Monday, 8 June 2026
Motor Starting Autotransformer: How Reduced-Voltage Starting Helps Protect Large Motors
Across-the-line starting works fine for small motors. For a 5 hp pump in ...
Control & Automation
Motor Starting Autotransformers
Rex Power Magnetics Motor Starting Autotransformers are designed to provide reduced-voltage starting for three-phase induction motors, lowering inrush current and mechanical stress during startup. This controlled acceleration minimizes electrical disturbances and extends the life of motors and driven equipment.
Built for demanding industrial and commercial environments, these transformers feature high thermal capacity, durable dry-type construction, and precise voltage tap settings for flexible starting torque control. Their robust design ensures consistent performance under frequent start-stop cycles, vibration, and heat.
Each unit is custom-engineered to meet specific voltage, kVA, and duty-cycle requirements, with options for open-core or enclosed configurations. All designs comply with CSA and UL standards and are suitable for common starting methods such as wye-delta and autotransformer start systems.
Rex Power Magnetics combines proven design expertise with manufacturing precision to deliver reliable, efficient motor starting solutions tailored to your application.
- Limits the inrush current of the motor significantly.
- Generates minimal losses durring starting.
- Suitable for long starting periods
- Starting voltages can be easily adjusted using the transformer taps.
- CSA certified: File No. LR 34493.
- UL listed: File No. E108255.
| Capacity | 15 to 400HP |
| Voltage Class | 1.2kV |
| Taps |
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| Cooling | Self Cooled (ANN / AN) |
| Frequency | 60Hz (50/60Hz Optional) |
| Conductors | Copper (Cu) Windings |
| Insulation System | 220°C (150°C Rise) (Optional 115°C or 80°C Rise Available) |
| Impregnation | Polyseter Resin Dipped and Baked |
| Enclosure Type | Open Core & Coil |
| Warranty | 12 Months (See Warranty and Limitations) |
| Quality System | ISO 9001:2015 Quality Management System |
| Certifications |
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| Reference Standards |
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| Optional Features & Accessories |
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Configure a motor starting autotransformer to build its Rex catalog part number. Codes follow the Rex catalog numbering system.
Options
Special Features / Accessories
Rex Part Number
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This tool assembles a catalog part number from the options you select, following the Rex Power Magnetics catalog numbering system. Not every combination of options is necessarily a standard or available product, and some configurations may require review or a custom quote. For a voltage not listed, the code ‘X’ is used and the value specified separately. Rex Power Magnetics makes no representations as to the accuracy of the information generated by this tool; part numbers should be confirmed with Rex before ordering. Contact Rex Power Magnetics to verify availability, pricing, and specifications.
Faqs
- General FAQs
- Technical Specification FAQs
- Installation & Application FAQs
- Performance & Product Selection FAQs
- Protection, Troubleshooting & Maintenance FAQs
A motor starting autotransformer is designed to reduce the voltage applied to an induction motor during startup, thereby limiting inrush current and mechanical stress. By providing a lower initial voltage, it allows the motor to accelerate smoothly before switching to full-line voltage. This results in reduced torque shock to mechanical systems and minimizes electrical disturbances on the supply network. Rex Power Magnetics motor starting autotransformers are commonly used in medium and large horsepower motor applications, particularly where direct-on-line starting would exceed allowable current limits or create voltage dips on the power system.
The autotransformer temporarily connects to the motor during the initial start sequence. It provides a reduced voltage — typically 50%, 65%, or 80% of line voltage — depending on the desired starting characteristics. Once the motor reaches a predetermined speed, contactors switch the motor to full-line voltage operation. The transformer is then isolated from the circuit. This stepped-voltage starting method limits current while still supplying sufficient torque for acceleration. Rex Power Magnetics designs its autotransformers for high short-duration duty cycles and robust mechanical strength to withstand the stresses of repeated starts.
