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Pud
15-11-2007, 07:16 AM
I know that we can control the speed of a single phase electric motor by changing the voltage.

Can some one tell me how changing the voltage alters the speed of the motor.

progzmaster
16-11-2007, 12:58 AM
I know that we can control the speed of a single phase electric motor by changing the voltage.

Can some one tell me how changing the voltage alters the speed of the motor.

Single phase induction motors are almost identical to three phase ones, the difference is how the windings spaced apart.

The rotation speed of rotor are slower than the rotation speed of magnetic field in the stator (synchronous speed). The differece is called 'slip', expressed in percent.

For small values of motor slip, the slip is proportional to the rotor resistance, stator voltage frequency, and load torque, and is in inverse proportion to the second power of supply voltage.

So, if you decrease the supply voltage, you increase the slip which result in a slower rotation speed.

Toosh
16-11-2007, 01:03 AM
hi, i might be wrong but only dc voltage can be controlled speed wise hence is why we have invertors now.

regards Norm

Pud
16-11-2007, 07:40 AM
Single phase induction motors are almost identical to three phase ones, the difference is how the windings spaced apart.

The rotation speed of rotor are slower than the rotation speed of magnetic field in the stator (synchronous speed). The differece is called 'slip', expressed in percent.

For small values of motor slip, the slip is proportional to the rotor resistance, stator voltage frequency, and load torque, and is in inverse proportion to the second power of supply voltage.

So, if you decrease the supply voltage, you increase the slip which result in a slower rotation speed.

Progzmaster,
Thank you .I suspected it had something to do with the slip but didn't know exactly how.

Regards,
Pud

Pud
16-11-2007, 07:48 AM
hi, i might be wrong but only dc voltage can be controlled speed wise hence is why we have invertors now.

regards Norm
Norm,
Thank you foir your reply.
We can control A.C. P.S.C and shaded Pole motors with Voltage control.
Did you read progzmaster's reply?

Regards,
Pud

paul_h
16-11-2007, 01:56 PM
hi, i might be wrong but only dc voltage can be controlled speed wise hence is why we have invertors now.

regards NormHae you never seen speed controllers on condenser fan motors?
Inverters are common to drive compressors since you still need power to pump.
But it's fairly common to have a rheostat device hooked up to a pressure control to drive condenser fan motors slower to stop over condensing at lower ambients.
edit: like these http://www.danfoss-saginomiya.com/?link=products&sublink=fan

progzmaster
16-11-2007, 05:56 PM
Progzmaster,
Thank you .I suspected it had something to do with the slip but didn't know exactly how.

I forgot to say, that the torque is also falling dramatically if you decrease the voltage. This speed control is only suitable for low-power and relatively constant torque apllications, like fan speed control.

NH3LVR
17-11-2007, 04:13 AM
I forgot to say, that the torque is also falling dramatically if you decrease the voltage. This speed control is only suitable for low-power and relatively constant torque apllications, like fan speed control.

Not my area of expertise, however I have noted that single phase fans with reduced voltage speed control is not noted for prolonging the life of motors.
But it is cheaper.

cristopher_uy
25-11-2007, 09:50 PM
i maybe new in these forums but as far as i knew
single phase a/c motors designed to be a ,1)SpeedControlled such applied on window and split type residential and some on commercial,2)Constant Speed motor.

But in my own experience ,especially dealings with PSC(PEMANENT SPLIT CAPACITOR)motor,we can change the speed by increasing the farad rating of the capacitor ,but be sure if change, amperage still
within the motor amperage range limit.

