# Please School Me On Electric Motors



## Martin W (Apr 17, 2016)

I have a question. What makes an electric motor have a specific RPM? I have two 5hp motors almost identical. One is 1730 rpm and one is 1725 rpm. Both are 230 volt single phase.
Are the rpm ,s marked on the tag accurate?
How much of a variance can you have for incoming voltage? With a voltmeter I have 243 volts at the panel
Thanks
Martin


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## Terrywerm (Apr 17, 2016)

There are others that may be able to explain the physics better than I can, but AC motor RPM ratings are 'nominal' or ballpark figures. AC Motor RPM is determined by a combination of motor design and the frequency of the alternating current that is used to power the motor. The motors you are looking at are more than likely designed for 60 Hz and would run at or very near to the listed RPM as long as the power is supplied at 60 Hz. If you took that motor to Europe, it would run somewhat slower as power there is supplied at 50 Hz. Alternating current frequency varies very little, as all of the generators on the grid have to run 'in phase' with each other. Power companies do try to keep voltage levels constant, but keeping the frequency constant is probably more important for them.

Voltage ratings are also 'nominal' because voltage will always vary somewhat at the panel. This is due to a number of factors that I will not go into right now, but it is important that you know that your incoming voltage will vary slightly. Motors rated for 220, 230, or 240 VAC single phase are pretty much interchangeable.

For all practical purposes, the two motors you have there can be considered interchangeable.


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## JimDawson (Apr 17, 2016)

The nominal RPM for a 4 pole motor running at 60Hz is 1800 RPM.  In a perfect world that is what your motors will turn when idling.  In reality, there is some slippage.  The RPM rating is at full load and at full load the slippage accounts for the RPM difference.

At 60Hz:
A 2 pole motor will turn 3600 RPM
A 6 pole motor will turn 900 RPM

230 Volts is a nominal voltage.  Motors are normally happy running at +/- 10% (or more) of that.  243 volts is fine.


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## Martin W (Apr 17, 2016)

Thank you

Martin


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## strantor (Apr 17, 2016)

The slippage JimDawson refers to is not to be confused with mechanical slippage. In electrical terms it's conventionally called "Slip." Slip is unique to induction motors. Inside the motor's stator (the outside part) there are pairs of coils (electromagnets, called "Poles") that alternate polarity every time the incoming AC changes polarity. For 60Hz, the poles change from (North-South) to (South-North) 60 times per second. When you take these poles and arrange them in a circle, you create a rotating magnetic field. It's this rotating outer magnetic field that the magnetic field of the rotor is "chasing" and that's why it spins. 

An induction motor has no magnets of it's own in the rotor. The rotor develops a magnetic field when the magnetic lines of flux from the rotating outer magnetic field cut through the rotor. So the stator creates a rotating magnetic field which_* induces *_(hence *induction* motor) a magnetic field in the rotor, which in turn opposes the original magnetic field of the stator, which results in the stator "chasing" the rotating magnetic field of the stator.

If we look at the math:


we see that for a 4 pole motor at 60hz, the rotating magnetic field of the stator is spinning at 1800rpm.
So why does the rotor not spin at 1800 rpm also?
Because if it did, then no magnetic lines of flux from the stator would cut through the rotor, no current would be induced in the rotor, and the rotor would have no magnetic field of its own. 
In order for there to develop any magnetic field in the rotor, the rotor must spin some RPM less than the stator.
This 1800 RPM is called "Synchronous Speed."
Since the rotor does not spin at synchronous speed, it is classified as an "Asynchronous Motor."
There are such things as synchronous AC motors. They employ permanent magnets in the rotor. 
Since Permanent Magnet AC motors do not rely on induction, they are not Induction Motors.

NOTE the difference between synchronous speed and rotor speed is called "Slip."
Slip is variable. An unloaded motor will have very little slip. With no load, your "1725" RPM motor may spin much closer to 1800RPM.
1725RPM is given as the rated RPM because that's the speed it will spin when loaded to it's rated load. if you overload the motor, it will spin even slower than 1725rpm.
1725RPM should never be assumed to be the actual speed of the motor, only a close guess. There is only once instance where the speed will be exactly 1725 RPM, and that is at rated load. If you need to make an induction motor go an exact speed, then you need a VFD with encoder feedback to force it to that speed, and even then, it will always be +/- some amount. That is why induction motors are not typically used in servo applications.


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## John Hasler (Apr 17, 2016)

strantor said:


> If you need to make an induction motor go an exact speed, then you need a VFD with encoder feedback to force it to that speed, and even then, it will always be +/- some amount.


You can get zero speed error with a PID controller in the loop.


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## strantor (Apr 17, 2016)

John Hasler said:


> You can get zero speed error with a PID controller in the loop.


