric motor rotation - 11/07/07 06:07 AM
Greg,
If you mean one chip with no support components, I don't recall seeing one of those. But you do just mean for sensing phase rotation on the input rather than shaft rotation, don't you?

I've repaired a Time-Mark PLM within the last couple of years and the circuit was fairly simple. By the time I was repairing disk drives, they were all single phase units. I remember the 5KW transmitters I used to install or service having a PLM to make sure the blower motor wasn't running the wrong way. That would've sucked.
Joe
Posted By: gfretwell Re: Electric motor rotation - 11/07/07 06:26 AM
There were some resistors and capacitors but only one logic chip. It was in the power on circuit so the motor wouldn't even bump.

I have had the blower running the wrong way problem. A squirrel cage blower almost works going backward. It just doesn't move enough air to keep things cool.
Posted By: JValdes Re: Electric motor rotation - 11/07/07 03:26 PM
Joe, Yes more channels are usually needed for direction sensing. Single channel can do it, but it is more difficult, and requires ajustment of the periferal devise. But most of all the encoders I use (on motors and servos) are dual channel with a extra reference channel. Servos are usually fitted with a resolver.
Let me clarify what I said regarding counting. You are correct that the encoder does not count. The devise receiving the signal does.
I think we both are correct in our statements, I may have not provided enough information to you to make sense of my statements.
I am not going to get into a "who knows more" match with you. I am not a testing engineer. I just have worked with VFD's and other equipment that employ encoders and resolvers. Also measurement equipment ect..., that incorporate encoders on the equipment. But my statement regarding direction is sensed by the postive or negative output of the encoder. Turn an encoder in one direction it is a postive square wave relative to zero. And turn the other direction it is a negative square wave. Or Visa Versa.
The encoder is not smart. The devise receiving the signal is the smart one. It just receives the signal and does what it is programmed to do. Maybe we should discuss the smart equipment and leave the simple encoder to rest.....John
Posted By: gfretwell Re: Electric motor rotation - 11/07/07 03:44 PM
I suspect you guys are talking about different kinds of "encoders". There are several ways to look at shaft rotation. The typical slotted wheel or magnetic pickup can't resolve direction without using two. If you have one that is like a little generator it can. (AKA tach/generator)
Posted By: JoeTestingEngr Re: Electric motor rotation - 11/08/07 01:39 AM
Greg,
The PLM that I repaired just used an old 741 op amp wired up as a comparator with hysteresis. I think one phase just used a cap for phase shift so it would be subtractive at a summing junction in the wrong rotation.

John,
I don't want to sound competitive or belittling, just want to make sure we're on the same page. If there is an encoder out there that behaves in a way I'm not familiar with, I want to be able to add it to my bag-o-tricks for the future. I had looked at your first 2 links and found them too generic. The 3rd referenced the type I'm already familiar with that had available quadrature outputs. I didn't get to the last link though. So I guess that in some instances you mean an encoder subsystem that might include the component that we would call an encoder. So are you saying that you know of one type that produces a zero to let's say +5 volt square wave in one rotation, and zero to -5 volt square wave in the other? If so, please direct me to it so I can research it further.
Thanks,
Joe
Posted By: JValdes Re: Electric motor rotation - 11/08/07 04:13 PM
Joe,
All of my comments were based on a basic quadrature or single channel encoder. Nothing you would be able to add to your bag of tricks. I think my wording may have been confusing you, as to what I was trying to say.

When I say zero I mean when looking at the encoder on a scope. Square wave up or down for polarity. Zero when the encoder is not turning. Flat DC line zero.
When I connect the encoder to the scope, I connect the encoder to a variable DC source, adjusting the voltage for the rating of the encoder. Then look at the square wave (with the time setting as low as possible) to verify the integrety of the square wave and the voltage. I do this to test the encoder before I realease the equipment to the customer. Thats it. If it's a quad I look for a positive square in one direction and a negative square in the other direction. I turn the encoder manually so I can see the square wave plainly. Single channel, naturally I will see only postive or negative. Not both.

gfretwell,
We are aware of the tach and encoder operation. Tach's or (magnetic pickups)" produce the same square wave output as the encoder does. But requires a gear or similar for the tach to see. As each tooth passes the tach it produces a square wave, relative to the tach input voltage.

Encoders use either glass or metal disc's within them that have holes, blank spots, or lines the the encoder light cannot pass through. Each time the light is blocked it produces the square wave.
Posted By: n1ist Re: Electric motor rotation - 11/08/07 05:14 PM
I think we are battling terminology here. Most quadrature encoders I have seen determine direction by which of the two square waves gets a rising (or falling) edge first. In firmware, I have done this by XOR'ing with the previous state.
The frequency of the square wave determines the rotation rate.

I haven't seen one that uses two different polarity (wrt ground) squarewaves, but there's a lot of stuff I haven't seen out there.
Posted By: JValdes Re: Electric motor rotation - 11/09/07 05:02 PM
n1ist, Exactly right on the first paragraph.

Second paragraph:
When you connect an encoder to a DC power source and the encoder is not turning you have a flat line on the screen of the scope that is zero volts. If you turn the encoder a square wave will appear either below the zero line or above the zero line. This is what I meant by polarity. Negative if the square wave is below the zero line and postive if it above the zero line. This is true on each channel.

When I say zero line I mean that there is no output from the encoder.

I was wrong when I said a single channel encoder is only negative or postive. It depends on the direction you are turning the encoder......John
Posted By: JoeTestingEngr Re: Electric motor rotation - 11/10/07 01:48 AM
John,
What you were saying still wasn't making sense until you just said the output would be zero volts on the scope unless it was turning. Of course it could be 0 or +5 depending on detector position relative to the slot. UNLESS, you just happen to have your scope switched to AC coupling. Then those things you said start making sense. As a rule, you're better off DC coupling on both your vertical and trigger inputs and using the 20 MHz bandwidth limit if you have one. You benefit by seeing a more accurate waveform and by being able to verify your margins with respect to your hi and lo input logic thresholds.

The really funny thing about this is the Deja Vu feeling I'm getting from a few months ago. My older brother was having interface problems between an encoder and a Precision Digital PD693. Turns out he was AC coupled and didn't notice the DC offset of his square wave source. I had asked about the leading edge overshoot evident in the scope picture and if ground was 0 or 1 division up from the bottom of the trace. When he said the middle, that solved the mystery. Then I was able to suggest that he check various p-p amplitude Vs DC offsets to characterize the input network on the device.

I know we've gotten way out in left field on this but it's nice to finally get all the dots connected.
Joe
© ECN Electrical Forums