NNTP-Posting-Date: Fri, 20 Dec 2002 03:00:21 -0600
From: "Big John"
Subject: Re: Help with motor speed control
Date: Fri, 20 Dec 2002 03:55:16 -0500
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The first thing you probably want to do is get some idea of the torque and
motor power you need. Measure the torque you need using a torque wrench and
calculate the motor power. The formula is:
Shaft Power [hp] = Speed [RPM] X Torque [lbft] / 5252
I'd multiply that by 2.5 or so for a safety factor and pick a motor
greater then or equal to that to work with. But I wouldn't go with more
than 5 times the peak power needed as you are getting into serious overkill.
As far as motor types and controllers there are perhaps thousands of
variations. Motors include induction, permanent magnet DC, wound field DC,
brushless DC, Synchronous, reluctance, universal, and stepper. Controllers
include PWM, flux vector, variable DC, SCR, rheostat, resonant mode, and a
whole bunch of other variations.
In industry the most common motors are polyphase induction motors. If you
need 10 hp or more and you have polyphase power available then that's the
way to go. If you are talking small lab glassware on a table top I doubt if
you would exceed 1/4 hp and you may not have polyphase power available, so
you have other choices. If you need 10 hp or more and are going with
induction I'd say go to a motor distributor. They have plenty of off the
shelf stuff. Perhaps a 1800 or 1200 RPM motor, a gear head, and a small
vector controller. You'll save yourself some mistakes this way.
Motors can be designed with very low shaft speeds but it is seldom done
because they tend to get heavy for their horsepower. Common speeds for
induction motors are 3600 RPM, 1800 RPM, and 1200 RPM sych (no-load) speed.
Full load speed is a bit lower due to slip. Very small PM DC motors may
have a shaft speed of 30,000 rpm. So, if you need something small and
homemade you'll have to slow it down. You can't do it in one step to 30
RPM. Say 1800 RPM to 30 RPM is a 60:1 ratio. Way too much for one set of
pulleys. You can get motors with gear heads. 10:1 is not an uncommon
ratio, then add a 6:1 pulley ratio and your set.
If your power needs are low, I'd recommend you look into PM DC motors,
perhaps for automotive applications. PM DC motors are easy to control. The
speed is directly proportional to the DC voltage (almost). They also have
plenty of starting torque. You can make a simple variable speed control by
making variable DC using a cheap car battery charger and a variac.
There are also wound field DC motors out there. But be careful. Wound
field DC motors will run-away if they lose field power. They need to be
operated with a field loss relay for safety. Talk to an electrician if you
go this route. The life you save maybe your own. There are also different
field configurations on these motors so you need to understand the
As far as how low a speed you can get high torque at - it depends on the
motor/controller design. DC motors tend to have naturally good low speed
torque. Induction motors generally have poor low speed torque, but modern
controllers can change this. With a good flux vector controller you can get
full load torque down to 0 RPM. But, the motor has to be made to handle it
because high torque equals high current and many motors will burn up because
they can't cool themselves if they not moving.
Also note that PWM drives generate high pitched noise. Then are usually
designed to run above 20kHz so it's not easy to hear, but the sound is there
and it is very annoying. If the motor is operating alone in a large noisy
shop floor - no problem. If it right next to people in a quiet lab - not
good. Consider this if you are going with a PWM motor drive.
Hope this helps,
Dave White wrote in message ...
>We have a device in our lab which is used to rotate glassware in order to
>mix the contents. The shaft needs to rotate at approximately 30 RPM, with
>some control allowance (maybe 20-40 RPM). The company (actually a lone
>engineer) provided this custom made device with a 1/15 HP AC motor
>controlled by a rheostat. We have found that in operation, there isn't
>enough torque to get the glassware to rotate at much more than about 10
>I have been asked to replace the motor and motor control circuit to give us
>more power. Looking at our local surplus store, there are a large number
>AC and DC motors, ranging from 1/2 to 2 HP and priced between $20 - $40.
>I'd like to use one of these to control the system. The rheostat control
>seems very inefficient, and I expect I'd use some kind of PWM circuit.
>The DC motor is 120V, 2 HP 4800 RPM and 11.2A. If I were to use PWM,
>the slowest I could run this motor without it stalling?
>There are plenty of washing machine motors rated at 1/2 HP, 1725 RPM.
>this be slowed down easily, and how slow could I go?
>I will be able to include some gearing, probably in the form of a pulley
>system, so all of the slowdown wouldn't need to come from the control
>Any help/advice regarding design, best motor to use, gearing, etc. will be
>GREATLY appreciated - my expertise lies in digital and low power analog
>circuitry, not high power industrial electronics.