Motor Electrical Testing and Motor Preventive Maintenance

What Is Motor Electrical Testing and Why Do Plants Need It?

Motor electrical testing measures the health of a motor's windings, insulation and supply so you can tell a sound motor from one that is weakening, long before it burns out. Plants use it to decide which motors to repair at the next shutdown, which to keep in service, and which spares to have on the shelf.

Electric motors drive almost everything that moves in a plant: conveyors, pumps, fans, compressors, mixers, extruders and chillers. Most fail for a small number of reasons. Winding insulation ages with heat, moisture, contamination and voltage stress. Connections loosen. Supply problems such as voltage unbalance or a lost phase overheat the windings. Bearings wear, and a hot bearing cooks nearby insulation. Many of these causes leave an electrical signature well before the motor trips or seizes.

We test motors at three levels: offline tests on the de-energized motor and its cable, online checks while it runs under load, and thermal scans of the motor, its connection box and the starter feeding it. Results go into a register for each motor, so this year's numbers can be compared with last year's. A single reading can look acceptable; a reading that has fallen steadily over three visits tells a different story.

The motor is only one part of the circuit. Starters, overloads and drives in your motor control centres are covered by MCC maintenance, and broader test programs by electrical testing services.

Which Motor Tests Do We Run, and What Does Each One Find?

No single test finds every motor fault, so we choose a set based on motor size, voltage, criticality and what is already known about it. The table shows the tests we use and what each one is good at catching.

Each test is carried out and judged under ANSI/NETA MTS (2023 edition) and the motor maker's data. On large or critical motors we add tests suited to their insulation system and agree the test voltages with you and, where relevant, the motor manufacturer. Thermal scanning of motors is done by our Level II thermographers as part of a wider electrical thermography survey or on its own.

Insulation Resistance and Polarization Index on Motor Windings

Insulation resistance testing applies a DC voltage between the windings and the motor frame and measures how much current leaks through the insulation. High, stable readings point to clean, dry insulation. Low or falling readings point to moisture, dirt, oil or ageing.

The reference document for this work is IEEE 43, the recommended practice for insulation resistance testing of rotating machinery. Its 2013 revision, published in 2014, is the edition test programs still cite. It is a recommended practice, so we use it as guidance on test voltages, temperature correction and acceptance values rather than as a rule.

Why we take readings over time

To get the polarization index, we take the reading at ten minutes and divide it by the reading at one minute. In clean, dry insulation the reading keeps climbing as the insulation polarizes, so the ratio is well above one. In wet or contaminated insulation, leakage dominates and the ratio stays flat. IEEE 43 offers guidance on reading the ratio, including cases where it adds little. We compare your results with that guidance, with the manufacturer's data, and with your motor's own history.

Temperature matters

Insulation resistance drops as temperature rises, so a reading taken on a warm motor straight after shutdown is not comparable with one taken on a cold motor. We record winding temperature with every test and correct readings to a common base, which is what makes year-on-year trending honest.

Winding Resistance, Surge Comparison and Running Tests Explained

Winding resistance and surge tests look for faults inside the copper, while running tests show how the motor behaves on its real load. Together they separate a motor problem from a supply or mechanical one.

Winding resistance

We measure the resistance of each phase with a low-resistance meter and compare the three. There is no universal ohm value for a motor of a given horsepower; resistance depends on the design, voltage and connection. What matters is balance: one phase reading noticeably different from the other two points to shorted turns, a broken strand or a poor connection in the terminal box. We also compare with the manufacturer's data and with earlier readings.

Surge comparison

A surge test sends short, fast-rising voltage pulses into each phase and compares the response waveforms. Healthy phases produce matching waveforms; a weak turn-to-turn insulation spot makes one phase's waveform shift. This is the test that finds the early stage of many winding failures, which a resistance test at low voltage can miss. Because it stresses the insulation, we agree test levels with you before we run it.

Running tests

With the motor on load, we measure current and voltage on each phase and compare them with the nameplate. Even modest voltage unbalance at the terminals shows up as a larger current unbalance and extra winding heat. Current above nameplate full-load amps points to overload or a mechanical problem in the driven equipment. A thermal scan at the same visit shows whether the heat is in the connections, the frame or the bearing housings.

