Three phase motors with two speeds of rotation are frequently used in air treatment plants to provide two different air flow rates depending on the operating conditions: Low Speed (LS) in winter mode and High Speed (HS) in summer mode (see: lack of evaporator capacity, some practical aspects, page 123).

Two speed motors are often fitted to water cooling towers (the operation of this type of equipment will be discussed on page 506). In the application shown, a 2-speed motor (point 1) drives the tower fan (point 2). When the fan is not running, if the compressor starts, the temperature of the water entering the tower starts to increase. The thermostat located at the tower outlet (point 4) detects this rise in temperature and starts the fan at LS. If the water temperature continues to increase, the thermostat changes the fan to HS and the tower provides its maximum cooling capacity...

Another type of dual speed motor, called the Dahlander motor is very widespread nowadays. The terminal box of this motor is shown below. It is a single-voltage motor with 6 terminals.

	For high speed, the three supply phases are connected to the terminals 2U, 2V and 2W, with the other three terminals (1U, 1V and 1W) being connected together.
	For low speed, the three supply phases are connected to 1U, 1V and 1W whilst the other three terminals (2U, 2V and 2W) must be left totally unconnected, with no supply attached.

Can you work out the internal wiring for the windings of this motor?

(Solution : Refrepair Manual pages 464 to 469)