Sure with a static load. But in a machine tool application where load might change from zero to 100% as fast as you can jab a cutter into a workpiece, PID can't keep up. It's a time-based algorithm and as such can only *re*act.


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## John Hasler (Apr 17, 2016)

strantor said:


> Sure with a static load. But in a machine tool application where load might change from zero to 100% as fast as you can jab a cutter into a workpiece, PID can't keep up. It's a time-based algorithm and as such can only *re*act.


Same is true with a DC motor.   Even a synchronous motor will be subject to a change in phase and therefor a transient change in speed when the load changes.


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## strantor (Apr 17, 2016)

John Hasler said:


> Same is true with a DC motor.   Even a synchronous motor will be subject to a change in phase and therefor a transient change in speed when the load changes.


True, but the loading effect is much less for a DC or PMAC and there is much less overhead in the error correction math. CNC machines do not typically utilize induction motors for that reason. I'm sure someone could dredge up an example, but I've never seen a CNC with induction motors on it.

On the other end of the spectrum is the series wound or universal motor. This type is never used for anything requiring tight speed control. Though if you're a firm believer in PID, there should be no reason for this, as in theory it should be just as controllable as an induction motor (which is just as controllable as a DC or PMAC motor).

I've experienced my share of PID loops and I'm not a big fan. I've had to design my own speed control algorithms in the past when PID failed me. For example, in a wire & cable processing plant I used to work in, the take up and payoff winders were particular about the size of reels they used. You put a reel too big in it, and the PID will be unstable; what it considers a "slight" adjustment of RPM results in a wild change in wire speed, because the circumference of the reel is much larger than the reel which was used to tune the PID. If the reel inserted is too small, then the "slight" adjustment is too slight, and the machine does not respond quickly enough. I had to move the PID out of the VFDs and into PLCs, where I could program a more elegant solution, which would look at an ultrasonic rangefinder to determine reel circumference and adjust the proportional constant appropriately.

I've worked with some old timers in the motion control industry and they say that for some applications there is no substitute for an old school setup with a DC servo, analog tachogenerator feedback, and an analog DC drive. The tach output is time-now; does not need to have any math performed for averaging encoder pulses. The drive circuitry is time-now; does not rely on a microcontroller running on a clock cycle. The output is time-now; no adjustment of PWM duty cycle, just instant correction of linear output. I've never encountered an application requiring this level of literally instantaneous response, but their explanation makes sense to me.


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## JimDawson (Apr 17, 2016)

strantor said:


> True, but the loading effect is much less for a DC or PMAC and there is much less overhead in the error correction math. CNC machines do not typically utilize induction motors for that reason. I'm sure someone could dredge up an example, but I've never seen a CNC with induction motors on it.
> 
> On the other end of the spectrum is the series wound or universal motor. This type is never used for anything requiring tight speed control. Though if you're a firm believer in PID, there should be no reason for this, as in theory it should be just as controllable as an induction motor (which is just as controllable as a DC or PMAC motor).
> 
> ...




Applications like you describe is why they invented motion controllers with 60 micro-second servo update times


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## John Hasler (Apr 17, 2016)

strantor said:


> True, but the loading effect is much less for a DC or PMAC and there is much less overhead in the error correction math. CNC machines do not typically utilize induction motors for that reason. I'm sure someone could dredge up an example, but I've never seen a CNC with induction motors on it.



I don't think it's the math anymore: optimal control can be implemented on a $10 DSP.  I think it's primarily the lack of full power speed range.  In any case there's no good reason to use an induction motor on an expensive CNC machine with such nice PMAC motors available.



> On the other end of the spectrum is the series wound or universal motor. This type is never used for anything requiring tight speed control. Though if you're a firm believer in PID, there should be no reason for this, as in theory it should be just as controllable as an induction motor (which is just as controllable as a DC or PMAC motor).



PID control of a series motor is tricky because there is an extra integrator in the loop.  I've done it though: I use one for the Z axis on my mill (it's what I had).  The DC treadmill motor on my lathe is also a series motor, but the controller is just proportional.  A series motor is not my first choice for precise speed control, though.



> I've experienced my share of PID loops and I'm not a big fan. I've had to design my own speed control algorithms in the past when PID failed me. For example, in a wire & cable processing plant I used to work in, the take up and payoff winders were particular about the size of reels they used. You put a reel too big in it, and the PID will be unstable; what it considers a "slight" adjustment of RPM results in a wild change in wire speed, because the circumference of the reel is much larger than the reel which was used to tune the PID. If the reel inserted is too small, then the "slight" adjustment is too slight, and the machine does not respond quickly enough. I had to move the PID out of the VFDs and into PLCs, where I could program a more elegant solution, which would look at an ultrasonic rangefinder to determine reel circumference and adjust the proportional constant appropriately.