Reduced-voltage autotransformer starters have historically been built with either two coils in an open-delta (V) connection or three coils connected in wye. Rex builds its motor starting autotransformers as three single-phase coils with their 0% ends joined at a common wye point, with the 50%, 65%, 80% and 100% taps brought out on each coil.
The three-coil design produces a balanced reduced voltage on all three motor phases. A two-coil open-delta unit leaves one phase fed directly from the line while the other two are transformed, so the motor sees a small voltage unbalance during the start; that unbalance shows up as a negative-sequence current that adds rotor heating and a slight torque pulsation on every start. On a motor started several times per hour, that extra heating is not trivial.
The three-coil design also shares the starting current between three windings instead of two, which lowers the current density in each coil and gives more thermal mass for the same frame.
A motor starting autotransformer is in circuit only for the few seconds of acceleration, so a continuous kVA rating would be meaningless. Rex rates each frame by the motor horsepower (15 to 400 HP) and system voltage it is designed to start, and the winding is dimensioned for the current that motor will draw on the available taps for a defined duty.
It is useful to convert that into short-time kVA to appreciate the duty. Take a 100 HP, 460 V motor with a locked-rotor current of about 744 A. On the 65% tap the motor current is 0.65 × 744 = 484 A and the line current is about 0.65 × 484 = 313 A. Apparent power on the line side is therefore √3 × 460 × 313 = roughly 250 kVA, and the autotransformer is passing that for perhaps 5 to 15 seconds. The equivalent across-the-line demand would be √3 × 460 × 744 = about 593 kVA.
The winding does not have to carry 250 kVA continuously; it has to absorb the I²R energy of one start without exceeding its 150°C rise (or the optional 115°C or 80°C rise) and then cool before the next. That is why the duty cycle, not the kVA, is what Rex needs on the order. The Transformer Rating Explained article contrasts continuous and short-time ratings.
No. Like every autotransformer, a Rex motor starting autotransformer has a single winding per phase with the line and motor connections tapped from it, so there is a direct metallic path from supply to motor. It does not create a separately derived system, and nothing on the motor side should be treated as isolated from the supply.
Three consequences follow. First, the motor branch circuit remains part of the same grounded or ungrounded system as the supply; the wye point at the 0% terminals is not a neutral and must not be bonded to ground or to the enclosure. Doing so would create a fourth-wire path that unbalances the autotransformer and defeats the ground-fault protection scheme. Second, the ground-fault relay or core-balance current transformer for the motor can be located on the line side of the starter, since fault current in the motor leads returns through the same source. Third, the autotransformer frame and any enclosure are bonded to the equipment grounding conductor in the normal way, sized to the branch-circuit overcurrent device.
On a Canadian 600 V high-resistance-grounded system, the same rule applies: leave the wye point floating. The Transformer Grounding and Bonding Explained article and the Autotransformer vs. Isolation Transformer article set out the distinction in detail.
Rex Power Magnetics mini power centers are available in primary voltages up to 600 V and secondary voltages typically of 208Y/120 V or 240/120 V, depending on system requirements. Standard ratings range from 3 kVA to 45 kVA, with custom sizes available. Three-phase and single-phase configurations can be specified. Tap settings, grounding arrangements, and panelboard circuit configurations can be customized to match project needs.
All Rex Power Magnetics motor starting autotransformers are designed and manufactured in accordance with CSA C22.2 No. 66 and IEEE C57 standards. They are CSA certified and built under the company’s ISO 9001-certified quality management system. Every unit undergoes dielectric, ratio, and resistance testing to ensure compliance with safety and performance specifications. The design is optimized for short-time duty cycles, ensuring mechanical strength, thermal endurance, and consistent performance across a wide range of motor starting conditions.
Motor starting imposes short-duration but high-stress conditions on transformer windings and insulation. Rex Power Magnetics designs its autotransformers with reinforced winding supports, high-temperature insulation, and mechanically braced cores to handle these stresses. The materials and construction are chosen to resist vibration, magnetic forces, and thermal expansion from frequent start cycles. Each design undergoes finite element and thermal analysis to ensure mechanical integrity and heat dissipation under repeated use. The result is a long-lasting transformer capable of performing reliably even in continuous-duty industrial applications.
