One of the disadvantages of DC motors is the high start-up or inrush current. When running, a DC motor generates an internal back-emf (electro-magnetic force) that acts to reduce the overall drive current. This back-emf arises from Faraday’s Law of Induction and is inherent in the motor construction, which is very similar to a generator. The generated back-emf opposes the driving emf and the difference represents the useful work that can be done by the motor.
Medical Grade Power Supply is Suitable for DC Motor Drives
Aug 23, 2024
In many medical applications, powerful DC motors are needed. For example, for heavy duty lifting, rotating, or tilting of medical beds and operating tables, to maneuver precision surgical robots or to control air pressure in ventilators. DC motors have the advantage of very high power-to-weight ratios, good reliability, and high torque at low speeds, so they are ideal for fitting inside robotic arms or bed mechanisms and, with suitable gearing, can also be used to pump fluids very accurately in dialysis and transfusion equipment.
However, at standstill, a stationary rotor offers no generator function, and the motor drive current is limited only by the DC resistance of the windings. The inrush current can easily be three times the running current, if only for a very short time until the motor spins up (Figure 1).
One of the ways to reduce the peak motor start-up current is to fit a PTC (positive temperature coefficient) resistor in series with the motor. PTCs are devices that have a higher resistance when cold than when they are hot, so they initially throttle the high inrush current until they warm up and then let the nominal current through with minimal impedance.
For the following example we will use a 24 VDC, 125W motor with a three times higher inrush current for 200ms and try to reduce the peak current by 50% (green curve in Fig. 1).
One of the ways to reduce the peak motor start-up current is to fit a PTC (positive temperature coefficient) resistor in series with the motor. PTCs are devices that have a higher resistance when cold than when they are hot, so they initially throttle the high inrush current until they warm up and then let the nominal current through with minimal impedance.
For the following example we will use a 24 VDC, 125W motor with a three times higher inrush current for 200ms and try to reduce the peak current by 50% (green curve in Fig. 1).
The necessary specification for the PTC can be found from the following relationships:
Step 1: find nominal current and inrush current:
Step 2: Calculate the energy rating of the PTC:
Step 3: Calculate the resistance of the PTC needed to reduce the peak inrush current by 50%:
A suitable inrush limiting device would be a 3 Ohm NTC with at least 100 J energy rating and a steady current rating of 6A or more. Even with the peak inrush current limited to only 50% of the peak inrush current, power supply still needs to be able to deliver this short duration start up current.
So, instead of a 125W power supply needed to deliver the steady power consumption, a 200W power supply would be needed to deliver the high inrush current. Using a PTC with a higher resistance to reduce the inrush current further is not a good idea because the increased series resistance will affect the performance of the motor and risk stalling it under high load conditions. A better alternative to over-dimensioning the power supply just to cope with the inrush current would be to select one that can deliver the temporary high inrush current and yet still be short-circuit protected with load faults.
RECOM’s new RACM140E series is just such a power supply. It can deliver 140W continuously and 210W boost power for up to 10 seconds, easily enough to handle the peak start-up and any short-term motor stall current. It also has full protection against continuous output short circuit and over current conditions. Another possible problem with the back-emf generated by DC motors is that under certain conditions such as a sudden disconnection of the load or harsh deceleration, the kinetic energy of the motor and geartrain turns the motor into a generator and the back-emf exceeds the driving emf and the voltage across the motor rises.
Step 1: find nominal current and inrush current:
Step 2: Calculate the energy rating of the PTC:
Step 3: Calculate the resistance of the PTC needed to reduce the peak inrush current by 50%:
A suitable inrush limiting device would be a 3 Ohm NTC with at least 100 J energy rating and a steady current rating of 6A or more. Even with the peak inrush current limited to only 50% of the peak inrush current, power supply still needs to be able to deliver this short duration start up current.
So, instead of a 125W power supply needed to deliver the steady power consumption, a 200W power supply would be needed to deliver the high inrush current. Using a PTC with a higher resistance to reduce the inrush current further is not a good idea because the increased series resistance will affect the performance of the motor and risk stalling it under high load conditions. A better alternative to over-dimensioning the power supply just to cope with the inrush current would be to select one that can deliver the temporary high inrush current and yet still be short-circuit protected with load faults.
