Hello all; infrequent poster.
I want to power a treadmill motor on 120v AC through a rectifier and choke, so I replaced the secondary on a MOT with another MOT primary that fit and removed windings until I got down to the 95v the motor requires. The motor runs fine but the new secondary gets too hot quickly even with no load on the motor. I've rewound MOT's before and not had this problem. Any suggestions? Thank you very much.

Rick A

 

Remove the secondary load completely and see if it still heats up, if it does, it points to a shorted turn!

 

What gauge is the wire in the secondary?
What is the motor current?


Is it a DC motor?

 

Thank you for your replies. It doesn't heat up with no load, suggesting the winding is good.
It's a standard DC treadmill motor, 14 ga wire and 21 amp rating, but I'd think it'd draw minimal current with no load on it.
Back to square one!

Rick A

 

The typical TM motor draws very low current when unloaded.
How many VA is the transformer?
Run it off of 120v and just limit the RPM! That is the way it is done on the TM system.

 

Using the information we have already: transformer output 95volts, Motor rating 21 amps. AND the translation: MOT= microwave oven transformer. First, motor power calculated : 21 amps X95 volts=2099.5 watts. THAT is quite a few watts. I have seen a very few microwave ovens rated up to 1200 watts. There might be a very few rated at 1500 watts.

IF the disassembly of the MWO transformer DID NOT reduce the efficiency at all , drawing 2000 watts from a 1200 watt transformer may cause a bit of over heating. This is guessing that both of the transformers came from 1200 watt microwave ovens. If they were 1000 watt ovens, that adds 200 watts to the overload. With the motor only drawing half of it's rated power that is still a full load fpr a 1000 watt transformer.. AND transformers DO get hot when operating with any real power load. How hot did the transformer actually get?? Too hot to touch would be overloaded. HOT to the touch could be 3/4 loaded.
A GOOD transformer OVERHEATING can only be one of three causes:: Excess primary current, Excess secondary current, or Wrong input frequency.

 

The typical TM motor does not run any where near the plate rated current in normal operation.
@Rick A , does it overheat with zero motor load?
Why not run it off of 120v and just limit the RPM, if you have RPM control.?

 

Thank you both. It's a standard MOT- I imagine in the wattage range you mention, but it seems that it shouldn't get so hot (too hot to touch after a minute or so) under little load. I'm controlling speed via a Reeves drive, which puts a little load on it but not that much- V-belt friction is all. It's on a drillpress, so intermittent load and seldom high current draw.

I could limit motor RPM but how would I go about that?

Rick A

 

One more thought: Did you accidentally get those windings exchanged?? 120 into the 95 side and about 150 into the motor?? AND, do you know that the motor is OK?? Are you using a rectifier to provide DC for the motor?? Can you measure the motor voltage while it is running??

 

I would see how it behaves with a direct full bridge supply off the 120v outlet, Use a simple PWM circuit to control it, use a opto isolator if neccessary, for the control.
.

 

I would see how it behaves with a direct full bridge supply off the 120v outlet, Use a simple PWM circuit to control it, use a opto isolator if neccessary, for the control.
.

THERE IS NOTHING SIMPLE about a 21 amp 120 volt PWM circuit. The schematic drawing could be simple, but not the realization of the the physical implementation of it.

 

The best way may be to pick up a MC2100 TM board, you have it all, just needs a simple PWM controller, a couple of $$'s, also has the isolation
In any case a drill press should not impose any kind of heavy continuise load.
I have used a few of these combo's in the past with no issues.

 

I tried house current direct to the bridge rectifier and the motor runs way too fast. I fear it wouldn't last too long that way.

I've looked at MC2100's on Ebay. Cheapskate that I am, I don't want to invest as much as they're asking. The Reeves drive works OK for variable speed. I could just use an AC motor but liked the idea of the TM motor- small, good power, easy to mount. I dug out a 130v DC motor from my junk and it runs well on rectified 120v. Maybe the transformer will work for something else that doesn't draw so much current. Nice try I guess.

Thank you both for your suggestions.

Rick A

 

A standard microwave oven transformer (MOT) is designed to operate for a short period - up to 20-30 minutes, after which it needs to be allowed to cool down. And it needs a fan to work properly. The MOT is designed with a high idle current, about 3 Amps, if it is a 230V transformer.
To reduce the idle current of the MOT, additional turns need to be added to the primary winding.
Or used the other way around – the secondary winding is used as the primary. If it were used this way, it is necessary to disconnect the end of the secondary winding from the transformer core.
In this case, the transformer’s power will be lower than it could deliver in the microwave.

 

I tried house current direct to the bridge rectifier and the motor runs way too fast. I fear it wouldn't last too long that way.

Obviously, but I did not mention running it direct, but if no transformer is used you still require a form of control, this way you can run it off of the 120v with no voltage reduction, just the way it is done on the TM!
The RPM is limited to the motor spec.

 

I have seen Microwave tansformers that have welded laminations in order to reduce physical noise from the transformer itself. This acts as a shorted turn with an iron conducter. anytime I used one there was a large power consumption with no secundery load

 

Mainly done to run a bead across the laminations to stop 'Buzzing' from the individual plate vibration, does not really affect the outcome in most cases.