understanding gear motor torque and speed relationship
The Relationship Between Torque and Speed in Geared Motors: Mastering the Core Formula for
Successful Selection
Torque and speed are the two most crucial parameters of a geared motor. Understanding
their inverse relationship
is fundamental to correct selection and preventing equipment from being unable to run
smoothly or too fast.
I. Core Formula: A Curve That Determines Everything
Under ideal conditions where power loss is negligible, the motor's output power, torque,
and speed strictly follow this formula:
[P = T × n = 9550]
- **P**: Power (kW)
- **T**: Torque (Nm)
- **n**: Speed (rpm)
**The Truth Revealed by the Formula:** When power is constant, torque is inversely
proportional to speed.
**Lower speed results in higher torque; higher speed inevitably leads to lower torque.**
This is an ironclad physical law for all rotating machinery.
II. What Does a Gear Reducer Do? —Trading Speed for Power
A speed reducer, through a gear ratio **i = n_in / n_out**, "exchanges"
the high speed of the motor
for the low speed and high torque required by the load.
Ignoring gear friction, the output torque is directly amplified:
[T_{out} \approx T_{motor} \times i
Considering efficiency:
[T_{out} = T_{motor} \times i \times \eta
Example: *A 4-pole motor with a rated torque of 10 Nm and a speed of 1450 rpm,
paired with a helical gear reducer
of i=10 (η≈0.94), can achieve an output torque of 10 × 10 × 0.94 = 94 Nm and
an output speed of approximately 145 rpm. A 90% reduction in speed yields
more than nine times the power.
III. Understanding Torque-Speed Characteristics
**1. Motor Side (Input)** AC induction motors or servo motors typically
operate in the constant torque region:
from zero speed to rated speed, the torque remains essentially constant.
Beyond the rated speed,
they enter the field weakening region, and the torque decreases
according to the curve.
**2. Gearbox Output Side** The output characteristic is the motor characteristic
"scaled" by the reduction ratio.
The constant torque region is amplified, while the constant power region
is compressed. Therefore,
**when actually using a geared motor, the long-term operating point
must fall within the constant torque region**,
and you cannot expect to output rated torque at the highest speed.
IV. Torque-Speed Conflicts in Selection
- **High Torque Requirement**: Either select a higher power motor or
increase the reduction ratio. However,
increasing the speed ratio will reduce the output speed, potentially failing to
meet the cycle time requirement.
- **High Speed Requirement**: The reduction ratio needs to be reduced,
but the output torque capability will decrease simultaneously.
- **Solution**: First, meet the torque requirement (enough to drive the motor),
then check the speed.
If both cannot be achieved simultaneously, the frame size or power
rating must be increased by one level.
V. Special Considerations for Servo Applications: Coupling of Inertia and Speed
When starting and stopping a servo motor at high speed, it's crucial to not only
verify the torque-speed curve but also
ensure inertia matching. Increasing the reduction ratio lowers the equivalent
inertia and improves dynamic response,
but sacrifices maximum speed. A balance must be struck between
bandwidth and motion time.
**In short:** Speed is the indicator, torque is the root. The fate of torque and
speed is determined the moment the reduction ratio is selected.
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Need specific torque-speed verification?
Send us your load torque, target speed, and operating cycle.
Our engineers will draw a complete
torque-speed envelope and accurately recommend a geared motor model.
**Contact us now for a 1-on-1 torque-speed matching and selection report.**
