Off to a Soft, or Hard, Start: Are soft and hard start kits necessary?

Published On
Aug 13, 2021

I recently had new air conditioner units and a new swimming pool circulation motor installed at my home. The installer asked if I wanted soft or hard start units installed on these motors for a nominal fee.

Not knowing that I was an electrician and well-acquainted with many different aspects of the electrical industry, he proceeded to tell me the many benefits of using these startup units for the new motors. He did a fairly good job of explaining their function, so when he finished, I explained that as an electrician, I was extremely familiar with the purposes of the startup kits. He stated that he normally had to do a hard sell and a thorough explanation for most homeowners to understand that he was trying to save them money and extend the lifetime of their motors.

A motor soft start kit is a device used with alternating current motors to temporarily reduce the startup current of the motor by reducing the torque, which aids the motor when starting from a locked rotor condition and improving the motor’s power factor. Adding a capacitor to the motor will reduce the startup current of the motor by giving it an extra boost of current from the capacitor, decreasing the stress on the motor and increasing its lifespan.

In many homes, when a large motor starts, such as an air conditioner, incandescent and LED lights often flicker or dim due to the reduction of system voltage from the motor startup. Installing a soft or hard start kit will often solve the issue of flickering lights. In water pump applications, a soft start can avoid pressure surges or water hammering and reduce the stress on the water piping system. Soft start units limit inrush current and improve the stability of the power source.

Hard and soft start kits consist of capacitors and relays, but a hard start kit is more often installed on an aging electrical motor in need of a little startup help and may be used to help extend its life. Most motor nameplates will have a locked rotor indicating the Code letter, based on 430.7(B) in the NEC , that indicates how much kilovolt-amperes (kVA) per horsepower (hp) a motor requires for startup. For the same size motor, a Code letter A has the smallest kVA per horsepower and a Code letter V has the highest kVA per horsepower.

For example, a 5-horsepower motor with a Code letter A has a kVA of 0.0–3.14. A Code letter V has 22.4 kVA and higher for the same 5-horsepower motor. The maximum startup current for a 5-horsepower motor, using the kVA of the motor and based on the Code letter A, would be calculated as such: 5 hp × 3.14 kVA = 15,700 VA. Then take 15,700 VA ÷ 240V, single-phase = 65.4A of startup current, with an actual running current of 25A. The same size 5-horsepower motor with a Code letter G would have a calculation at the highest of 6.29 kVA x 5 hp = 31.45 kVA, at a startup current of 31,450 VA ÷ 240 single-phase = 131A.

The soft or hard start kits can help motors start much faster and be more efficient than those without these kits, since they provide power-factor correction for motors with low power factor. In other words, installing shunt capacitors in parallel with a motor will increase the efficiency of the motor. Motor efficiency is the ratio of the energy output to the energy input expressed as a percentage, such as wattage divided by volt-amperes.

The lower the power factor, the less efficient the motor, with 100% being the best and anything lower being less efficient and requiring correction for improved efficiency. This correction applies to all motors, even those in appliances where the motor and nameplate may not be readily available or visible.

Some power companies charge the customer more money every month for power consumed where the facility uses low power-factor motors. This penalty is not normally applied to residential customers, but increasing the efficiency of any motor used in a home, or anywhere, is certainly desirable since the motors use less energy, cost less to operate and have a longer lifespan.

Understanding these basics and being able to explain this phenomenon to your residential customers in an easy-to-understand way is invaluable and creates new opportunities for your business.

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and Mark.C.Ode@ul.com.

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