Will a bad run capacitor prevent a motor from starting?

Asked By: Yone Neundorf | Last Updated: 2nd June, 2020
Category: home and garden indoor environmental quality
4.9/5 (799 Views . 21 Votes)
Electric Motor Starting Capacitor or Run Capacitor FAQs. If either or both start and run capacitors are defective the motor may try to start but will hum and won't keep running. When the circuit breaker trips or fuse blows at compressor start-up, we suspect a bad compressor motor.

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Considering this, will a motor start with a bad run capacitor?

There are also a number of symptoms that will tell you if the capacitor on a motor is faulty: A motor with a short-circuited capacitor often will start and run, but with less starting torque and lower full-load rpm than normal.

Subsequently, question is, why does a motor need a start capacitor? With a capacitor connected in series to the secondary winding, the magnetic field it produces lags behind the magnetic field generated by the primary winding. This difference in phases creates a starting torque and the motor starts to rotate. Capacitors that allow a motor to start rotating are called start capacitors.

Then, what causes a motor start capacitor to fail?

Many single-phase compressors require a start capacitor to assist in starting the motor. These capacitors will occasionally fail, causing a compressor to fail to start. Overheating is a primary cause of a failed start capacitor. If a start capacitor stays in the circuit too long, it will overheat and fail.

What happens if a run capacitor fails?

The most common problem that bad capacitors can cause is “hard starting.” This is when the compressor of an AC has difficulty starting up, stutters trying to turn on, and then shuts off a short while later. In most cases of capacitor problems, such as damage or a loss of charge, the capacitor will need to be replaced.

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What are the signs of a bad capacitor?

The most common signs and symptoms of a bad AC capacitor include:
  • AC not blowing cold air.
  • AC takes a while to start once you turn it on.
  • Humming sound coming from your air conditioner.
  • AC shuts off on its own.
  • AC won't turn on.

What are the symptoms of a bad run capacitor?

Signs of Capacitor Problems
  • Cracks or bulging of the capacitor.
  • A burning odor coming from the capacitor.
  • Smoke coming from the capacitor.
  • Oozing fluid around the capacitor and circuit board.
  • Insufficient charge when checked with a multimeter.

Will a motor run without a capacitor?

Capacitor start motors use capacitor only for starting, after which the capacitor is switched out by a centrifugal switch. Motor thereafter runs without capacitor on one winding.

How do you check if a capacitor is working?

To test the capacitor with a multimeter, set the meter to read in the high ohms range, somewhere above 10k and 1m ohms. Touch the meter leads to the corresponding leads on the capacitor, red to positive and black to negative. The meter should start at zero and then moving slowly toward infinity.

How can you tell the difference between a run and a start capacitor?


A start capacitor is found in the circuit of start windings when the motor is starting. This capacitor contains a higher capacitance than a run capacitor. The start capacitor creates a current to voltage lag in the separate start windings of the motor.

How do you know if your electric motor is bad?

With a multimeter set to low ohms (usually 200), test between each winding terminal and the metal casing of the motor. If there is any reading on any of these then the motor is bad, do not use it. You may find that when it runs ungrounded that the casing becomes live at up to supply voltage.

Can a bad capacitor ruin a motor?

Capacitor Failures Can Ruin Your Day . . . and Motors! Using the wrong capacitor rating or a poor quality capacitor can adversely affect the operation of the motor, the compressor or an entire HVAC system. The motor run capacitor is an essential component in HVAC systems, but all capacitors are not created equal.

What happens if capacitor fails in motor?

A bad motor capacitor may cause starting problems or could shut off the motor while running. Motor capacitors store electrical energy for the motor to use. The higher the capacitance of the capacitor the more energy it can store. If it is not attached to the motor at two points it needs to be replaced.

How do you size a capacitor for a motor?

Multiply 0.5 times the square of the voltage. Call this result "x.". Continuing the example, you have 0.5 times 11.5 volts times 11.5 volts, or 66.1 square volts for "x". Divide the start-up energy requirement, in joules, of the motor by "x" to arrive at the capacitor size needed in farads.

What causes capacitor failure?


Capacitors are major causes of failure in electronic systems. Capacitors fail for various reasons, such as wear out, aging, reverse polarity voltage caused damage, mechanical damage during assembly, dielectric failure, high temperature induced damage during soldering, operation, testing, etc.

Can a single phase motor run without a capacitor?

no, not necessary for every single -phase AC motor to have a capacitor, the function of the capacitor in the motor is to draw current which is leading to the current drawn by the motor main winding,so that a phase displacement will occur and a rotating field is produced by the resultant current.

Can you replace a capacitor with a higher UF?

You can almost always replace a capacitor with one of a higher voltage. This is the limiting factor of a capacitor due to dielectric breakdown voltages that the manufacturer chose. Varying capacitance gets a little trickier.

Can I put a bigger capacitor on a motor?

A motor that requires a 7.5 mfd capacitor will not work with a 4.0 mfd capacitor. This is not to imply bigger is better, because a capacitor that is too large can cause energy consumption to rise. In both instances, be it too large or too small, the life of the motor will be shortened due to overheated motor windings.

Can I bypass a capacitor?

In this case, the bypass capacitor is a first line of defense. It eliminates voltage droops on the power supply by storing electric charge to be released upon the occurrence of a voltage spike. Moreover, it accomplishes this task at a wide range of frequencies by creating a low-impedance path to ground for the supply.

What is the price of a capacitor?


Actual capacitor cost can run from $10 - $100 depending on the brand and model and if single or dual-voltage - usually around $40 - $60, though some original equipment manufacturers charge up to $200.

How do you size a capacitor for a compressor?

How to Size Capacitors for a Compressor
  1. Consult the owner's manual for your air conditioner to see if it lists the capacitor size it requires.
  2. Multiply the full load amps by 2,650.
  3. Multiply the working power of the motor (it should be listed in kilowatts in your owner's manual) by the power factor in table 1 included in the Resource below.

How do you wire a capacitor to a motor?

Connect the positive terminal of a small hobby motor to the first terminal of a resistor. Connect the second terminal of the resistor to the positive terminal of the capacitor. Use a resistor in the 10K-ohm to 100K-ohm range at first. The capacitor should be in the 1- to 100-farad range.