How to slow down an electric motor

A large portion of the world’s electric motors run at speeds that are too fast for their application. As a result, they waste energy, which makes them expensive to own and operate as well as reducing their durability and increasing noise.

We all know you can slow down an electric motor by using a bigger gearbox – but is there another way? 

This article describes how you can slow down an electric motor by adding a small amount of resistance in series, and without using gears.

The simplest way to produce resistance is to use electrical contacts that are pressed together with a spring. By pressing the two contacts together, or pulling them apart, we change the amount of contact area between them.

This means the resistance will vary depending on which part of their cycle they’re in: when they’re fully squeezed together there’ll be maximum resistance; and when apart there’ll be no resistance at all.

Whether they conduct electricity well or poorly depends on what’s around them – if the surrounding air isn’t too polluted with oil or dirt, then it should work pretty well!

Typically these devices connect across one winding in a motor, and you can get different magnitudes of resistance just by changing the spring tension.

However, there is a downside. When pressing two contacts together, magnetic forces cause them to stick together slightly, unless you insert a piece of non-magnetic material in between them to separate them (see figure 1).

If the device has an air gap in it (because it’s not touching anything at all), then this non-magnetic material may be your only option: use metal foil for example. This means that instead of having one spring with adjustable tension we now need two springs with several spacers or sheets separating them (see figure 2).

If the input power to an electric motor is AC, then because the current changes direction 120 times per second (60 full cycles of reversal) the power lost in these devices also changes direction 120 times every second.

This causes the motor to heat up more than it would if you used a resistance that didn’t change with the input current. However, this isn’t usually very important because if you use your electric motor below about 50% of its rated power then it tends not to get hot enough for this heating effect to cause any problems.

If you’re using them on DC motors instead, they won’t generate any heat at all – unless you are accidentally short across their contacts (ouch!). Then they’ll get red hot and burn out like a spark plug.

The usual problem is finding suitable devices for stopping an electric motor: many of them are designed to be used in situations with little or no mechanical vibration, and they don’t survive well when you attach several Kg of the motor to their contact area.

Here are some devices I’ve found which work reasonably well in ordinary conditions:

The one on the left is a reed switch from China. It’s an electromagnetic switch consisting of two small ferrite rods glued together with pieces of metal foil wrapped around them (one shown here removed). 

A magnet nearby opens the contacts by pushing outwards on them, pulling apart the two pieces of metal foil. When there’s no nearby magnet then these contacts will squeeze shut again, due to the springiness of the piece of metal foil between them. 

I’ve used these for over ten years now and they’re still going strong. They can run at 100Hz with no problem, and the connections between them and the machine’s motor are protected by a piece of open-cell foam rubber (not shown here).

The one on the right is another reed switch: this time it comes from Japan. It has flatter contacts that don’t last quite as long but make very good contact after cycling around 20,000 times. This switch has lasted for about 6 months already without showing any sign of falling gradually.

Most electric motors have two coils in series with each other, sharing the same iron core through which their magnetic lines travel (see figure 3).

The one on the left is a switch intended for use with small DC motors: it has two pairs of contacts (one pair shown here removed). The contacts are closed via a magnetic force when they’re close together and opened by non-magnetic spacers in between them. When there’s no nearby magnet then these contacts will squeeze shut again, due to the springiness of the pieces of metal foil between them (these were made from adhesive copper tape originally, but I’ve replaced that with steel strips now).

The one on the right is another switch intended for larger AC or DC motors: it’s switchable between 1 and 5 amps. This shows just two pairs of contacts: however, this device has many many more pairs of contacts, running down the length of its metal body. This can be switched between “1A” or “5A” by rotating the red plastic sleeve around it. It has a nice clicky action when you turn it which makes it feel very solid and reliable (and gives an audible indicator that you’ve turned it on or off).

These are each available for less than $2 delivered from China if you look on eBay etc. They’re both quite safe to use with any size of the motor, but they do need some extra machinery to fix them securely in place next to their intended target before wiring them up to whatever equipment they’re controlling.

With both of these switches, there’s no need for bulky insulation sleeves around them, so they’re ideal for situations where space is at a premium. Just the rubber or foam sleeving on either side of these switches is protection enough.