RECOM’s new RACM140E series is just such a power supply. It can deliver 140W continuously and 210W boost power for up to 10 seconds, easily enough to handle the peak start-up and any short-term motor stall current. It also has full protection against continuous output short circuit and over current conditions. Another possible problem with the back-emf generated by DC motors is that under certain conditions such as a sudden disconnection of the load or harsh deceleration, the kinetic energy of the motor and geartrain turns the motor into a generator and the back-emf exceeds the driving emf and the voltage across the motor rises.
The excessive energy can be absorbed by a capacitor placed across the motor (C1), but this will also have the negative effect of increasing the inrush current on motor start-up, so it cannot have too much capacitance. A better solution would be to dissipate the back-emf voltage in a braking resistor placed across the motor that is switched into circuit by Q1 only when the motor voltage exceeds the driving voltage (Figure 2).
The AC/DC power supply must be able to handle the back-emf generated motor voltage with either the simple motor capacitor or the more complex switched braking resistor solution.
The RACM140E series has output over-voltage protection which is set significantly higher than the nominal output voltage so that it can safely cope with motor-generated back-emf voltages (Table 1).
The “M” in the RACM140E-K name stands for ‘Medical’ as this series is certified for use in medical applications, meaning that it has 2MOPP (two means of patient protection) reinforced isolation as defined by the medical safety standards EN/IEC 60601-1 and ANSI/AAMI ES 60601-1 and it complies with the medical EMC standard EN 60601-1-2. The output leakage current to ground is below 300µA making is suitable for Body Floating (BF) patient connection, and the touch current is less than 100µA in normal operation.
The AC/DC power supply must be able to handle the back-emf generated motor voltage with either the simple motor capacitor or the more complex switched braking resistor solution.
The RACM140E series has output over-voltage protection which is set significantly higher than the nominal output voltage so that it can safely cope with motor-generated back-emf voltages (Table 1).
| Parameter | Model | Output Voltage | OVP Level |
|---|---|---|---|
| Over-Voltage Protection (Hiccup mode) |
RACM140E-12SK | 12V | 30V |
| RACM140E-15SK | 15V | 30V | |
| RACM140E-24SK | 24V | 40V | |
| RACM140E-36SK | 36V | 48V | |
| RACM140E-48SK | 48V | 65V |
Table 1: RACM140E series output Over Voltage Protection (OVP) levels
The “M” in the RACM140E-K name stands for ‘Medical’ as this series is certified for use in medical applications, meaning that it has 2MOPP (two means of patient protection) reinforced isolation as defined by the medical safety standards EN/IEC 60601-1 and ANSI/AAMI ES 60601-1 and it complies with the medical EMC standard EN 60601-1-2. The output leakage current to ground is below 300µA making is suitable for Body Floating (BF) patient connection, and the touch current is less than 100µA in normal operation.
The RACM140E power supply is available with Molex-type connectors, push-in fittings, or spade terminals and can be panel or chassis-mounted. The low height of 40mm or less means they also fit inside 1HE rack-mount enclosures. Besides medical certification, the RACM140E is also certified to the household and industrial safety and EMC standards with overvoltage category OVC III at up to 2000m or OVC II for operation up to 5000 meters altitude.
Dimensions:
To simplify system integration, the RACM140E series features ample margins to EN55032 ‘Class B’ EMC limits with increased surge and burst immunity protection levels. The input voltage range is universal, from 80-264 VAC or 120-370 VDC, and the nominal output voltages of 12, 15, 24, 36, or 48 VDC can be adjusted by up to +20% via an on-board trim potentiometer.
With its compact size, cost-effectiveness, high peak load capability, and comprehensive range of medical, household, and industrial safety and EMC certifications, the RACM140E will find many uses besides powering DC motors, such as building and home automation, medical robots, data and telecom, industrial automation, intralogistics, and test & measurement applications.
Dimensions:
- Enclosed: 147 x 81.5 x 40 mm
- Open Frame: 147 x 81.5 x 38 mm
To simplify system integration, the RACM140E series features ample margins to EN55032 ‘Class B’ EMC limits with increased surge and burst immunity protection levels. The input voltage range is universal, from 80-264 VAC or 120-370 VDC, and the nominal output voltages of 12, 15, 24, 36, or 48 VDC can be adjusted by up to +20% via an on-board trim potentiometer.
With its compact size, cost-effectiveness, high peak load capability, and comprehensive range of medical, household, and industrial safety and EMC certifications, the RACM140E will find many uses besides powering DC motors, such as building and home automation, medical robots, data and telecom, industrial automation, intralogistics, and test & measurement applications.
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