Introduction
the prime mover, such as a fixed speed electric motor or a diesel/petrol engine, drives a
hydraulic pump to transfer fluid to a hydraulic motor. The output speed can be adjusted
by controlling the fluid flow rate or pressure. The two different types outlined below are
characterized by the method employed to achieve the speed control.
1.6.1 Hydrodynamic types
Hydrodynamic variable speed couplings, often referred to as fluid couplings, are
commonly used on conveyors. This type of coupling uses movable scoop tubes to adjust
the amount of hydraulic fluid in the vortex between an impeller and a runner. Since the
output is only connected to the input by the fluid, without direct mechanical connection,
there is a slip of about 2% to 4%. Although this slip reduces efficiency, it provides good
shock protection or soft-start characteristic to the driven equipment. The torque
converters in the automatic transmissions of motor cars are hydrodynamic fluid
couplings.
The output speed can be controlled by the amount of oil being removed by the scoop
tube, which can be controlled by manual or automatic control systems. Operating speed
ranges of up to 8:1 are common. A constant speed pump provides oil to the rotating
elements.
1.6.2 Hydrostatic type
This type of hydraulic VSD is most commonly used in mobile equipment such as
transportation, earthmoving and mining machinery. A hydraulic pump is driven by the
prime mover, usually at a fixed speed, and transfers the hydraulic fluid to a hydraulic
motor. The hydraulic pump and motor are usually housed in the same casing that allows
closed circuit circulation of the hydraulic fluid from the pump to the motor and back.
The speed of the hydraulic motor is directly proportional to the rate of flow of the fluid
and the displacement of the hydraulic motor. Consequently, variable speed control is
based on the control of both fluid flow and adjustment of the pump and/or motor
displacement. Practical drives of this type are capable of a very wide speed range,
steplessly adjustable from zero to full synchronous speed.
The main advantages of hydrostatics VSDs, which make them ideal for earthmoving
and mining equipment, are:
• High torque available at low speed
• High power-to-weight ratio
• The drive unit is not damaged even if it stalls at full load
• Hydrostatics VSDs are normally bi-directional
Output speed can be varied smoothly from about 40 rev/min to 1450 rev/min up to a
power rating of about 25 kW. Speed adjustment can be done manually from a hand-wheel
or remotely using a servo-motor. The main disadvantage is the poor speed holding
capability. Speed may drop by up to 35 rev/min between 0% and 100% load.
Hydrostatic VSDs fall into four categories, depending on the types of pumps and
motors.
• Fixed displacement pump – fixed displacement motor
The displacement volume of both the pump and the motor is not adjustable.
The output speed and power are controlled by adjusting a flow control valve
located between the hydraulic pump and motor. This is the cheapest solution,