Tuesday, December 2, 2014

Drive motors

 Drive motors

Before installing or replacing a drive motor, it must be established that the voltage and frequency of the motor (as stated on the nameplate) agree with those of the supply.

A motor must not be connected to a supply outside the range stated on the nameplate. If connected to a low voltage supply it may not start. If it does start it may not run to its full speed; the windings will then overheat, and may burn out if continued starting attempts are made by the action of the overcurrent protector.

It is therefore important that the overload protector is of the correct rating if a replacement is made. Those fitted to open drive motors should be rated as near as possible to 125 per cent of the nameplate current.

Installation details

The motor frame must always be connected to the local earth circuit. It is most important to ensure that the flexible earthing strip between the motor frame and the cradle is not broken and that the correct connection is made.

When water cooled systems are involved, under no circumstances must the water supply tubing be used for earthing.


Motor bearings should be lubricated at the time of installation or replacement. For sleeve bearings use high grade light mineral oil; for ball bearings use pure mineral grease.

Do not apply an excess of oil or grease, because excess lubricant can damage motor windings and switch mechanisms. Do not allow oil to contact the resilient rubber motor mounting pads. A small drop of oil should be applied to the swivel pivot.


Worn bearings will probably be evident in the form of a hum or rattle from the motor.

The end play should not exceed 0.25 mm. If shims are used to correct the clearance or take up wear, they must not be fitted at one bearing only. This will result in the rotor being out of centre with the stator and will cause humming.

Thermal overloads

These are usually self-resetting after cooling. They are set to cut out when the temperature of the motor windings reaches approximately 90 °C (194 °F).

Nuisance tripping of a motor may be the result of poor ventilation of the motor or the load on the compressor. This should be checked before any replacement is made.

Direction of rotation

To reverse the rotation of a capacitor start motor, the direction of current flow in either the start or the run winding must be reversed (but not both windings). This entails reversing the internal connections of a winding, as shown in Figure 102. For three phase motors it is only necessary to reverse any two electrical leads to the motor terminals.





Terminal board connections

Suppliers of drive motors are numerous and the terminal identification is not standard. A variety of connections for motors in common use is provided in Figure 103.

Three phase electrical connections

Smaller compressor motors incorporate overload protectors, either internally or externally mounted, and use various small starting devices. By contrast the three phase motor, being larger, requires more sophisticated methods of automatic starting and overload protection, provided by contactors or starters.

Contactors and starters

A contactor is a power operated switch suitable for frequent making and breaking of an electrical circuit. A starter is a power operated switch with inbuilt overcurrent features, employed for starting a motor and protecting it during running. Both controls enable large currents to be switched by means of a system control (thermostat) with low current rating.



Suppliers of these controls are numerous. Each has its own options with regard to overload protection, single phasing protection and accessibility for component replacement. Wiring diagrams for the controls are normally supplied or are available on demand. These should be referred to before any attempt is made to install, make electrical connections to or diagnose faults on

the controls. Because of the different types of control, only typical examples are dealt with here. Figure 104 shows connections for single phase starting, and Figure 105 shows a particular three phase starter.

Direct-on-line starter

With the standard squirrel cage induction motor, it is advantageous to have DOL starting. This allows a cheaper motor starter and control gear to be used, and also imposes less strain on motors during starting.



However, electrical supply authorities will normally restrict starting currents. DOL starters should only be specified for motors within the supply authority limits.

Figure 106 shows the electrical connections for three phase DOL starters.