the rotation rate of the stator's magnetic field, , where is the frequency of the power supply, is the number of magnetic poles, and is the synchronous speed of the machine. For in hertz and synchronous speed in RPM, the formula becomes: . For example, for a four-pole, three-phase motor, = 4 and = 1,500 RPM (for = 50 Hz) and 1,800 RPM (for = 60 Hz) synchronous speed. The number of magnetic poles, , is equal to the number of coil groups per phase. To determine the number of coil groups per phase in a 3-phase motor, count the number of coils, divide by the number of phases, which is 3. The coils may span several slots in the stator core, making it tedious to count them. For a 3-phase motor, if you count a total of 12 coil groups, it has 4 magnetic poles. For a 12-pole 3-phase machine, there will be 36 coils. The number of magnetic poles in the rotor is equal to the number of magnetic poles in the stator. The two figures at right and left above each illustrate a 2-pole 3-phase machine consisting of three pole-pairs with each pole set 60° apart. Slip Slip, , is defined as the difference between synchronous speed and operating speed, at the same frequency, expressed in rpm, or in percentage or ratio of synchronous speed. Thus where is stator electrical speed, is rotor mechanical speed. Slip, which varies from zero at synchronous speed and 1 when the rotor is stalled, determines the motor's torque. Since the short-circuited rotor windings have small resistance, even a small slip induces a large current in the rotor and produces significant torque. At full rated load, slip varies from more than 5% for small or special purpose motors to less than 1% for large motors. These speed variations can cause load-sharing problems when differently sized motors are mechanically connected. Various methods are available to reduce slip, VFDs often offering the best solution. Torque Standard torque The typical speed-torque relationship of a standard NEMA Design B polyphase induction motor is as shown in the curve at right. Suitable for most low performance loads such as centrifugal pumps and fans, Design B motors are constrained by the following typical torque ranges: Breakdown torque (peak torque), 175–300% of rated torque Locked-rotor torque (torque at 100% slip), 75–275% of rated torque Pull-up torque, 65–190% of rated