Speed Regulation of Hydraulic Motors - Part 1
FLUID POWER - Design Data Sheet 51
Speed regulation must not be confused with speed control of a
hydraulic motor. Speed regulation refers to the undesirable slow
down of the motor when it picks up a load. It is an indicator of
how well the motor will perform under changing loads and how wide
an adjustable speed range can be covered satisfactorily.
Speed regulation is not a fixed characteristic of a motor which
can be catalogued. Rather, it is a measure of the ability of a
motor to resist slowing down as load is increased. It is the result
not only of the motor characteristic but of the circuit in which it
is used. It depends on the speed control method, the brand or type
of flow control valve,and sometimes the characteristics of the
associated pump may influence speed regulation. If any of these
factors should change, the speed regulation also changes. So, while
the internal slippage of the motor may be the most influential
factor, other circuit components or operating conditions can also
contribute to good or poor speed regulation. Good circuit design
involves an understanding of these factors and how their effect can
be minimized when it is important to have the best possible speed
regulation. It is the purpose of this issue and the one following
to examine these factors.
Speed regulation is expressed as the percentage of speed loss
between no-load and full-load operation, and is different at each
no-load speed, being best at maximum speed and becoming
progressively worse as the no-load ·speed is reduced. Finally, it
becomes so poor at low speeds that the motor can no longer operate
A speed regulation of 0% would mean there is no loss of speed
whatsoever as load increased from zero to full. All motors, in
practice, suffer a speed loss under load increases, and the purpose
of good design is to hold this loss of speed to an acceptable
A speed regulation of 100% would mean that the motor would stall
as the load was increased.
How Important is Good Speed Regulation?
This depends on how important it may be on the particular
application to prevent motor speed from dropping below an
acceptable minimum as load is applied.
If the motor is running a load which remains constant at a given
speed, the speed regulation is of little importance. It is on
applications where the load may abruptly change, causing an abrupt
speed change, that good speed regulation may be one of the more
important design considerations.
How Does Speed Regulation Affect Speed Range?
If motor operation will be at a fixed speed, then speed regulation
will be less important than if an adjustable range of speed must be
covered, although it still may be important.
Speed regulation becomes of utmost importance on applications
where a wide range of adjustable speed must be covered and where
the load may change abruptly, particularly at lower speeds. Not
only must the hydraulic motor be carefully selected for these
applications, but the method of speed control and type of speed
control valve are critical.
Evaluation of the Application
To decide how important speed regulation may be on a given
application, these questions should be considered:
- Will the load on the hydraulic motor be reasonably constant or
will it be subject to abrupt changes of major magnitude?
- If abrupt load changes can occur, will they be a regular part
of the machine operation or cycle, or will they occur only
occasionally and under abnormal circumstances?
- Will these abrupt changes in load be more likely to occur or
will they occur more often in the upper range of adjustable speed,
or in the lower range of speed?
- If and when these abrupt load changes occur, how important is
it to machine operation or to the product being produced that speed
changes be minimized?
- If a speed change should occur as a result of an abrupt change
in load, and if this results in motor stall, how important would
- How wide a range of adjustable speed is to be covered, and will
the majority of operation be at maximum speed or at a greatly
llustration of Motor Speed Regulation
The above graph shows the performance of a hypothetical gear-type
hydraulic motor over a speed range of 0 to 2,000 RPM, and over a
load range of 0 to 100% of its rated torque (or horsepower).
Percentage torque loading is plotted along the left side of the
graph, speed along the bottom. Motor RPM is in direct proportion to
GPM. Lines curving upward to the left show motor performance at
several GPM flows in various tests. The leftward curvature shows a
loss of speed as torque load increases. This is due to internal
slippage. Notice that curvature of all GPM lines is about the same.
This indicates that internal slippage, in GPM, remains about the
same at all speeds but increases with load increase and is a higher
percentage at lower no-load speeds.
To interpret the graph, start at Point A. When a flow of 50 GPM
is supplied to the motor, it will run at a no-load speed of 2,000
RPM. As load is increased to 100% (full load), operation is at
Point B. With pick-up of full load, motor speed has dropped from
2,000 to 1,800 RPM because of internal loss of working oil through
According to the definition of speed regulation given on the
opposite side of this sheet, regulation would be:
200 (speed loss) ÷ 2,000
(no-load speed) = 0.10 or 10%
PLOT OF SPEED REGULATION FOR
||A to B
||C to D
||E to F
||G to H
||J to K
||L to M
Note from the chart that the actual amount of speed loss, 200
RPM, is approximately constant at all speeds. A loss of 200 RPM
does not affect motor performance much while it is running at 2,000
RPM, but has a tremendous effect on performance when the motor is
operating at speeds less than 800 RPM.
Speed Range of a Hydraulic Motor
To show how speed regulation becomes progressively worse at the
lower end of the speed range, the regulation has been calculated
for each of the GPM/Speed lines, and is shown in the far right
column of the chart below.
If performance charts like the above are available for the motor
under consideration for a certain application, its drop in speed
under various load increases can be estimated, and the range of
adjustable speed over which its performance will be acceptable, can
also be determined.
As a rule, a motor should be evaluated at the maximum load
change which can occur at minimum speed.
Example: Assume that on a certain application,
and using the motor graphed above, that a speed loss of 200 RPM is
all that can be tolerated from zero to full load when operating at
a reduced speed of 800 RPM. What is the maximum adjustable speed
range over which this motor can be operated with acceptable
Solution: Since 800 RPM is the minimum
acceptable speed of this motor under any range of adjustable speed
for the conditions stated, the only way to get a wider range of
adjustable speed is to raise the top speed. From 800 minimum to
2,000 RPM maximum is a range of 2½:1. From 800 up to 2,400 (if
possible) is a range of 3:1, etc.
Although motor performance is of prime importance for obtaining
good speed regulation, the type of speed control circuit and the
type of flow control valve used are very important also. In the
next issue we will give recommendations for improving speed
regulation when as wide a range of adjustable speed as possible
must be covered.
Download a PDF of Fluid
Power Design Data Sheet 51 - Speed Regulation of Hydraulic Motors -
© 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