How to Measure Motor RPM with UNI-T UT372D Tachometer Malaysia
Checking a motor’s actual rotational speed can help maintenance technicians confirm whether the machine is operating according to its rated speed or control settings.
The **UNI-T UT372D 2-in-1 Tachometer** supports both non-contact optical measurement and contact measurement. For most running motors, the non-contact method is the safer and more convenient starting point.
## Why Measure Motor RPM?
A motor may continue running even when its actual speed is different from the expected value.
RPM measurement may be required during:
- Motor installation and commissioning
- Preventive maintenance
- Variable-frequency drive setup
- Belt and pulley inspection
- Pump and fan troubleshooting
- Motor replacement
- Production-speed verification
An abnormal reading does not automatically mean that the motor is defective. The actual speed may also be affected by load, drive frequency, belt condition or the driven equipment.
## Check the Motor Information First
Before measuring, inspect the motor nameplate and record:
- Rated speed in RPM
- Rated frequency
- Number of motor poles, if stated
- Rated voltage
- Whether a variable-frequency drive is installed
- Expected operating load
A standard induction motor normally operates slightly below its theoretical synchronous speed when carrying a load. Therefore, a small difference between rated and measured speed may be normal.
The measurement should be compared with the motor specification and operating conditions, not judged by RPM alone.
## Method 1: Non-Contact RPM Measurement
Non-contact measurement uses the UT372D optical sensor and a reflective target.
This method is suitable when the motor shaft, coupling, pulley or fan hub is visible while the machine is operating.
### Step 1: Stop the Machine Safely
Switch off and isolate the machinery before applying reflective tape.
Never attempt to place tape on a rotating shaft, coupling or pulley.
### Step 2: Clean the Target Area
Select a visible part that rotates at the same speed as the component being checked.
Clean a small area so the reflective tape can attach securely. The surface should not be oily, dusty or wet.
### Step 3: Apply One Reflective Mark
Apply one small piece of reflective tape to the rotating component.
Using one reflective mark allows the tachometer to recognise one pulse as one complete revolution.
If several reflective marks are detected, the displayed RPM may be multiplied and become incorrect.
### Step 4: Restart the Machine
Ensure that all guards, tools and personnel are clear before restarting the equipment.
Do not operate machinery without its required safety guards merely to obtain an easier measurement.
### Step 5: Select Non-Contact RPM Mode
Set the **UNI-T UT372D** to the optical RPM measurement mode.
Aim the sensor at the reflective mark. The UT372D supports a target distance of approximately **50 to 500 mm**, but the actual usable distance can depend on the target size, surface condition and surrounding light.
### Step 6: Hold the Instrument Steady
Keep the tachometer aligned with the reflective mark until the reading stabilises.
Record several readings instead of relying on a single momentary value. The UT372D provides maximum, minimum, average and last-reading functions that can help assess speed stability.
## Method 2: Contact RPM Measurement
Contact measurement is performed by placing the correct tachometer attachment against the end of an accessible rotating shaft.
This method may be useful when:
- Reflective tape cannot be applied
- The shaft end is accessible
- Optical reflection is unstable
- The component rotates at a suitable speed
- Contact can be made without entering a hazardous area
The UT372D contact measurement range is **1 to 19,999 RPM**.
## How to Perform Contact Measurement
Before using contact mode:
1. Inspect the shaft and confirm that it is suitable for contact.
2. Install the correct rotational-speed attachment.
3. Select contact RPM mode.
4. Maintain a stable and straight position.
5. Apply only enough pressure to maintain reliable contact.
6. Wait for the reading to stabilise.
7. Remove the instrument carefully from the rotating component.
Do not perform contact measurement if the shaft is inaccessible, unstable, unguarded or unsafe to approach.
**Use non-contact mode whenever physical contact creates unnecessary risk.**
## Why Is the RPM Reading Unstable?
An unstable RPM reading may be caused by:
- Reflective tape that is too small or damaged
- More than one reflective mark on the target
- Excessive ambient light
- Incorrect measuring angle
- Hand movement
- Changing motor load
- Variable-frequency drive speed changes
- Belt slipping
- An unstable rotating component
Repeat the measurement from a stable position and confirm that only one clear reflective target is detected.
If the motor speed itself is fluctuating, inspect the electrical supply, motor controller, mechanical load and driven equipment.
## Understanding the Result
The RPM reading should be compared with:
- Motor nameplate information
- Variable-frequency drive setting
- Pulley or gearbox ratio
- Machine operating specification
- Previous maintenance records
- Actual production speed
For belt-driven equipment, motor RPM and driven-shaft RPM may be different because of the pulley ratio.
For geared equipment, the output shaft may rotate much more slowly than the motor. Always confirm which component is being measured.
## RPM Measurement Does Not Diagnose Every Fault
The UT372D can confirm whether a motor or shaft is rotating at the expected speed. It cannot independently identify:
- Bearing damage
- Shaft misalignment
- Mechanical imbalance
- Electrical insulation deterioration
- Abnormal motor temperature
- Internal winding problems
For a more complete motor assessment, technicians may also require:
- Vibration meter
- Clamp meter
- Insulation resistance tester
- Infrared thermometer or thermal imager
- Power-quality or motor-analysis equipment
The required instruments depend on the observed symptom and the importance of the machine.
## Typical Malaysian Factory Applications
The UNI-T UT372D can support motor-speed checks for:
- Water pumps
- Cooling-tower fans
- Air-handling units
- Exhaust and ventilation fans
- Air compressors
- Conveyor systems
- Packaging machinery
- Printing and textile equipment
Factories in Selangor, Kuala Lumpur, Johor and Penang can use RPM measurements as part of commissioning, preventive maintenance and fault investigation.
## Choosing a Suitable Tachometer
Choose the **UNI-T UT372D** when both optical and contact measurements may be required.
Before purchasing, send MTM Precision a photo of the motor, shaft, pulley or machine to be measured. We can help determine whether non-contact RPM, contact RPM, surface-speed measurement or another testing instrument is more appropriate.
**MTM Precision Sdn Bhd**
**Showroom & Service Centre:** No. 29-1 & 29-2, Jalan Bandar 18, Pusat Bandar Puchong, 47160 Puchong, Selangor, Malaysia
**Tel:** 03-8080 7172
**WhatsApp:** +6016-660 7346
**Email:** mtmpre@yahoo.com
**Website:** www.mtmpre.com.my
28 Sep 2026
