Sunday, December 20, 2009

Capacitor Start – Capacitor Run Motor

by Roger Desrosiers

Capacitor Start-Capacitor Motors or (CSR) are used almost exclusively on Hermetic and Semi-Hermetic motors compressors. Rarely will this motor be used on an open type motor because of the cost of the components necessary to produce it. Most open–type motors do not use a starting relay, but use a centrifugal switch instead. Open type motors are usually built as permanent split-capacitor or capacitor start motors.


The CSR motor begins operation on a phase displacement between the start winding and the run winding, which allows rotation to begin. The run capacitor adds a small amount of starting torque but its main function is to increase the running efficiency of the motor. The run capacitor is wired in the circuit to provide the most efficient phase angle between the current and voltage when the motor is running. The run capacitor is in the circuit any time the motor is running. Both the start and run capacitors are wired in series with the start winding but are in parallel with each other. Figure 1 shows a schematic diagram of the motor with the starting components.

The microfarad rating of the run capacitor is much lower than that of the start capacitor for any given motor application. The capacitances of the two capacitors in parallel are additive, the same as resistors are in series. If the run capacitor has a capacitance of 10 mfd and the start has a capacitance of 110 mfd their total capacitance is 120 mfd. During startup this combined capacitance in series with the start winding causes a greater phase angle between start and run winding, which gives the motor more starting torque. When the potential relay opens the start capacitor is taken out of the circuit, however the run capacitor and start winding stay in the circuit. The reason the start winding can stay in the circuit is because the run capacitor limits the current going through the start winding. If the run capacitor fails because of an open circuit within the capacitor, the motor may start, but the running amperage might be about 10 % too high and the motor will get too hot if operated at full load. The motor is actually a PSC motor when running. The start capacitor is only used to help start the motor and at 75% of rpm it is disconnected from the start winding.


This motor is sometimes difficult to troubleshoot because of the number of components it has. For example, the windings, bearings, potential relay, starting capacitor, and running capacitor must all be checked. The windings of the CSR motor are easily checked with an ohm meter to determine if the windings are shorted, open, or grounded. The windings will be enclosed in a hermetic casing and the terminals will be on the outside of the casing. The technician must know how to determine the correct terminals to check. The bearings of a CSR can be worn so badly that the motor will seize or be very noisy with a knocking sound and the bearings are enclosed, therefore harder to check. A high amp draw can sometimes indicate bad bearings but consideration must be given to other possibilities such as an over charge or a high heat load.

The starting relay can be checked by diagnosing the condition of the contacts and the coil. The contacts can be checked with an ohm meter or be visual inspection. The ohm meter should show zero resistance across the contacts which are normally closed on a potential relay. If you have doubts about the integrity of the relay you can disassemble the relay. Then the condition of the contacts can be determined (sticking, pitting and misalignment). The coil can be checked like the windings of a motor. The starting and running capacitors are easily checked with an ohm meter to determine their condition.

Trouble shooting a CSR motor is done by checking all the components of the motor. These motors must be checked thoroughly to prevent other components from being destroyed. For example, the starting capacitor will be destroyed if the contacts or coil of the start relay are bad.

Roger J. Desrosiers

About the Author: Roger is a contributing faculty member of He has over 40 years experience in Air Conditioning and Refrigeration. He is also a member of R.S.E.S., CM, The Association of Energy Engineers, Certified Energy Manager, ASHRAE, Certified Pipe Fitter United Association and is 608 Universal Certified.