Liquid Resistance Starter

LRS for large motors up to 12 MW

  • Rugged and Reliable

  • Cheapest option for large drives

  • Power Savings

  • Customized for each application

  • Smooth progressive acceleration


Pioneer Electrical Works, started manufacturing slipring motor starters, in a modest way, in 1967. Nearly a quarter century later, PIONEER has for the first time in India, manufactured 5000 kW liquid resistance slip ring motor rheostats.

Products developed at PIONEER must represent genuine technical advance. Due to this philosophy no challenge has been too great for PIONEER, and like a David, we have always risen to challenges however Goliath in size or complexity be the drive to be controlled.

This web-page gives you details of our range of liquid resistance rheostats type LRS, however, it is not meant as a substitute for a personal meeting. It is intended to set the stage to create an interest in our products, leading to personal interaction, thereby enabling us to extend to you the pre and post engineering services, which are the hallmark of PIONEER.

Not only are high quality competitive range of products offered by PIONEER, but out traditional strengths are the engineering services that back our products.


The range of PIONEER liquid rheostats – type LRS – is designed for controlling of large KW slip ring motors and extends, up to 10,000 kW, or any higher rating required.

PIONEER type LRS rheostats unique design, with horizontal movement of the moving contact system, enable a wide variation of the resistance. This coupled with current sensing relay which synchronizes the change of resistance to the motor acceleration, ensures smooth progressive acceleration of the drive.

With the use of Computer Aided System Engineering (CASE), wide range of specific requirements can be created to including twin drive rheostats for large drives and speed variation by control slip.

PIONEER type LRS rheostats incorporate:

– Tank for electrolyte – available in discrete frame sizes, according to KW rating,

– Horizontal movement contact system, and

– Control panel,

All mounted on a common frame.

Electrolyte Tank

The tank for the electrolyte is of welded construction, made from mild steel of adequate thickness and strengthened by gussets – where needed. The inner surface of the tank is lined with FRP or Epoxy coating to provide greater corrosion resistance.

The tank can be filled from the top and drained through a drain plug provided at the bottom of the tank. Level gauge, with over flow spout, is provided for visual indication of the electrolyte level.

Rheostat System

Three mild steel electrodes, mounted on a channel to form star point, move on two parallel guide rails between the mild steel fixed electrodes. The fixed electrodes are connected by copper bus to the rotor terminals. The area of the electrode system plates are designed for low current density of 1 amps/cm2. A 0.37 kW, induction motor coupled by a worm reducer to a chain and sprocket arrangement moves the electrode carriage on the horizontal guide rails. The motor is coupled through a clutch arrangement which enables emergency manual operation of the electrode carriage by hand wheel provided for this purpose.

On completion of the contact carriage system are signaled by limit switches, which also serve to interlock the carriage movement so that it remains in the resistance shorted position, so long as the controlled motor is operating, and returns carriage corresponding to greatest resistance position, when motor is switched off.

Polypropylene barriers between phase fixed to one end of the tank, increase insulation levels and reduce cross currents. The other end of the barrier being free allows circulation of liquid to maintain temperature and concentration of electrolyte uniform.

Adequately dimensioned cable box houses the rotor terminals which can conventionally accept two or four runs of cable terminated on L shaped tinned copper terminal.

Control System:

The control system is housed in a control panel constructed out of sheet and having ingress protection IP54/55. The pilot motor speed of the moving contact system control by Variable Frequency Drive. The acceleration time of main motor can be controlled by varying the frequency of piolot motor.

A supervisory timer is provided to switch off the main motor in case the starting time is abnormally prolonged. Low level & High Temperature indication and interlock with main motor starting are also provided. Optional control equipment can also be incorporated on request.



Fitted as Standard:

  • ✓ Auto/Manual operation

  • ✓ Rmax and Rmin monitoring

  • ✓ VFD to set precise acceleration time of main motor

  • ✓ Gearbox with clutch-declutch arrangement

  • ✓ Hand wheel for manual operation

  • ✓ Temperature and Level monitoring

  • ✓ Control and Contactor panel at least IP54 compliant

Accessories/Spare list:

  • Fix and Moving Electrode set

  • Phase barrier set

  • Insulators

  • Pilot motor with gear box

  • Power contactor’s contact kit

  • Temperature and Level switch


  • Circulating pump (Electrolyte mixer)

  • Slip/buffer resistors (useful for Crusher/chopper, etc)

  • Heat exchanger for high number of starts (electrolyte-water or electrolyte-air)

  • Anti freeze heaters in tank for lower ambient

Required details to quote:

  • Motor Power (P, KW)

(The largest driving factor for the size of the LRS is the motor Power)

  • Rotor Voltage (U2, V)

(According to motor datasheet/nameplate. This data is required for the layout of the switchgear and bus bars)

  • Rotor Current (I2, A)

(According to motor datasheet/nameplate. This data is required for the layout/selection of the switchgear and bus bars)

  • Motor rated Speed

  • Type of Driven machine

  • Single or Twin Drive

  • Load Torque curve of driven machine

  • Number of consecutive starts (z)

  • Desired number of starts per hour (h)

  • Starting torque and starting time (ta, s)

Please specify special requirements, if any.


To start the slipring motors of following:

  • Milling (Cement mill, Ball mill, SAG mill, Vertical Roller Mill, hammer mill, etc..)

  • Crusher (jaw crushers, cone crushers and gyratory crushers)/Chopper/Shredder

  • Ventilation (all type of fans/blowers)

  • Pumps

  • Compressors

  • Conveyors


  • Cement & Mineral

  • Mining

  • Steel

  • Sugar

Do's and Don'ts:


  • Take the control trial first, then no load trial and after successful completion of no load trail proceed for load trial

  • Use Sodium carbonate (Na2CO3) with water for electrolyte


  • Don’t use caustic soda (Sodium hydroxide / NaOH) to prepare the electrolyte.