We applied these on our storage room evaporator fan motor ,since inside temp is cold which help motor not to overheats thats why we make such waybut still the amperange within its amperage range limits,but if u applied to another application,where motor is expose to high ambient temp ,i suggest not to attempt such one.

thanks

Peter_1
26-11-2007, 02:45 PM
edit: like these http://www.danfoss-saginomiya.com/?link=products&sublink=fan

The Danfoss (Saginomya in fact) ones works as far as I've read not on a voltage drop but on a cut-off of the sinewave llike this

http://www.d.umn.edu/~mharvey/dimmingcurve2.gif

or another picture

http://static.howstuffworks.com/gif/dimmer-switch-diagram-6.gif


Therefore, you see in most speed controllers a cooled thyristor which is in fact a very fast electronic on/off switch

So the peak to peak value
http://www.sayedsaad.com/fundmental/4_Alternating%20Current_files/image002.gif

stays the same but the average voltage drops, we don't have the full amplitude any longer.
If you measure this with a standard analog or a digital multimeter (which measure rarely True RMS), you will see a voltage drop which is in fact not a voltage drop. (peak to peak is still the same but true RMS is reduced)
You can only measure this with a more expensive multimeter

http://www.dytran.com/img/tech/a6-fig-5.gif

Peter_1
27-11-2007, 08:27 AM
Copy of Tektronix : TRMS/AVG

True RMS or TRMS reading DMMs populate the high end of available handheld instruments. They are assuming greater importance today in light of the heightened concern over power line harmonic distortion caused by widespread use of switching power supplies in office computers and todays proliferation of electronic devices. TRMS is an advantage when measurements of AC waveforms other than sinousoidal are required.

RMS measurements are a measure of the equivalent heating effect produced by a voltage and, to be accurate, must include any DC component present along with the AC component that most users associate with RMS readings. Many TRMS DMMs are not capable of measuring the TRMS value of the combined AC+DC and errors may result if there is any DC superimposed on the AC voltage.

Conventional non-TRMS DMMs measure the average of the absolute value of AC voltage and are calibrated so that the readings are corrected to that of the RMS value for a sine wave. This works well but errors occur if harmonics are present with the effect becoming progressively worse as the harmonic content increases. Table 2 shows the readings that would be obtained when average and TRMS responding DMMs are used to measure sine, triangular, square, and pulse waveforms and DC.
Table 2: Average VS. TRMS Comparison of Typical Waveforms

First, it can be seen from the data that average responding DMMs can have a substantial error when measuring squarewaves with their rich harmonic content. TRMS is not always the best, especially if millivolt amplitude sine waves are to be measured. Average responding instruments are faster settling to the final value or for adjusting levels near zero volts than equivalent TRMS DMMs and may have less offset or zero errors. This is common to TRMS meters since most use the same type TRMS converter integrated circuit.

Second, a "TRMS" DMM that does not read AC+DC will exhibit a -42.2% error under the AC with DC offset condition shown in the table. In this case, if TRMS readings are required, it will be necessary to compute it from separate DC and TRMS AC measurements using the equation:

VTRMS = (VDC2 + VAC2)-2

In both of the previously described instances, understanding what the DMM is actually responding to will help to prevent misinterpretation of measurements. A few DMMs offer the ability to select average, AC TRMS, or AC+DC TRMS thus allowing the user to choose the optimum mode for the desired measurement.

Link at http://www.tek.com/Measurement/cgi-bin/framed.pl?Document=/Measurement/App_Notes/dmmspecs/newunderstand.html&FrameSet=mbd

jamal adam
06-12-2007, 02:28 PM
Hi
thank you peter1 for your information very good information and excently

philjd26
09-12-2007, 01:39 AM
what?:eek: sounds good

m_i_mahmoud
15-04-2009, 08:32 AM
Hi All
[I know that we can control the speed of a single phase electric motor by changing the voltage.]

[Can some one tell me how changing the voltage alters the speed of the motor.]
:p
Regards
MIM

TRASH101
15-04-2009, 08:59 AM
Here is a link that compliments Peters post on phase chopping

http://www.aircareautomation.com/data/article1.pdf

nike123
17-04-2009, 03:50 PM
AC motor rpm is determined by frequency of power supply, number of poles and motor slip.
By chopping portion of voltage sinusoid we control amount of slip and, in that way, actual speed of motor.
Therefore, we actually "changing voltage" by chopping portion of sinusoid and not by lowering actual voltage (peak to peak voltage).