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newSchoolWorks.com › VoTechWorks › Building & Construction › HVAC/R › Electrical Controls for HVAC/R Systems › Lesson 7
BLD-HVAC07-03 · Lesson 7 · 11.5 clock hours · lab

Capacitors: Run, Start, and Dual-Round

Objective. Students will safely discharge, test, and evaluate run capacitors and start capacitors using a capacitance meter, and replace an out-of-tolerance capacitor while following discharge and LOTO procedures.

What Capacitors Do in HVAC/R Systems

Capacitors store an electrical charge and release it to create a phase shift in the current supplied to single-phase motors. Without that phase shift, a single-phase AC motor cannot develop enough starting torque or run efficiently. HVAC/R equipment relies on two types of capacitors for motor operation: run capacitors and start capacitors. Many condensers also use a dual-round capacitor, which combines both functions in one cylindrical can.

Run Capacitors

A run capacitor stays in the circuit the entire time the motor operates. It is an oil-filled, metal-can component rated in microfarads (µF) and voltage — typically in the range of 3 µF to 70 µF for HVAC motors. Because it runs continuously, it must be a high-quality, low-loss type. Run capacitors are usually rated at 370 VAC or 440 VAC. A run capacitor that drifts out of tolerance forces the motor winding to work harder, raises amperage draw, and shortens motor life.

Start Capacitors

A start capacitor is switched into the circuit only during the brief start-up period — usually by a start relay or potential relay — and then removed once the motor reaches approximately 75–80 percent of running speed. Start capacitors are electrolytic, rated in much higher µF values (often 88 µF to several hundred µF), and are not designed for continuous-duty use. They are identifiable by their plastic or Bakelite case and the bleed resistor wired across their terminals, which slowly dissipates charge when the capacitor is switched out.

Dual-Round Capacitors

A dual-round (also called a dual-run) capacitor replaces two separate run capacitors with a single cylindrical component. One section serves the compressor motor; the other serves the condenser fan motor. Terminals are labeled C (common), HERM (hermetic compressor), and FAN. When replacing a dual-round capacitor, both µF ratings — printed separately on the label — must match the equipment specifications.

Stored-Charge Hazard — Read Before Touching Any Capacitor

A capacitor can hold a lethal charge even after the unit has been de-energized. Never touch capacitor terminals with bare hands or uninsulated tools before discharging. The discharge must be performed before any hands-on work, every single time.

Two-resistor discharge procedure:

1. Apply LOTO — lock out, tag out the disconnect, and verify zero voltage at the unit with a DMM.

2. Obtain the 10,000 Ω, 5 W discharge resistor tool with insulated leads.

3. With insulated-grip tools, touch one lead to one terminal and the other lead to the second terminal simultaneously. Hold contact for at least five seconds.

4. For a dual-round capacitor, discharge HERM-to-C, FAN-to-C, and then HERM-to-FAN.

5. Verify with a DMM set to DC volts that terminal voltage reads zero before proceeding.

Using a screwdriver blade to short capacitor terminals is prohibited — the arc can damage the capacitor, destroy the tool, and injure you. Use only the resistor discharge tool.

Testing Capacitors with a Capacitance Meter

Set the meter to capacitance (µF) mode. After discharging, touch the meter probes to the capacitor terminals. For a dual-round, measure each section separately (HERM-to-C, then FAN-to-C). Record the measured value and compare it to the nameplate-rated µF.

Industry-accepted tolerance for run capacitors: ±6% of nameplate rating. Some manufacturers publish ±5%; use the tolerance stated on the equipment documentation when available. A capacitor reading outside that range is considered failed and must be replaced.

Start capacitors have a wider rated tolerance range printed on the component itself (for example, 88–108 µF). The measured value must fall within the printed range.

Replacing a Dual-Round Capacitor

1. With LOTO applied and capacitor discharged, photograph the existing wiring or sketch the wiring diagram before removing any wires.

2. Remove wires one at a time, labeling each with tape if needed.

3. Release the mounting strap or bracket and remove the old capacitor.

4. Install the new capacitor — same µF ratings for each section and equal or greater voltage rating.

5. Reconnect wires: C (common) to the common wire, HERM to the compressor start/run wire, FAN to the fan motor wire. Verify against the unit wiring diagram before re-energizing.

6. Remove LOTO per procedure, energize the unit, and monitor starting amperage and running amperage to confirm normal operation.

Service Documentation

Record the nameplate µF rating, measured µF value, percent deviation, pass/fail determination, and action taken on the service report form. Good documentation protects you legally and helps the next technician.

PPE for This Lab

Safety glasses are required at all times. Insulated-grip tools (1,000 V rated) must be used whenever handling capacitors. Do not handle capacitors with wet hands. If a capacitor is visibly bulged, leaking oil, or has a ruptured bleed vent, treat it as hazardous and inform the instructor before touching it.

Key terms8

Run capacitorAn oil-filled capacitor that remains in the motor circuit during normal operation to improve efficiency and power factor; rated in µF and AC voltage.
Start capacitorAn electrolytic capacitor switched into the circuit only during motor start-up to boost starting torque; removed from the circuit once the motor reaches operating speed.
Dual-round capacitorA single cylindrical capacitor housing two independent run-capacitor sections — one for the compressor and one for the fan motor — with terminals labeled C, HERM, and FAN.
Microfarad (µF)The unit of capacitance used to rate HVAC capacitors; one millionth of a farad. Nameplate and measured values are both expressed in µF.
Percent deviationThe difference between the measured capacitance and the nameplate-rated capacitance, expressed as a percentage of the rated value; used to determine whether a capacitor is within tolerance.
Discharge resistorA resistor (commonly 10,000 Ω, 5 W) used to safely bleed stored charge from a capacitor before handling; prevents dangerous arcing and electric shock.
Lockout/Tagout (LOTO)An OSHA-required safety procedure that physically locks an energy-isolating device in the off position and attaches a tag, ensuring the equipment cannot be energized while work is in progress.
ToleranceThe allowable range of variation from a rated value. Run capacitors in HVAC service are generally accepted within ±5% to ±6% of nameplate µF rating.

