12.2 Motors, Drives & Electrical Fundamentals
Key Takeaways
- Ohm's law states that voltage equals current times resistance, and power in a DC or single-phase resistive circuit equals volts times amps.
- Three-phase power is calculated as volts times amps times power factor times the square root of three, or about 1.732.
- Single phasing occurs when one leg of a three-phase supply is lost, causing the remaining legs to draw excessive current and overheat the motor.
- Reversing any two of the three leads on a three-phase motor reverses its direction of rotation.
- A variable frequency drive changes motor speed by varying frequency, and the affinity laws show that power varies with the cube of speed.
12.2 Motors, Drives & Electrical Fundamentals
The WPI Need-to-Know Criteria list general electrical principles including troubleshooting breakers, relays, and circuits as supporting knowledge at the intermediate level across nearly every content area. Operators are not electricians, but they are expected to read a nameplate, interpret an amperage reading, and know when to stop and call one.
1. The Fundamental Relationships
Ohm's law
where E is voltage in volts, I is current in amperes, and R is resistance in ohms.
Power
| Circuit | Formula |
|---|---|
| DC or single-phase resistive | $P = E \times I$ |
| Single-phase with power factor | $P = E \times I \times \text{PF}$ |
| Three-phase | $P = E \times I \times \text{PF} \times \sqrt{3}$, where $\sqrt{3} \approx \textbf{1.732}$ |
Worked example. A three-phase motor draws 42 amps at 460 volts with a power factor of 0.86.
Converting to horsepower drawn: 28,780 ÷ 746 ≈ 38.6 hp of electrical input.
Power factor
Power factor is the ratio of real power (kW) to apparent power (kVA). Inductive loads such as motors lag, producing a power factor below 1.0. A low power factor means the utility must deliver more current for the same useful work, and utilities commonly impose a power factor penalty. Correction uses capacitor banks. Motors running lightly loaded have notably poor power factor, which is one reason oversizing motors is costly.
2. Three-Phase Induction Motors
The workhorse of the plant. Key nameplate data an operator must read:
| Nameplate item | Meaning |
|---|---|
| Horsepower | Rated mechanical output |
| Voltage | Design supply voltage, e.g., 230/460 V |
| Full load amps (FLA) | Current at rated load — compare your clamp meter reading to this |
| RPM | Full-load speed (slightly below synchronous speed; the difference is slip) |
| Service factor (SF) | Permissible overload multiplier, e.g., 1.15 means 15% above rated hp continuously |
| Frame size | Physical dimensions for replacement |
| Insulation class | Temperature capability: A, B, F, H in ascending order |
| Enclosure | ODP (open drip proof), TEFC (totally enclosed fan cooled), explosion-proof |
| Efficiency | Nominal efficiency; premium efficiency motors pay back quickly on continuous duty |
Synchronous speed and slip
At 60 Hz: 2-pole = 3,600 rpm, 4-pole = 1,800 rpm, 6-pole = 1,200 rpm. Actual full-load speed is a few percent lower — that difference is slip, and it is what produces torque in an induction motor.
Reversing rotation
Swap any two of the three phase leads. This is the single most-tested electrical fact in operator exams, and it matters because a centrifugal pump running backward still moves some water and can fool an operator into chasing a hydraulic problem.
3. Motor Failures and Protection
| Failure | Cause | Indication |
|---|---|---|
| Overload / overheating | Mechanical binding, excessive load, high ambient temperature, clogged cooling fins, low voltage | Amps above FLA, overload relay trips, burnt smell, discolored windings |
| Single phasing | Loss of one leg of the three-phase supply — a blown fuse, an open contact, a broken conductor | Motor hums but will not start, or continues running while drawing very high current on the remaining legs. Rapidly destroys the windings |
| Voltage imbalance | Unequal phase voltages | A small percentage imbalance produces a much larger current imbalance and significant heating |
| Insulation failure | Moisture, heat, age, contamination | Low megohm reading on insulation resistance test; ground fault |
| Bearing failure | Lubrication, misalignment, belt tension, shaft currents from VFDs | Noise, heat, vibration |
| Moisture intrusion | Failed seals in submersibles | Seal-fail alarm; low insulation resistance |
Protective devices
| Device | Protects against |
|---|---|
| Fuses and circuit breakers | Short circuits and ground faults — fast, high-current events |
| Overload relays (thermal or electronic) | Sustained overcurrent — slower, heat-based; sized to motor FLA and service factor |
| Phase monitor / phase failure relay | Single phasing, phase reversal, voltage imbalance |
| Ground fault protection | Current leaking to ground |
| Moisture and over-temperature sensors | Built into submersible motors |
The distinction that gets tested: a breaker or fuse protects the circuit from short circuits; an overload relay protects the motor from running too hot. They are not interchangeable, and a repeatedly tripping overload should never be "solved" by installing a larger heater or a bigger breaker.
4. Starting Methods
An induction motor draws locked rotor current of roughly 6 to 8 times full-load amps at start. On large motors that inrush causes voltage dip and mechanical shock.
| Method | Character |
|---|---|
| Across-the-line (full voltage) | Simplest and cheapest; full inrush and full starting torque shock |
| Reduced voltage (autotransformer, part winding, wye-delta) | Lower inrush, lower starting torque |
| Solid-state soft starter | Ramps voltage smoothly; reduces both electrical inrush and mechanical/hydraulic shock (helps prevent water hammer) |
| Variable frequency drive | Ramps frequency and voltage; full speed control |
5. Variable Frequency Drives and the Affinity Laws
A VFD rectifies incoming AC to DC and inverts it back to AC at a variable frequency, changing motor speed. Because a centrifugal machine's behavior follows the affinity laws, this is where the energy savings live:
- Flow varies directly with speed
- Head varies with the square of speed
- Power varies with the CUBE of speed
Worked example. A pump delivering 1,400 gpm at 60 ft using 40 hp is slowed to 80% speed.
- Flow: 1,400 × 0.80 = 1,120 gpm
- Head: 60 × 0.80² = 60 × 0.64 = 38.4 ft
- Power: 40 × 0.80³ = 40 × 0.512 = 20.5 hp
A 20% speed reduction cuts power almost in half. This is why VFDs replace throttling valves wherever flow must be varied — throttling wastes the energy as heat across the valve, while slowing the pump never generates it.
VFD cautions
- Minimum speed limits — pumps must still overcome static head, and motors need cooling airflow. Below roughly 25–30 Hz, a TEFC motor's shaft-mounted fan may not cool it adequately.
- Shaft currents can pit bearings; shaft grounding rings or insulated bearings are used on larger drives.
- Harmonics feed back into the electrical system and may require line reactors or filters.
- Heat — VFD enclosures need ventilation and clean filters.
6. Electrical Safety at the Plant
- Lockout/tagout is mandatory before any work on electrically driven equipment — covered fully in Section 13.3.
- Test before you touch. Verify de-energization with a meter you have proven on a known live source.
- Arc flash is a serious hazard at motor control centers; boundaries, labeling, and appropriate arc-rated PPE are required.
- Only qualified persons open energized enclosures. An operator's job is to observe, report, and isolate — not to troubleshoot inside a live MCC bucket.
A three-phase pump motor rotates backward after a rewiring job. How is rotation corrected?
A pump running at full speed uses 60 hp. If a variable frequency drive slows it to 70 percent speed, approximately what power will it require?
A three-phase motor hums but will not start, and when it does run it draws very high current on two legs. What has most likely occurred?
What does a motor nameplate service factor of 1.15 indicate?