Copyright Goodheart-Willcox Co., Inc. 260 Essential Electrical Skills for HVACR: Theory and Labs The following are examples of applying the formula to different pole numbers and power values: • Two-pole motor operating at 60 Hz power: Ns = 120 × 60 Hz = 3600 RPM 2 • Two-pole motor operating at 50 Hz power: Ns = 120 × 50 Hz = 3000 RPM 2 • Four-pole motor operating at 60 Hz power: Ns = 120 × 60 Hz = 1800 RPM 4 As shown in these examples, speed decreases as more poles are added. The four- pole motor rotor completes only a half revolution per ac cycle compared to motors with two poles. Lower frequency also reduces rotor speed. Note that synchronous speed is not the actual rotor speed. The rotor lags the rotating magnetic field since a mechanical load is attached. The difference between the synchronous and the actual speed is called the slip. A motor’s actual design speed is given as the rated full-load speed. The rated speed occurs only when the rated load is applied in addition to the rated voltage and frequency. For example, a 1/4 hp motor with a rated full-load speed 3450 RPM has a 1/4 hp load attached, and the voltage and frequency match the rated values and operates at approximately 3450 RPM. Even if the motor is running without a load, the RPM does not match the synchronous value. There are losses to the speed due to the bearings and magnetic field losses in the form of heat. Not all motor applications require the high speed produced by a two-pole motor. Therefore, HVACR motors use between two and eight poles depending on required speeds and torque. More poles reduce speed and increase torque, or rotational force. 13.3.1 Resistance-Start, Induction-Run A resistance-start, induction-run (RSIR) motor uses a high resistance (dc) to start winding to split the phase. See Figure 13-14. Ideally, the phase is split by 90° to gain the most starting torque. The RSIR falls short of that by creating only about half the phase shift. As a result, this motor is limited to low starting torque applications, such as frac- tional horsepower refrigeration compressors used in compact dorm room refrigerators. S R L2 C L1 RSIR Centrifugal switch Goodheart-Willcox Publisher Figure 13-14. RSIR motor schematic diagram. Note that the centrifugal switch inside the motor is shown outside the circle, representing the motor housing.