3-Phase, Induction-Type, Electric Motors for Pump Drive
FLUID POWER - Design Data Sheet 33
Data Sheet No. 3 shows how to determine motor HP
required to drive a hydraulic pump rated for (so many) GPM at a
certain PSI level. Additional information in the present issue
covers other important areas which may affect the selection of the
best motor type for a specific job.
Motor housings, fusing, thermal
overload protection, and motor starters will be covered in a later
The motor type used on most hydraulic
pump drives is the 3-phase, squirrel cage, induction motor, of
integral HP in the range of 1 to 500 HP. Information in this issue
applies only to this type and may not be applicable to other
3-Phase Squirrel Cage
This type motor has a rotor made up of iron laminations but does
not have a winding on the rotor; therefore, it has no brushes,
commutator, or slip rings. All windings are on the stator which is
also constructed of iron laminations with var ous numbers of north
and south poles (in pairs). The motor runs at a constant speed
determined by the line freqequency (Hertz) and by the number of
pairs of magnetic poles which it has. Except for a small amount of
speed slip at full load condition, it will not run at slower speeds
with out severely overheating.
|Design B Motor
Speeds - Synchronous and Full Load
@ 60 Hz
@ 60 Hz
@ 50 Hz
@ 50 Hz
Full load RPM in the chart has been
calculated on a drop in speed (slip) of about 3% from the
theoretical or synchronous speed.
Current and Voltage
Motor Current. Torque is produced by current
flow; the higher the current the greater the torque output. Current
is also responsible for temperature rise in the windings. Any
operating condition such as low voltage, wrong frequency, or torque
overload, which causes current in excess of nameplate rating to
flow, will cause an abnormal temperature rise.
Design B motors (most often used on
pump drives) can start under full load, but if they must be started
frequently, the pump should be unloaded until the motor starts, to
prevent high starting current from overheating the motor.
Effects of Low
Voltage. Nameplate HP rating is based on full voltage
being available. HP output is a combination of voltage times
current. If the voltage is too low, then to produce rated HP the
current becomes too high, and this causes an abnormal temperature
rise. Motors can usually accommodate as low as 90% of rated
voltage, and although there will be an abnormal temperature rise,
it will not be great enough to damage insulation. For permanent
operation on a voltage source known to be low, the HP rating should
be reduced by the same percentage that the voltage is low.
Example: A 25-HP, 220
volt meter on a 208-volt line has only 94½% of its rated voltage.
Therefore, it should be derated to 0.945 × 25 = 23.6 HP (plus
service factor if applicable).
Effects of High
Voltage. If the motor is not loaded beyond nameplate HP,
the full load current will be lower than rating and the motor will
run cooler than its rating. However, its starting current and
breakdown current (at stall) will be higher than normal. The
wiring, fusing, and thermal overload protection will have to be
sized accordingly. Also, the motor noise will greatly increase, and
may be objectionable.
Voltage Test. On
installations where the motor s running at or very near full HP, an
unbalance of as little as 3½% between the highest phase voltage and
the average of all three voltages may result in a temperature rise
of about 25% above the normal rated rise, causing damage to the
If the voltage, at full load, is
unbalanced between phases, either the motor is defective or the
power line is unbalanced. To determine where the fault lies, first
measure the voltage of all phases. Then, advance all power lines by
one phase and repeat the measurements. If the higher voltage
advances with the re-connection, the power line is unbalanced.
Corrective measures may be taken as follows:
Check for voltage unbalnce of each
phase where the power line enters the building. If unbalanced more
than 3½% at that point, call the utility company for an inspection
and corrective measures.
With the motor running at full load,
compare the voltage of each phase at the motor with voltage
readings taken at the power line entrance. If the voltage loss in
any phase is more than 3% check for hgh resistance in wiring,
connections, fuses, circuit breaker, or disconnect switch.
table shows nominal voltage for which polyphase motors are usually
made, and the maximium voltage range over which they can be
operated (10% variation from nominal rating).
||104 to 126
||1 to 15
||180 to 220
||1 to 500
||207 to 253
||1 to 500
||207 to 253
||414 to 506
||414 to 506
||1 to 500
||518 to 632
||1 to 500
||444T & Up
|*Over-voltage (at higher noise) can be tolerated
better than under-voltage provided current is limited to nameplate
The motor magnetic structure and- windings are designed to obtain
certain desired characteristics of torque and speed. Four NEMA
designs are available as follows:
Design B. This type
is the most often used for hydraulic pump drives but does have some
limitations: Starting torque required by the load should not exceed
50% of the motor rated torque; the load reaction should have little
or no torque pulsation; load inertia should be no greater than the
inertia of the motor rotor; the motor should work against a fairly
steady load with infrequent starting and stopping.