Rex uses copper windings on all motor starting autotransformers. For a short-time-rated device this is not primarily a conductivity decision; it is a thermal-mass and mechanical decision.
During a start the winding carries several times the motor’s full-load current for several seconds, and the heat generated in that period has nowhere to go except into the mass of the conductor and the surrounding insulation. The temperature rise of the conductor during the start therefore depends on its specific heat per unit volume and its resistivity. Copper has both a lower resistivity and a higher volumetric heat capacity than aluminum, so for the same cross-section it generates less heat and its temperature rises less for a given start. An aluminum winding would need considerably more cross-section to match, increasing frame size.
Copper is also stronger and less prone to creep at connections, which matters for a coil that experiences a mechanical shock at every start and whose tap terminals are periodically reconnected in the field. Copper terminals also accept the standard lugs used in starter wiring without the bimetallic precautions aluminum requires.
The combination of copper, the 220°C insulation system and the polyester impregnation is what gives the unit its high thermal capacity for long or repeated starts. The Transformer Windings Explained article covers conductor selection in more depth.
The cooling designation ANN/AN on a Rex motor starting autotransformer means the unit is cooled by natural convection of air over the core and coils, with no fans. For a continuous-duty transformer that sets the steady-state rating. For an MSA it sets something different: the rate at which the unit sheds the heat deposited during a start.
Each start raises the winding temperature almost adiabatically, because a few seconds is far too short for convection to matter. After the run contactor bypasses the autotransformer, the winding cools toward the enclosure ambient along an exponential curve whose time constant depends on the thermal mass of the copper and core and on the convective surface area. The permissible starting duty is therefore a balance: the energy per start, multiplied by the number of starts per hour, must not exceed what the natural convection can remove in that hour while the winding stays below its rated rise.
This is why an MSA that is fine at three starts per hour can overheat at eight, and why the lower-rise options (115°C or 80°C) add copper and surface area rather than fans. It is also why an open core-and-coil unit must be mounted with free air around it and not packed against contactors or cable bundles. The Thermal Management in Dry-Type Transformers article explains the cooling classes.
Rex offers three levels of thermal monitoring on motor starting autotransformers:
• Thermal sensing and indication: a temperature sensor embedded in the winding, with the signal brought out to terminals so the starter controller or a plant SCADA system can read winding temperature.
• Thermometers (analog or digital): a local dial or display reading, typically mounted on the enclosure, for inspection without instruments.
• Thermostat alarm and trip contacts (N.O. or N.C.): fixed-setpoint switches that change state when the winding reaches an alarm temperature and a higher trip temperature.
The thermostat contacts are the option most panel builders use. The normal arrangement is an N.C. trip contact wired in series with the start command so that a hot autotransformer cannot be started, together with an N.O. alarm contact wired to a pilot light or a PLC input. Because the contacts are in the control circuit, they are powered from the industrial control transformer that supplies the starter, not from the MSA.
Where the starter includes a restart timer, the trip contact should be used as a permissive that holds the timer off, so that after a trip the operator cannot override the cooling interval simply by pressing start again. Specify the contact form and the required setpoints on the order; the Product Configurator has a field for special features.
Rex builds motor starting autotransformers for 60 Hz as standard and offers 50 Hz nominal frequency as an option. Selecting it is not a nameplate change; the core is designed for the lower frequency.
At a given voltage, core flux is inversely proportional to frequency, so a 60 Hz design operated at 50 Hz runs at roughly 120% of its intended flux density. In a short-time-rated device that shows up as a much larger magnetizing current at the instant the start contactor closes and, during a long acceleration, as higher core loss and noise. Rex corrects this by increasing the core cross-section or turns for the 50 Hz design, so the unit behaves at 50 Hz exactly as the 60 Hz unit does at 60 Hz.
