Sizing DC motors

[stebbo]

I need a bit of a refresher here as I've managed to avoid motor theory somehow since Uni.

Anyway, I'm just about to start prototyping a treadmill optimised for continual use, maybe 10-12 hours a day, but only running at slow speed, between 1 and 2 mph controllable by a pot.

PWM controlled DC PM motors seem to be the go, brushed from what I can tell. A few calcs reveal these motors rotate up to about 700rpm, but I'll be more in the 100rpm region.

6.5HP peak motors (about 3.5 continuous it appears) tend to be the biggest that powerful home units run, and apparently can even handle 500 pound users. These motors can also drive the treadmill up to about 14mph. I do not know how well such motors handle such long sessions of use.

I'm only aiming at 250 pound maximum but wish the motor to be as economical powerwise as possible. 10-12 hours a day at slow speeds where I'm thinking the load on the motor will be the greatest (person's weight on deck for longer and they won't be self propelling the treadmill as much) might cost a couple of dollars a day of electricity and discourage purchase.

I'm assuming just throwing the largest treadmill motor I can find at the job won't be the most cost-effective. Will theory allow me to size an optimal motor, or will loading be simply too hard to estimate and trial and error the only practical solution? Thanks.

 

[gar]

110512-2130 EDT

stebbo:

First, you need to make measurements on your treadmill to determine power required vs speed and loading. You could take an existing treadmill and use input measurements of voltage and current to an existing brushed motor to get some useful data.

Today conventional DC motors will not be low speed. I would expect 2,500 to 10,000 RPM to be a base speed. Torque is determined by the field magnetic flux and the armature ampere-turns. Thus, if a given motor is torque limited by these factors and thermal constraints, then you increase HP by increasing speed, and that means greater armature voltage.

Get some useful data before making decisions.

.

 

[stebbo]

Thanks, it's all starting to come back. I like your idea of testing an existing treadmill as I'm not too sure how accurately I could estimate the loading of a walking person.

Checking things out a bit further and seeing the sort of gearing used in some machines, it looks like the motors run at about 4000rpm at top speed, but around the 350-700rpm mark at 1-2mph. I'm guessing the PWM controller simply duty cycles down the supply voltage to scale down the speed.

I saw a loading graph for one of the motors and (after startup), they appear pretty much 90% efficient over their entire range of safe loads. But I haven't come across a graph yet of efficiency at different operating voltages so I'm unsure as yet whether I'll need to optimize efficiency via motor choice or voltage and gearing. I'm being charged a fortune for internet via my cellphone now and for the next two weeks but I continue looking deeply into it then.

But in the meantime, would I be correct in thinking that a larger motor is not going to cost more to run than a smaller motor doing the same job?

 

[gar]

110514-1401 EDT

Any electric motor will have to be run at a lower torque level at speeds lower than base speed, unless an external blower (fan) is used for cooling. Current is torque and I^2*R losses are heat to be removed. Also obviously as speed or torque is reduced the efficiency goes down.

If you do not need more than a certain speed, then design for that speed with the motor at its base speed. Base speed is the speed of the motor at full normal armature voltage and normal field excitation. With a wound field motor the motor can be run above base speed with normal armature voltage by decreasing the field excitation. PM motors do not have this capability unless mechanically the gap can be increased. When using reduced field excitation the torque is less for a given armature current.

PWM us used to adjust average armature voltage.

Don't get too hung up on efficiency.

As RPM is reduced frictional losses are reduced, but relatively speaking electrical resistance losses will increase.

Before anything else define your maximum required treadmill speed. Then gear your selected motor to get that speed at the motor base speed. Now you can size the motor for the required torque.

But before anything else get actual data from a similar treadmill. From an experimental starting point in the development of your treadmill I would start with a motor double the size you might predict from the similar treadmill experiments.

Treadmill motors are cheap on the surplus market.

.

 

[Besoeker]

PWM controlled DC PM motors seem to be the go, brushed from what I can tell. A few calcs reveal these motors rotate up to about 700rpm, but I'll be more in the 100rpm region.

Don't know where you get the 700 pm.
Considered much huger rotational speeds and reduction gearboxes?

 

[stebbo]

Don't know where you get the 700 pm.
Considered much huger rotational speeds and reduction gearboxes?

It was a mistake--I was basing the calcs off photos and what I thought was the motor drive pulley was actually the flywheel! :roll:

 

[stebbo]

Ok, I beginning to get a decent picture now, thanks Gar. The more powerful home units do run PM DC motors with a cooler fan mounted to the motor. However as soon as one goes to commercial treadmills they employ AC motors, so I'm a bit fearful at this stage a DC motor just isn't going to handle hours and hours of continuous operation. AC motors I'm thinking the noise will be too excessive for a home environment, possibly too the operating costs.

 

[gar]

110514-2214 EDT

There is nothing wrong with a PM DC motor vs an AC motor other than brush and commutator wear. The DC motor controller is likely simpler. A DC motor system will be simpler and easier for you to use initially. I think a motor and control may be in the $50 range surplus. These surplus motors are are inexpensive and convenient for modification of small light duty drill presses.

.

 

[SuperE]

If it were me I would go with a 3/4 HP, 3-phase motor and use a VFD controller that takes single phase 230 VAC 50/60 Hz (since you live in Australia) and transforms it into variable frequency, variable voltage 3-phase AC. For the VFD use a KB Electronics KBVF-24 motor controller which costs $174.25 that you can get from Solid State Drives. The 3-phase motor should be an industrial type like a Leeson 110034 rated at 1725 RPM base speed at 3/4 HP. It costs $221.82 from the Electric Motor Warehouse site.

You will need to get the pot for the VFD which is also available from the same site where you can purchase the VFD controller. You could market your treadmill as being super reliable since there are no brushes to worry about replacing. The average Joe is really not going to be competent enough to know how to do a brush replacement anyways, so you definitely have an advantage in the consumer home market.

Btw, those treadmill motors are most likely "hype HP" instead of the real HP. Retailers are not held to the standard of correctly stating the actual HP like an electric motor manufacturer is. If an electric motor manufacture lies about the HP rating then they can get into some serious trouble versus the retailer.