Motor Controls & Starters

Motors drive pumps, conveyors, compressors, fans and mixers. A motor cannot protect itself or decide when to run, so it needs a set of parts around it to start it, stop it and shut it down before it burns up. Those parts are called motor controls. This guide covers the hardware and the basic control circuits. Once you understand them, most starters look the same, no matter who built them.

What You Will Be Able to Do

• Read a motor nameplate and pick out the numbers that matter.
• Explain what a contactor does, and what an overload relay adds.
• Choose between across-the-line, reversing and reduced-voltage starting.
• Explain two-wire and three-wire control, seal-in and jogging.
• Spot the signs of a worn contactor or a failing coil.

Warning: Motor circuits commonly run at 480 volts, and a control circuit can be fed from a separate source. Only qualified people work on them energized. Lock out and prove dead before you touch conductors, and treat every part of a starter as live until it has been tested.

Motor Basics

Most industrial motors are three-phase induction motors. A rotating magnetic field in the stationary windings (the stator) induces current in the rotating part (the rotor). There are no brushes to wear out, which makes them simple and rugged.

Reading the Nameplate

A service factor of 1.15 means the motor can run at 115 percent of rated horsepower. Doing that on a regular basis shortens its life, so treat it as a margin and not as extra capacity.

Enclosures

• ODP (open drip-proof): openings let air through the motor. It suits clean, dry indoor locations.
• TEFC (totally enclosed fan-cooled): the housing is closed, and a fan on the shaft blows air over the outside. It suits dusty, dirty and wet areas.
• TENV (totally enclosed non-ventilated): closed, with no fan, used mainly on small motors.
• Explosion-proof: built for classified hazardous locations.

Inrush Current

A motor at rest acts almost like a short circuit. Starting at full voltage, it draws about 6 to 8 times its full-load current until it reaches speed. This is often called locked-rotor current. A motor with an FLA of 27 amps can pull about 160 to 215 amps for a few seconds. That is why overload devices must allow the start, and why large motors often use a reduced-voltage method.

Contactors

A contactor is a switch operated by an electromagnet. It lets a small control signal switch a large motor current.

• Coil: an electromagnet that pulls the moving part (the armature) in when energized. Coils come in voltages such as 24 V DC, 120 V AC and 480 V AC. The coil voltage has no relation to the motor voltage.
• Main contacts: heavy silver-alloy contacts that carry motor current.
• Auxiliary contacts: small contacts that move with the armature. They are normally open (NO) or normally closed (NC), and are used for control logic.

NEMA Sizes

NEMA contactors and starters are rated by size. Each size has a maximum horsepower that depends on voltage:

IEC contactors are smaller and rated by current and duty. They are often less forgiving, so match them to the motor and the application using the manufacturer's tables.

Contactor Problems

Contact wear. Pitting and darkening are normal. Replace contacts when they have worn down to about half their thickness, or as the manufacturer directs. Replace them as a set. Do not file silver contacts. Filing strips off the silver and shortens their life. The dark oxide on silver still conducts.

Buzz and chatter. An AC coil that buzzes loudly, or a contactor that will not fully close, means the armature is not seating. Check these causes:

A coil that cannot seat draws too much current and burns out quickly.

Heat. Darkened insulation or a burned smell at the coil means overheating. Loose power connections cause heat too, so torque them to the manufacturer's value.

Overload Relays

An overload relay protects the motor from sustained overcurrent, such as a jammed conveyor or a failing bearing. It watches for heat over time, and it allows the brief inrush during a start.

An overload relay is not the same as a breaker. A breaker or fuse protects against short circuits and ground faults. It is sized well above motor current so that it does not trip on inrush, so it cannot protect the motor from a slow overload. You need both.

When an overload trips, its NC contact in the control circuit opens and drops out the coil. The overload does not break the motor current itself. The contactor does.

Types

• Bimetallic: a heater element warms a strip of two bonded metals, which bends and trips the relay. Heaters are chosen from the manufacturer's table, using the motor's FLA. Many have a manual or automatic reset.
• Eutectic (solder pot): a special alloy melts at a set temperature and releases a ratchet. It must cool before reset. You will find these on older equipment.
• Electronic: current transformers and a processor model the motor's heat. They need no heaters, and many add phase-loss and ground-fault protection.

Trip Classes

The class number tells you the longest time, in seconds, the relay takes to trip at 6 times its rating.

Setting the Relay

Use the motor's nameplate FLA to set the relay, not the breaker or the wire size. NEC 430.32 sets a maximum, based on the motor:

For a motor with 27 FLA and a 1.15 service factor, the most the relay can be set at is 27 x 1.25 = 33.75 amps. Many plants set it lower, closer to the FLA, to protect the motor better. Heater tables from a manufacturer already include their own margin, so follow the table rather than adding a percentage on top.

Repeat trips mean something is wrong. Find the cause before you reset it.

Motor Starters

A motor starter is a contactor plus an overload relay. A contactor by itself has no motor protection. A starter switches the motor and protects it.