The 50 Hz option is normally combined with an IEC system voltage such as 380 V, 400 V or 415 V, and with a motor rated in kW. Convert kW to HP (1 kW = 1.34 HP) for the Part Number Builder, and state both on the order. Export starters may also require CE marking of the assembly, which Rex supports by designing to the applicable IEC clearances on request. Machinery for mining and marine duty projects outside North America are the usual cases. The Custom vs. Standard Transformers article describes how such variants are handled.
Every Rex motor starting autotransformer is built to IEEE C57.12.01 and tested to IEEE C57.12.91, the test code for dry-type transformers. Routine tests carried out on each unit include:
• Turns ratio at each tap, verifying that the 50%, 65% and 80% points deliver the correct fraction of line voltage on all three coils
• Winding resistance, which confirms conductor cross-section and joint quality and provides the reference for later field checks
• Polarity and phase relationship between coils
• Applied-voltage (dielectric withstand) test from the winding to core and frame
For the 1.2 kV insulation class, IEEE C57.12.01 specifies a power-frequency applied test in the low kilovolt range and a corresponding basic impulse level; the exact values used on the unit are recorded on the Rex test report, which can be requested with the order. Because an autotransformer has a single winding per phase, there is no primary-to-secondary test; the applied test is winding-to-ground.
Design (type) tests on the frame include temperature rise under the rated starting duty, which is what validates the 150°C rise claim, and mechanical integrity under repeated starting current. Rex performs all testing under its ISO 9001:2015 quality system.
Motor starting autotransformers are available as open core-and-coil assemblies for panel mounting or as enclosed units in NEMA 1, 3R, or 12-rated housings. Enclosures provide protection against dust, moisture, and physical impact while maintaining accessibility for wiring and maintenance. Floor-mounted or wall-mounted configurations can be specified based on installation requirements. Ventilated designs allow heat dissipation during short-time operation, ensuring safe temperature rise during repeated starts. Compact, robust construction ensures ease of integration into motor control systems or retrofit installations.
The standard Rex motor starting autotransformer is an open core-and-coil assembly intended to bolt to the back plate or floor of a starter enclosure. Several points govern a good installation.
Mount the unit with the coils oriented so that convection air can rise freely through them, and leave a clear space above and around the coils; the unit sheds the heat of each start by natural convection between starts, and a coil boxed in by contactors, cable bundles or a bus duct cools far more slowly. Place it low in the enclosure so that its heat does not rise past the contactors and control components, and keep the industrial control transformer and any electronic timer away from the warmest zone.
Because an MSA is dense and experiences a mechanical impulse at every start, the mounting surface must be stiff. On thin back plates, add a stiffener or mount to the enclosure floor, and use the external anti-vibration pads Rex offers where the enclosure sits on a steel structure or a skid. Torque the mounting bolts and check them at commissioning.
Keep the tap terminal board accessible so the tap can be changed at commissioning by moving one lug per phase. Enclosure outlines are on the Enclosure Drawings page, and the Transformer Clearance Requirements article covers ventilation spacing.
Yes, and this is one of the most common reasons for specifying one. A generator set has a much higher source impedance than a utility connection, so an across-the-line start on a large motor causes a deep voltage dip, a frequency dip as the governor responds, and often an under-voltage trip. A Rex motor starting autotransformer reduces the starting kVA the generator sees by roughly the square of the tap fraction.
For example, a 100 HP, 460 V motor demands about 593 kVA across the line but only about 250 kVA on the 65% tap. Generator manufacturers rate their sets for a maximum motor-starting kVA at a stated voltage dip. Reducing the demand by nearly 60% either allows a set roughly half the size to start the motor within its dip limit, or keeps the dip on an existing set small enough that other loads ride through.
Two cautions apply.
1. Torque on the 65% tap is about 42% of the across-the-line value, so the load must start unloaded, such as a pump against a closed valve or a fan with closed dampers.
