# How can I test if a 400VAC, 0.54uf capacitor is working?

#### babaliaris

Joined Nov 19, 2019
115
I can only supply 5vDC input to the capacitor. Will this help?

Currently, I have no multimeter, or a wave generator or anything.

Can I just charge the damn thing using 5vDC and then use a LED to see if it light up?

The capacitor does not seem to be damaged or blown.

Joined Jul 18, 2013
25,469
Rather difficult without equipment to measure a value!
Why do you suspect it? What circuit is it operating in? AC or DC?
Max.

#### Alec_t

Joined Sep 17, 2013
12,890
Can I just charge the damn thing using 5vDC and then use a LED to see if it light up?
Getting the LED to light (albeit extremely briefly) with the cap charged to 5VDC should be possible, but would be no guarantee that the cap would 'work' at higher AC voltages.

#### babaliaris

Joined Nov 19, 2019
115
Rather difficult without equipment to measure a value!
Why do you suspect it? What circuit is it operating in? AC or DC?
Max.
The capacitor is in a AC circuit and it's connected in series with a motor. The motor is doing a weird noise and it's not functioning properly. This capacitor from what I read is creating a phase change so the motor will rotate.

If it's not the capacitor, then probably is something wrong with the motor.

#### rsjsouza

Joined Apr 21, 2014
356
As @MaxHeadRoom said, finding the value itself will be challenging without even a multimeter. This capacitor does not hold much charge at 5V and therefore you would need a way to discharge it and measure the elapsed time with accuracy.

For example, if you have a meter with 10 MΩ input impedance, you can charge it to 5 V and measure the time it takes to reach 1.85 V (the time constant of the circuit). If the capacitor is in spec, it would somewhere around C (0.54µ) x R (10 M) = 5.4s

Details at:
https://www.electronics-tutorials.ws/rc/rc_2.html
Disclaimer: the capacitance will vary greatly according to its tolerance specs and the voltage applied to its terminals. The ideal scenario in your case (without any equipment) is to either find someone that has a test setup or simply replace with a compatible one and see if the whole setup works better.

Joined Jul 18, 2013
25,469
Are you sure it is only .54uf?
Is that if it is a start/run capacitor that is really small value.
What are the details of the motor? HP etc?
Is the cap Chinese origin?
Max.

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,088
Synchronous motors will tend to rock the shaft from side to side when the cap is out of circuit. Like the motor does not know what direction to turn.

Most have 3 or 4 visible wires. A common and CCW and CW. The cap is placed across CW and CCW. Ac power is applied to Common and (CW or CCW).

See http://www.hurst-motors.com/papbdirectdrive.html

Caps are in the right order of magnitude for this type of motor. They are usually used in clocks and ceiling fans. Motro speed varies with line frequency.

Short/open tests with DVM is not definative, Short is always bad.

Use an ohmmeter on a fixed scale and watch it charge, flip the leads and watch the ohmmeter again. This was best done with an analog meter,
Having something to compare too even better.

#### KeithWalker

Joined Jul 10, 2017
2,403
What kind of weird noises is the motor making? The most likely cause for noise is dry bearings, especially if it has sleeve bearings.

Joined Jul 18, 2013
25,469
The capacitor is in a AC circuit and it's connected in series with a motor. The motor is doing a weird noise and it's not functioning properly. This capacitor from what I read is creating a phase change so the motor will rotate.

If it's not the capacitor, then probably is something wrong with the motor.
Motors of this type are often have a PM field and are essentially Bi-polar stepper motors, and as mentioned are designed to run at a fixed RPM according to the applied frequency of the supply.
Superior Electric used to be a large manuf of these as well as L.V. DC stepper motors.
As I mentioned, Chinese origin caps do not fare well for AC motor applications.
If replacing you need AC motor run capable types.
Max.