I know from reading the posts about motors, that there are many experts here, & I have got a situation that I need some help with. Rewired a average sized horse barn, simple job. Owner asks if I would look at a motor control that operates a 6 position horse exerciser, turned by a "Leeson" 1HP DC motor. He tells Me that "One day it just took off, & gradually sped up until it was too fast for the horses to walk." I looked at the controller, seemed simple enough, 120 volt supply Stop/Start Run/Brake Forward/Reverse switches, & a speed control. I called the Mfg. "Leeson", they suggested replacing the controller, checking & or repairing the old one wouldn't be worth it,since a new unit was around 200.00 Owner said to go ahead & order it. When it arrived, I replaced it, went through the setup instructions exactly. It acted the same, ran too fast, & was unable to slow it with the speed control. I called the "Leeson" Rep, talked to some Tech support Guys, & they said "Sometimes a DC motor can act that way if it's going bad" This I didn't understand, but then again, I don't have much experience with motors & their controls, so I bit. (This is the moment I wish I hadn't got involved with this) I explained all of this to the Farm Owner, said He would replace the motor Himself, so I only charged Him for the Control & shipping, & was on My way. Well, He called Me the other day, paid 450.00 for a new 'Leeson" DC motor,replacecd it & nothing changed. Still runs too fast, & unable to slow it with the controller. I am stumped, there's nothing left to change. This setup worked perfectly for years, Owner took it apart & moved it from Texas to Kentucky, & it worked correctly for about a year....until that day it started "running too fast." Now, does Anyone have a clue what may be wrong here ? The control is 120 or 240, but has always worked on a 120 volt supply. I want to try & help this Customer correct this, but after a new control & motor, I don't know where to start, ...but I do know where to look for the answer, I told the farm Owner about this Group, & said if it could be done, these People would know how.
There's usually a 5k or 10k ohm speed pot (potentiometer) that is used to adjust the speed of the motor, I'd check that first. What brand of controller is it, are the Start/Stop control buttons built into the controller or are they separate? If their not built into the dc drive controller, I'd check them. You could bench test the motor and controler and eliminate any existing electrical conditions. Any mechanical reasons? Good luck.
If the motor has separate leads for the field windings and rotor, check the connections (ALL of them) very carefully. A loss of field/rotor or shorted rotor can cause the motor to run away. Since you've already replaced the motor and control, I'd check all the wiring for any shorts, opens or unintentional grounds.
Sounds like a series wound dc motor, because only series motors run away like this when lightly loaded. Which means that the load may have been reduced in some way, with a commensurate reduction in the field winding excitation. Therefore I suspect, since you have replaced both the motor and controller, that there may be some sort of 'brake device' fitted to this machine, to keep it loaded when there's no horse on board, and that it may not now be working. The simplest 'brake' would be a fan, or blower perhaps. Also, speed control is often done with these motors by shunting the motor armature with a suitable variable resistance, and mxslicks advice to check all the wiring is good to field windings and to the armture's pot shunting connections particularly, is correct. Alan
edited to remove a double negative.
[This message has been edited by Alan Belson (edited 08-09-2005).]
Hmmm. A perm. magnet motor. "Suddenly one day" it ran away too fast for the horse. It would have to be shunt wound, (no field windings to series). If the pot* was dead-shorted, it's got a new one now. A wiring problem causing a short-circuit of the potentiomer* perhaps? Difficult to envisage how, but it is possible. For this use though, traction, I would have thought a 'series' type the obvious motor choice. Alan ps. * Afterthought. More a rheostat than a pot if all the current has to flow through a resistance for speed control.
[This message has been edited by Alan Belson (edited 08-09-2005).]
Alan, the reason I bring this up is one thought that came to mind.
A remote pot, and switch in another location?
Or is all this at just one location?
Could it be possible that another speed adjustment pot is somewhere else, and that pot is turned up, or shorted, and that is driving the turntable fast. And no-one remmembered to tell anyone there is another set of controls.
I have been on a few calls where the guy says,"it was working fine and then all of a sudden", but he fails to tell you he did something before hand.
Leeson makes controllers for both permanent-magnet and shunt-wound motors.
Shunt-wound is required for 4-quadrant, or "regenerative" controllers that perform active braking (applying reverse torque) for speed regulation. Since the device isn't intended to slow horses down, I'm guessing that Dnk has got it right, and it's a 90-volt PM motor with just two wires.
I'm also guessing that the controller is the 174308, which has the switches and pot built into the NEMA-1 box, in which case it's really hard to imagine a fault condition that wasn't cleared with the replacement of both the controller and the motor. But then, stranger things have happened, so I would:
1) Check that the wire to the CCW pin on the pot is not broken, and
2) Clip a DVM across control board pins S2(+) and S1(-) and check that the voltage varies between 0 and +1.4 volts over the range of the pot.
3) Obviously, check that the "MIN SPD" adjustment is full CCW (zero).
4) While you're at it, see if turning the "MAX SPD" adjustment down reduces the speed at all.
I'll have to agree with mxslick -- If everything else has been replaced, the wiring is all that's left. One possibility is that there's a leak to ground on one of the motor wires, which could have two effects:
1) Depending on the controller circuit design, it could effectively bypass the SCR control. Hint: Does it also run too fast when reversed?
2) If the controller is very clever, it measures the motor speed in the off-time between drive pulses by measuring the open-circuit voltage it generates. Leakage to ground on a motor lead could result in an under-speed measurement, and the controller would apply more torque to compensate. This is also how a motor that's "going bad" can fool the controller into running it too fast.
As a test, I'd consider disconnecting the existing wires and run a temporary 2-wire cable directly from the controller to the motor. (How far is that, by the way?)
Please post the controller and motor model numbers if you've got 'em, and good luck.
I thank all of You that have offered some advice, & I don't have the model # of the motor, but it was a 90V dc, permanent mag motor. Th model number for the control is exactly as John Crighton guessed Leeson # 174308, but Alan, the braking device is built into the control, & there isn't any thing else at the motor location. The distance to the motor is about 200 ft. The Farm Owner had installed NMB to serve the motor, but it looks to be OK. As far as setting the control, I couldn't get the motor to slow with the individual adjustments at all. I didn't keep the manual, but if I remember correctly, You are supposed to set the POT to 100%, then slowly increase the speed control setting until you reach the max speed You need. I couldn't slow the motor with the speed setting, or the potentiometer ? It was almost like the control wasn't effecting the motor at all. Also, even with the motor stopped, reversing it causes the 20 amp SP breaker to trip ? Before, & after the control was changed. I have thought about running a temp wire out to the motor, just to bypass the existing wiring. Thanks again !