You do not say what type of drive ? You don’t say what speed you need ? Beltdrive is easy, use different size pulleys ! Chain the same ! A speed changing gearbox ! A 4 pole will be about 1430RPM, Single phase will not be happy on simple VFD. Change motor to required speed or go to 240v 3phase motor and VFD

Give us more clues ! Noel

If the motor is running on AC, the speed will be related to the frequency of the suppy, and cannot be altered, unless the power supply is through a VFD.

A brushed DC motor speed can be controlled by varying the supply voltage, but at the cost of reduced powere as the voltage is reduced.

Brushless DC motor speeds can be controlled electronically.

If the existing motor is AC; Sounds like it , being 4 pole. On 50 Hz mains, I would expect a shaft speed circa 1470 rpm.

If the objective is to vary the speed of the machine driven by the motor, this can be achieved in a variety of ways, but only one will even approach being infinitely variable.

This is a variator, usually in the form of two, opposed conical belt pulleys, one of which can be used to move the belt up and down the cone, thus varying the ratio.

The other means provide speed changes for the output shaft, in fixed steps. Gears, Chain and sprockets, or belts and pulleys. Gears and chains provide a positive drive, which unless cogged belts and pulleys are used, V or flat belts do not.

I hope that this clarifies your thoughts

Howard.

On 8 December 2023 at 20:59 T.Swinfield Said:

Hi, what is the best way to reduce the speed of a 240v  4 pole electric motor.

…

As always in engineering, much depends on the purpose.   This can influence the type of electric motor used, and then choice of speed control.

Assuming the 4-pole motor is 3-phase of moderate power output, the best way of providing continually variable speed control is a VFD.  Variable Frequency Drives can generate 3-phase power from a single-phase supply, provide soft-start, run synchronous motors faster and slower than nominal,  and electronically boost torque at slow speeds.  Cost between £60 and a few hundred quid.   Disadvantages: need to understand how to wire them up, the manuals tend to be written for experts, not home enthusiasts, and they cover lots of  complicated options.   Good news, many of the cheaper variety come pre-configured with a diagram covering basic set-up, which is often all that’s needed.  Expensive industrial units are more likely to be unfriendly – expecting the customer to be or to have access to an  experienced professional.

Belts and pulleys are a good choice is a single speed reduction is needed, or maybe 3 fixed speeds achieved by 2 pairs of 3 pulleys.  Disadvantage: some way of adjusting belt tension is needed, and belts tend to slip.  They may not be suitable for transferring high power, or when the motor and driven object have to remain in sync.   Toothed belts are available but they require matching toothed pulleys.

Sprockets and chains are often used when non-slip high power or timing are needed over a distance.  The chains have to be lubricated and are noisier than belts – if that matters.

Not the law exactly, but belts and chains usually require the machine to be stopped for speed changes.   Doesn’t matter on most machine tools, but no good for vehicles.

Gears, perhaps arranged in a gearbox that allows several ratios, are the preferred solution over short distances.   Gear spacing is part of the design, so multi-speed is probably better done with a commercial unit than home-made, though a simple crash gearbox with isn’t rocket science.  Synchromesh and a clutch add complexity.   Gearboxes tend to be very noisy.

All the mechanical systems provide good torque at low speed, but the common methods only operate at fixed speeds, not continuously over a range like a VFD.   The few mechanical systems that provide continuous speed control, such as those involving cones, work, but are fiddly to adjust, relatively unreliable, and high maintenance.  Not recommended unless something special is needed.

Another possibility is a fluid torque converter like the automatic gearbox in a car.  Expensive and complicated. They’re good at matching  motors to loads for maximum power transfer, not quite the same thing as speed control, and over the top if a belt and two pulleys will do the job!

Belts and pulleys are a good answer for most purposes and so is a VFD.  Using both together is very flexible.  (With a VFD a mid-range pulley ratio is good for almost everything over a wide-ish speed range, but the operator can drop to a lower pulley ratio when high torque is needed at slow speed.)

Horses for courses…

Dave

“Not the law exactly, but belts and chains usually require the machine to be stopped for speed changes. Doesn’t matter on most machine tools, but no good for vehicles.”
Except that some vehicles do use a variable belt drive. It means that you can vary the speed without changing the engine speed, thus keeping maximum power.

Any reduction of speed from the  motor’s initial design, even if possible, is probably not desirable- especially a big reduction which will compromise the torque available.

I used to have a home made power hacksaw that was given to me. This used a timing type belt and pulleys that gave a bigger reduction ratio than would be possible with V belts.

Rod

As Rod and Noel say, running a motor below its normal speed can result in problems.

The slower speed will mean that less air is forced through the motor by the fan, so it could overheat.

Also, slow running is likely to result in the windings  generating a lower back EMF, so that the current draw increases, and with it the heat input.

This would be a double whammy, making damage even more likely.

IMO, keep the motor at designed speed, and use belt and pulleys, or chain and sprockets to reduce the speed of the hacksaw.

Howard