The most demanding moment in driving a large crusher or mill electrically is the first start after the machine has stopped. Tonnes of rotating mass, the flywheel and the crusher jaw are all at rest, and to overcome this enormous inertia the motor must produce a very high starting torque. Moreover this is usually a quarry, a mine or a rural plant; the grid is weak, and the high starting current a normal motor draws lowers line voltage, adversely affecting both its own start and other loads nearby. This is exactly where a slip-ring (wound rotor) asynchronous motor driven by a liquid resistance starter (LRS) comes in. In this article we discuss conceptually how a soft start is achieved at high inertia and on a weak grid.

Slip-ring motor and liquid resistance starter LRS on a large crusher drive

Why Is High Inertia (GD²) a Problem?

The resistance a rotating system shows against being moved from rest is measured by the moment of inertia (usually expressed as GD² or J). In crushers, ball mills and crushers with large flywheels this value is very high, because heavy masses are gathered far from the axis of rotation. High inertia means the motor must produce high torque for a long time to bring the machine from standstill up to rated speed. This long and demanding start causes both excessive winding heating and prolonged high current in a normal squirrel-cage motor.

We covered the logic of motor selection for impact and high-inertia loads, and the flywheel-inertia relationship, in motor selection under impact load: flywheel and inertia. The way to select crusher motor power by crusher type is in crusher motor kW selection. We examined the high starting torque requirement of the main drive motor in ball and rod mills in ball and rod mill main drive motor.

Slip-Ring (Wound Rotor) Motor: Access to the Rotor Circuit

In a squirrel-cage motor the rotor consists of a short-circuited cage that cannot be accessed from outside; the starting behaviour cannot be adjusted externally. In a slip-ring motor the rotor consists of a three-phase winding whose ends are brought out through slip-rings. This makes it possible to add resistance to the rotor circuit from outside. The key point is this: when resistance is added to the rotor circuit, the motor produces high torque even at low speed while the starting current drawn from the grid drops. So a slip-ring motor can satisfy two normally conflicting demands at the same time: "high starting torque + low starting current".

We compared the basic difference between squirrel-cage and slip-ring motors, and which suits which load, in detail in squirrel-cage vs slip-ring asynchronous motor. You can find the relationship between speed, slip and actual speed in an asynchronous motor in speed, slip and actual speed, and the difference between rated and starting torque in starting torque and rated torque.

How Does a Liquid Resistance Starter (LRS) Work?

A Liquid Resistance Starter (LRS) is a resistance made of electrodes moving in a liquid electrolyte (usually a solution of water and salt/soda), connected to the rotor circuit of a slip-ring motor. At the moment of starting the electrodes are far apart; the resistance in the rotor circuit is high; so the motor starts with high torque but low current. As the motor accelerates the electrodes are slowly brought closer together and the liquid resistance decreases steplessly. When rated speed is reached the resistance is near zero and the rotor winding is short-circuited so the motor continues to run like a normal asynchronous motor.

The greatest advantage of the LRS is its ability to change resistance steplessly and smoothly. Unlike fixed-resistance stepped starters, because the liquid resistance decreases continuously, torque and current follow a smooth curve without jumps. This soft start reduces sudden stress on mechanical elements such as belts, couplings, gears and gearboxes, extending machine life. The liquid also absorbs and stores the heat released during starting; this is a major advantage especially in long starts at high inertia.

Electrodes of a liquid resistance starter and stepless resistance in an electrolyte solution

The Importance of Low Starting Current on a Weak Grid

Quarries, mines and rural plants are often at the far end of the grid; the line is long and the short-circuit power is low. On such a grid, the 5-7 times starting current a large motor draws in direct-on-line starting causes a serious dip in line voltage. This voltage dip can strain other motors on the same transformer, dim the lighting and even prevent the start from completing. We covered why the starting current (LRA) is high and the methods of reducing it in starting current (LRA) and its reduction.

The slip-ring motor + LRS combination brings the starting current down to levels close to the rated current, because the rotor resistance naturally limits the current. This both prevents voltage collapse on a weak grid and allows the large machine to start smoothly. If the crusher plant runs on a generator, low starting current is even more critical; we examined the role of starting current in generator selection in generator kVA and motor kW matching.

Comparison With DOL and Soft Starter

Direct-on-line (DOL) starting is the simplest and cheapest method, but it is not suitable for large machines with high inertia because of both excessive starting current and winding heating. A soft starter lowers the starting current somewhat by gradually raising the supply voltage; however, when the soft starter reduces the voltage the torque also falls (torque is proportional to the square of voltage). So a soft starter struggles with a high-inertia load that demands high starting torque; it may not bring the motor up to rated speed.

