In a crushing and screening plant, almost all energy is consumed by electric motors: crushers, screens, conveyors, feeders, pumps, screw washers and fans. Motors that are correctly selected, correctly started and regularly maintained both reduce energy costs and prevent unplanned stoppages. Conversely, an incorrectly sized or poorly maintained motor can stop the whole line.
In this guide we cover comprehensively the selection of electric motors in mining and crushing and screening applications, efficiency and protection classes, starting methods, motor protection, maintenance and fault diagnosis.
Types of Motor Used in Mining
| Motor type | Characteristics | Typical use |
|---|---|---|
| Squirrel cage induction motor | Robust, economical, easy to maintain | Conveyors, screens, pumps, most crushers |
| Slip ring induction motor | Controlled through rotor resistance, high starting torque | Very large crushers and mills (older and larger plants) |
| Vibrating motor | Unbalanced weights on the shaft ends produce vibration | Dewatering screens, vibrating feeders, bin vibrators |
| Drum motor | Motor and gearbox inside the pulley | Short and medium conveyors |
| Geared motor | Motor and gearbox in one unit | Conveyors, screws, feeders |
| Synchronous and permanent magnet motors | High efficiency, precise speed control | Large mills, applications needing high efficiency |
Efficiency Classes: IE2, IE3, IE4
The energy efficiency of electric motors is expressed by the international IE classes: IE1 (standard), IE2 (high), IE3 (premium) and IE4 (super premium). In many countries, a minimum efficiency class is a legal requirement in certain power ranges. On a crusher or conveyor motor running continuously, a difference of a few points in efficiency can save far more in energy over the motor’s life than its purchase price. For energy-saving methods across the plant, see our energy efficiency guide.
Protection Class (IP) and Environmental Conditions
The IP code shows a motor’s protection against dust and water. The first digit is protection against solids/dust and the second against water. For the dusty and sometimes wet environment of crushing and screening plants, IP55 or higher is usually preferred (IP65/IP66 where there is heavy dust and water jets). In addition:
- Insulation class: Class F insulation with class B temperature rise gives a safety margin for motor life.
- Altitude and temperature: At high altitude and high ambient temperature, motor power should be calculated with derating.
- Cooling: If the housing fins and fan cover are covered with dust, cooling deteriorates; regular cleaning is essential.
Determining Motor Power
Motor power should be chosen according to the equipment’s requirement and operating conditions. An undersized motor overheats and trips frequently; an oversized motor runs inefficiently at low load and lowers the power factor. In crushers, motor power depends on capacity, reduction ratio and rock properties (rock crushability guide). For conveyors, belt length, lift, capacity and friction losses are taken into account.
Starting Methods
| Method | Advantage | Disadvantage | Application |
|---|---|---|---|
| Direct-on-line (DOL) | Simple, cheap, full starting torque | Very high starting current, mechanical shock | Small motors |
| Star-delta | Reduces starting current, economical | Starting torque falls; current peak at changeover | Medium motors starting unloaded (e.g. an empty crusher) |
| Soft starter | Controlled starting and stopping, reduced shock | No speed control | Crushers, pumps, conveyors |
| Frequency inverter (VFD) | Speed control, soft starting, energy savings | Higher cost, harmonics, cooling requirement | Feeders, conveyors, pumps, VSI crushers |
| Slip ring motor + rotor resistance | High starting torque, low starting current | Maintenance (slip rings, brushes) | Large crushers and mills |
Important note: Crushers should be started empty (with an empty crushing chamber) wherever possible. Trying to start a full crusher strains both the motor and the mechanical parts.
Frequency inverters are also used to adjust feeder speed automatically according to crusher load (choke feeding). We describe these automation applications in our automation and digital twin guide.
Motor Protection
- Motor protection circuit breaker and thermal overload relay: Protection against overcurrent.
- Thermistor (PTC): Temperature sensors in the windings; recommended especially for inverter-fed motors and motors with frequent starts.
- Phase protection relay: Protection against phase loss, phase sequence errors and imbalance.
- Electronic motor protection relay: Monitoring of current, voltage, earth leakage and load; common on large motors.
- Bearing temperature sensors: Give early warning of bearing failure on large motors.
Maintenance Practices
| Maintenance | Method | Frequency |
|---|---|---|
| Visual check and cleaning | Fan cover, fins, terminal box | Weekly |
| Bearing greasing | On motors with grease nipples, by the quantity and interval on the nameplate | Manufacturer’s instructions |
| Vibration measurement | Detecting bearing and balance problems | Monthly or continuous |
| Thermography | Hot connections, bearing and housing heating | Every three months |
| Insulation resistance measurement (megger) | Condition of winding insulation; especially after long stoppages and in humid environments | Periodically and before commissioning |
| Current measurement | Phase balance and load | Monthly |
| Connection check | Tightness of terminal and cable connections | Yearly (more often on vibrating equipment) |
For motor bearing selection and failure analysis, see our bearings guide, and for predictive maintenance methods, our predictive maintenance guide.
