Motor Capacitors Selection Guide
A run capacitor stays in the circuit as long as the motor runs, while a start capacitor drops out once the motor has started; selection rests on this difference.
This guide covers what to look at when choosing products in the Motor Capacitors category.
The difference between a run capacitor and a start capacitor
The two types do different jobs in the same motor, and their construction is also different. IEC 60252-1 defines a run (operating) capacitor as a capacitor that, together with the motor's auxiliary winding, helps with starting, improves torque during operation and mostly stays in the circuit as long as the motor runs. A start capacitor, on the other hand, supplies a leading-phase current to the auxiliary winding and is taken out of the circuit after the motor has started; the requirements for this type are given in IEC 60252-2.
- Run Motor Capacitors: made for continuous duty and mostly polypropylene film capacitors. Some of the Hawcon models in this group are CBB 60 series, with a plastic tube housing.
- Start Motor Capacitors: non-polarized aluminum electrolytic capacitors designed only for a short starting period; their capacitance is mostly larger than that of run types.
For this reason, one is not fitted in place of the other. In motors that need high starting torque, the two types can be present together; in this case the start capacitor works in parallel with the run capacitor during starting.
How to read the label
- Capacitance (µF): run capacitors carry a single value, for example 12.5 µF or 40 µF. Start capacitors carry two numbers such as 124-149 µF; the capacitance lies between these two limits. The value of the new capacitor must be the same as the value on the motor label or on the removed capacitor.
- Rated voltage (V AC): the Hawcon run models in this category are labeled 400-450 V and the Hawcon start models 220-250 V; Tense product names do not state the voltage. The voltage of the capacitor fitted as a replacement must not be lower than that of the one removed; IEC 60252-1 states that operating above the rated voltage shortens capacitor life.
- Housing dimensions (diameter x length, mm): dimensions such as 35x65 or 50x106 show whether the capacitor will fit the seat and clamp on the motor. The same capacitance can be listed in different housing dimensions; for example, 80 µF exists in both a 60x120 and a 55x123 mm housing.
- Connection and mounting: look at whether the leads of the removed capacitor are terminals or wires and how the housing is fastened to the motor. Hawcon CBB 60 models are listed as "screw, with terminals"; verify the detail on the product card.
- Safety class: IEC 60252-1 defines protection classes from S0 to S3 for run capacitors. If the motor or equipment manufacturer requires a particular class, look for this marking on the label.
Failure signs and replacement
The capacitance of a run capacitor drops over the years; the motor struggles to start or cannot reach full power. Swelling or bulging may be visible on the housing of a faulty capacitor, but the absence of visible damage does not show that the capacitor is sound. Capacitance is read with a measuring instrument that can measure capacitance and compared with the label value.
If the switch in the start circuit stays closed, the start capacitor remains in the circuit continuously; in such a fault, replacing only the capacitor is not enough, and the centrifugal switch or start relay must also be checked. A capacitor can hold a charge even after the power is cut; measurement and replacement should be done according to the manufacturer's instructions, by people competent in this work.
Relationship to power capacitors and lighting capacitors
A motor capacitor is rated in µF, while the Power Capacitors in a power factor correction panel are rated in kVAr, and they are subject to separate standards: IEC 60252-1 excludes low-voltage shunt power capacitors from its scope and refers to IEC 60831-1. The product to look for to compensate the reactive power of a single-phase load is in the Single-Phase Power Capacitors group.
Some Hawcon run models are listed under the name "motor and lighting capacitor". Fluorescent and discharge lamp capacitors are also outside the scope of IEC 60252-1 and are left to IEC 61048; before using one in a luminaire circuit, verify the area of use in the product data.
Brands in this category
- Hawcon: 400-450 V run capacitors with the CBB 60 series plastic tube housing and the "motor and lighting" type, and 220-250 V starting (start) capacitors. Run names state the capacitance, voltage and housing dimensions; start names state the capacitance range and voltage.
- Tense: run and starting capacitors; the product names include only the capacitance value or range.
Order notes
Capacitors are sold by the piece. Capacitance is written in product names as "40 uF" for Hawcon and "40uf" for Tense; try both spellings in the site search. For Tense models, verify the rated voltage and housing dimensions before ordering. For a product listed as made to order, the lead time is confirmed before ordering.
Comparison and selection table
| Selection Criterion | Permanent Split (Motor Run) Capacitor | Intermittent (Motor Start) Capacitor | Replacement & Sizing Rules |
|---|---|---|---|
| Duty Cycle | Remains continuously in circuit with auxiliary winding | Engaged only for 1–3 seconds during starting, then switched out | Never substitute a start capacitor for a continuous run capacitor |
| Capacitance & Voltage | 1.5 µF – 100 µF, 400 V / 450 V AC (self-healing MKP film) | 50 µF – 400+ µF, 250 V / 330 V AC (electrolytic construction) | Match the nameplate capacitance (µF) within ±5% |
| Terminals & Mounting | Flying wire leads or dual 6.3 mm Faston tabs, M8 base stud | Bakelite/plastic cylinder paired with centrifugal switch/relay | Verify cylinder diameter fits inside motor terminal box |
| Typical Application | Pumps, fans, blowers, burners, HVAC compressors | Loaded air compressors, refrigeration & high-torque starts | A 450 V AC capacitor can safely replace a 400 V AC unit |
Frequently asked questions
Can a start capacitor be fitted in place of a run capacitor?
No. A start capacitor is a non-polarized electrolytic capacitor designed only for a short starting period, and after the motor has started it is taken out of the circuit by the centrifugal switch or start relay. A run capacitor, on the other hand, stays in the circuit as long as the motor runs and is made for continuous duty. The capacitance level and rated voltage of the two types are also different; choose the type with the same duty as the removed capacitor.
What happens if the µF value of a motor capacitor is chosen larger or smaller than the label?
Capacitance determines the phase difference between the main winding and auxiliary winding currents. When the value is off, the magnetic field around the rotor becomes unbalanced; the motor turns unevenly, runs noisily, loses performance, draws more energy and heats up. For this reason, the idea that a higher value will start the motor more strongly is not correct. Take the µF value on the motor label or on the removed capacitor exactly.
Can a 450 V capacitor be used in place of one marked 400 V?
A capacitor whose rated voltage is the same as or higher than the removed one can be used; one with a lower voltage is not used. IEC 60252-1 states that operating above the rated voltage shortens capacitor life. The capacitance value, however, must stay the same. Because the housing of a higher-voltage model may be larger, check from the diameter and length whether it fits the seat on the motor.
How many µF capacitor is fitted to a water pump or hydrophore motor?
There is no single answer by motor power; the value depends on the motor's winding design, and two motors of the same power may need different capacitors. The correct value is written on the motor label or on the removed capacitor. If the label is unreadable, consult the data of the pump or motor manufacturer. After finding the value, also match the duty (run or start), the rated voltage and the housing dimensions.
What does a double value such as 124-149 µF on a start capacitor mean?
Because start capacitors are electrolytic in construction, their capacitance is labeled with a range instead of a single value; 124-149 µF shows that the capacitance of the capacitor lies between these two limits. When choosing a new one, look for the same range as on the removed capacitor. The voltage value must also be checked separately; the Hawcon start models in this category are labeled 220-250 V.