2. The generator’s voltage regulator response affects the actual dip, so the set supplier should confirm the figures.
It can, provided the duty cycle is defined around the whole group rather than one motor. A single Rex motor starting autotransformer can be switched by selector contactors to start motor A, transfer it to line, then start motor B, and so on. Pumping stations, compressor houses and multi-fan cooling installations sometimes use this arrangement to avoid an autotransformer for each motor.
The thermal design must reflect the worst-case sequence. If three 150 HP pumps are started one after another following a power interruption, the MSA absorbs three starts’ worth of energy within perhaps a minute and must be rated for that cumulative duty. Rex would normally size such a unit for the largest motor at the 115°C or 80°C rise option, or select a larger frame, based on the number of consecutive starts and the minimum interval stated on the order.
The control logic also needs care: only one motor may be connected to the tap terminals at a time, each transition must be complete before the next start is permitted, and the thermostat trip contact should block the sequence if the winding is too hot. Provide the full sequence and timing through the Product Configurator so the frame and rise can be confirmed, and see the Role of Design, Environment and Load in Transformer Lifespan article for how cumulative duty affects insulation life.
Replacement of the autotransformer in a legacy starter is a routine engineered-to-order job for Rex, but starters from different manufacturers and eras vary in taps, terminal arrangement and size. Collect the following before requesting a quotation for a Rex motor starting autotransformer:
• Motor nameplate: HP, voltage, full-load current, locked-rotor current or code letter, and speed
• The existing autotransformer nameplate, including manufacturer, taps, duty and temperature rise
• Whether the starter is open or closed transition, and whether the existing unit has two or three coils
• The tap in use, and whether the motor has struggled to accelerate on it
• The available mounting footprint and height, and the position of the tap terminals relative to the contactors
• Starts per hour and the enclosure ambient
Rex will match the taps to the existing wiring where possible; if the old unit had a non-standard tap or was a two-coil design, the connection drawing will show the changes needed. Three-coil units can usually replace two-coil units with minor rewiring, since the third coil simply adds a phase to the wye point. Submit the data through the Get a Quote form or a local representative found through the Agent Locator; the Transformer Failures article helps diagnose why the original unit failed so that the replacement is not undersized in the same way.
A motor starting autotransformer may sit de-energized for weeks between starts, and a cold coil collects condensation whenever the air is warmer than the metal. Moisture in a dry-type winding lowers its insulation resistance and, in the worst case, causes a flashover at the first start. Standby pumps, seasonal HVAC equipment and temporary power skids are the typical cases.
Rex offers anti-condensation strip heaters as an option. These are low-wattage resistive heaters mounted near the coils and powered from the control supply whenever the motor is not running, keeping the winding a few degrees above ambient. They should be wired through a normally-closed auxiliary contact of the run contactor so that they switch off when the motor runs.
For outdoor, washdown or marine sites, specify a NEMA Type 3R, 4 or 4X enclosure, in 304 or 316L stainless steel where salt or chemicals are present, and keep the enclosure drains clear. Before a first start after a long idle period, measure insulation resistance from winding to ground; a low reading means the unit must be dried, usually by running the heaters, before energizing. The polyester resin impregnation resists moisture absorption but does not prevent surface condensation. The Low-Ambient Temperature Considerations for Dry-Type Transformers article covers cold and damp environments, and the marine duty page describes Rex’s approach to shipboard equipment.
Using a motor starting autotransformer significantly reduces inrush current, mechanical shock, and voltage fluctuations on the electrical system. This method provides a smoother start compared to direct-on-line operation, improving motor and driven equipment longevity. It also enables larger motors to be started on limited-capacity electrical networks without triggering protection devices. Additionally, the reduced current draw during startup minimizes flicker in lighting circuits and improves system voltage stability. The result is improved overall system performance, energy efficiency, and mechanical reliability in high-demand industrial environments.
