Pressure and flow ranges in the table will cover most
applications, but intermediate values can be estimated by combining
values in the table. Example: HP required at 4500 PSI is the sum of
values shown in the 2000 and 2500 PSI columns. At 73 GPM, for
example, HP will be the sum of values in the 3 and 70 GPM lines. At
10,000 PSI, power is twice the value at 5000 PSI at the same
For pressures less than 500 PSI, HP calculations
tend to become inaccurate because mechanical friction and fluid
flow losses are a greater percentage of applied pressure.
Approximate power requirements can be estimated with
the "Rule of 1500" which states:
1 HP is required for each 1 GPM at 1500 PSI or any
multiple of 1500 such as 3 GPM at 500 PSI, 2 GPM at 750 PSI, 1/2
GPM at 3000 PSI, etc.
Another handy rule-of-thumb is that it takes
about 5% of the pump maximum rated HP to drive the pump when
unloaded and pressure is very low. This power is consumed in
bearing losses and fluid flow losses.
Overloading of a Motor
The motor most often used to drive a hydraulic pump
is a 3-phase, induction-type, Design B motor. The service factors
on most open frame sizes is 0.15 which means that the motor can be
overloaded about 15% above current shown on its nameplate, assuming
it is being operated in a normal temperature environment. TEFC and
explosion proof motors have a service factor of 1.0.
An electric motor can be overloaded for short
periods during the cycle provided the average horsepower is no
greater than its nameplate rating plus service factor where this
The amount of intermittent overloading is up to
the user, but we suggest the overload be no more than 25% above its
nameplate current rating sustained no longer than about 10% of the
time required for a complete cycle.
Operation on Incorrect
Most A-C 60 Hz motors can be operated on a 50 Hz
line and vice versa, but adjustments will have to be made in the
current, HP, and speed ratings. The important thing to remember is
that it is the current which causes heating. The HP which can be
produced will be related to its current draw, and may be more or
less than its nameplate rating.
||60 Hz Motor on 50 Hz Line
||50 Hz Motor on 60 Hz Line
|HP will be:
|Adjust voltage to:*
|Full load torque:
|Locked rotor torque:
|Locked rotor current:
|Max. service factor:
*Voltage adjustment is to maintain current at
rated value, to produce rated shaft torque. Current is always a
limiting factor on a variation in rated Hz (frequency) or
Effects of Low
Nameplate HP is based on full voltage being
available. HP output is a combination of voltage times current. If
voltage is too low, then to produce rated HP the current must be
too high, and this overheats the motor. Motors can usually
accommodate as low as 90% of rated voltage and still produce
nameplate HP although temperature rise in the windings will be
greater than rated rise. For permanent operation on a voltage
source known to be low, the HP load should be limited, and reduced
by the same percentage that the voltage is low.
Effects of High Voltage
If motor load does not exceed nameplate HP
rating, full load current will be lower than nameplate rating and
the motor will run cooler than rating. However, its starting and
breakdown current (at stall) will be higher than normal. The
wiring, fusing, and thermal overload protection will have to be
sized accordingly. Motor noise will increase.
Using an Oversize Motor
Using a 20 HP motor on a system which requires
only 10 HP, for example, will give good results for running the
pump but will consume more electricity than a 10 HP motor and will
cause the power factor of the plant electric system to be poorer,
especially during periods the motor is idling. Idling current of a
20 HP motor is about half the full load current of a 10 HP motor.
This is an extra power waste during periods in the cycle when the
pump is idling.
Using an Undersize Motor
Using a 20 HP motor on a system which requires 25 HP
for brief periods is quite possible, but during overload periods
the current of such a motor maybe-about twice the current of a 25
HP motor. There will be an extra waste of power during peak periods
in the cycle. But the smaller motor could more than make this up
during periods in the cycle when less than 20 HP is required.
Some power companies charge for power by the
number of kilowatt-hours used. But they may base their rate on the
maximum current flow at any moment during the metering period. A
poorer power factor in the plant increases current flow without
increasing HP delivered. Therefore, either oversizing or
undersizing a pump-drive electric motor will increase the cost of
running the system.
NEMA Standard Terminal Markings and
Connections for 9-Lead, 3-Phase Motors
These diagrams show which motor leads must be
wired together inside the motor junction box for operation of the
motor on its highest or lowest voltage as stated on its nameplate.
Normally, on a star connected motor the center point of the
windings is brought out for connection to ground.
||1 & 6 & 7
||2 & 4 & 8
||3 & 5 & 9
||4 & 7, 5 & 8, 6 & 9
© 1988 by Womack Machine Supply Co. This company assumes no
liability for errors in data nor in safe and/or satisfactory
operation of equipment designed from this information.