2 speed for one single phase motor

I'd think one could make a simple 25Hz AC-DC-AC inverter for this purpose -- and for a fractional HP motor, it could be pretty inexpensive. I'm posting mainly to see what others come up with, but in the mean time I'll look around.

Something like this is what I had in mind.

I built a unit for single phase motors but it had an out put of 2 phases at 90 deg separation and eliminated the cap/inductor . The control is more complex to achieve good efficiency and low end torque is lousy below 6 to 8 Hz on a 60 Hz motor. It worked well for variable torque applications (fans , pumps etc.) But I could get no interest in it from industry. It would make a good converter dedicated to dc power to ac motors in refrigerators and other single phase motor devices. the trend today is to use 3 phase motors supplied by a std 3 phase variable speed drive and fed with single phase input to the front end rectifiers the VFD has to be oversized and programmable to ignore the missing 3rd leg , but all is readily available in the market.

Jon

The easiest thing to do may be to get a two speed motor. A half horse motor isn't that expensive and you could easily eat up the additional cost of a two speed motor trying to make a single speed motor two speed.

fundamental fact is the motor speed depends on 1)the windings and 2) the power frequency. -assuming load is within motor capacity. Higher loads mean more 'slip' and higher current draw (more heat).

With an existing motor, one has to vary the frequency of the power input.

The Shenyan Tongren Electric Company manufactures a thrysistor control for AC motors.

Mark

In one respect you are lucky that you have a fan load where the load varies as the square of the speed, so at half speed (700) the load decreases to a quarter of the load at 1400rpm. However changing the speed from a 4 pole (1400 rpm) to 700 rpm (8 pole) is not so easy.

The equation for calculating the synchronous speed of an induction motor is n=120f/p where n is revs per minute, f is frequency in hertz and p is the number of poles that the motor winding has. In your case this calculates out at 1500rpm which reduces to 1400 when it is running at full load. So to vary the speed you have to either;

1 Change the frequency, which you say is too expensive for your budget.

2 Rewind or reconnect the motor windings to create a 4 and 8 pole winding. This is not easy as your presnt motor arrangement would probably not suit taking an 8 pole winding even for a fan drive that reduces as the square of the speed reduction. You would also need to have a good understanding of induction motor winding design to ensure that the modified winding would give you the results you were expecting. It may be worthwhile considering reconnecting your winding to create "consequent" poles when in the 8 pole configuration. This is done by connecting the windings to be all say north poles which forces the magnetic field to create its own south poles - called consequential poles. This may work as the load you require at the 700rpm is only one quarter of the 1400rpm load - but it is a gamble. You also need to keep in mind that you have both a starting and a running winding to consider and, if the running winding is controlled by a centrifugal switch, it may not cut out the starting winding before 700 rpm and it will probably burn out if its switching is not taken into consideration. Also the speed (winding) change over switch would be quite complicated and would be best done using a multi pole relay or contactor rated accordingly.

Nice summary of the options.

Re changing frequency, it's interesting that there is doesn't appear to be a simple, cheap speed changer on the market. The sine wave synthesis technology is widely available in inverters (for boats, RV's etc.) The rectification prior to synthesizing would be simple. I'd be willing to bet that the time base in the inverter could be changed by swapping just one resistor (or maybe a resistor and a capacitor). (I wouldn't expect the timing to be entirely integrated into a chip -- because a manufacturer would at least want to provide either 50 or 60 HZ.)

In the AC world, two speed and variable speed control of fans and compressors has become more common for energy efficiency. I wonder if there might be an of-the-shelf frequency halver in the AC world?

why u r not thinking how normal ceiling fan works for different speeds which is single phase motor.

I did. Its called "Get a motor that is designed to run at two speeds." Just like a ceiling fan motor is designed to run at different speeds by using nothing more exotic than a switch.

Well you wind the motor for 4 pole & 8 pole connections & use pole changing switch for speed change