Building a Motor Preventive Maintenance Program That Fits Your Plant

A motor preventive maintenance program tests each motor at an interval set by how critical it is and what condition it is in, rather than testing every motor on the same calendar. Critical motors with no installed spare get the closest attention; small, easily swapped motors may only need a thermal scan and a running check.

We borrow that logic from NFPA 70B: a motor that looks worn, sits in a wet or dusty room, or would stop the line if it failed is serviced sooner, and the harshest of those three factors decides. NFPA 70B's own interval table covers the motor control equipment that feeds your motors at five, three or one year cycles depending on condition. We carry the same thinking to the motors themselves, inside a program that also satisfies CSA Z463 and uses ANSI/NETA MTS test methods.

1. Motor register: we list each motor with its nameplate data, location, load, starter or drive, and whether a spare exists.
2. Criticality ranking: we agree with your maintenance team which motors stop production, affect safety or food safety, or cost the most to replace.
3. Baseline tests: the first visit sets reference values for each motor tested.
4. Intervals: critical motors in harsh rooms are tested most often; stable, low-risk motors less often.
5. Trending and review: after each round we compare readings, flag motors moving the wrong way, and adjust intervals.

Motor testing pays off most as part of a wider plan. See predictive electrical maintenance for how trending drives decisions across your equipment, and factory electrical maintenance for whole-plant programs.

Testing Motors Around Production Schedules and Shutdowns

Most running tests and thermal scans happen during normal production, with no downtime. Offline tests need the motor stopped and isolated, so we group them into your planned shutdowns, changeovers and sanitation windows.

Before offline work, the motor's supply is disconnected, locked out and tagged under s.42 of Ontario's Reg. 851, and absence of voltage is verified. Where a variable frequency drive feeds the motor, we isolate the drive and make sure stored energy in its DC bus has discharged before connecting test leads. We disconnect drives before any insulation or surge test so the drive electronics are not exposed to the test voltage.

Reg. 851 recognizes that testing and troubleshooting sometimes has to happen on live equipment; s.42.1 covers that case and sets the protective equipment and procedures required. Our electricians work under CSA Z462 for any energized measurement, wearing the PPE the risk assessment calls for.

We service plants across Ontario, with most motor work in the industrial areas of Mississauga, Brampton and Vaughan. Night and weekend shutdowns are common, and we plan crews around them. When a motor fails between visits, contract clients get emergency response from the same team that knows their equipment.

What a Motor Test Report Tells Your Maintenance Team

The motor test report gives each motor a clear status, the readings behind it, and a recommended action, so your team can plan repairs, rewinds or replacements with evidence rather than guesswork.

• Motor identification: tag, location, nameplate data and the starter or drive feeding it.
• Test conditions: winding temperature, ambient conditions, test voltages and whether the cable was included.
• Readings: insulation resistance corrected to a common temperature, polarization index, phase resistances and balance, surge comparison result, running currents and voltages.
• Thermal images: the motor frame, terminal box and starter, with temperature rise noted.
• Trend: this round's values beside earlier rounds where we have them.
• Status and action: serviceable, monitor, plan repair, or remove from service, with the reason in plain words.

Where testing points to a supply problem, such as unbalanced voltage or a failing starter contact, the report says so and our licensed electricians can correct it. Where the problem is inside the motor, the report gives your repair shop the readings it needs.

When Motor Testing Is Not the Right First Step

Motor electrical testing is not always the answer. If a motor is vibrating, noisy or running hot at the bearings but its electrical readings are normal, the cause is usually mechanical: alignment, balance, lubrication or the driven load. The report will say when results point that way, so you can bring in the right trade.

• Small, low-cost motors with spares on the shelf are often cheaper to run to failure and swap. A yearly thermal scan is usually enough for these.
• A motor that tripped today needs fault finding before scheduled testing. Book electrical troubleshooting and we will trace whether the cause is the motor, the cable, the starter or the supply.
• Brand-new motors should have acceptance values recorded at installation, using the ANSI/NETA ATS (2025 edition) tests; that baseline is more useful than a test a few weeks later.

For plants with many motors, the right first step is usually a walkdown with your maintenance lead to rank criticality. From there we can recommend which motors to test, which to scan, and which to leave alone. See also our work for manufacturing plants and food processing plants.