Electric Motor RPM?

Hi, I have 2 electric, single-phase hysteresis synchronous motors, one is 1/100HP, 1800RPM and the other one 1/500HP, 600RPM. Both are 117V/60Hz motors.

The 60Hz, I'm guessing, means that there are 60 electrcal pulses/second that "control the motor. The bigger motor does 30 revolutions in a second. During that second, the motor is controlled 60 times by electrical pulsation (60Hz), so every revolution is controlled by 2 pulsations. The smaller motor, on the other hand, rotates 10 times every second, so every revolution should in this case be controlled by 6 electrical pulsations, i.e. being better controlled by the 60Hz AC. I know this might sound like a stupid reasoning, but I felt I had to get my head around this, since I got both motor options available for a project, and felt that the smaller motor might be the more "accurate" one, however academic this might be in real life. Grateful for any kind of constructive input - Thanks!!!

Edited By Peter Andersson 1 on 10/05/2012 15:10:10

The speed accuracy of synchronous motors is determined by the supply frequency. Both your motors will have the same accuracy as each other so the choice of which one to use governed by which is the most suitable for the application.

Regardless of how many pulses/rev (or revs/pulses) at the end of the day the number of revolutions done by the motor will depend on how many puses the utility company sends out (there may be short term variations but the everage over the day/hour or whatever will be correct.

Synchronous motors are used in electric clocks and timers etc but its unusual to see motors of such low power rated in HP, is there something special about the ones you have?

Ian

Thanks Ian, yes; they are both controlled by the 60Hz, but one of them will rotate 3 times as many as the other one in any given time. If the controlling factor of the motors is the alternating current, i.e. the pulsation of the mains supply, then my take on this would be that the motor that receives the most pulsations per revolution, is the better controlled one...or is this simple logic flawed? the higher the pulsation per revolution ratio, the better...

Your question, 'which is the better controlled?' answered accurately would be that they must be identical.

The1800 RPM motor need 30 cycles for one rotation and the 600RPM needs 10 but if you fitted external gearing so that the output shafts both revolved at the same speed they would still do the same number of revolutions over an hour/day/year etc.

The speed constancy is totally dependent on the stability of the 60Hz supply.

ian

do we assume that you have a supply of 60hz as in the UK and Europe we have 50Hz - that could make a difference! Oh and at 230/240 volts.

Hi, I might be able to explain this

the motors are called synchronous because the speed is synchronised with the frequency of the supply. you can change the speed of these motors by changing the frequency with a variable frequency drive (VFD). A 60 Hz motor will run slower on a 50 hz supply (provided the voltage is correct) what changes the design speed in different motors on the same frequency is the number of magnetic poles the motor has. In a 2 pole motor the rotor will rotate one half turn with every alternation in current. At 50 Hz this will give 50/2= 25 multiply by 60 (seconds) =1500 RPM the most common speed. Actually because the rotor is always playing "catch up" with the stator the actual rpm is usually between 1425 and 1440 RPM.. This is known as "slip" and is induced so that the magnetic attraction of the pole produces torque. In a 4 pole motor the rotor will turn 1/4 turn with every alternation so we have 50/4=12.5 multiply by 60=750rpm Therefore, more poles= slower speed.

Phil.

Hmmmm, some confusion here methinks. A synchronous motor is just that, the rotation speed is synchronised to the frequency of the line voltage. A synchronous motor does not have slip, nor does it need slip to produce torque. In contrast an induction motor does need slip to produce torque, which is why an induction motor is classified as an asynchronous machine.

Regards,

Andrew

Correct Andrew, I was giving a simplified version. The synchronous motor is locked to the frequency, and can only be speed controlled by a VFD, whereas the asynchronous can be designed with a different number of poles to give different speeds, but these are also dependant on the frequency. TBH I havent had a lot to do with smaller motors, I mostly dealt with 3 phase motors 20hp to 250 hp!

I heard once that at power stations (here in the US) they had a synchronous clock in the turbine control room driven from their generator. In the early morning (around 4:00 eastern time) all the Power stations would compare their time to a nationally broadcast time clock. The time diference would indicate how many extra or fewer their particular turbine/generator sets turned. The speed was then altered to zero the count difference.

It seems hard to believe that that actually works, considering the Power stations are all connected via the Grid. Perhaps this was before the Grids.

Does anyone know if this was actual the case or an urban legend ?

Phil: Thanks for the elucidation. However, synchronous motors can be designed with varying numbers of poles to vary the speed, in a similar fashion to an induction motor. See for instance:

Although if you need to fine tune the speed then I agree that a VFD is required, in both cases.

Bruce: A question; is it an urban myth that the US doesn't have a national grid, in the same way as the UK? As far as I recall when there was a huge power outage on the US East coast some years ago the smug commentators on the BBC put it down to the 'fact' that the US electrical network wasn't a true grid.

Regards,

Andrew

Edited By Katy Purvis on 01/06/2015 12:27:45

this is very interesting, even if off topic. I can understand how a power station gets synchronised, but what about all the wee wind generators? There is a rash of them sprouting round here ( N Aberdeenshire), but looking out my window now I can see 5 or6, and despite a good wind they are all stationary. One a few weeks ago had a large lorry mounted generator running next to it for several days. All I can find out is "we are just balancing the grid " Any body any idea? BTW I've a NC in elect eng. so do have some idea. Apologies to Ed. for off topic.

Thanks for that Michael, I am generally in favour of renewables, having once lived next to a nuclear one, but there does seem to be some fairly clueless hopefuls. The answer I was given was probably **** as you say, but sounds better than "dunno".