Design D. This design
may be preferred if starting torque is greater than 50% of rated
motor torque. Also, when there may be severe and frequent changes
in the torque load.
There are several variations of Design
D motors, but all of them have a slip in speed of more than 5% (as
compared with less than 3% on a Design B motor). Those having a 5
to 8% slip are reasonably obtainable, but those having a higher
slip, up to 13%, should be considered as special order items and
may require extended delivery time.
Design D motors are sometimes used to
"peak out" a hydraulic pump at a pressure which would severely
overload and damage a Design B motor. The slip in speed under full
load or overload reduces the input HP and the line current.
Designs A and C.
These are seldom used for pump drives. They are capable of starting
full torque loads, but line current may be extremely high,
requiring special and expensive starting equipment.
Effects of Incorrect
Most hydraulic systems are operated from a utility-company power
line on which the frequency is closely controlled. Where operation
is from a small, isolated power source, the frequency must be
accurate to within 5% of the motor rating to obtain full motor
If a 60 Hz motor is to be operated
from a 50 Hz power source, or vice versa, significant sacrifices
must be made in motor performance as shown in this chart:
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. Motor current is always a
limting factor on a variation in rated frequency or voltage.
Any 3-phase induction motor can be switched directly across full
line voltage for starting but this produces a very high current
surge in the line. Utility companies have regulations which limit
the current surge and voltage fluctuation which can be imposed on
the power line during motor starting. Usually, motors of 50 or more
HP must be started at reduced voltage to limit the current
transient. Several types of reduced voltage starters are
In addition to the current surge
produced wh!m a motor is connected directly across the line, the
starting shock may be too severe for some types of loads, and
reduced voltage starting may be necessary even on small motors.
The published service factor (usuaily 1.15 × nameplate HP on
continuous duty for motors up to 200 HP) may be used, but only if
operating on the correct frequency and on no more than 3% above or
below rated voltage, and if operating under all normal
environmental conditions as follows:
- In an ambient temperature
no higher than 40°c, nor colder than 0°c.
- At an altitude no higher
than 3,300 feet, nor lower than sea level, nor in a pressurized or
evacuated space which results in pressures outside those
- Installed properly on a
rigid base, in a location which provides free and unrestricted
circulation of clean, dry, cooling air, and where it can be
periodically inspected for lubrication, and given proper
Operation of a motor under conditions causing a higher than
rated temperature rise in the windings may shorten the life of the
insulation by one-half for a 10°C extra rise.
In addition to the usual precautions against electric shock the
motor frame should be earth grounded. If ground is not carried in
with the power wiring a separate ground wire, connected to the
motor frame should be run to an outside ground rod. It is not good
practice to ground to a water or gas pipe.
Guards should be placed over rotating
parts such as couplings, sheaves, or gears connected to the motor
shaft, to prevent clothing of personnel from entanglement.
A motor may be overlOaded forsnort periods. Data Sheet No. 3
suggests limits for overloading. Excessive line current, far out of
proportion to the increase in HP output flows during overloads. For
example, a Design B motor overloaded to 150% rated HP may draw
about 4 times its normal full load current.
Overheating. It is the current through the
windings which causes a temperature rise. Motor will not overheat
even if run on abnormally high or low voltage or on an incorrect
frequency if current is kept to the maximum shown on the nameplate.
This means that if voltage and frequency are not within specified
limits, the HP load must be reduced as much as necessary to limit
the current to nameplate value.
Motor may overheat from being started
too frequently, or from being "plugged" for quick stop or
Insulation breaks down prematurely under conditions of voltage,
frequency, or load which cause an abnormally high temperature rise
in the windings.
with sleeve or roller bearings must be mounted with shaft within 5
to 10 degrees of horizontal. Motors with shaft vertical must have
ball bearings. Unusually heavy side loads, especially when using
small diameter gears or sheaves, will reduce bearing life. Motors
carrying heavy side loads should have roller bearings.
Download a PDF of
Fluid Power Design Data Sheet 33 - 3-Phase, Induction-Type,
Electric Motors for Pump Drive.
© 1990 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.