www.wwtonline.co.uk | OCTOBER 2018 | 11
on the network to improve quality.
With submersible pump systems,
when using a VSD and running the motor
at low speed in systems with a high static
head, users have to be careful not to move
too close to the shut-off point as this risks
stopping the flow through the pump.
This may typically happen to pumps that
pump out of a tank which has a variable
input.
It may be tempting to implement
the very simple control philosophy of
maintaining a fixed level in the tank, but
this will only work if the inflow meets
the minimum flow requirements of the
pump. If the inflow does not meet the
minimum flow requirement, the pump
could be severely damaged. Avoid this
by determining the minimum acceptable
running speed of the pump and then use
this as the minimum speed setting for the
drive.
Some other points to observe:
Drives enable 'over-speed': Drives are
generally used to reduce speed, but they
may also be used to increase speed above
the motor's nominal level, for example if
greater extraction is required due to water
being unavailable from other sources. It is
therefore necessary to check the limits for
both pump and motor.
Reverse rotation can cause tripping:
Unless a non-return valve (NRV) is
installed in the discharge, reverse rotation
may occur on shut-down. By using the
ramp stop feature of the VSD, mechanical
stresses as a result of the NRV slamming
shut can be significantly reduced.
Dimension according to motor plate
data: A VSD is a source of current and
must always be selected based on the
motor name plate current, with special
attention being given where output filters
or long cables are involved.
Motor insulation: Variable speed
operation puts a higher voltage stress
on the motor insulation system. The
insulation system must be checked and, if
it is not suitable for VSD use, a filter must
be placed between the VSD and motor.
Remote monitoring reduces
maintenance costs: Motors are o‰en
equipped with sensors to enable remote
condition monitoring, which must
operate without disturbance from the
drive. Monitoring of motor winding
temperatures is particularly important, as
ageing and lifetime are closely related to
temperature. Operating at 10°C above the
thermal class temperature will halve the
winding lifetime.
