What is a general purpose motor?

What is a general purpose motor

What is a general purpose motor?

General Purpose motors are for everyday use in normal environments. They may be used in applications which include compressors, pumps, fans, conveyors, material handling, machine tools and many others.

1.Why are general purpose motors not suitable for high-altitude areas?

Altitude has adverse effects on motor temperature rise, motor corona (for high-voltage motors), and commutation of DC motors. The following aspects should be considered:

(1) At higher altitudes, the general purpose motor temperature rise increases while the output power decreases. However, if the decrease in ambient temperature with increasing altitude compensates for the effect of altitude on temperature rise, the rated output power of the motor can remain unchanged.
(2) High-voltage motors used at high altitudes require measures to prevent corona.
(3) Altitude adversely affects the commutation of DC motors, so the selection of carbon brush materials should be considered.

2.Why does a motor generate shaft current?

The current flowing through the shaft-bearing pedestal-base circuit of a motor is called shaft current.

Reasons for the generation of shaft current are:

(1) Magnetic field asymmetry;
(2) Presence of harmonics in the power supply current;
(3) Manufacturing and installation defects resulting in uneven air gap due to rotor eccentricity;
(4) Gaps between two halves of a detachable stator core;
(5) Improper selection of the number of stator core segments arranged in a fan shape.

Harmful effects: Shaft current can erode the surface of motor bearings or balls, causing the bearing performance to deteriorate, increasing frictional losses and heat generation, and ultimately leading to bearing failure.


(1) Eliminate pulsating magnetic flux and power supply harmonics (e.g., by adding AC reactors on the output side of a variable frequency drive);
(2) In motor design, insulate the bearing pedestal and base for motors with sliding bearings, and insulate the outer ring and end cover for motors with rolling bearings.

3.Why is it not advisable to operate a general purpose motor under light load?

Operating a general purpose motor under light load can result in:

(1) Low power factor of the motor;
(2) Low motor efficiency, leading to equipment waste and uneconomical operation.

4.What are the reasons for motor overheating?

The reasons for motor overheating are:

(1) Overloading of the motor;
(2) Phase loss;
(3) Blockage of the cooling air path;
(4) Excessive running time at low speeds;
(5) Excessive power supply harmonics.

5.What preparations should be made before starting a motor that has been idle for a long time?

Before starting a general purpose motor that has been idle for a long time, the following steps should be taken:

(1) Measure the insulation resistance between the stator windings, phase-to-phase, and between windings and ground.

The insulation resistance R should satisfy the following equation:

R > Un / (1000 + P/1000) (MΩ)

Un: Rated voltage of the motor winding (V)
P: Motor power (kW)

For a motor with Un = 380V, R > 0.38 MΩ.

If the insulation resistance is low, the following methods can be used:
a) Dry the motor by running it unloaded for 2-3 hours.
b) Apply low-voltage AC to the windings or connect the three-phase windings in series and use DC voltage for drying, keeping the current at 50% of the rated current.
c) Introduce hot air using a fan or heating element.

(2) Clean the motor.

(3) Replace the bearing grease.

What is a general purpose motor

6.Why should motors in cold environments not be started arbitrarily?

Extended operation of motors in low-temperature environments can result in:

(1) Cracking of motor insulation;
(2) Freezing of bearing lubricating grease;
(3) Oxidation of soldering flux in wire connections.

Therefore, motors in cold environments should be stored with heating, and the windings and bearings should be inspected before operation.

7.What are the reasons for three-phase current imbalance in motors?

The reasons for three-phase current imbalance in motors are:

(1) Unbalanced three-phase voltages;
(2) Poor or faulty connections in internal motor phases;
(3) Interturn short circuit or short circuit between windings or to ground in the motor;
(4) Wiring errors.

8.Why can’t a motor designed for 50Hz be used with a 60Hz power supply?

When a motor is designed, the silicon steel sheets are typically operated in the saturation region of the magnetization curve. When the power supply voltage is constant, reducing the frequency increases the magnetic flux, excitation current, and motor current, leading to increased copper losses and ultimately causing the motor temperature to rise. In severe cases, the coil may overheat and damage the motor.

Asynchronous Motor-general purpose motor

9.What are the reasons for motor phase loss?

Reasons for motor phase loss can include:

Power supply-related causes:

(1) Poor contact of switches;
(2) Transformer or line break;
(3) Blown fuses.

Motor-related causes:

(1) Loose screw connections in the motor terminal box;
(2) Poor internal wiring connections;
(3) Broken motor windings.

10.What are the causes of abnormal vibration and noise in motors?

Mechanical factors:

(1) Poor lubrication of bearings or bearing wear;
(2) Loose fastening screws;
(3) Foreign objects inside the motor.

Electromagnetic factors:

(1) Overloading of the motor;
(2) Three-phase current imbalance;
(3) Phase loss;
(4) Short circuit faults in stator or rotor windings;
(5) Open-circuited bars in squirrel cage rotors.

11.What preparations should be made before starting a motor?

Before starting a motor, the following preparations should be made:

(1) Measure the insulation resistance (should not be lower than 0.5 MΩ for low-voltage motors).
(2) Measure the power supply voltage. Check the motor wiring for correctness and ensure that the power supply voltage meets the requirements.
(3) Check the starting equipment.
(4) Verify the suitability of fuses.
(5) Check the grounding and bonding of the motor.
(6) Check the transmission device for defects.
(7) Ensure the motor environment is appropriate and remove flammable substances and other debris.

12.What are the reasons for motor bearing overheating?

Causes related to the motor itself:

(1) Excessive tightness of the bearing inner and outer races;
(2) Defective dimensional tolerances of components, such as misalignment between the motor frame, end covers, and shaft;
(3) Improper bearing selection;
(4) Poor lubrication of bearings or presence of foreign particles in the grease;
(5) Shaft currents.

Causes related to usage:

(1) Improper installation, such as misalignment between the motor shaft and the driven shaft;
(2) Over-tightening of belt pulleys;
(3) Inadequate maintenance of bearings, insufficient lubrication, or aging of the lubricating grease.

13.What are the reasons for low insulation resistance in motors?

Reasons for low insulation resistance can include:

(1) Moisture or water ingress into the windings;
(2) Accumulation of dust or oil contamination on the windings;
(3) Insulation aging;
(4) Insulation damage in motor lead wires or terminal boards.


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