Single-phase motors protection

Manual and Magnetic Starters Manual motor starters are operated by hand. The simplest type of manual starter is a snap switch with no overload protection, used only for motors of 1,492 kW (2 hp) and smaller, usually single-phase motors with integral overload protection. 

Some overload-protection schemes measure motor-winding temperature directly various methods are used. Small single-phase motors are available with built-in overload protection, A thermostat built into the motor senses motor-winding temperature directly. 

Adequate single-phase protection is provided on low-voltage ac motor starters by three overload relays, which are now standard. Rotor heating is not particularly a problem on smaller motors which have more thermal capacity, but it is important to protect the stator windings of these machines against burnout. 

This part deals w ith three- and single-phase a.c. machines, and their protective switchgears. However, reference is inade to and comparisons drawn of a d.c, motor with an a.c. motor, to assist a user to inake a proper choice of machine. 

Overload devices in current use are typically thermal overload relays to BS 4941, motor starters for voltages up to and including 1000 V A.C. and 1200 V D.C. or BS 142 electrical protection relays. Relays to BS 4941 generally provide overload and single-phasing protection. Those complying with BS 142 are also frequently fitted with instantaneous earth fault and over-current trips. 

Freeze thermostat (low water temperature limit) Single-phase protection (preventing attempts to start with loss of one phase of a three-phase electrical supply) Time out (preventing too-frequent starting and motor burn). 

The purpose of negative phase sequence protection is the same as for high voltage motors. It is not normally needed for motor ratings less than approximately 22 kW. In the simpler designs of moulded case circuit breakers and relays the negative phase sequence detection is more in the form of single-phase protection, wherein a phase is completely lost. 

The devices are not restricted to single-phase systems. Figure 3.4 illustrates a three-phase RCD connected into the supply from a three-phase distribution board to a motor. In this particular case, the RCD may be set to operate at a leakage current of perhaps 500 mA since it is providing protection... 

Rotating electrical machines starting performance of single-speed three-phase cage induction motors for voltages up to and including 690 V Electrical apparatus for explosive gas atmosphere. Increased safety protection e 8789/1996 6381/1991 BS EN 60034-12/1996 BS 5501-6/1977... 

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