Motor temperature
MichaelMouse,
It is a very popular misunderstanding about dual voltage motors running cooler when wired for 240 volts as opposed to 120 volts, but it doesn't work that way. As far as the wires in the motor are concerned there is no difference.
Here is a very brief and simplified explanation of the way that dual voltage motors are wound and operate. The field windings (i.e., the outer fixed part of the motor -- also called the stator) of an AC induction motor consist of a number of separate windings or coils. Some of these coils are connected in series and groups of these series connected in parallel. When a motor is wound so that it can be operated on either 120 or 240 volts, leads are brought out to the junction box for two sets of coil groups. Each of these groups consists of a number of series and/or parallel coils, but I will refer to each coil group as a winding. If 120 volts is available to run the motor, then the two windings are connected in parallel so that half of the current will flow through each winding. On the other hand, if 240 volts is available to run the motor, the two windings are connected in series and thus the same total current will flow through both windings. And because the impedance of both windings is the same, the voltage across each winding will be half of the total, or 120 volts. The result is that there is no difference in power used nor efficiency of the motor. The rotor does not know the difference.
Here is a practical example:
A dual voltage motor is rated at 10 Amps for 120 volts and 5 Amps for 240 volts. When the motor is configured for 120 volts, the voltage across each set of windings is 120 volts since the windings are in parallel. Since the windings have the same impedance, the current will be divided equally between the two windings so 5 Amps will flow in each winding. When the motor is configured for 240 volts, the windings are in series so 5 Amps will flow in each winding and since the impedance of both windings are the same, the voltage across each winding will be 120 volts.
Now, the only difference in operating the motor will depend upon the wiring from the SE (service entrance) panel to the wall outlet. If it has been wired to code, there should not be any problem, but if someone had a run of 250 feet of 14 AWG wiring on a 15 Amp breaker (which, by the way is not allowed by NEC) to this motor, there would be enough voltage drop at 120 volts to cause the motor to draw more current than normal to maintain speed.
While I am on the subject of motors, I feel like I should mention that a motor does not draw its nameplate current except when it is loaded to full capacity. When a motor is running completely unloaded the current that it draws is roughly 60 to 70 percent of FLC (full load current). The unloaded current is referred to as the magnetizing current, but it also takes into account things like bearing friction; copper losses (resistance of the copper wires); iron losses (eddy currents and hysteresis in the iron laminations); and viscous drag of the bearing, cooling fan, and the air inside the motor. The motor current increases roughly proportionally to the torque load on the motor up to the maximum load. Beyond that point, the motor speed decreases rather dramatically with increased torque load and the load current increases dramatically until it reaches what is known as the locked rotor current (LRC) which is normally about 6 times the FLC. The LRC is exactly what its name implies -- the motor does not rotate and this condition should not be allowed to continue very long or else bad things will start to happen (like opening up the Grainger catalog to the motors section).
Bill Boehme
p.s. My description ignored start windings since the same thing applies to them as to the run windings or more specifically, they normally are not in the circuit long enough to do more than get the motor started.