Electrical Test & Measurement · Electrical Test Equipment
Benchtop tester measuring the three winding resistances of a three-phase motor from 10 µΩ to 20 kΩ, calculating spread and unbalance for star or delta connection and sorting against limits.
CXT3513 ·
A three-phase winding is symmetrical by design, and almost every fault that matters breaks that symmetry. The tester exists to measure the three resistances the same way, at the same moment, and to turn the comparison into a decision.
One instrument, three Kelvin channels, one colour screen. Each channel forces a current through a winding and senses the voltage across it separately, so what appears on the display is the winding and not the leads.
The screen is arranged around the comparison rather than around a single reading: line resistances in one column, phase resistances in the next, and the spread and unbalance for both underneath. The sorting result is on the same screen, so an operator does not look away to find it.
The case is a bench instrument with the input sockets along the lower edge and the keypad to the right. It is meant to sit at a test station, not to be carried to a motor in the field.
Resistance is measured by forcing a known current and reading the voltage that appears across the winding. The four-terminal connection is what makes this work at small values: one pair of leads carries the current, a second pair senses the voltage directly at the winding terminals, and the voltage drop along the current leads never enters the reading.
Three channels are measured in turn or in a scan. Because the same current source and the same conversion path are used for all three, the readings are comparable with each other in a way that three separate measurements are not. That comparability is the point: the unbalance is a difference between readings, and a difference is only meaningful if the readings share a reference.
The instrument converts between line and phase resistance according to the connection setting, computes the spread as the difference between the highest and lowest of the three, and expresses the unbalance as a percentage. Which of the two is judged is a setting, because different acceptance documents use different ones.
Temperature compensation works from a measured or entered ambient and converts the reading to a stated reference temperature. Winding resistance rises with temperature, so a motor tested warm will read high against a cold specification unless the reading is referred back.
The range decides whether the instrument fits at all. A small servo winding and a large stator are at opposite ends of the same scale, and the bottom of the range is where the four-terminal connection and the zeroing behaviour matter most.
The acceptance criterion decides the comparator setup. If the drawing states a percentage unbalance, judge on unbalance; if it states a maximum difference in ohms, judge on spread. Setting both and judging on one is a common source of argument at the station.
The line decides the interfaces. A manual station needs the display and the buzzer. A station with a handler needs the sorting signals. A line that records results needs the serial link or a USB stick, and a line that scans more than three channels needs the optional RS485 with several instruments cascaded.
The throughput decides the rate. The fast setting suits a line that tests every motor; the slow setting suits a laboratory check where the reading is recorded.
The instrument measures resistance, not inductance or insulation. A shorted turn in a winding may barely change its DC resistance, especially in a large motor, so a resistance unbalance within limits does not clear a winding of every fault. An impulse or surge test is the method for turn-to-turn insulation, and it answers a different question.
The reading depends on temperature, and the compensation only removes that dependence if the temperature used is the winding temperature rather than the air temperature. A motor that has just run is warmer than the room, and the difference is large enough to matter.
Kelvin clips have to be on clean metal. Paint, oxide and varnish at the terminal add resistance in the current path or, worse, in the sense path, and a bad sense connection shows up as a reading that drifts or jumps between scans.
Finally, the unbalance figure is only as good as the three readings being simultaneous in condition. A winding that is still cooling between the first and third channel will show an unbalance that is an artefact of the test sequence rather than of the motor.
