As an Electrical engineer I agree with everything that Duane stated about dual voltage single phase motors. I especially appreciated the safety comment about using a double pole switch for 240 volt motors. In the USA, 240 VAC is supplied with two lines while 120 VAC is supplied with one line and ground. While a single pole switch will turn the 240 VAC motor on or off, it will only switch on/off one of the two lines. This will leave the other line directly connected to the motor and the motor will have voltage applied to it even if the switch is off. UL will not allow any manufacturer of a product that uses a 240 VAC motor that is built into the product to use a single pole switch to turn off the motor.
On capacitor start single phase motors, there is a start winding that has a capacitor in series with the winding. The purpose of the capacitor is to provide a phase shift on the start winding. This will help the motor start by providing extra starting torque due to the phase shift. These motors also include a centrifugal switch that is internal to the motor that will open its contact and will remove this capacitor and start winding from the motor circuit once the motor has reached an adequate speed. If this switch goes bad and stays switched on, the motor will quickly overheat and will likely also burn up the start capacitor. If the capacitor is open or the centrifugal switch doesn't engage, the motor will not start and will also quickly overheat.
Some manufactures of products will incorporate dual voltage/dual frequency motors into their design. Typically these are 120/240 VAC, 50/60 Hz, 2875/3450 RPM. These motors have been specifically designed to run on either 50 or 60 Hz without burning up. This is a compromise. The motor actually will only run at optimum efficiency at one frequency. In the case of a dual frequency motor, the motor manufacturer will design the motor to be optimum at 55 Hz, but will run sufficiently efficient at either 50 Hz or 60 Hz to not overheat. The reason that the product manufacturer has included a multi-frequency product is so that it can be used around the world, without having to have different stocked items for different voltage products. A similar thing is often done on voltages. Some motors are listed as 208/240 VAC, 50/60 Hz, 2875/3450 RPM. Again this is a compromise. A motor can only be efficient at one voltage point for the winding. If the motor has a voltage range instead of a fixed voltage, it just isn't as efficient as it could be. In this case, the motor will have a maximum efficiency at 224 VAC and 55 Hz. But it will run cool enough to work well at both voltages and at both frequencies. The reason I am telling you about this is that while these types of motors help the manufacturer of a product reduce the number of SKU's (stocking units) that he has to produce to sell products around the world, it is costing you in inefficiency. These motors will draw more current than fixed voltage/fixed frequency motors. Usually this is about 10 - 15% more power consumption. So don't go for these range of voltage/range of frequency motors if you can avoid them, unless of course you don't care about spending more money for power than you need to.
Last edited by oldwoodenshoe; 12-03-2014 at 04:16 PM.