You fixed a design error.  



> I've worked with some old timers in the motion control industry and they say that for some applications there is no substitute for an old school setup with a DC servo, analog tachogenerator feedback, and an analog DC drive. The tach output is time-now; does not need to have any math performed for averaging encoder pulses. The drive circuitry is time-now; does not rely on a microcontroller running on a clock cycle. The output is time-now; no adjustment of PWM duty cycle, just instant correction of linear output. I've never encountered an application requiring this level of literally instantaneous response, but their explanation makes sense to me.



PID control is older than digital encoders and microcontrollers.  It was developed to improve on the performance of simple proportional controllers such as you describe.  And it did.  A modern system with a high-resolution encoder and a fast DSP can do better, though.


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## CraigB1960 (Apr 17, 2016)

strantor said:


> I've worked with some old timers in the motion control industry and they say that for some applications there is no substitute for an old school setup with a DC servo, analog tachogenerator feedback, and an analog DC drive. The tach output is time-now; does not need to have any math performed for averaging encoder pulses. The drive circuitry is time-now; does not rely on a microcontroller running on a clock cycle. The output is time-now; no adjustment of PWM duty cycle, just instant correction of linear output. I've never encountered an application requiring this level of literally instantaneous response, but their explanation makes sense to me.



I'm an old-timer (not in motion control but in transmission protection relaying and automation) and rejoiced when DSP's became available to implement good algorithms and filters in real-time.


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## stevemetsch (Apr 18, 2016)

there is a pretty good site for learning about motors  generally.
http://www.centuryelectricmotor.com/Motor-Mastery-University.aspx


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## jeff_g1137 (Apr 18, 2016)

Hi
All way over my head.


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## markba633csi (Jun 19, 2016)

Don't worry Jeff, just hang out here for a while you'll be an expert in no time.
About series wound or "universal" motors: they don't have great starting torque which somewhat limits their use for machine tools.
Mark S.


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## John Hasler (Jun 20, 2016)

markba633csi said:


> Don't worry Jeff, just hang out here for a while you'll be an expert in no time.
> About series wound or "universal" motors: they don't have great starting torque which somewhat limits their use for machine tools.
> Mark S.


Series motors have very high starting torque.  That is why they are used as starting motors and traction motors.  They do not have good inherent speed control.


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## markba633csi (Jun 20, 2016)

I defer to John; I was under the impression they had less than many types of capacitor run/start motors tho.
MS


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## Downunder Bob (Dec 27, 2016)

John Hasler said:


> Series motors have very high starting torque.  That is why they are used as starting motors and traction motors.  They do not have good inherent speed control.


Yes John is correct, That series motors do not have good inherent speed control, is a slight understatement. they should never be run without having a load connected because they can run away very quickly, A series motor such as a car starter motor if run without engaging the engine can run away to the point of destroying itself in just a few seconds.. It will accelerate to such a speed within a few seconds that centrifugal force will cause the rotating armature to fly apart.


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## Superburban (Dec 27, 2016)

strantor said:


> In order for there to develop any magnetic field in the rotor, the rotor must spin some RPM less than the stator.
> This 1800 RPM is called "Synchronous Speed."



Can someone help me understand this better?

I'm picturing a race track, with the field and rotor racing around. If the rotor is going slower then the field, then why does the field not out run or even out lap the rotor? I can understand one lagging behind the other, but I should think they would have to match each others speed somewhere, or the field would get way ahead of the rotor.

Thanks. Not many forums where one can ask basic questions, and not get treated like an idiot.


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## markba633csi (Dec 28, 2016)

Super: The field DOES out lap the rotor,  that's the nature of the slip.  The more load on the motor, the more slip, the more the rotor is lagging behind the field.
Does that make sense now?
Mark S.


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## JimDawson (Dec 28, 2016)

The best analog I can think of is a paddle wheel on a boat.  There is always some slippage, but it still moves the boat forward.


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## Superburban (Dec 28, 2016)

Ok, am I correct that the induced magnetic field in the rotor follows the stater field, and is not in one physical spot on the rotor?


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## markba633csi (Dec 28, 2016)

Sort of, the rotor has "conductor bars" spaced evenly around it which "grab the flux" so to speak and turn it.  Very layman's here but that's essentially it.
Jim's description is a good one- like a paddle wheel, always slipping some.
Mark S.


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## Superburban (Dec 28, 2016)

Thankyou I'm good with mechanical and basic electrical stuff, but electronics throw me in a tail spin.


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## KBeitz (May 12, 2018)

Another good example of slip is watching a V-belt travel around a pulley with a strobe light.


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