What causes horsepower and torque to remain the same in a dual speed (single winding) motor?
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.




  Also, how do I distinguish one from the other just by reading the nameplate?


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.