What is the left vertical line of a ladder diagram?

The left vertical line is a supply rail, labeled L1 on a line-voltage diagram. The right rail is L2. On a transformer-fed circuit the rails are X1 and X2.

Each horizontal line in a ladder diagram is identified as a ___?

Rung. Each rung runs across the two supply rails: L1 and L2 on a line-voltage diagram.

How to read a ladder diagram

Read like a book: start at the top left, then left to right and top to bottom. Trace each rung from its supply rail to its return rail through a load.

Explain why each completed rung needs at least one power-consuming device

Every rung needs a load, such as a coil or pilot light. Without one, closing the control contacts would short the two supply rails together.

Normally open contact

A normally open contact is open at rest. No current passes until the device actuates it.

A normally open set of relay contacts will close when:

The relay coil is energized. Normally open contacts are open with no coil current and close as soon as the coil is energized.

Normally closed contact block

A normally closed contact is closed at rest and opens when actuated. STOP is wired NC in series with the parallel pair of START (NO) and the holding contact (M). A break in that series stop path de-energizes the coil and stops the motor.

Normally open vs normally closed

Both describe the rest state. NO is open at rest and closes when actuated. NC is closed at rest and opens when actuated.

A NEMA-rated motor starter consists of ___ assembled and wired in the same enclosure.

A magnetic contactor and an overload relay, mounted in one enclosure.

How do motor starters work?

The contactor lets a remote control device start and stop the motor. The overload relay protects the motor from overload conditions.

How to wire a motor starter

Forward starter: L1 to T1, L2 to T2, L3 to T3, through the contactor and overload relay. Three-wire control: STOP (NC) in series, START (NO) in parallel with the seal-in.

What is a jog switch or jog button?

A jog button is a momentary start with no seal-in, so the motor runs while the button is held and stops when it is released.

A motor is connected L1 to T1, L2 to T2, and L3 to T3. To reverse the motor, the leads need to be connected ___.

Interchange any two phase leads. Options: swap L1 and L3 at the starter, or T1 and T3 at the motor. Both give L1 to T3, L2 to T2, L3 to T1.

How to reverse a 3-phase motor rotation

Interchange any two phase leads, which reverses the direction of rotation. Some books swap L1 and L3 at the line side, others T1 and T3 at the motor leads.

What happens if both contactors close at the same time?

Both contactors closing would short L1 to L3 through the swapped wiring. Interlocks prevent the second contactor from closing while the first is closed.

Overload trip meaning

An overload trip opens the relay's normally closed control contact, stopping the motor. The relay operates on excessive current, and it does not necessarily protect against short circuits.

How to reset an overload relay

Find the cause first. Manual reset requires operator action, such as pressing a button. Automatic reset restores the overload contact, usually after cooling; the motor may restart if the control circuit still calls for it. NEC 430.43 prohibits an automatic-restart overload device if restarting can injure persons.

Motor overload trip troubleshooting

Let it cool and reset. If it trips soon again, check for motor shorts or grounds, and compare full-load amps to the relay setting.

What causes an overload to trip constantly?

A load that is too high or jammed, high ambient temperature, a defective overload protector, or windings that are short-circuited or grounded can each cause an overload to trip continually.

Which side of the coil does the overload go on? (NEMA vs NFPA 79)

A common NEMA-style starter drawing puts the overload contact on the return side, between the coil and L2. In a grounded control circuit, NFPA 79 allows that return-side contact where the conductors between the contact and coil do not extend beyond the control enclosure.

Generally, control transformers step down the main line voltage to a control voltage of ___ or ___.

120 volts or 24 volts. Most industrial control circuits operate at one of these, and control transformers step line voltage down to them.

What current is used to calculate a control transformer's primary fuse limit?

Rated primary current: VA divided by primary voltage, for a single-phase transformer. The applicable NEC protection method sets the allowed percentage.

How do you size a control transformer, and what does VA mean?

VA means volt-amperes, apparent power. Choose a VA rating at least equal to the sealed VA of operating coils plus other loads. Then confirm the manufacturer's inrush value for that rating exceeds the inrush VA of coils starting together plus running loads.

What is a VFD, and is there a VFD simulator online?

A VFD controls motor speed by changing the frequency of the power it delivers to the motor. Lower frequency means slower rpm.

VFD won't accept a run command from the terminal strip even though keypad run works

With Start Source 1 active, P046=1 selects keypad run. In this VFD sim, set P046 to 2 (DigIn TrmBlk) and confirm t062 is 48 (2-Wire FWD).

Questions from the bench

These are common motor control questions, in the words people search with. Most answers come with a step to try on a simulator.

Where a step names a simulator or an article, the link opens it. Try the step, then check the answer against what the circuit does.