Yes. Rex Power Magnetics offers custom motor starting autotransformers with special voltage taps, duty cycles, enclosure types, and integration options. Customers can specify insulation class, temperature rise, and mechanical design to match their control scheme. Options include NEMA-rated enclosures, thermal sensors, and wiring provisions for automatic control systems. Engineering teams work closely with OEMs and panel builders to optimize performance for unique motor characteristics or operating environments. Every custom unit undergoes the same quality control and testing as standard production models.
Both approaches add thermal capacity to a Rex motor starting autotransformer, but they do it differently and suit different situations.
Selecting the next HP frame increases the conductor cross-section and core size, so each start produces a smaller temperature rise and the unit has more surface area to shed the heat. This suits long individual starts, for example a high-inertia fan or centrifuge that takes 20 to 30 seconds to accelerate, or a motor whose locked-rotor current is unusually high for its horsepower. The tap percentages are unchanged, so nothing changes in the starter wiring.
Specifying the 115°C or 80°C rise option keeps the HP rating but designs the unit to reach a lower temperature at the end of the rated duty. More starts per hour can then be accommodated before the limit is reached, and insulation life is extended. This is the better choice for frequent starts of normal duration, high enclosure ambients, and enclosures that cannot be ventilated.
For the most severe cases, such as several consecutive starts after a power interruption, Rex may recommend both. The Transformer Insulation Classes article explains how rise relates to insulation life, and the duty-cycle data on the order lets Rex make the recommendation rather than guessing.
No. Once the run contactor closes and the start contactor opens, the Rex motor starting autotransformer is completely out of the circuit. The motor runs across the line at full voltage with the efficiency, power factor and speed shown on its nameplate, and the autotransformer draws no current and generates no loss. This is a genuine advantage over methods that remain in circuit.
A solid-state soft starter without a bypass contactor continues to conduct through its SCRs at full speed, adding a small conduction loss and requiring cooling; with a bypass contactor it approaches the autotransformer’s zero running loss. A VFD remains in circuit permanently and, while it offers speed control, it introduces conduction and switching losses of a few percent and injects harmonic current into the supply that may require an AC line reactor or a harmonic mitigating transformer to meet IEEE 519 limits.
During the few seconds of the start, the autotransformer is a linear magnetic device and produces no harmonics beyond the brief magnetizing transient at energization. Its only lasting effect on power quality is beneficial: the reduced starting current lowers the depth and duration of the voltage dip seen by the rest of the plant. For sites with existing power quality concerns, this makes the MSA the least intrusive reduced-voltage method. The Understanding Harmonics in Power Systems article explains what the VFD alternative introduces.
The tap percentages on a Rex motor starting autotransformer are fractions of the actual line voltage at the moment of starting, not of the nominal system voltage. If the supply is already depressed, the effects compound, because motor torque varies with the square of the voltage at its terminals.
Suppose the bus is at 92% of nominal before the start because the plant is heavily loaded or the feeder is long. On the 65% tap the motor sees 0.65 × 0.92 = 60% of nominal voltage, so its torque is 0.60² = 36% of the across-the-line value instead of the 42% expected at nominal. The starting current then causes a further dip, so the terminal voltage during acceleration may be lower still. A load with a comfortable torque margin on paper can fail to reach transition speed, and the motor and autotransformer then heat for the full timer period.
The practical rules are to base tap selection on the minimum expected pre-start voltage, to allow an accelerating torque margin of at least 10% to 15% of full-load torque at that voltage, and to check the supply voltage during commissioning. Where the bus regularly runs low, correcting the supply with tap adjustment on the upstream distribution transformer or a buck-boost autotransformer is often better than moving the MSA to the 80% tap. The Transformer Tap Changers article explains the upstream adjustment.
These transformers are low-maintenance, but periodic inspection ensures long-term reliability. Maintenance involves checking terminal tightness, cleaning ventilation openings, and inspecting insulation and leads for wear or discoloration. In systems with high starting frequency, thermal monitoring or regular infrared scanning can help identify hot spots early. Rex Power Magnetics’ robust construction minimizes the risk of loosening or vibration-related failures. With proper installation and routine checks, motor starting autotransformers provide decades of dependable service in demanding industrial environments.
