I have a single phase asynchronous motor rated 230V, 500W. It has three terminals labeled U, V, W. I connected L1 to U and N to V (other way round makes no difference though). Between V and W is a 500V 18µF capacitor.

Without load the motor starts spinning and reaches top speed within two seconds. However when I put light load it starts rotating very slowly and is accelerating only slowly too. Once it reaches maybe ~ 2/3 of it's top speed of 2340 RPM (labeled, not measured) it get's much more torque.

Is there anything I can do so the motor has more torque when starting, without affecting torque at top speed? Maybe replacing the capacitor but to what value?

  • \$\begingroup\$ single phase doesn't have U,V,W connections. It has to have 4 wires, ie two phases. I guess you have a three phase motor. \$\endgroup\$ Commented Nov 6, 2018 at 20:15
  • \$\begingroup\$ It certainly is single phase. It is also very old. Maybe at that times an existing wire connecting plate was used? \$\endgroup\$
    – JohnDoe
    Commented Nov 6, 2018 at 20:22
  • \$\begingroup\$ Synchronous motors, whether single-phase or three-phase, inherently have very little load at low speed. Is this a synchronous motor? \$\endgroup\$
    – Hearth
    Commented Nov 6, 2018 at 20:29
  • \$\begingroup\$ Single phase motors must have some way of generating a phase shift to develop starting torque. Usually this is done with a capacitor and a start winding. \$\endgroup\$
    – τεκ
    Commented Nov 6, 2018 at 20:30
  • \$\begingroup\$ @Felthry it is asynchronous without any slip rings. \$\endgroup\$
    – JohnDoe
    Commented Nov 6, 2018 at 20:47

2 Answers 2


This is the usual behaviour of asynchronous motors. See their torque-speed characteristic:


The acceleration depends on the difference between driving machine torque and working machine torque at any particular speed. If your working machine has a pretty high torque at low speeds, you may even run into this situation:


Here, you cannot even reach the green operating point. The machine is stuck on the left. You need a clutch to run this working machine with this driving machine.

Using a bigger starter cap may improve things a little bit but don't expect wonders. The problem isn't the cap but the machines you have.

  • \$\begingroup\$ Thank you, I will accept that as the correct answer. While it doesn't fully answer my question I have learnt about torque characteristics and that this is a somewhat inherent limitation. \$\endgroup\$
    – JohnDoe
    Commented Nov 7, 2018 at 20:35

Maybe the cap is just worn. It happens, you won't see by eye. The polymer caps are self healing, when the high voltage punches the plates trough a polymer isolates the shorted dots. So after time the cap has no capacitance. Other types caps like electrolyte dries over time and has no capacitance.

Now, basically you have two types of capacitors: run and starting. The starting cap is engaged by means of centrifugal switch mounted on a motor shaft, when it reaches certain speed it is disconnected. Meanwhile the run capacitor is always connected. All combinations are possible: run, run + start, start.

A higher capacitance will give you extra torque and more current load (motor heating), but if reving at high speeds - ie. motor unloaded it will be destroyed due to high voltage present on the terminals. Lower capacitance may give you not enough torque to start, so that's why an extra run cap isneeded.

It all depends on the type of load - a pump does not need a start cap, but only a run cap because it never spins freely. A circular saw might needs both of them or just a starting one, since the timber shall not be present when starting a saw.

If yo don't own a multimeter with capacitance measurement, then the best thing to do is to buy a new capacitor of the same capcitance, knowing that the load type is still the same. Else you might search a standard capcitance value knowing the rated power and speed, (current,..).

On this link you have all three types combinations and reference capacitances vs. speed and power. Motors datasheet


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