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I've been researching how to control a smallish AC-motor using a relay, and more specifically, how to turn an AC motor off using a relay.

This is a subject that has been up many times at this site, but the answers given vary quite remarkably.

The fundamental problem is that an AC motor will "kick back" when an attempt is made to interrupt the current flowing to it. When the relay opens, the voltage across the motor will increase dramatically. If this is not dealt with, there will be arcing at the relay terminals, which increases wear and may also cause EMI problems.

This question: How to calculate resistor and capacitor size for snubber circuits has a very well written answer, including a calculation suggesting the use of a 40uF capacitor in series with a 160 ohm resistor, for a 220mA motor with 1H of inductance. The capacitor stores the energy in the "kickback", and while it does charge up to some voltage, this voltage is much much lower than the voltage caused by an unmanaged kickback.

This question: Relay protection with MOV and RC Snubber , has an answer suggesting a 0.1uF capacitor in series with a 100 ohm resistor, without having any details about the motor.

This question: How to design an RC snubber for a solenoid relay driving an inductive load?, has a an answer indirectly suggesting that 0.1uF and 120 ohm is reasonable for a 10A 1500W 230V motor.

In this question: Using an RC snubber with a triac. Is this design safe? (simulation included) , the author wonders why a 10nF cap in series with 39 ohm is recommended, when 100nF gives better results in a simulation, for a 0.3H 220V motor.

This question: Resistance and Inductance of a ceiling fan motor needed to spec snubber has an answer suggesting a 1uF cap without resistor for an unknown 230V ceiling fan.

The calculation in the answer to the first question above seems very reasonable. The cap should be sized so that it can hold all the energy stored by the inductance of the motor at max current. To do this sizing, it is necessary to know the motor inductance.

The answers suggesting a 0.1uF or even 10nF cap, usually don't contain any information about motor inductance. It seems to me that such a low capacitance would yield a very large voltage transient for most AC motors.

Could it be that the often cited 0.1uF or 10nF cap typically is way too small to protect a relay from arcing? And also that such a small cap would need a very high voltage rating (perhaps impracticably high) to have decent longevity of the cap itself?

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  • \$\begingroup\$ I would expect that the answers could be very different for AC induction motors vs universal motors... \$\endgroup\$ – SomeoneSomewhereSupportsMonica Jan 4 '18 at 0:34
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My guess is that all the 10nF or 100nF recommendations are for zero-crossing TRIAC applications, and that one has to be really careful to not apply that recommendation blindly to relay-based solutions.

So, I propose the following possible solutions:

1: RC snubber designed according to the calculations in How to calculate resistor and capacitor size for snubber circuits . This protects the relay from arcing and reduces EMI problems.

2: 100nF and 100 ohm RC-snubber in parallel with a MOV. The snubber doesn't protect the relay, but may reduce EMI-problems since such a snubber forms an RC low-pass filter with a 100kHz cutoff.

3: Just a MOV. May cause EMI-problems?

MOVs have a limited lifetime and must be sized appropriately to have enough longevity for the application. They may not be a suitable solution if the motor is to be turned off frequently. Placing the MOV across the relay protects the relay better, but means the MOV is always energized even when the motor is off. A short-circuit failure of the MOV will then start the motor.

The MOV may be changed to a bidirectional TVS diode. IEC 60950-1 (if it is applicable to the application) explicitly forbids TVS diodes for surge suppression. If the TVS is connected across the relay, it is basically connected the same way as a surge suppressor, and may thus not be allowed(?). IEC 60950-1 also states that MOV:s, if used, must be used together with "an interrupting means having an adequate breaking capacity", presumably a fuse.

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