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DC Drive Repair; understand the basics first

From our DC Drive repair service experience, there appears to be fairly common misunderstandings about the basics of DC Drives and DC motors.. so here’s a short blurb that I hope will be helpful.

EMA has been doing DC Drive repairs for over 20 years, so we’ve seen plenty of issues with DC Drives Repair and Motors. Let’s start with DC Motors.. everyone knows that a DC motor has an armature, and in most cases, a shunt field. Although

Old BBC DC Drive

most applications use DC motors in a speed control mode, DC motors are inherently torque machines. The torque, or if you prefer, the magna-motive force of a motor is the armature flux times the field flux. Flux, in general, is the lines of magnetic force produced by current flowing through the armature and field windings.

Here’s the first point.. IF you reduce the lines of flux being produced by either the armature or the field, then overall, the magna-motive force of the motor is reduced. In other words, the motor’s ability to produce torque (turning force) will be reduced.

In a typical DC motor or DC drive application, the field flux is held constant, and as the motor load changes, the current into the armature will change.

Here’s the second point… IF the field flux is held constant, then motor torque is directly proportional to the armature current. i.e. If a motor is rated at 100 ft lbs of torque, and full load amps is 100 amps, then if the motor is drawing 50 amperes, we know that motor is producing 50 ft lbs of torque. AGAIN AS LONG AS THE FIELD IS HELD CONSTANT.. (Remember my first point) If the field flux is reduced, then that relationship is no longer valid.

Now, lets talk about DC Drives and speed.

Reliance MaxPak

The speed of a DC motor IF THE FIELD FLUX IS HELD CONSTANT is directly proportional to the armature voltage. The speed of a DC motor has no relationship to the armature current. You can be drawing 100% full load amps on the armature, with the machine stopped. The motor is producing 100 % torque, but not moving. Again, the speed of the motor has no relationship to armature current, but it does to armature voltage.

This means, IF THE FIELD IS HELD CONSTANT, that a motor rated for 1800 rpm and a 500 volt armature, will be doing 900 rpm if you have 250 volts on the armature. This is why DC drives can operate as voltage regulators, and control the speed of a DC motor. It makes DC Drive repair much easier if you understand this. It keeps you from condemning a DC Drive, and going through a prolonged troubleshooting procedure, when there’s in fact nothing wrong with the drive to begin with. And, that happens a lot more than you might suspect. Attempting a DC Drive repair, when the DC Drive is fine.. is well.. useless.

DC Motors are inherently generators.. if we take this same motor, excite the field to its rated value, and spin it by external means to 1800 rpm, then the motor armature will have 500 volts present. If we lower the field, the voltage on the armature will decrease; if we raise it, the voltage will increase. (until roughly 750 -800 volts at which point it will likely flash over)

DC Drives can also be operated as speed regulators (as opposed to a voltage regulator) and can use a tachometer mounted on the motor shaft to give tighter speed control than what can be accomplished by armature voltage regulation alone.

So, armed with that bit of information.. let’s look at a couple of real life DC Drive repair examples involving both DC Motors and DC Drives.


DC Drive Repair example 1:

A motor is drawing much higher armature amps than it should. You measure the 3 phase input to the drive, and the current is generally balanced. (by the way, this is always a good first check.. if the current is severely imbalanced, you probably have SCRs not firing in the drive output. We call this condition “single phasing” and the decreased ripple frequency can cause issues with the DC motor)

If the amps into the drive are balanced, the DC Drive is probably fine, and does not need repair. In general, drives supply voltage and motors draw current. So if the motor is drawing higher amps, it usually means that either the torque required to move the load has increased, OR, the DC motor’s ability to produce torque has decreased.

What’s the first check? Remember my first point at the beginning of this blog.. motor torque is the armature flux times the field flux. Since we’re seeing high armature amps, we assume there’s plenty of armature flux, so, what’s left?

CHECK THE FIELD! This is a simple step that’s often overlooked in DC Drive repair scenarios, and results in a lot of un-necessary downtime and trouble. If the field is OK, then its probably a mechanical problem with the load.


DC Drive Repair example 2:

The motor will not reach top speed. (let’s assume the DC Drive is set up as a voltage regulator)

First check… armature current.. are you drawing more than full load amps? If so, then you’re back to the situation we just discussed prior to this. If you’re not, is the current limit setting in the drive set too low? The drive’s current limit setting will limit the armature current. If too low, it may prevent the motor from reaching top speed. Usually, it will also result in an unstable speed, but not always.

If current is within ratings, and the drive is not in current limit, then check Armature Voltage.. IF it’s reading 500 volts, and your speed is too low, what could that mean?

Remember that a DC motor’s speed is directly proportional to armature voltage PROVIDING THE FIELD FLUX IS CONSTANT.. so what’s the next check? How about the field?

IF the field is too high, and this happens, the speed will be lower for any given armature voltage. High field amps can also cause other issues, such as motor overheating and at some point a burned field, but in the short term, it will affect speed in a voltage regulated DC Drive. (by the way.. the motor will have INCREASED torque abilities.. at least in the short term)

EMA regularly performs DC Drive repairs.. we also sell and apply new drives and motors. Contact us!

See your nearest EMA facility at the bottom of this page, or contact us via the form on the right side, OR, just click the Live Help button. We’re happy to talk with you on the phone about your DC Drive repair issue.

Visit our Field Service page, or our Shop Service page.


2 Responses

  1. Troubleshooting a Fincor 5126-75-3cx . Intermittent speed fluctuation. Speed drops off then overdrives then settles at set point again. Your website gave me some new ideas towards narrowing down to the problem. Thank You.
    P.S. motors are driving a Goss Metro Liner Offset Printing Press

    1. Your series field could be bucking in one of the motors. BRING ONE ON AT A TIME BY CLUTCHING UNITS IN AND OUT. or your master trigger signal could be dropping. Try switching triggers in master cabinet, or switching the drive feedback selector in master with press at a standstill. STILL NO LUCK THEN SELECT EVERYTHING TO LEFT OR RIGHT FOLDER. CHECK 3 PHASE POWER AND AMPS SHOULD BE BALANCED.

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