Across-the-Line Starters

An across-the-line starter applies full voltage to the motor all at once. It is also called full-voltage or direct-on-line (DOL) starting. It is simple, low in cost and gives the most starting torque. The downsides are the high inrush current, a voltage dip on the system and a mechanical jolt to belts, gears and couplings. Utilities may limit it on large motors.

Reversing Starters

Swapping any two of the three phases reverses a motor, so a reversing starter uses two contactors. One feeds the motor as-is, and the other swaps two phases.

If both contactors closed at once, they would put a phase-to-phase short across the line. So a reversing starter uses two interlocks:

• Mechanical interlock: a physical link that stops the second contactor from closing.
• Electrical interlock: an NC auxiliary contact from each contactor in series with the other coil.

They back each other up, and you need both. One can fail or be defeated, and the result of both contactors closing is an arc inside the starter.

Reduced-Voltage Starting

These methods cut inrush and mechanical shock. Each one also reduces starting torque, so the load has to be able to start with less.

Wye-delta. In wye, each winding sees line voltage divided by 1.732, or about 58 percent. Current and torque fall by the square of that, to about one-third. After a timed delay, the starter switches to delta and full voltage. The motor must have all six leads brought out. In an open transition, power is briefly cut during the switch, which can cause a current spike. A closed transition avoids this.

Soft starters use silicon-controlled rectifiers (SCRs, a kind of thyristor) to raise voltage gradually over a set time. A soft stop lets a pump slow gently, which prevents water hammer. Many include a bypass contactor that takes over at full speed, to reduce heat in the SCRs.

Variable frequency drives also start a motor gently, and can run it at any speed. They are a separate subject, because they work in a very different way.

Control Circuits

The control circuit tells the starter when to run. It usually runs at 120 V AC or 24 V DC.

A control transformer, or control power transformer (CPT), steps line voltage such as 480 volts down to 120 volts or less, so that pushbuttons and coils work at a lower voltage. It has its own fuses. It can also be fed from a separate source, which matters whenever you lock out.

Two-Wire Control

Two-wire control uses a maintained device, such as a float switch, pressure switch or thermostat. When the switch closes, the motor runs. When it opens, the motor stops.

After a power loss, the motor restarts by itself when power returns, if the switch is still closed. That suits a sump pump or a ventilation fan. It can injure someone on a machine with exposed moving parts.

Three-Wire Control

Three-wire control uses momentary pushbuttons, and it gives low-voltage protection. After a power loss, the motor stays off until someone presses Start.

 L1                                                   L2
  |                                                    |
  +--[ STOP NC ]--+--[ START NO ]--+--( M )--[ OL NC ]--+
                  |                |
                  +----[ M aux ]---+

1. At rest, STOP is closed, START is open, and the M auxiliary contact is open.
2. Pressing START energizes coil M.
3. The M auxiliary contact closes in parallel with START.
4. Releasing START does not drop the coil, because the auxiliary now carries the current.
5. Pressing STOP opens the circuit. The coil drops out and the auxiliary opens.

That auxiliary is called a seal-in or holding contact. A power loss, an overload trip, a stop button or a broken wire all open the circuit in the same way. That is the safety value of this design.

Jogging

Jogging, or inching, runs a motor only while the button is held. It is used for small movements, such as lining up a belt or a machine. A jog circuit stops the seal-in from engaging, usually with a selector switch or a jog relay. Frequent jogging heats the motor and wears contacts, so starters used this way are often sized up.

Common Additions

• Pilot lights: often red for running and green for stopped, though color meaning varies by plant.
• Hand-Off-Auto selector: Hand runs the motor directly, Off stops it, and Auto lets a device or PLC control it.
• Timers: on-delay and off-delay timers handle sequences, such as starting a lube pump before a main motor.
• Sequence interlocks: an auxiliary contact from one starter is wired in series with another, so that a feeder cannot start until the conveyor is running.
• PLC control: a PLC output energizes the coil, with hardwired overload and safety contacts kept in the circuit.

Common Mistakes

Check Your Understanding

1. A conveyor jams, and the motor draws 130 percent of its rated current. Which device should act, the breaker or the overload relay?

The overload relay. The breaker is sized to carry starting current, so it will not trip on a moderate overload.

2. A motor nameplate shows 42 FLA, service factor 1.0, and no temperature rise marking. What is the highest overload setting?

115 percent of FLA: 42 x 1.15 = 48.3 amps.

3. Which NEMA size would you use for a 40 HP motor at 460 volts?

NEMA Size 3, rated up to 50 HP at 460 volts.

4. After a brief power outage, an operator is surprised that a mixer started up by itself. What kind of control does it probably have?

Two-wire control. Three-wire control would need someone to press Start again.

5. Why does a wye-delta starter need access to six motor leads?

The starter has to connect the windings first in wye and then in delta, so each end of each winding must be available.

Key Takeaways

• A starter is a contactor plus an overload relay. A breaker protects against faults, and an overload protects against slow overheating.
• Set overloads from the nameplate, up to 125 or 115 percent depending on the motor.
• Inrush is about 6 to 8 times FLA, so reduced-voltage methods are used to soften a start.
• Reversing starters need mechanical and electrical interlocks.
• Three-wire control stays off after a power loss. Two-wire control restarts.
• Never file silver contacts, and always find the cause of a trip.