These drives should be used for applications that only require a single rotational
direction. The torque speed curve for a typical induction motor is shown in Figure 15.10.
When the motor is used with a fixed frequency AC source the synchronous speed of the
motor will be the frequency of AC voltage divided by the number of poles in the motor.
The motor actually has the maximum torque below the synchronous speed. For example a
2 pole motor might have a synchronous speed of (2*60*60/2) 3600 RPM, but be rated for
3520 RPM. When a feedback controller is used the issue of slip becomes insignificant.
Figure 1 Torque Speed Curve for an Induction Motor
• Wound rotor induction motors use external resistors. varying the resistance
allows the motors torque speed curve to vary. As the resistance value is increased the
motor torque speed curve shifts from the Class A to Class D shapes.
• The figure below shows the relationship between the motor speed and applied
power, slip, and number of poles. An ideal motor with no load would have a slip of 0%.