Student notebook · Motor control

Ladder basics

What is the left vertical line of a ladder diagram?
The left vertical line is a supply rail, labeled L1 on a line-voltage diagram. The right rail is L2. On a transformer-fed circuit the rails are X1 and X2.
Try it →
In Basic Motor Starter's learn phase, open the ladder drawer. On this line-voltage drawing, the left rail is labeled L1 and the right rail is labeled L2.
Basic Motor Starter
Each horizontal line in a ladder diagram is identified as a ___?
Rung. Each rung runs across the two supply rails: L1 and L2 on a line-voltage diagram.
Try it →
In Basic Motor Starter's learn phase, open the ladder drawer. The control circuit appears as one rung, labeled 1, running between L1 and L2.
Basic Motor Starter
How to read a ladder diagram
Read like a book: start at the top left, then left to right and top to bottom. Trace each rung from its supply rail to its return rail through a load.
Try it →
In Basic Motor Starter's learn phase, follow the steps. Click a highlighted term in a step to light the ladder element it names in the ladder drawer.
Basic Motor Starter
Explain why each completed rung needs at least one power-consuming device
Every rung needs a load, such as a coil or pilot light. Without one, closing the control contacts would short the two supply rails together.
Try it →
On paper, draw two rungs, one with a coil and one with no load. Close the contacts on each, trace the path from supply rail to supply rail, and mark the rung that shorts.

Contacts (NO / NC)

Normally open contact
A normally open contact is open at rest. No current passes until the device actuates it.
Try it →
In Basic Motor Starter's commission phase, press START and release it. The SEAL-IN readout shows HELD, and the motor keeps running.
Basic Motor Starter
A normally open set of relay contacts will close when:
The relay coil is energized. Normally open contacts are open with no coil current and close as soon as the coil is energized.
Try it →
In Basic Motor Starter's commission phase, press START. The M COIL readout shows ENERGIZED, and the SEAL-IN readout shows HELD.
Basic Motor Starter
Normally closed contact block
A normally closed contact is closed at rest and opens when actuated. STOP is wired NC in series with the parallel pair of START (NO) and the holding contact (M). A break in that series stop path de-energizes the coil and stops the motor.
Try it →
In Basic Motor Starter's commission phase, press STOP with the motor running, and the motor stops. The sim does not model a broken wire, so STOP is the closest test it offers.
Basic Motor Starter
Normally open vs normally closed
Both describe the rest state. NO is open at rest and closes when actuated. NC is closed at rest and opens when actuated.
Try it →
In Basic Motor Starter's commission phase, the START and STOP checks test a NO contact and an NC contact in the same run.
Basic Motor Starter

Starters and contactors

A NEMA-rated motor starter consists of ___ assembled and wired in the same enclosure.
A magnetic contactor and an overload relay, mounted in one enclosure.
Try it →
In Basic Motor Starter's learn phase, open the Inside the Starter section. The auxiliary contact, coil, and overload are shown inside the starter enclosure.
Basic Motor Starter
How do motor starters work?
The contactor lets a remote control device start and stop the motor. The overload relay protects the motor from overload conditions.
Try it →
In Basic Motor Starter, build the control circuit in the wiring phase, then press START in the commission phase. The motor runs.
Basic Motor Starter
How to wire a motor starter
Forward starter: L1 to T1, L2 to T2, L3 to T3, through the contactor and overload relay. Three-wire control: STOP (NC) in series, START (NO) in parallel with the seal-in.
Try it →
In Basic Motor Starter's wiring phase, the task list runs Power Circuit, Control Circuit, Stop Circuit, Start Circuit, Overload Protection, then Seal-In Circuit. Work through them in that order.
Basic Motor Starter
What is a jog switch or jog button?
A jog button is a momentary start with no seal-in, so the motor runs while the button is held and stops when it is released.
Try it →
In Selector Switch Jog's learn phase, the act titled Three Ways to Jog has you operate the circuit in both modes. When it asks, turn the selector to JOG, then press and release START. The green light comes on while START is held and goes out when you release it.
Selector Switch Jog

Reversing

A motor is connected L1 to T1, L2 to T2, and L3 to T3. To reverse the motor, the leads need to be connected ___.
Interchange any two phase leads. Options: swap L1 and L3 at the starter, or T1 and T3 at the motor. Both give L1 to T3, L2 to T2, L3 to T1.
Try it →
In Reversing Hoist's learn phase, open the step named Power Circuit: Reverse & Motor. The lesson shows the load-side swap: REV T1 to OL L3, T2 to OL L2, and T3 to OL L1.
Reversing Hoist
How to reverse a 3-phase motor rotation
Interchange any two phase leads, which reverses the direction of rotation. Some books swap L1 and L3 at the line side, others T1 and T3 at the motor leads.
Try it →
In Reversing Hoist's learn phase, open the step named Power Circuit: Reverse & Motor. The lesson shows one convention, which crosses T1 and T3 on the load side.
Reversing Hoist
What happens if both contactors close at the same time?
Both contactors closing would short L1 to L3 through the swapped wiring. Interlocks prevent the second contactor from closing while the first is closed.
Try it →
In Reversing Hoist's learn phase, the act titled Why These Protections Exist starts with a short operating check, then removes the interlock wires. Press UP, then press DOWN while the hoist runs, and the sim shows the short circuit. In the commission phase of your own wired circuit, the FWD interlock blocks REV check has you press UP and then DOWN: DOWN does nothing until you press STOP, which drops the forward contactor.
Reversing Hoist