An autotransformer starter is a motor branch circuit, so the protection rules of the Canadian Electrical Code (Section 28) and NEC Article 430 apply, not the transformer rules. The branch-circuit short-circuit and ground-fault device, a fused disconnect or circuit breaker, is placed on the line side of the Rex motor starting autotransformer and sized on the motor’s full-load current and starting characteristics. The autotransformer needs no separate primary protection; the branch-circuit device and the overload relay protect it.
The overload relay belongs in the motor leads where the start and run paths join, so that it carries motor current both on the tap and at full voltage. On the 65% tap the motor current is about 65% of locked-rotor current for the acceleration time, so the overload relay must be a class (10, 20 or 30) that rides through that period. An overload relay mistakenly placed on the line side would see only about 42% of locked-rotor current during starting and would under-protect the motor.
If a circuit breaker with an adjustable instantaneous trip is used, set it above the peak current at transition in a closed-transition starter, and well above the reconnection transient in an open-transition starter. Time-delay fuses sized per the code tables usually handle the starting profile. The Transformer Fuse Sizing article covers coordination principles.
Tripping at transition rather than at initial start is a signature of the transition itself, not of the Rex motor starting autotransformer. The usual causes and remedies are:
• Open-transition surge. If the starter disconnects the motor before reconnecting it at full voltage, the reconnection transient can exceed locked-rotor current. Convert to closed transition using the wye-point contactor and the 0% terminals, or raise the instantaneous setting if the code permits.
• Timer set too short. If transition occurs before the motor nears synchronous speed, the run contactor closes on a motor still drawing high current, and the overload relay accumulates heat from both the start and the second surge. Lengthen the timer beyond the measured acceleration time, or use a current-sensing transition.
• Timer set too long. The motor runs on the tap after reaching speed; the extra time heats the overload relay, which is near its trip point by transition.
• Wrong tap. A tap that is too low leaves the motor short of transition speed. Move up one tap.
• Contactor overlap. If the wye-point and run contactors overlap because an interlock has failed, the coils are shorted and the branch-circuit device trips instantly. Check the interlocks.
Record the actual acceleration time and line current with a recording meter during commissioning so that settings are based on measurement. The Transformer Commissioning article describes the measurement approach.
A Rex motor starting autotransformer is designed to reach its rated 150°C rise only at the end of its rated starting duty and to be cool before the next start. Overheating, whether detected by the optional thermostat, discoloured insulation or a smell of hot varnish, means more energy is going into the winding than the design assumed. Check the following in order.
- Starting duty. Count the starts per hour and the acceleration time. Level-controlled pumps and pressure-controlled compressors often start far more often than specified. If the nameplate duty is exceeded, add a restart timer or specify a lower-rise unit.
- Transition failure. If the run contactor is not closing, the motor is running continuously through the autotransformer on the tap. The unit will overheat within minutes; continued hum after the start is the tell-tale sign.
- Stalled or slow starts. A motor that does not reach transition speed within the timer period holds starting current for the whole period. Check the tap, supply voltage and mechanical load.
- Ventilation and ambient. Blocked openings, cable bundles against the coils, or an enclosure in direct sun raise the starting temperature of every cycle.
- Winding condition. Measure each coil’s resistance against the Rex test report; a high or unequal reading indicates a damaged joint or coil.
The Transformer Failures article covers the failure mechanisms, and Rex support is available through the Contact page.
Rarely. A Rex motor starting autotransformer that delivers the correct tap voltage on all three phases is doing its job; a motor that will not accelerate on that voltage has a torque, supply or wiring problem. Work through the following.
Measure the voltage at the motor terminals on all three phases while the motor is on the tap. It should be the tap percentage of line voltage, balanced within a percent or two. A low reading on one phase points to a wiring error, a contactor pole not closing, or one coil on a different tap. Balanced but low readings point to a depressed supply or the wrong tap.