Common Failures and Their Causes
| Failure | Possible cause | Prevention |
|---|---|---|
| Winding burnout | Overload, phase loss, insufficient cooling, moisture | Correct protection, cleaning, thermistors |
| Bearing failure | Lubrication error, belt tension, misalignment, shaft current | Greasing routine, alignment, insulated bearings |
| Excessive vibration | Imbalance, soft foot, misalignment | Alignment, connection checks |
| Insulation deterioration | Moisture, dust, ageing | Megger tracking, drying, protection class |
| Burnt terminals | Loose connection | Periodic tightening, thermography |
Shaft currents in inverter-fed motors: Frequency inverters can create voltage on the motor shaft, causing current to pass through the bearings. This produces fluted marks on the bearing raceways (a “washboard” appearance). Proper earthing, filters and insulated bearings prevent this problem.
Points to Watch on Belt-Driven Motors
Most crushers and screens are driven by V-belts. Excessive belt tension strains the motor bearings; insufficient tension causes slipping, heating and belt wear. Pulley alignment should be checked with a laser or straight edge, and belts replaced as sets. For belt drives and flywheels on jaw crushers, see our flywheel guide.
A Field Example
Suppose the motor of an impact crusher at a plant trips on thermal overload frequently in the summer months. Measurements show the motor is not running far above its rated current; but thermography reveals the motor housing is excessively hot and the fan cover is clogged with rock dust. In addition, the sun shines directly on the motor enclosure. When the fan cover is cleaned and a sunshade and ventilation are added to the enclosure, the problem disappears. This example shows that motor failures often have environmental rather than electrical causes.
Frequently Asked Questions
Does buying an IE3 motor always make sense?
It usually makes sense for motors that run continuously for long periods; the energy savings can quickly cover the extra cost. For motors that run little, the payback period is longer.
Soft starter or inverter?
If only the starting current and mechanical shock need to be reduced, a soft starter is sufficient. If speed control is needed, an inverter is preferred.
When should a motor be rewound and when replaced?
Rewinding is often economical for large motors; for small motors, buying a new, more efficient motor may make more sense. Efficiency loss and lead time should also be considered.
Why is a megger measurement important?
It shows deterioration of winding insulation before a failure; it is recommended especially before starting a motor after a long stoppage.
Motor and Drive Parts by Equipment
| Equipment | Typical drive | Parts to keep as spares |
|---|---|---|
| Jaw crusher | Motor + V-belts + flywheel | Belt set, pulley, motor bearings (jaw crusher spare parts) |
| Cone crusher | Motor + V-belts or coupling + countershaft | Belt set, coupling elements (cone crusher spare parts) |
| Impact crusher | Motor + V-belts | Belt set, pulleys (impact crusher spare parts) |
| Vertical shaft impactor | Twin motors + V-belts, often inverter-fed | Belt set, motor bearings (VSI crusher spare parts) |
| Vibrating screen | Motor + V-belts or cardan shaft; vibrating motor | Belts, vibrating motor (vibrating screen spare parts) |
| Conveyor | Geared motor or drum motor | Coupling, gearbox parts (belt conveyor spare parts) |
| Feeder | Vibrating motors or inverter-fed motor | Vibrating motor, weights (feeder spare parts) |
| Screw washer | Geared motor | Gearbox and coupling (screw washer spare parts) |
First Checks After a Motor Failure
- Determine why the protection device tripped (overcurrent, thermistor, phase, earth leakage).
- Check that the equipment turns freely mechanically; there may be a jam in the crusher or a blockage on the conveyor.
- Measure the supply voltage and phase balance.
- Check the terminal connections and cable.
- Measure winding and insulation resistance.
- Check bearing noise and temperature.
- Confirm that the cooling paths are clear.
Restarting the motor repeatedly without understanding the cause of failure can increase winding damage.
Glossary
- IE class: Motor efficiency class (IE1–IE4).
- IP code: Degree of protection against dust and water.
- Soft starter: A device that reduces starting current and mechanical shock by increasing voltage in a controlled way.
- Frequency inverter (VFD): A device that controls motor speed by changing frequency.
- Megger: An insulation resistance tester.
- Thermistor (PTC): A sensor that monitors winding temperature.
More Questions
How hot can a motor housing get?
Motors are designed for a certain temperature rise and the housing can be too hot to touch; what matters is deviation from the normal value. Regular comparisons should be made with thermography.
How are the weights on vibrating motors adjusted?
The unbalanced weights at the shaft ends are set equally at both ends according to the manufacturer’s scale. On twin-motor systems, both motors must have the same setting.
Is it necessary to keep spare motors?
Keeping spares for the motors of critical equipment such as the primary crusher and main conveyor prevents long stoppages. Standardising motor types reduces stock costs.
Support from CSP Mühendislik
CSP Mühendislik supports the sizing of motor and drive systems for crushing and screening plants, panel and automation infrastructure, the supply of belt, pulley and drive parts, and service, maintenance and repair. See our spare parts page for spare parts for your equipment and our products page for all our products.