Overloads

Overload trip meaning
An overload trip opens the relay's normally closed control contact, stopping the motor. The relay operates on excessive current, and it does not necessarily protect against short circuits.
Try it →
In Basic Motor Starter's commission phase, click the OL trip indicator. The motor stops, and the OL readout shows TRIPPED.
Basic Motor Starter
How to reset an overload relay
Find the cause first. Manual reset requires operator action, such as pressing a button. Automatic reset restores the overload contact, usually after cooling; the motor may restart if the control circuit still calls for it. NEC 430.43 prohibits an automatic-restart overload device if restarting can injure persons.
Try it →
In Basic Motor Starter's commission phase, trip the overload, then click the reset control. The sim does not show a cooling period or which reset type applies.
Basic Motor Starter
Motor overload trip troubleshooting
Let it cool and reset. If it trips soon again, check for motor shorts or grounds, and compare full-load amps to the relay setting.
Try it →
In Basic Motor Starter's troubleshooting phase, two scenarios report the same symptom: "Motor won't start." One is a stuck STOP contact and the other is an open overload contact. This phase shows no overload readout, so use your multimeter to tell them apart. The sim does not model full-load current, relay setting, or supply voltage.
Basic Motor Starter
What causes an overload to trip constantly?
A load that is too high or jammed, high ambient temperature, a defective overload protector, or windings that are short-circuited or grounded can each cause an overload to trip continually.
Try it →
Read Reading the Trip Pattern in Overload Relay Keeps Tripping: Diagnose It Now. It lists the possible causes for each trip pattern, and the simulators show the trip state, not these causes.
Overload Relay Keeps Tripping: Diagnose It Now
Which side of the coil does the overload go on? (NEMA vs NFPA 79)
A common NEMA-style starter drawing puts the overload contact on the return side, between the coil and L2. In a grounded control circuit, NFPA 79 allows that return-side contact where the conductors between the contact and coil do not extend beyond the control enclosure.
Try it →
In Basic Motor Starter's learn phase, read the Return Path section, which says both overload placements are electrically valid and that NEMA practice uses the return side. Then, in the wiring phase, delete the factory jumpers from A2 to 95 and from 96 to L2, and wire the coil from A2 to L2, L1 to 95, and 96 to the STOP. Do not leave a direct L1 to STOP wire in place. The sim accepts the hot-side overload.
Basic Motor Starter

Transformers and control voltage

Generally, control transformers step down the main line voltage to a control voltage of ___ or ___.
120 volts or 24 volts. Most industrial control circuits operate at one of these, and control transformers step line voltage down to them.
Try it →
In Control Transformer Starter's learn phase, open the step named Secondary Fuse & Control Power. It says the transformer outputs 24V between X1 and X2. The sim does not show 120 volts.
Control Transformer Starter
What current is used to calculate a control transformer's primary fuse limit?
Rated primary current: VA divided by primary voltage, for a single-phase transformer. The applicable NEC protection method sets the allowed percentage.
Try it →
In Control Transformer Starter's explorations, work Primary-Only Fuse Protection first, then Full Protection — Add Secondary Fuse.
Control Transformer Starter
How do you size a control transformer, and what does VA mean?
VA means volt-amperes, apparent power. Choose a VA rating at least equal to the sealed VA of operating coils plus other loads. Then confirm the manufacturer's inrush value for that rating exceeds the inrush VA of coils starting together plus running loads.
Try it →
Read How to Size a Control Transformer in Control Transformer Sizing: VA Load and NEC 430.72(C). Its example adds the sealed loads to 40 VA, then sums the inrush surge to about 190 VA and says to confirm the size against published inrush data.
Control Transformer Sizing: VA Load and NEC 430.72(C)

Variable-frequency drive

What is a VFD, and is there a VFD simulator online?
A VFD controls motor speed by changing the frequency of the power it delivers to the motor. Lower frequency means slower rpm.
Try it →
Open the VFD simulator and read the opening section. The narration says the drive controls motor speed by changing the frequency of the power it delivers.
Variable Frequency Drive
VFD won't accept a run command from the terminal strip even though keypad run works
With Start Source 1 active, P046=1 selects keypad run. In this VFD sim, set P046 to 2 (DigIn TrmBlk) and confirm t062 is 48 (2-Wire FWD).
Try it →
In the VFD simulator's troubleshooting phase, open the first scenario, where the keypad START runs the pump but the hardwired RUN toggle does nothing. The scenario loads with the disconnect off, so turn it on. With the drive stopped and the RUN toggle off, set P046 to 2, confirm t062 is 48, then turn the RUN toggle on.
Variable Frequency Drive