If the voltage is correct, the motor is not producing enough torque for the load. Confirm the load is in its unloaded starting condition: discharge valve closed, dampers closed, conveyor empty, compressor unloader working. Check the driven machine for binding. If the torque margin at the reduced voltage is thin, move to the next higher tap.
Finally, check that the transition timer is not so short that the starter transfers before the motor can accelerate; the symptom is a surge at transition. If all three coils measure the same resistance and the tap voltages are correct, the autotransformer can be ruled out.
A Rex motor starting autotransformer is silent when de-energized, which is nearly all the time, and produces a distinct 120 Hz hum during the few seconds of a start. That hum is louder than a distribution transformer of similar size because the core is being driven at starting current, and it is normal. It should stop abruptly when the run contactor closes and the start contactor opens.
Abnormal noise falls into three categories. Hum that continues after the start means the autotransformer is still in circuit; the run contactor has not closed or the start contactor has not opened, and the unit will overheat if the condition persists. Stop the motor and check the contactor sequence.
A rattle or buzz during the start that was not present at commissioning usually indicates loosened mounting bolts, core clamping or terminal hardware; thousands of starting impulses will eventually loosen fasteners that were not torqued or locked. De-energize, lock out, and re-torque. Rex’s external anti-vibration pads reduce the transmission of vibration to the enclosure.
A sharp crack or arcing sound at the tap board during start is a loose or corroded tap connection or a contactor pole failing to seat. It will erode the terminal and must be corrected before the next start. The Transformer Humming Noise article explains the magnetostriction behind the normal hum.
Commissioning a starter built around a Rex motor starting autotransformer is best done in a fixed sequence, with results recorded for the maintenance file:
• Confirm the nameplate HP, voltage, frequency and taps against the motor and drawings, and check for shipping damage.
• Measure insulation resistance from each winding to frame; a low reading after storage means the unit must be dried first.
• Measure each coil’s resistance and compare the three readings with each other and with the Rex test report.
• Verify that all three coils are on the same tap, that the three 0% terminals go to the wye-point contactor, and that no 0% terminal is grounded.
• Torque the mounting bolts, core clamps and terminal hardware to the drawing values.
• With the power circuit de-energized, check the wye-point/run contactor interlock and the thermostat trip contact if fitted.
• Set the transition timer from the estimated acceleration time.
• Perform the first start unloaded with a recording meter on the line, note the acceleration time and peak current, and adjust the timer and tap.
• Perform a second start only after the winding has cooled for the interval stated for the duty.
Record the tap, timer setting and measured acceleration time on the starter. The Transformer Commissioning article expands on each check; the connection drawing and test report are available through the Product Documentation page.
Because a Rex motor starting autotransformer is stressed mechanically and thermally at every start rather than continuously, its maintenance is driven by the number of starts and the environment rather than calendar time alone. A reasonable programme is an inspection every 12 months, shortened to 6 months for units that start more than a few times per hour, operate outdoors, or sit in dusty or corrosive atmospheres.
At each inspection, with the starter locked out:
• Clean dust from the coils and ventilation openings; dust slows cooling between starts.
• Inspect the coil surface for resin discolouration or cracking, which indicates overheating.
• Check the tap, line and 0% connections for discolouration, pitting or looseness, and re-torque.
• Re-torque mounting bolts and core clamping hardware, which loosen under repeated starting impulses.
• Measure insulation resistance to frame and the resistance of each coil, and record them.
• Test the thermostat contacts and strip heaters, if fitted, and confirm the timer setting.
Trend both resistances from commissioning onward. A steady fall in insulation resistance points to moisture or contamination; a rise in one coil’s resistance points to a deteriorating joint. Either, caught early, is a repair rather than a failure. The Transformer Maintenance article provides the general checklist this list builds on, and replacement units can be sourced through the Get a Quote form using the nameplate part number.