Every value below is taken from source documentation. Where a parameter is not stated in the available material it is marked as information required rather than estimated.
| Range | 10 µΩ to 20 kΩStandard |
|---|---|
| Basic accuracy | 0.05%, with the detailed specification given in the operating manualStandard |
| Test current | ≤ 100 mAStandard |
| Connection | Four-terminal Kelvin, one pair of current and one pair of voltage leads per channelStandard |
| Channels | Three, with single-channel or three-channel scan measurementStandard |
| Zeroing | Self-stabilising zero, so a residual offset can be ignored; manual zero available when neededStandard |
| Line resistance | Rab, Rbc and RcaStandard |
|---|---|
| Phase resistance | Ra, Rb and RcStandard |
| Derived values | Spread and unbalance for line resistance and for phase resistance, for star and delta connectionStandard |
| Comparator | Pass or fail judged on the spread or on the unbalance, with absolute upper and lower limitsStandard |
| Buzzer | Pass, fail or offStandard |
| Temperature compensation | Converts the measured resistance to the value at a set ambient temperature, using the temperature input on the rear panelStandard |
| Stored setups | Five sets of measurement parametersStandard |
| Trigger | Internal, external or manualStandard |
| Screen | 4.3 inch IPS colour LCD, 480 × 272, 24-bit colourStandard |
|---|---|
| Reading resolution | 4½ digitsStandard |
| Fast rate | 15 readings per second, single channelStandard |
| Slow rate | 8 readings per second, single channelStandard |
| Screen content | Three winding readings, line and phase values, spread, unbalance and the sorting result togetherStandard |
| RS232C | Remote control and data uploadStandard |
|---|---|
| HANDLER | Sorting and control signals for an automatic stationStandard |
| USB host | USB stick for recording results, and firmware update from a fileStandard |
| RS485 | Optional, with the instrument’s own Modbus protocol for cascading several unitsOptional |
| Keyboard lock | Locks every key except the test function; released by holding the keyStandard |
| Temperature input | Rear-panel connector for the temperature compensation functionStandard |
| Operating temperature | 0 to 40 °CStandard |
|---|---|
| Operating humidity | Up to 90% RH, without condensationStandard |
| Mains | 198 to 242 V AC at 47 to 63 Hz; the rear panel marking states 198 to 240 V at 50 or 60 Hz and 30 VAStandard |
| Body | 215 × 86 × 330 mm, about 4.0 kg, benchtop caseStandard |
| Model | Configuration | Key ranges | Availability |
|---|---|---|---|
| CXT3513 | Benchtop three-channel winding resistance tester with a 4.3 inch colour display and Kelvin inputs on three channels | 10 µΩ to 20 kΩ; 0.05% basic accuracy; ≤ 100 mA test current; 15 readings per second | Supplied with a CXT25035 four-terminal Kelvin clip set |
| CXT3513 + RS485 | The same instrument with the optional RS485 interface fitted | As CXT3513, with Modbus cascading of several units | For a station that has to scan more channels than the three on board |
Certifications are only published once a certificate copy can be produced, so this section is deliberately empty rather than populated with assumed approvals. Tell us which approvals your application requires and we will confirm what is available for this instrument.
The following are required for a complete specification but are not stated in the documentation supplied to us. They are listed rather than estimated:
Because a motor is usually specified one way and measured the other. With the winding connected in star or delta, the resistance at the terminals is a combination of the phase resistances, and the conversion depends on which connection is used. Showing both lets the inspector compare the reading with the drawing without doing the conversion by hand, and it makes an error in the connection setting visible immediately.
It turns three numbers into one judgement. A winding fault, a wrong turn count, a poor joint or a wrong wire gauge shows up as one phase differing from the other two, and the percentage unbalance is what the acceptance criterion is usually written against. The spread in absolute terms is the same information without the normalisation, which is why both are offered as comparator bases.
That depends on the resistance being measured. The accuracy is a percentage of reading, so at the bottom of the range the absolute error is small in micro-ohms; at the top of the range it is larger in absolute terms but still a small fraction of the reading. What matters more for a small winding is the four-terminal connection, since the lead and contact resistance would otherwise be a large part of the result.
The resistance of the windings, or the motor rating from which it can be estimated; the connection the motor is wound for, star or delta; the acceptance criterion, whether a percentage unbalance or an absolute spread; the number of motors per shift, which decides whether the fast or slow rate matters; and how the result has to leave the instrument, by sorting signal, serial link or USB stick. Quantity and destination complete the enquiry.
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