2.6 Motor Connections, Control Circuits & Troubleshooting

Why connections and controls are tested

DLI's knowledge-area table lists motor and transformer connections (115/230 V and 230/460 V connections, wye and delta), motor control circuits (three-wire start-stop, start-stop-jog, control circuit transformers), and troubleshooting common electrical problems for the journeyworker exam. These questions test practical shop knowledge more than code lookups.

Dual-voltage three-phase motors

Most general-purpose motors have nine leads (T1 to T9) and can run on 230 V or 460 V. Each phase has two winding halves. For the higher voltage, the halves are in series; for the lower voltage, they are in parallel. Always follow the motor's connection diagram, but the standard NEMA connections are:

Connecting the series (high-voltage) arrangement to 230 V puts only half of rated voltage on each winding half, so the motor develops about one-quarter torque and may stall. Connecting the low-voltage arrangement to 460 V doubles the voltage on each winding and burns the motor out quickly.

Single-phase 115/230 V motors

A dual-voltage single-phase motor has two main windings (T1-T2 and T3-T4) and a start winding. For 230 V, the main windings are connected in series (T2 joined to T3). For 115 V, they are connected in parallel (T1 with T3, T2 with T4). The start winding connects across one main-winding section so it always sees about 115 V.

Reversing rotation

• Three-phase: interchange any two line leads. Reversing starters use two contactors that swap L1 and L3, with mechanical and electrical interlocks so both cannot close at once.
• Single-phase split-phase or capacitor-start: reverse the start winding leads relative to the run winding. Swapping the two line leads does not reverse a single-phase motor.

Wye-delta (star-delta) starting

A motor built to run in delta can be started in wye. In wye, each winding sees $1 \div \sqrt{3}$ (58%) of line voltage, so starting current and starting torque drop to about one-third of their across-the-line values. After acceleration, the starter switches the windings to delta. The method requires a six-lead (or 12-lead) motor designed for delta running.

Two-wire and three-wire control

The three-wire start-stop circuit

Read the circuit from L1 (or the control transformer X1) to L2 (X2):

1. The stop button (normally closed) is in series with everything.
2. The start button (normally open) is in series after stop.
3. A normally open auxiliary holding (seal-in) contact of the starter, labeled M, is wired in parallel with the start button.
4. The starter coil M follows, then the overload relay contact (OL), normally closed, in series.

Pressing start energizes M; the M auxiliary contact closes and keeps the coil energized after the button is released. Pressing stop, opening an overload contact, or losing control voltage drops out the coil, and the holding contact opens. The motor will not restart until someone presses start again.

Multiple stations: to add pushbutton stations, wire additional start buttons in parallel with each other and stop buttons in series with each other.

Adding jog (inch)

Jogging runs the motor only while a button is held, with no seal-in. Common methods:

• A selector switch in series with the holding contact: in the "jog" position it opens the seal-in path, so the start button acts as a jog button.
• A jog pushbutton with a normally open contact that energizes the coil and a normally closed contact that opens the holding circuit at the same time.
• A control relay arrangement that seals in only for run.

The design goal is the same: during jog, the holding contact must not be able to keep the coil energized.

Control circuit transformers and protection

Control circuits are often fed through a control circuit transformer (CCT) that steps line voltage (such as 480 V) down to 120 V. NEC 430.72 requires overcurrent protection for motor control circuit conductors and for the transformer. Many CCTs have secondary fuses and primary fuses. Section 430.74 requires the control circuit to be arranged so that an accidental ground in the control circuit will not start the motor or bypass manually operated shutdown devices or automatic safety shutdown devices. This is why the coil is typically connected directly to the grounded side (X2) and the switching contacts are placed on the ungrounded side (X1).

Troubleshooting common problems

DLI expects you to reason from symptoms to causes. A practical sequence:

1. Verify supply voltage at the equipment terminals with the load running. Low voltage under load but normal voltage at no load points to a high-resistance connection or undersized conductors (voltage drop).
2. Check control voltage at the coil. If the coil is energized and the contactor does not close, the coil or contactor is defective; if there is no coil voltage, trace through the stop, start, overload, and safety contacts.
3. Check for tripped overloads and find out why: mechanical binding, low voltage, a stalled load, or single-phasing.
4. Measure current in all three phases. Unequal currents point to unbalanced voltage, a high-resistance connection, or a winding problem. A small voltage imbalance produces a much larger current imbalance and extra heating.
5. Test insulation with a megohmmeter after locking out and disconnecting the motor. A ground fault shows as low resistance from a winding to the frame.

Classic symptoms