The slip-ring motor + LRS achieves the opposite: without reducing voltage, the rotor resistance both produces high torque and limits the current. That is why this is the classic solution preferred on very large crushers, ball mills and crushers with large flywheels. At smaller powers and low inertia, a soft starter or star-delta may be enough; we covered crusher motor starting options in crusher motor starting: soft starter and star-delta, and the general comparison of star-delta and soft starter in star-delta or soft starter.

The Starting Curve: Torque and Current Dancing Together

The best way to understand a motor's starting behaviour is to consider the torque-speed and current-speed curves together. In a squirrel-cage motor the current is very high at the moment of starting while the torque is relatively limited; as the motor approaches rated speed the current falls. On a high-inertia load this creates a contradiction: high torque is needed to turn the machine, but high torque only comes with high current, which both heats the winding and strains the grid. In a slip-ring motor, by adjusting the rotor resistance we can shift the peak of the torque curve to the speed we want. At the moment of starting, with high resistance, we obtain maximum torque right at zero speed; so the machine starts moving with strong torque from the very first instant. We covered the high torque and starting need on long-distance belt conveyors in mine overland belt conveyor motors, and the screen, feeder and belt motors other than the main crusher in screen, feeder and belt drive motors.

Field Conditions: Dust, Heat and Maintenance

A crusher plant is a dusty, humid and vibrating environment, and these cannot be ignored in motor selection. The slip-ring and brush system of slip-ring motors requires periodic maintenance; since dust can damage this system, protection and cleaning are important. We covered the dust sealing of the motor and IP65/IP66 protection in dust sealing in crusher motors, and the topic of dust-humidity-impact protection on the quarry site in motor protection on the quarry and mine site.

Cooling and overheating are also critical in a crusher motor running continuously at full load; we examined this in cooling and overheating in crusher motors, and bearing life under impact and dust in bearing life in crusher and mill motors. We gathered the importance of spare planning to reduce the downtime cost of a motor failure in motor failure and downtime cost on crushers. You can reach our robust cast-iron heavy-duty motors via our efficient electric motors page.

Frequently Asked Questions

Does every crusher need a slip-ring motor and LRS?

No. The slip-ring motor + LRS solution makes sense especially on large machines with high inertia and on weak grids. On small and medium-power crushers, where the grid is strong, a squirrel-cage motor with star-delta or a soft starter is often enough. The decision is made by evaluating the machine inertia, the required starting torque, the strength of the grid and the starting frequency together.

Is an LRS better than a soft starter?

It is more accurate to say "suited to a different job" than "better". A soft starter reduces current by lowering voltage but also reduces torque; so it can struggle with high-inertia loads that demand high starting torque. The LRS, without lowering voltage, both produces high torque and limits current through rotor resistance; that is why it is the classic choice on very large crushers and mills. On low-inertia loads with low torque demand, a soft starter is a simpler solution.

Is the maintenance of a slip-ring motor harder than a squirrel-cage one?

Yes, the slip-ring and brush system requires periodic maintenance, cleaning and brush replacement over time; a squirrel-cage motor has no such part. This extra maintenance is the price of the soft start and low starting current advantages the slip-ring motor provides at high inertia. In a dusty crusher environment, protecting and regularly cleaning the slip-ring area is especially important for motor life.

Get a Quote

If you want to correctly determine the motor and starting solution for your crusher, mill or high-inertia heavy drive application, our expert team is at your side. Share the machine inertia, required starting torque, grid condition and starting frequency; let us select the right motor together. Call +90 (532) 345 49 86 now or request a quote through our contact page. For heavy-duty motors, visit our efficient electric motors page and our hemmotor.com homepage.

Crusher Starting Solution Checklist

  • Have you determined the machine's moment of inertia (GD²) and the required starting torque?
  • Is the grid strong enough to carry the starting current, or is it weak?
  • Has the starting frequency (how many starts per day) been considered in method selection?
  • If high inertia and low starting current are needed, has a slip-ring motor + LRS been evaluated?
  • At low inertia, is a soft starter or star-delta enough?
  • If running on a generator, has low starting current been prioritised?
  • For a dusty environment, are the motor protection class (IP) and slip-ring protection planned?
  • Has a periodic maintenance plan been created for the slip-ring and brush system?