A basic characteristic of asynchronous motors is that they operate properly only at one rated voltage for a rated frequency and connection. For the 240 volt connection and the rated voltage (usually 50 Hz or 60 Hz) the motor should have only 240 volts applied. The applied voltage is allowed to vary by about 5%, so about 228 to 252 volts would be ok.
If an inverter is designed to control the motor speed, it must keep the voltage proportional to the frequency. If the motor is designed for 240 volts and 50 Hz, it can operate at 120 volts and 25 Hz or 360 volts and 75 Hz. The speed will vary in proportion to the frequency with the voltage having little effect on speed but a more significant effect on current and torque capability. The speed in RPM is equal to (120 X frequency / motor poles) minus slip. Slip at full load is about 3% of the rated 50 Hz speed and is proportional to load torque.
The can probably operate between 10 Hz and 75 Hz if the ratio of voltage to frequency is maintained at 240/50 = 4.8 V/Hz. For the motor to be capable of producing the same torque at all speeds, the ratio will need to be increased somewhat at lower speeds.
This is a basic summary of asynchronous motor speed control by changing frequency. It is not possible to present a the complete theory as an answer to a question here.
It is possible to reduce the speed by reducing just the voltage below the rated voltage, but that method is very limited and much affected by the nature of the load. Increasing the voltage above the rated voltage increases the speed only slightly and is also much affected by the load. Increasing the voltage by much more than 5% will cause excessive current and overheat the motor.
Added Details Regarding Voltage
Increasing voltage increases magnetizing current thus increasing magnetic flux. Increasing flux will allow the motor to develop more torque at a given slip or to have less slip at a given load. Less slip means higher operating speed. At or near the rated voltage, the motor reaches a minimum current for a given torque. Because of magnetic saturation of the iron, the increase in flux for a given increase in current is reduced to the point that the current is increasing faster than the torque is increasing. The current increase increases internal heating with little or no benefit.
Slip is directly proportional to the torque transmitted to the load. Once the load has been accelerated to a stable operating speed, the torque transmitted to the load is the torque that is required to drive the load at that speed. Since the full-load slip of a normal design induction motor is about 3% of rated speed, the motor speed only changes by about 3% between no-load and full-load. The stable operating point is the point at which the speed vs. torque demand curve of the load crosses the speed vs. torque capability curve of the motor. The result of any change in the motor curve is influenced by the magnitude and slope of the load curve.
Increasing Speed Using a VFD
Most variable frequency drives (VFDs) are factory set with the maximum output set to the power frequency in the region where they are sold. However most have configuration settings that allow the maximum frequency to be increased.
In this case, the motor can be connected for either 240 volts or 480 volts and the rated motor frequency is assumed to be 60 Hz. The there are two supply voltages available, 380 and 470. VFDs do not normally have an internal voltage boost feature, so we will assume that the maximum output voltage is limited to the input voltage. Let us first assume that the VFD is set for 50 Hz output and it is desired to operate at the 60 Hz speed with no decrease in torque capability. To do that, connect the 470 volt supply to the VFD and connect the motor for 480 volts. Configure the VFD for normal use with a 480 volt, 60 Hz motor. The input voltage is 2% low, but the motor and VFD should be able to tolerate that with no difficulty.
There are other alternatives. The maximum frequency could be extended above 60 Hz with constant output voltage. That would result in the torque capability dropping as speed increases. The torque capability would follow a constant power curve to about 90 Hz. Above that, torque would be further limited.
If the VFD output current rating is sufficient to supply the motor current required for the 240 volt connection, there are other alternatives using that motor connection.
Best Answer
The change in the winding connection shown, reverses the orientation of half of the windings of each phase with respect to the other half. That results in each pole being divided into two poles thus doubling the number of poles in the motor and halving the synchronous speed.
If the flux produced by a given voltage with the low-speed connection is much higher than that produced with the high speed connection, the motor is designated as a constant horsepower motor. The motor's torque capability is directly proportional to the flux produced. Since horsepower is torque multiplied by speed, a constant horsepower motor has high torque capability at low speed such that torque multiplied by speed is constant.
If the flux produced is about the same for both connections, the torque capability is the same for both and the motor is designated a constant-torque motor.
Assume that the line-to-line voltage is 400 volts. For the series delta connection the voltage applied across each winding is 200 volts. For the parallel wye connection, the voltage applied across each winding is 230 volts. Thus the series delta connection voltage per winding is 87% of the voltage for the parallel wye connection. The torque capability is approximately proportional voltage squared. That would make the torque for the high-speed connection approximately 75% of the torque for the low-speed connection, whereas the torque should be 50% at the higher speed for constant power at 200% speed. The performance capability of the motor is actually somewhere between constant-torque and constant-horsepower, but such motors are designated constant-horsepower motors even though their torque capability exceeds the constant-horsepower limit.
In estimating the comparative torque capability of the low vs. high speed connections, no allowance has been made for the increased losses at the higher speed and the inability to optimize the winding configurations for the number of slots vs. poles.
The diagram below shows how the low-speed and high speed connections are implemented. Note that the current flows from on end to the other in a phase winding (T1 to T2) for the high speed connection and from the center to both ends (T4 to T1/T2) for the low-speed winding.
There are three types of single-winding, three-phase, two-speed induction motors. There may not be a standard that requires the type to be marked on the nameplate, but it may be. The three types are:
Constant Horsepower (or Constant Power)
Constant Torque
Variable Torque
There are NEMA and IEC standards that require the rated speed and output power ratings to be marked on the nameplate. If a single-winding, two-speed motor conforms either of those standards it should have two speeds marked, the higher speed will be very close to twice the lower speed.
The output power marked on the nameplate could be stated in horsepower watts or kilowatts. It is usually marked in horsepower for NEMA motors.
A constant-power motor should have only one power rating marked or about the same power rating for both the low and the high speed.
A constant-torque motor should have two power rating marked. The power rating for the low speed should be very close to half the power rating for the high speed.
A variable torque motor should have two power ratings marked. The power rating for the low speed should be significantly less than half the power rating for the high speed, probably about a quarter of the power rating for the high speed.