# Help Please! DC variable speed motor always running at max speed

Thread Starter

#### gvp

Joined Mar 26, 2016
10
Hello,
I have almost zero electrical experience and wonder if anyone can help me resolve an issue, I tried searching the forum but this question might be too basic. I have a 115v 3600 rpm DC motor with a speed control unit. Initially I needed the motor to run the opposite direction so I went in and swapped the polarity. The motor worked perfectly when I finished but shortly after this the motor only ran at maximum speed when turned on. I opened the potentiometer and everything seemed in order. Can someone suggest what else I might check? Could the problem only be in the speed control unit or could I have messed something up when changing polarity? If the pot is working fine is there something else on the speed control unit I should check?
Thanks in advance for any help!

#### crutschow

Joined Mar 14, 2008
29,484
Sounds like the device on the heatsink that controls the voltage to the motor (as determined by the pot) shorted for some reason.
I don't see how reversing the motor leads would cause that.
What's the motor current rating?
What is the power source?

gvp
Thread Starter

#### gvp

Joined Mar 26, 2016
10
Thank you crutschow!
The motor is 0.35 amps and the power supply is just a 10A 125V AC cord.
Not sure why it would have shorted but it could have been just too much finagling with the thing to get it back in the case
So if I've got a short should I just replace the whole speed controller module?

#### crutschow

Joined Mar 14, 2008
29,484
..............
So if I've got a short should I just replace the whole speed controller module?
If you don't want to try to fix it, then replacement is the easiest option.
Is there a part number on the device attached to the heatsink?

#### Alec_t

Joined Sep 17, 2013
12,733
The motor is 0.35 amps
That is almost certainly the free-running current. The start-up current could be ten times that! What is the current/voltage rating of the controller?

Thread Starter

#### gvp

Joined Mar 26, 2016
10
If you don't want to try to fix it, then replacement is the easiest option.
Is there a part number on the device attached to the heatsink?
I would prefer to fix it if possible, since I don't know what specs are important when finding a replacement, then again given that this is the first time I've messed with anything electrical maybe full replacement would be easier?
The device on the heat sink has no part number on it unless it is on the back side which is attached to the heatsink with thermal epoxy :/

Thread Starter

#### gvp

Joined Mar 26, 2016
10
That is almost certainly the free-running current. The start-up current could be ten times that! What is the current/voltage rating of the controller?
Hi Alec_t, sorry for my ignorance, I only know what is marked. The motor has a 115V and 0.35 amp rating, there are no markings on the controller itself although the pot is marked for 250K resistance - is this the rating of the controller?
If you are wondering about mismatch in the ratings for the motor & controller I bought the whole motor & controller as a unit which is sold for hobby grinders so I assume everything matches up.
Thanks

#### MaxHeadRoom

Joined Jul 18, 2013
24,986
Appears to be a very el-cheapo controller, even more so than the PWM versions off ebay for $5.00. also it appears to be a simple SCR/Triac version, but it would have to be a Universal motor and would not reverse by simply swapping the leads unless a bridge is fitted?. Also the size of that motor appears more than a 50w motor?. What is your application that the motor is mounted on? What is the part number on the semiconductors? Max. Last edited: #### Abdul Wali Joined Mar 19, 2016 35 make sure the field is not open circuit this problem normally occours becouse of leaving the field open else just connect a rheostat or a resistive load in series with the field windings speed should be controlled. Thread Starter #### gvp Joined Mar 26, 2016 10 Appears to be a very el-cheapo controller, even more so than the PWM versions off ebay for$5.00. also it appears to be a simple SCR/Triac version, but it would have to be a Universal motor and would not reverse by simply swapping the leads.
Also the size of that motor appears more than a 50w motor?.
What is your application that the motor is mounted on?
What is the part number on the semiconductors?
Max.
I'm sure the whole thing is a cheap as it gets. The motor reversed successfully by swapping the leads - and worked fine for a day, I just subsequently lost speed control (always at max now).
I apologize for being a total novice here (literally my first attempt at anything electrical). If you tell me where to look for the semiconductors and wattage I could give you more detail - I don't see any part numbers on anything on the controller except the pot. The motor is only labeled with the amperage, voltage. torque, and rpm.
The motor is mounted in a plastic case which is being bolted inside an aluminum box. I am using it for a diamond lap/hone for sharpening engraving tools so I need decent torque, and some speed adjustability, but would never be using it at its max of 3600 rpm.
Thank you

#### MaxHeadRoom

Joined Jul 18, 2013
24,986
Look at the part number mounted to the aluminum heat sink (U1)?. this is the major component, and any other diode or rectifier or other semi's that have any number on them, this will indicate the nature of the controller.
Max.

#### MaxHeadRoom

Joined Jul 18, 2013
24,986
make sure the field is not open circuit this problem normally occours becouse of leaving the field open else just connect a rheostat or a resistive load in series with the field windings speed should be controlled.
I am betting it is a P.M. motor not wound field.
Max.

Thread Starter

#### gvp

Joined Mar 26, 2016
10
Look at the part number mounted to the aluminum heat sink (U1)?. this is the major component, and any other diode or rectifier or other semi's that have any number on them, this will indicate the nature of the controller.
Max.
Okay, turns out it was just so faint I couldn't see it. The part on the heatsink has the following markings:
VU
BTA08
6006
MAR 228
There are no numbers that I could see on the other components.

Thread Starter

#### gvp

Joined Mar 26, 2016
10
I was also able to find some specs from a website for this motor - so it is a P.M. 245/154 Watt
• Motor Type: Permanent Magnet DC
• Motor Torque: 48 0z-in (.339 N-m)
• Watts In: 245 / Watts Out: 154
• Motor RPM: 2, 800
• Integrated Speed Control for near infinite adjustment
• Arbor OD: 1/2"
• Uses a 4A, 250V fuse.

Thread Starter

#### gvp

Joined Mar 26, 2016
10
Yes, I swapped the destination for the outputs from the rectifier bridge in order to change the polarity

#### MaxHeadRoom

Joined Jul 18, 2013
24,986
I was also able to find some specs from a website for this motor - so it is a P.M. 245/154 Watt
• Motor Type: Permanent Magnet DC
• Motor Torque: 48 0z-in (.339 N-m)
• Watts In: 245 / Watts Out: 154
• Motor RPM: 2, 800
• Integrated Speed Control for near infinite adjustment
• Arbor OD: 1/2"
• Uses a 4A, 250V fuse.
Makes more sense, I guess the fuse did not do its job!
Max.

#### MaxHeadRoom

Joined Jul 18, 2013
24,986
Yes, I swapped the destination for the outputs from the rectifier bridge in order to change the polarity
That in itself should not have been a problem, unless something was shorted in the process!
Max.

Thread Starter

#### gvp

Joined Mar 26, 2016
10
Thank you so much for your help Max!
I'll order the piece today and hope that is the problem. There are so many BTA08 to choose from, will any of them work?
Thanks again!

#### MaxHeadRoom

Joined Jul 18, 2013
24,986
Triacs are usually very forgiving, as long as the max voltage and current is satisfied.
What do you mean by so many? There are often many manuf. but the spec should be close.
Max.