Worked examples4

1. A run capacitor is rated at 45 µF. Your capacitance meter reads 42.3 µF. Is the capacitor within the ±6% tolerance?
  1. Calculate the allowed deviation in µF: 45 µF × 0.06 = 2.7 µF.
  2. Determine the acceptable range: 45 − 2.7 = 42.3 µF (lower limit); 45 + 2.7 = 47.7 µF (upper limit).
  3. Compare the measured value to the range: 42.3 µF falls exactly on the lower limit of 42.3 µF.
  4. Calculate percent deviation to confirm: (45 − 42.3) ÷ 45 × 100 = 2.7 ÷ 45 × 100 = 6.0%.
Answer: The capacitor reads exactly at the ±6% lower boundary (6.0% deviation). It is at the edge of acceptable; document it carefully and flag it for monitoring, but it is technically within the 6% limit. Many technicians would replace it given it is at the limit — record your reasoning on the service form.
2. A dual-round capacitor is rated 35/5 µF. You measure 32.1 µF on the HERM section and 4.8 µF on the FAN section. Apply a ±6% tolerance. Which section, if any, has failed?
  1. HERM section — allowed deviation: 35 × 0.06 = 2.1 µF. Acceptable range: 32.9 µF to 37.1 µF. Measured: 32.1 µF. Deviation: (35 − 32.1) ÷ 35 × 100 = 8.3%. This is outside ±6%.
  2. FAN section — allowed deviation: 5 × 0.06 = 0.3 µF. Acceptable range: 4.7 µF to 5.3 µF. Measured: 4.8 µF. Deviation: (5 − 4.8) ÷ 5 × 100 = 4.0%. This is within ±6%.
  3. Because both sections share one component, and the HERM section has failed, the entire dual-round capacitor must be replaced.
Answer: The HERM section has failed (8.3% low, outside ±6%). The FAN section is serviceable (4.0% low). Replace the entire dual-round capacitor.
3. A run capacitor is rated at 10 µF. The technician measures 10.7 µF. What is the percent deviation, and is it serviceable?
  1. Calculate percent deviation: (10.7 − 10) ÷ 10 × 100 = 0.7 ÷ 10 × 100 = 7.0%.
  2. Compare to ±6% tolerance: 7.0% exceeds 6%.
  3. The capacitor is high by 7.0%, which is outside the accepted tolerance.
Answer: Percent deviation is +7.0%. The capacitor is out of tolerance (high) and should be replaced.
4. A start capacitor is labeled 88–108 µF. The meter reads 95 µF. Is it serviceable?
  1. Identify the rated range printed on the start capacitor: 88 µF to 108 µF.
  2. Compare the measured value: 95 µF.
  3. 95 µF is greater than 88 µF and less than 108 µF, so it falls within the printed range.
Answer: The start capacitor reads 95 µF, which is within the 88–108 µF rated range. It is serviceable.

The math in this lesson1

Learners compute percent deviation of measured microfarads from rating against a ±6% tolerance.
7.RP.3
Brush up in the math halls at newSchoolWorks.com:
In class: Safety briefing: instructor demonstrates stored-charge hazard and correct two-resistor discharge procedure before any student handles a capacitor; students practice discharge on a charged test capacitor before proceeding. · Capacitor evaluation lab: students use a capacitance meter to measure µF values of a set of run and start capacitors labeled with rated values, calculate percent deviation, and apply the industry-accepted tolerance range to grade each as serviceable or failed. · Replacement exercise: students remove an out-of-tolerance dual-round capacitor from a condenser training unit (de-energized and LOTO applied), install a same-rated replacement, verify connections against the wiring diagram, and record findings on a service report form.
Materials: Assortment of run capacitors (various µF ratings), dual-round capacitors, start capacitors, capacitance meter, two-resistor discharge tool (10,000 Ω, 5 W), insulated-grip screwdrivers, digital multimeter, condenser training unit, LOTO hardware, service report forms
Check: Discharge procedure performed before every capacitor contact, capacitance meter readings documented with pass/fail notation, replacement installed correctly and matching rated µF ±5% = pass. Any contact with an undischarged capacitor = immediate stop and safety review.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Electrical fundamentals / motor circuits
1. What is the primary purpose of a run capacitor in a single-phase HVAC motor circuit?
Safety / electrical hazards
2. Before touching a capacitor that has been de-energized, a technician MUST:
Troubleshooting / electrical testing
3. A run capacitor is rated at 40 µF. A capacitance meter reads 36.8 µF. Applying a ±6% tolerance, this capacitor is:
Electrical components / controls
4. On a dual-round capacitor, the terminal labeled HERM connects to the:
Electrical components / controls
5. Which of the following describes a start capacitor compared to a run capacitor?
Safety / regulatory compliance
6. LOTO (Lockout/Tagout) is required before replacing a dual-round capacitor because:
Installation and replacement
7. When replacing a dual-round capacitor, a technician discovers the failed component is rated 45/7.5 µF at 440 VAC. Which replacement is acceptable?
Safety / electrical hazards
8. A technician is discharging a dual-round capacitor. After discharging HERM-to-C and FAN-to-C, what additional discharge step is required?
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